Animal Ecology the Dispersion of Communities of Animals

Animal Ecology The dispersion of communities of animals Every ecosystem has a distinct animal community that may be classed in a number of ways. The reason for this diversity and richness is because most animal species have evolved to be relatively specialized. Studying the zonation of these communities along major environmental gradients, like the one from the poles to the equator, is convenient. It demonstrates the dominant influence of plants on animal distribution by creating unique local conditions and by creating habitat boundaries that cause animal communities to be more sharply divided than they otherwise would be. The vertical zones of communities on a mountainside, which also demonstrate the idea that (9) each community's members can be separated into those who are “exclusive,” (8) amply demonstrate this. and (10) into those "characteristics" of it, whilst the majority of the remaining species—which make up the community—occur in several associations. (11) The light in the sea and the salts in water are two other examples of vertical gradients. (13) