Mimicry or pseudo-sematic colours

The fact that animals with distant affinities may more or less closely resemble each other was observed long before the existing explanation was possible. Its recognition is implied in a number of insect names with the termination -formis, usually given to species of various orders which more or less closely resemble the stinging hymenoptera. The usefulness of the resemblance was suggested in Kirby and Spences Introduction to Entomology, London, 1817, ii. 223. H.W. Bates (Trans. Linn. Soc. vol. XXiII., 1862, p. 495) first proposed an explanation of mimicry based on the theory of natural selection. He supposed that every step in the formation and gradual improvement of the likeness occurred in consequence of its usefulness in the struggle for life. This was one of the first attempts to apply the theory of natural selection to a large class of phenomena up to that time well known but unexplained. Numerous examples of mimicry among tropical American butterflies were discussed by Bates in his paper; and in 1866 Wallace extended the hypothesis to the butterflies of the tropical East (Trans. Linn, Soc. vol. xxv., 1866, p. 19). The term mimicry is used in various senses. It is often extended, as indeed it was by Bates, to include all the superficial resemblances between animals and any part of their environment. Wallace, however, separated the cryptic resemblances already described, and the majority of naturalists have followed this convenient arrangement. In cryptic resemblance an animal resembles some object of no interest to its enemy (or prey), and in so doing is concealed; in mimicry an animal resembles some other animal which is specially disliked by its enemy, or some object which is specially attractive to its prey, and in so doing becomes conspicuous. Some naturalists have considered mimicry to include all superficial likenesses between animals, but such a classification would group together resemblances which have widely different uses.
The resemblance of a mollusc to the coral on which it lives, or an external parasite to the hair or skin of its host, would be procryptic; that between moths which resemble lichen, syncryptic; between distasteful insects, synaposematic; between the Insectivor mole and the Rodent mole-rat, syntechnic; the essential element in mimicry is that it is a false warning (pseud-aposematic) or false recognition (pseudepisematic) character.
Some have considered that mimicry indicates resemblance to a moving object; but apart from the non-mimetic likenesses between animals classified above, there are ordinary cryptic resemblances to drifting leaves, swaying bits of twig, etc., while truly mimetic resemblances are often specially adapted for the attitude of rest. Many use the term mimicry to include synaposematic as well as pseudo-sematic resemblances, calling the former Müllerian, the latter Batesian, mimicry. The objection to this grouping is that it takes little account of the deceptive element which is essential in mimicry. In synaposematic colouring the warning is genuine, in pseudaposematic it is a sham. The term mimicry has led to much misunderstanding from the fact that in ordinary speech it implies deliberate imitation. The production of mimicry in an individual animal has no more to do with consciousness or taking thought than any of the other processes of growth. Protective mimicry is here defined as an advantageous and superficial resemblance of one animal to another, which latter is specially defended so as to be disliked or feared by the majority of enemies of the groups to which both belong. Resemblance which appeals to the sense of sight, sometimes to that of hearing, and rarely to smell, but does not extend to deep-seated characters except when the superficial likeness is affected by them. Mutatis mutandis, this definition will apply to aggressive (pseudepisematic) resemblance. The conditions under which mimicry occurs have been stated by Wallace.

1. that the imitative species occur in the same area and occupy the same station as the imitated;
2. that the imitators are always the more defenceless;
3. that the imitators are always less numerous in individuals;
4. that the imitators differ from the bulk of their allies;
5. that the imitation, however minute, is external and visible only, never extending to internal characters or to such as do not affect the external appearance.

It is obvious that conditions 2 and 3 do not hold in the case of Müllerian mimicry. Mimicry has been explained, independently of natural selection, by the supposition that it is the common expression of the direct action of common causes, such as climate, food, etc.; also by the supposition of independent lines of evolution leading to the same result without any selective action in consequence of advantage in the struggle; also by the operation of sexual selection.
It is proposed, in conclusion, to give an account of the broad aspects of mimicry, and attempt a brief discussion of the theories of origin of each class of facts (see Poulton, Linn. Soc. Journ. Zool., 1898, p. 558). It will be found that in many cases the argument here made use of applies equally to the origin of cryptic and sematic colours. The relationship between these classes has been explained: mimicry is, as Wallace has stated (Darwinism, London, 1889), merely an exceptional form of protective resemblance. Now, protective (cryptic) resemblance cannot be explained on any of the lines suggested above, except natural selection; even sexual selection fails, because cryptic resemblance is especially common in the immature stages of insect life. But it would be unreasonable to explain mimetic resemblance by one set of principles and cryptic by another and totally different set. Again, it may be plausible to explain the mimicry of one butterfly for another on one of the suggested lines, but the resemblance of a fly or moth to a wasp is by no means so easy, and here selection would be generally conceded; yet the appeal to antagonistic principles to explain such closely related cases would only be justified by much direct evidence. Furthermore, the mimetic resemblances between butterflies are not haphazard, but the models almost invariably belong only to certain sub-families, the Danainae and Acraeinae in all the warmer parts of the world, and, in tropical America, the Ithomiinae and Heliconiinae as well. These groups have the characteristics of aposematic species, and no theory but natural selection explains their invariable occurrence as models wherever they exist. It is impossible to suggest, except by natural selection, any explanation of the fact that mimetic resemblances are confined to changes which produce or strengthen a superficial likeness. Very deep-seated changes are generally involved, inasmuch as the appropriate instincts as to attitude, etc., are as important as colour and marking. The same conclusion is reached when we analyse the nature of mimetic resemblance and realize how complex it really is, being made up of colours, both pigmentary and structural, pattern, form, attitude and movement. A plausible interpretation of colour may be wildly improbable when applied to some other element, and there is no explanation except natural selection which can explain all these elements. The appeal to the direct action of local conditions in common often breaks down upon the slightest investigation, the difference in habits between mimic and model in the same locality causing the most complete divergence in their conditions of life. Thus many insects produced from burrowing larvae mimic those whose larvae live in the open. Mimetic resemblance is far commoner in the female than in the male, a fact readily explicable by selection, as suggested by Wallace, for the female is compelled to fly more slowly and to expose itself while laying eggs, and hence a resemblance to the slow-flying freely exposed models is especially advantageous. The facts that mimetic species occur in the same locality, fly at the same time of the year as their models, and are day-flying species even though they may belong to nocturnal groups, are also more or less difficult to explain except on the theory of natural selection, and so also is the fact that mimetic resemblance is produced in the most varied manner. A spiderant, by a modification of its body-form into a superficial resemblance, and by holding one pair of legs to represent antennae; certain bugs (Hemiptera) and beetles have also gained a shape unusual in their respective groups, a shape which superficially resembles an ant; a Locustid (Myrmecophana) has the shape of an ant painted, as it were, on its body, all other parts resembling the background and invisible; a Membracid (Homoptera) is entirely unlike an ant, but is concealed by an ant-like shield. When we further realize that in this and other examples of mimicry the likeness is almost always detailed and remarkable, however it is attained, while the methods differ absolutely, we recognize that natural selection is the only possible explanation hitherto suggested. In the cases of aggressive mimicry an animal resembles some object which is attractive to its prey. Examples are found in the flower-like species of mantis, which attract the insects on which they feed. Such cases are generally described as possessing alluring colours, and are regarded as examples of aggressive (anticryptic) resemblance, but their logical position is here. resembles its model, an
Darwin suggested the explanation of these appearances in his theory of sexual selection (The Descent of Man, London, 1874). The rivalry of the males for the possession of the females he believed to be decided by the preference of the latter for those individuals with especially bright colours, highly developed plumes, beautiful song, etc. Wallace did not accept the theory, but believed that natural selection, either directly or indirectly, accounts for all the facts. Probably the majority of naturalists follow Darwin in this respect. The subject is most difficult, and the interpretation of a great proportion of the examples in a high degree uncertain, so that a very brief account is here expedient. That selection of some kind has been operative is indicated by the diversity of the elements into which the effects can be analysed. The most complete set of observations on epigamic display was made by George W and Elizabeth G Peckham upon spiders of the family Attidae (Nat. Hist. Soc. of Wisconsin, vol. i., 1889). These observations afforded the authors conclusive evidence that the females pay close attention to the love-dances of the males, and also that they have not only the power, but the will, to exercise a choice among the suitors for their favour. Epigamic characters are often concealed except during courtship; they are found almost exclusively in species which are diurnal or semi-diurnal in their habits, and are excluded from those parts of the body which move too rapidly to be seen. They are very commonly directly associated with the nervous system; and in certain fish, and probably in other animals, an analogous heightening of effect accompanies nervous excitement other than sexual, such as that due to fighting or feeding. Although there is epigamic display in species with sexes alike, it is usually most marked in those with secondary sexual characters specially developed in the male. These are an exception to the rule in heredity, in that their appearance is normally restricted to a single sex, although in many of the higher animals they have been proved to be latent in the other, and may appear after the essential organs of sex have been removed or become functionless. This is also the case in the Aculeate Hymenoptera when the reproductive organs have been destroyed by the parasite Stylops. Wallace suggested that they are in part to be explained as recognition characters, in part as an indication of surplus vital activity in the male. More recent theories by the likes of W. D. Hamilton and Amotz Zahavi (see handicap principle) have also been proposed.

Warning and signalling (semantic colours)

Warning colouration is the exact opposite of camouflage, its function being to render the animal conspicuous to its enemies, so that it can be easily seen, well remembered, and avoided in future.

Warning colours are associated with some quality or weapon which renders the possessor unpleasant or dangerous, such as unpalatability, an evil odour, a sting, the poison-fang, etc. The object being to warn an enemy off, these colours are also called aposematic.

Recognition markings, on the other hand, are episematic, assisting the individuals of the same species to keep together when their safety depends upon numbers, or easily to follow each other to a place of safety, the young and inexperienced benefiting by the example of the older. Episematic characters are far less common than aposematic, and these than cryptic; although, as regards the latter comparison, the opposite impression is generally produced from the very fact that concealment is so successfully attained.

Warning or aposematic colours, together with the qualities they indicate, depend, as a rule, for their very existence upon the abundance of palatable food supplied by the animals with cryptic colouring (the models). Unpalatability, or even the possession of a sting, is not sufficient defence unless there is enough food of another kind to be obtained at the same time and place (Poulton, Proc. Zool. Soc., 1887, p. 191). Hence insects with warning colours are not seen in temperate countries except at the time when insect life as a whole is most abundant; and in warmer countries, with well-marked wet and dry seasons, it will probably be found that warning colours are proportionately less developed in the latter.

In many species of African butterflies belonging to the genus Junonia, including the subgenus Precis, the wet-season broods are distinguished by the more or less conspicuous under sides of the wings, those of the dry season being highly cryptic. Warning colours are, like cryptic, assisted by special adaptations of the body-form, and especially by movements which assist to render the colour as conspicuous as possible. On this account animals with warning colours generally move or fly slowly, and it is the rule in butterflies that the warning patterns are similar on both upper and under sides of the wings.

Many animals, when attacked or disturbed, sham death (as it is commonly but wrongly described), falling motionless to the ground. In the case of well-concealed animals this instinct gives them a second chance of escape in the earth or among the leaves, etc., when they have been once detected; animals with warning colours are, on the other hand, enabled to assume a position in which their characters are displayed to the full (J. Portschinsky, Lepidopterorum Rossiae Biologia, St Petersburg, 1890, plate i. figs. 16, 17). In both cases a definite attitude is assumed, which is not that of death.

Other warning characters exist in addition to colouring: thus sound is made use of by the disturbed rattlesnake and the Indian Ec/jis, etc. Large birds, when attacked, often adopt a threatening attitude, accompanied by a terrifying sound. The cobra warns an intruder chiefly by attitude and the dilation of the flattened neck, the effect being heightened in some species by the spectacles. In such cases we often see the combination of cryptic and sematic methods, the animal being concealed until disturbed, when it instantly assumes an aposematic attitude. The advantage to the animal itself is clear: a poisonous snake gains nothing by killing an animal it cannot eat; while the poison does not cause immediate death, and the enemy would have time to injure or destroy the snake.

In the case of small unpalatable animals with warning colours the enemies would only first become aware of the unpleasant quality by tasting and often destroying their prey; but kin of the organism killed may gain by the experience thus conveyed, even though the individual might suffer. An insect-eating animal does not come into the world with knowledge: it has to learn by experience, and warning colours enable this education as to what to avoid to be gained by a small instead of a large waste of life. Furthermore, great tenacity of life is usually possessed by animals with warning colours. The tissues of aposematic insects generally possess great elasticity and power of resistance, so that large numbers of individuals can recover after very severe treatment.

The brilliant warning colours of many caterpillars attracted the attention of Charles Darwin when he was thinking over his hypothesis of sexual selection, and he wrote to AR Wallace on the subject (C Darwin, Life and Letters, London, 1887, uI. 93). Wallace, in reply, suggested their interpretation as warning colours, a suggestion since verified by experiment (Proc. Ent. Soc. Lond., 1867, p. lxxx; Trans. Ent. Soc. Loud., 1869, pp. 21 and 27). Although animals with warning colours are probably but little attacked by the ordinary enemies of their class, they have special enemies which keep the numbers down to the average. Thus the cuckoo appears to be an insectivorous bird which will freely devour conspicuously coloured unpalatable larvae. The effect of the warning colours of caterpillars is often intensified by gregarious habits. Another aposematic use of colours and structures is to divert attention from the vital parts, and thus give the animal attacked an extra chance of escape. The large, conspicuous, easily torn wings of butterflies and moths act in this way, as is found by the abundance of individuals which may be captured with notches bitten symmetrically out of both wings when they were in contact. The eye-spots and tails so common on the hinder part of the hind wing, and the conspicuous apex so frequently seen on the fore wing, probably have this meaning. Their position corresponds to the parts which are most often found to be notched. In some cases (e.g. many Lycaenidae) the tail and eye-spot combine to suggest the appearance of a head with antennae at the posterior end of the butterfly, the deception being aided by movements of the hind wings (see automimicry). The flat-topped tussocks of hair on many caterpillars look like conspicuous fleshy projections of the body, and they are held prominently when the larva is attacked. If seized, the tussock comes out, and the enemy is greatly inconvenienced by the fine branched hairs. The tails of lizards, which easily break off, are to be similarly explained, the attention of the pursuer being probably still further diverted by the extremely active movements of the amputated member. Certain crabs similarly throw off their claws when attacked, and the claws continue to snap most actively. The tail of the dormouse, which easily comes off, and the extremely bushy tail of the squirrel, are probably of use in the same manner. Animals with warning colours often tend to resemble each other superficially.

This fact was first pointed out by Henry W. Bates in his paper on the theory of mimicry (Trans. Linn. Soc. vol. xxiii., 1862, p. 495). He showed that the conspicuous, presumably unpalatable, tropical American butterflies, belonging to very different groups, which are mimicked by others, also tend to resemble each other, the likeness being often remarkably exact. These resemblances were not explained by his theory of mimicry, and he could only suppose that they had been produced by the direct influence of a common environment. The problem was solved in 1879 by Fritz Müller (see Proc. Ent. Soc. Lond., 1879, p. xx.), who suggested that life is saved by this resemblance between warning colours, inasmuch as the education of young inexperienced enemies is facilitated. Each species which falls into a group with common warning (synaposematic) colours contributes to save the lives of the other members. It is sufficiently obvious that the amount of learning and remembering, and consequently of injury and loss of life involved in the process, are reduced when many species in one place possess the same aposematic colouring, instead of each exhibiting a different danger-signal. These resemblances are often described as Mullerian mimicry, as distinguished from true or Batesian mimicry described in the next section. Similar synaposematic resemblances between the specially protected groups of butterflies were afterwards shown to exist in tropical Asia, the East Indian Islands and Polynesia by F Moore (Proc. Zool. Soc., 1883, p. 201), and in Africa by EB Poulton (Report Brit. Assoc., 1897, p. 688). R Meldola (Ann. and Mag. Nat. Hist. X., 1882, p. 417) first pointed out and explained in the same manner the remarkable general uniformity of colour and pattern which runs through so many species of each of the distasteful groups of butterflies; while, still later, Poulton (Proc. Zool. Soc., 1887, p. 191) similarly extended the interpretation to the synaposematic resemblances between animals of all kinds in the same country. Thus, for example, longitudinal or circular bands of the same strongly contrasted colours are found in species of many groups with distant affinities.

Certain animals, especially the Crustacea, make use of the special defence and warning colours of other animals. Thus the English hermit-crab, Pagurus bernhardus, commonly carries the sea-anemone, Sagartia parasitica, on its shell; while another English species, Pagurus pridauxii, inhabits a shell which is invariably clothed by the flattened anemone, Adamsia palliata.

The white patch near the tail which is frequently seen in the gregarious ungulates, and is often rendered conspicuous by adjacent black markings, probably assists the individuals in keeping together; and appearances with probably the same interpretation are found in many birds. The white upturned tail of the rabbit is probably of use in enabling the individuals to follow each other readily. The difference between a typical aposematic character appealing to enemies, and episematic intended for other individuals of the same species, is well seen when we compare such examples as (1) the huge banner-like white tail, conspicuously contrasted with the black or black and white body, by which the slow-moving skunk warns enemies of its power of emitting an intolerably offensive odour; (2) the small upturned white tail of the rabbit, only seen when it is likely to be of use and when the owner is moving, and, if pursued, very rapidly moving, towards safety.

Animal colouration

Animal colouration has been a topic of interest and research in biology for well over a century. Colours may be cryptic (functioning as an adaptation allowing the prevention of prey detection; aposematic (functioning as a warning of unprofitability) or may be the result of sexual selection. Colouration may also be function in mimicry of other organisms. The subject may be investigated in terms of both the chemical and physical basis of the colours (proximate cause) and the evolution of colouration (ultimate cause).

Camouflage is generally viewed as a result of natural selection, and involves an organism's colour blending in with its biotic (e.g. moss) or abiotic (e.g. sand) surroundings. Camouflage is often accompanied by behavioural adaptations that make the most of it, such as landing on areas of similar colour, and aligning the body correctly. It may involve costs as well as benefits, such as the cost of finding a suitable resting spot. Colour may change during the seasons, during an organism's life cycle, or even over very brief intervals, such as with the chameleon. Polymorphism may also occur, allowing individuals of the same species to have different camouflage, and making prey detection more difficult for predators. Organisms living in the same environment may come to have similar colouration through convergent evolution. Colours are an aspect of only one of the senses, and although the visual system is most important for humans, some animals cannot even see (such as those living in caves, underground, in the deep sea, or those active at night) and their colour may be of little or no adaptive value. These organisms rely primarily on other senses, such as olfaction and hearing, and even electroreception.Contents.

A camouflaged Jumping spider can easily capture prey.

The Goldenrod Crab Spider (Misumena vatia) has the capacity of changing colour by secreting a liquid yellow pigment into the outer cell layer of the body.
Main article: Camouflage

Cryptic colouration has evolved in many species that have been subjected to the pressures of predation and also in predatory species. Such colours help predators (aggressive resemblance or anticryptic colouring) and prey (protective resemblance or procryptic colouring). Protective resemblance is far commoner among animals than aggressive resemblance, in correspondence with the fact that predaceous forms are as a rule much larger and much less numerous than their prey. In the case of insectivorous vertebrates and their prey such differences exist in an exaggerated form. Cryptic colouring, whether used for defence or attack, may be either general or special. In general resemblance the animal, in consequence of its colouring, produces the same effect as its environment, but the conditions do not require any special adaptation of shape and outline. General resemblance is especially common among the animals inhabiting some uniformly coloured expanse of the Earth's surface, such as an ocean or a desert. In the former, animals of all shapes are frequently protected by their transparent blue colour; on the latter, equally diverse forms are defended by their sandy appearance. The effect of a uniform appearance may be produced by a combination of tints in startling contrast. Thus the black and white stripes of the zebra blend together at a little distance, and their proportion is such as exactly to match the pale tint which arid ground possesses when seen by moonlight (F Galton, South Africa, London, 1889).

Special resemblance is far commoner than general, and is the form which is usually met with on the diversified surface of the earth, on the shores, and in shallow water, as well as on the floating masses of algae on the surface of the ocean, such as the Sargasso Sea. In these environments the cryptic colouring of animals is usually aided by special modifications of shape, and by the instinct which leads them to assume particular attitudes. Complete stillness and the assumption of a certain attitude play an essential part in general resemblance on land; but in special resemblance the attitude is often highly specialized, and perhaps more important than any other element in the complex method by which concealment is effected. In special resemblance the combination of colouring, shape and attitude is such as to produce a more or less exact resemblance to some one of the objects in the environment, such as a leaf or twig, a patch of lichen, or flake of bark. In all cases the resemblance is to some object which is of no interest to the enemy or prey respectively. The animal is not hidden from view by becoming indistinguishable from its background, as in the cases of general resemblance, but it is mistaken for some well-known object.

In the past these effects were explained as a result of the direct influence of the environment upon the individual (GLL Georges-Louis Leclerc, Comte de Buffon), or by the inherited effects of effort and the use and disuse of parts (JEP Jean-Baptiste Lamarck), but natural selection, which can accumulate any and every variation which tends towards survival, has been the accepted explanation now for almost a century. A few of the chief types of methods by which concealment is effected may be briefly described. The colours of large numbers of vertebrate animals are darkest on the back, and become gradually lighter on the sides, passing into white on the belly. Abbott Handerson Thayer (The Auk, vol. xiii., 1896) has suggested that this gradation obliterates the appearance of solidity, which is due to shadow.

The colour-harmony, which is also essential to concealment, is produced because the back is of the same tint as the environment (e.g. earth) bathed in the cold blue-white of the sky, while the belly, being cold blue-white bathed in shadow and yellow earth reflections, produces the same effect. Thayer has made models (in the natural history museums at London, Oxford and Cambridge) which support his interpretation in a very convincing manner. This method of neutralizing shadow for the purpose of concealment by increased lightness of tint was first suggested by EB Poulton in the case of a larva (Trans. Ent. Soc. Loud., 1887, p. 294) and a pupa (Trans. Ent. Soc. Loud., 1888, pp. 596, 597), but he did not appreciate the great importance of the principle. In an analogous method an animal in front of a background of dark shadow may have part of its body obliterated by the existence of a dark tint, the remainder resembling, e.g., a part of a leaf (W Müller, Zool. Jahr. JW Spengel, Jena, 1886).

A camouflaged Orange Oak Leaf butterfly (centre)

This method of rendering invisible any part which would interfere with the resemblance is well known in mimicry. A common aid to concealment is the adoption by different individuals of two or more different appearances, each of which resembles some special object to which an enemy is indifferent. Thus the leaf-like butterflies (Kallima) present various types of colour and pattern on the under side of the wings, each of which closely resembles some well-known appearance presented by a dead leaf; and the common British yellow under-wing moth (Tryphaena pronuba) is similarly polymorphic on the upper side of its upper wings, which are exposed as it suddenly drops among dead leaves. Caterpillars and pupae are also commonly dimorphic, green and brown. Such differences as these extend the area which an enemy is compelled to search in order to make a living. In many cases the cryptic colouring changes appropriately during the course of an individual life, either seasonally, as in the ptarmigan or Alpine Hare, or according as the individual enters a new environment in the course of its growth (such as larva, pupa, imago, etc.). In insects with more than one brood in the year, seasonal dimorphism is often seen, and the differences are sometimes appropriate to the altered condition of the environment as the seasons change. The causes of change in these and Arctic animals are insufficiently worked out: in both sets there are observations or experiments which indicate changes from within the organism, merely following the seasons and not caused by them, and other observations or experiments which prove that certain species are susceptible to the changing external influences. In certain species concealment is effected by the use of adventitious objects, which are employed as a covering. Examples of this allocryptic defence are found in the tubes of the caddis fly larvae (Trichoptera), or the objects made use of by crabs of the genera Hyas, Stenorhynchus, etc. Such animals are concealed in any environment. If sedentary, like the former example, they are covered up with local materials; if wandering, like the latter, they have the instinct to reclothe. Allocryptic methods may also be used for aggressive purposes, as the ant-lion larva, almost buried in sand, or the large frog Ceratophrys, which covers its back with earth when waiting for its prey. Another form of allocryptic defence is found in the use of the colour of the food in the digestive organs showing through the transparent body, and in certain cases the adventitious colour may be dissolved in the blood or secreted in superficial cells of the body: thus certain insects make use of the chlorophyll of their food (Poulton, Proc. Roy. Soc. liv. 417). The most perfect cryptic powers are possessed by those animals in which the individuals can change their colours into any tint which would be appropriate to a normal environment. This power is widely prevalent in fish, and also occurs in Amphibia and Reptilia (the chameleon affording a well-known example). Analogous powers exist in certain Crustacea and Cephalopoda. All these rapid changes of colour are due to changes in shape or position of superficial pigment cells controlled by the nervous system. That the latter is itself stimulated by light through the medium of the eye and optic nerve has been proved in many cases. Animals with a short life-history passed in a single environment, which, however, may be very different in the case of different individuals, may have a different form of variable cryptic colouring, namely, the power of adapting their colour once for all (many pupae), or once or twice (many larvae). In these cases the effect appears to be produced through the nervous system, although the stimulus of light probably acts on the skin and not through the eyes. Particoloured surfaces do not produce particoloured pupae, probably because the antagonistic stimuli neutralize each other in the central nervous system, which then disposes the superficial colours so that a neutral or intermediate effect is produced over the whole surface (Poulton, Trans. Ent. Soc. Lond., 1892, p. 293).

Cryptic colouring may incidentally produce superficial resemblances between animals; thus desert forms concealed in the same way may gain a likeness to each other, and in the same way special resemblances, e.g. to lichen, bark, grasses, pine-needles, etc., may sometimes lead to a tolerably close similarity between the animals which are thus concealed. Such, likeness may be called syncryptic or common protective (or aggressive) resemblance, and it is to be distinguished from mimicry and common warning colours, in which the likeness is not incidental, but an end in itself. Syncryptic resemblances have much in common with those incidentally caused by functional adaptation, such as the mole-like forms produced in the burrowing Insectivora, Rodentia and Marsupialia. Such likeness may be called syntechnic resemblance, incidentally produced by dynamic similarity, just as syncryptic resemblance is produced by static similarity.

Nature Platyzoa

The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors. A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora. These groups share the presence of complex jaws, from which they are called the Gnathifera.

Lophotrochozoa

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida. The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented. Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore. These were traditionally grouped together as the lophophorates. but it now appears they are paraphyletic, some closer to the Nemertea and some to the Mollusca and Annelida. They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Bryozoa or moss animals.

Deuterostomes

Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes, the initial opening (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as starfish, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.
In addition to these, the deuterostomes also include the Hemichordata, or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group.
The Chaetognatha or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.

Groups of animals

The sponges (Porifera) were long thought to have diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores. Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum. However, a phylogenomic study in 2008 of 150 genes in 21 genera revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 genera. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.
Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny placozoans are similar, but they do not have a permanent digestive chamber.
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the deuterostomes and the protostomes, the latter of which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well.

Origin and fossil record

Animals are generally considered to have evolved from a flagellatedchoanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella. eukaryote. Their closest known living relatives are the
The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia. These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.
The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.
Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.
Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms. During the beginning of the Tonian period around 1 billion years ago, there was a decrease in Stromatolite diversity, which may indicate the appearance of grazing animals, since Stromatolites diversity increased when grazing animals went extinct at the End Permian and End Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.

Animal nutrition

All animals are heterotrophs, meaning that they feed directly or indirectly on other living things. They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.
Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey. The other main category of consumption is detritivory, the consumption of dead organic matter. It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.
Most animals feed indirectly from the energy of sunlight. Plants use this energy to convert sunlight into simple sugars using a process known as photosynthesis. Starting with the molecules carbon dioxide (CO₂) and water₂O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose₆H₁₂O₆) and releases oxygen (O₂). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as glycolysis. (H (C
Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight. Instead chemosynthetic archaea and bacteria form the base of the food chain.

Structure

With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development, it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

Reproduction and development

Nearly all animals undergo some form of sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova. These fuse to form zygotes, which develop into new individuals.
Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation.
A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.

Animal

Animals

Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdomAnimalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms for sustenance.

Most known animal phyla appeared in the fossil record as marine species during theCambrian explosion, about 542 million years ago.

The word "animal" comes from the Latin word animalis (meaning with soul, from anima, soul). In everyday colloquial usage, the word usually refers to non-human animals.^[1] Frequently, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.^[2] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.^[3]

Characteristics

Animals have several characteristics that set them apart from other living things. Animals areeukaryotic and mostly multicellular, which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae.^[6] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.All animals are motile, if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.

Forest Hills, Queens

Forest Hills
—  Neighborhoods of New York City  —
Station Square
Country	United States
State	New York
County	Queens
Population (2000)
- Total	70,204
- Density	29,252/sq mi (11,294.3/km2)
Diversity
- White	64.4%
- Asian & Pacific Islander	20.0%
- African American	2.0%
- Hispanic	10.4%
- Foreign-born	47.7%
Time zone	EST (UTC-5)
- Summer (DST)	EDT (UTC-4)
ZIP code	11375
Area code(s)	718, 347, 917

Forest Hills is a neighborhood in the borough of Queens in New York City.

Neighborhood

The neighborhood is home to upper-middle class residents, of whom the wealthier residents often live in the neighborhood's Forest Hills Gardens area. Historically, Forest Hills has been home to a large Jewish population, with more than ten thousand of them located in the area.
The community of Forest Hills was founded in 1906; before that, the area was known as Whitepot. In 1909, Margaret Olivia Slocum Sage, who founded the Russell Sage Foundation, bought 142 acres (0.57 km²) of land from the Cord Meyer Development Company. The original plan was to build good low-income housing and improve living conditions of the working poor. Grosvenor Atterbury, a renowned architect, was given the commission to design Forest Hills Gardens. The neighborhood was planned on the model of the garden communities of England. As a result, there are many Tudor-styleradio commercial offered homes in Forest Hills. homes in Forest Hills, most of which are now located in Forest Hills Gardens. However, there are currently a number of Tudor homes in particular areas of Forest Hills outside of the Gardens. What is credited as the world's first
The southern part of Forest Hills contains a particularly diverse mixture of upscale housing, ranging from single-family houses, attached townhouses, and both low-rise and high-rise apartment buildings. South of the Long Island Rail Road, the Forest Hills Gardens area is a private community that features some of the most expensive residential properties in Queens County. It was subject to restrictive covenants until the mid-1970s.
Forest Hills Gardens was named "Best Community" in 2007 by Cottage Living Magazine. The adjacent Van Court community also contains a number of detached single-family homes. There are also attached townhouses near the Westside Tennis Center and detached frame houses near Metropolitan Avenue. Finally, there are a number of apartment buildings scattered throughout the community. The most notable high-rise apartment buildings are The Continental on 108th St, Kennedy House, the Pinnacle, and the Windsor.
The north side of Forest Hills is home to the Cord Meyer community, which contains detached single-family homes. Teardowns and their replacement with larger single family residences has had a significant impact on the architectural integrity of the area. However, the Bukharian Jewish community advocating the changes say the bigger homes are needed for their large extended families.
Northern Forest Hills is a combination of low-rise apartment buildings and detached single-family homes. The majority of these buildings are owner-occupied co-operatives and condominiums.
On the northwestern edge of Forest Hills, on 62nd Drive, immediately adjacent to the Long Island Expressway is a NYCHA (New York City Housing Authority) low-income housing project that provoked controversy.among the residents in the more prestigious areas of Forest Hills when it was constructed in the early 1970s.
The main thoroughfare is the twelve-lane-wide Queens Boulevard. Metropolitan Avenue is known for its antique shops. Forest Hills is easily accessible by subway, rail, bus and car. The commercial heart of Forest Hills is a mile-long stretch of Austin Street between Yellowstone Boulevard and Ascan Avenue, where many restaurants, boutiques, and chain stores are established. Restaurants are diverse; diners can find nearly any cuisine they desire.
Forest Hills has the multiple-link Forest Hills – 71st Avenue subway station (E F M R trains) at the intersection of Continental Avenue and Queens Boulevard. The local 75th Avenue stop (E F trains) is also in the area, and some entrance/exits of the express Kew Gardens – Union Turnpike station (E F trains) service the southeastern portion of Forest Hills. The neighborhood also has a commuter train station, the Forest HillsLong Island Railroad, where Continental Avenue and Austin Street meet. station of the
Forest Hills was once the home of the U.S. Open tennis tournament. The event was held at the West Side Tennis Club before it moved to the USTA National Tennis Center in Flushing Meadows Park, about four miles away. When the Open was played at the tennis stadium, the tournament was commonly referred to merely as Forest Hills, just as All-England Lawn Tennis Association Championships are referred to, simply, as Wimbledon. In the 2001 motion picture, The Royal Tenenbaums, Luke Wilson's character plays a tennis match at the West Side Tennis Club in Forest Hills. Gene Hackman's character is also shown cruising on the premises. A pivotal scene in Alfred Hitchcock's 1951 film Strangers on a Train, in which the main character (played by Farley Granger) is a professional tennis player, features a lengthy championship game at the Club, with distinctive shots of the surrounding community.
Two monuments are erected in Forest Hills Gardens: a tribute to the victims of World War I, the "Great War"; and the mast of the Columbia, the winner of the America's Cup yacht races in both 1899 and 1901.

Forest Education

The first dedicated forestry school was established by Georg Hartig at Dillenburg in Germany in 1787, though forestry had been taught much earlier in central Europe.
In 1886, the first issue of Revista Pădurilor (Forestry Review) was published in Romania.
The first in North America, the Biltmore Forest School was established near Asheville, North Carolina, by Carl A. Schenck on September 1, 1898, on the grounds of George W. Vanderbilt'sBiltmore Estate. Another early school was the New York State College of Forestry, established at Cornell University just a few weeks later, in September 1898. Early 19th century North American foresters went to Germany to study forestry. Some early German foresters also emigrated to North America.
In South America the first forestry school was established in Brazil, specifically in Viçosa, Minas Gerais, and later moved to Curitiba, Paraná.
Today, an acceptably trained forester must be educated in general biology, botany, genetics, soil science, climatology, hydrology, economics and forest management. Education in the basics of sociology and political science is often considered an advantage.
In India, forestry education is imparted in the agricultural universities and in Forest Research Institutes (deemed universities). Four year degree programmes are conducted in these universities at the undergraduate level. Masters and Doctorate degrees are also available in these universities.
In the United States, postsecondary forestry education leading to a Bachelor's degree or Master's degree is accredited by the Society of American Foresters.
In Canada the Canadian Institute of Forestry awards silver rings to graduates from accredited university BSc programs, as well as college and technical programs.
The International Union of Forest Research Organizations is the only international organization that coordinates forest science efforts world-wide. Organizations such as the Forest Policy Education Network are dedicated to facilitating international forest politics and exchanging information on the subject.

Forestry plans

Foresters work for the timber industry, government agencies, conservation groups, local authorities, urban parks boards, citizens' associations, and private landowners. The forestry profession includes a wide diversity of jobs, with educational requirements ranging from college bachelor's degrees to PhDs for highly specialized work. Industrial foresters plan forest regeneration starting with careful harvesting. Urban foresters manage trees in urban green spaces. Foresters work in tree nurseries growing seedlings for woodland creation or regeneration projects. Foresters improve tree genetics. Forest engineers develop new building systems. Professional foresters measure and model the growth of forests with tools like geographic information systems. Foresters may combat insect infestation, disease, forest and grassland wildfire, but increasingly allow these natural aspects of forest ecosystems to run their course when the likelihood of epidemics or risk of life or property are low. Increasingly, foresters participate in wildlife conservation planning and watershed protection. Foresters have been mainly concerned with timber management, especially reforestation, maintaining forests at prime conditions, and fire control.

Foresters develop and implement forest management plans relying on tree inventories showing an area's topographical features as well as its distribution of trees (by species) and other plant cover. Plans also include roads, culverts, proximity to human habitation, hydrological conditions, and soil reports. Forest management plans include the projected use of the land and a timetable for that use. Traditional forest management plans focus on providing logs used for timber, veneer, plywood, paper, wood fuel or other industries. Hence, considerations of product quality and quantity, employment, and profit have been of central, though not always exclusive, importance. Foresters frequently develop post-harvest site plans for reforestation, weed control, fertilization, or thinning. The objectives of landowners and leaseholder influence plans for harvest and subsequent site treatment. In Britain, plans featuring "good forestry practice" must always consider the needs of other stakeholders such as nearby communities or rural residents living within or adjacent to woodland areas. Foresters consider tree felling and environmental legislation when developing plans. Plans instruct the sustainable harvesting and replacement of trees. They indicate whether road building or other forest engineering operations are required.
Agriculture and forest leaders are also trying to understand how the climate change legislation will affect what they do. The information gathered will provide the data that will determine the role of agriculture and forestry in a new climate change regulatory system.

Forest Today

Today a strong body of research exists regarding the management of forest ecosystems and genetic improvement of tree species and varieties. Forestry also includes the development of better methods for the planting, protecting, thinning, controlled burning, felling, extracting, and processing of timber. One of the applications of modern forestry is reforestation, in which trees are planted and tended in a given area.
In many regions the forest industry is of major ecological, economic, and social importance. Third-party certification systems that provide independent verification of sound forest stewardship and sustainable forestry have become commonplace in many areas since the 1990s. These certification systems were developed as a response to criticism of some forestry practices, particularly deforestation in less developed regions along with concerns over resource management in the developed world. Some certification systems are criticised for primarily acting as marketing tools and lacking in their claimed independence.
In topographically severe forested terrain, proper forestry is important for the prevention or minimization of serious soil erosion or even landslides. In areas with a high potential for landslides, forests can stabilize soils and prevent property damage or loss, human injury, or loss of life.
Public perception of forest management has become controversial, with growing public concern over perceived mismanagement of the forest and increasing demands that forest land be managed for uses other than pure timber production, for example, indigenous rights, recreation, watershed management, and preservation of wilderness, waterways and wildlife habitat. Sharp disagreements over the role of forest fires, logging, motorized recreation and others drives debate while the public demand for wood products continues to increase.

Forestry

Forestry is the art and science of managing forests, tree plantations, and related natural resources. The main goal of forestry is to create and implement systems that allow forests to continue a sustainable continuation of environmental supplies and services. The challenge of forestry is to create systems that are socially accepted while sustaining the resource and any other resources that might be affected.
Silviculture, a related science, involves the growing and tending of trees and forests. Modern forestry generally embraces a broad range of concerns, including assisting forests to provide timber as raw material for wood products, wildlife habitat, natural water quality management, recreation, landscape and community protection, employment, aesthetically appealing landscapes, biodiversitywatershed management, erosion control, and preserving forests as 'sinks' for atmospheric carbon dioxide. A practitioner of forestry is known as a forester. Note that the word "forestry" can also refer to a forest itself. management,
Forest ecosystems have come to be seen as the most important component of the biosphere, and forestry has emerged as a vital field of science, applied art, and technology.

History

The use and management of many forest resources has a long history in China, dating from the Han Dynasty and taking place under the landowning gentry. It was also later written of by the Ming DynastyXu Guangqi (1562–1633). In the Western world, formal forestry practices developed during the Middle Ages, when land was largely under the control of kings. Control of the land included hunting rights, and though peasants in many places were permitted to gather firewood and building timber and to graze animals, hunting rights were retained by the members of the nobility. Systematic management of forests for a sustainable yield of timber is said to have begun in the 16th century in both the German states^{[citation needed]} and Japan. Typically, a forest was divided into specific sections and mapped; the harvest of timber was planned with an eye to regeneration. Chinese scholar
The practice of establishing tree plantations was promoted by John Evelyn; it had already acquired some popularity in the British Isles. Louis XIV's minister Jean-Baptiste Colbert's oak forest at Tronçais, planted for the future use of the French Navy, matured as expected in the mid-19th century: "Colbert had thought of everything except the steamship," Fernand Braudel observed. Schools of forestry were established after 1825; most of these schools were in Germany and France. During the nineteenth and early twentieth centuries, forest preservation programs were established in the United States, Europe, and British India. Many foresters were either from continental Europe (like Sir Dietrich Brandis), or educated there (like Gifford Pinchot).
The enactment and evolution of forestry laws and binding regulations occurred in most Western nations in the 20th century in response to growing conservation concerns and the increasing technological capacity of logging companies.
Tropical forestry is a separate branch of forestry which deals mainly with equatorial forests that yield woods such as teak and mahogany. Sir Dietrich Brandis is considered the father of tropical forestry.

Forest loss and management

Main articles: Forestry, Logging and Deforestation
The scientific study of forest species and their interaction with the environment is referred to as forest ecology, while the management of forests is often referred to as forestry. Forest management has changed considerably over the last few centuries, with rapid changes from the 1980s onwards culminating in a practice now referred to as sustainable forest management. Forest ecologists concentrate on forest patterns and processes, usually with the aim of elucidating cause and effect relationships. Foresters who practice sustainable forest management focus on the integration of ecological, social and economic values, often in consultation with local communities and other stakeholders.
Anthropogenic factors that can affect forests include logging, urban sprawl, human-caused forest fires, acid rain, invasive species, and the slash and burn practices of swidden agriculture or shifting cultivation. The loss and re-growth of forest leads to a distinction between two broad types of forest, primary or old-growth forestsecondary forest. There are also many natural factors that can cause changes in forests over time including forest fires, insects, diseases, weather, competition between species, etc. In 1997, the World Resources Institute recorded that only 20% of the world's original forests remained in large intact tracts of undisturbed forest^. More than 75% of these intact forests lie in three countries - the Boreal forests of Russia and Canada and the rainforest of Brazil. In 2006 this information on intact forests was updated using latest available satellite imagery. and
Canada has about 4,020,000 square kilometres (1,550,000 sq mi) of forest land. More than 90% of forest land is publicly owned and about 50% of the total forest area is allocated for harvesting. These allocated areas are managed using the principles of sustainable forest management, which includes extensive consultation with local stakeholders. About eight percent of Canada’s forest is legally protected from resource development (Global Forest Watch Canada)(Natural Resources Canada). Much more forest land — about 40 percent of the total forest land base — is subject to varying degrees of protection through processes such as integrated land use planning or defined management areas such as certified forests (Natural Resources Canada).
By December 2006, over 1,237,000 square kilometers of forest land in Canada (about half the global total) had been certified as being sustainably managed (Canadian Sustainable Forestry Certification Coalition). Clearcutting, first used in the latter half of the 20th century, is less expensive, but devastating to the environment and companies are required by law to ensure that harvested areas are adequately regenerated. Most Canadian provinces have regulations limiting the size of clearcuts, although some older clearcuts can range upwards of 110 square kilometres (27,000 acres) in size which were cut over several years. China instituted a ban on logging, beginning in 1998, due to the destruction caused by clearcutting. Selective cutting avoids the erosion, and flooding, that result from clearcutting.
In the United States, most forests have historically been affected by humans to some degree, though in recent years improved forestry practices has helped regulate or moderate large scale or severe impacts. However, the United States Forest Service estimates a net loss of about 2 million hectares (4,942,000 acres) between 1997 and 2020; this estimate includes conversion of forest land to other uses, including urban and suburban development, as well as afforestation and natural reversion of abandoned crop and pasture land to forest. However, in many areas of the United States, the area of forest is stable or increasing, particularly in many northern states. The opposite problem from flooding has plagued national forests, with loggers complaining that a lack of thinning and proper forest management has resulted in large forest fires.
Old-growth forest contains mainly natural patterns of biodiversity in established seral patterns, and they contain mainly species native to the region and habitat. The natural formations and processes have not been affected by humans with a frequency or intensity to change the natural structure and components of the habitat. Secondary forest contains significant elements of species which were originally from other regions or habitats.
Smaller areas of woodland in cities may be managed as Urban forestry, sometimes within public parks. These are often created for human benefits; Attention Restoration Theory argues that spending time in nature reduces stress and improves health, while forest schools and kindergartens help young people to develop social as well as scientific skills in forests. These typically need to be close to where the children live, for practical logistics.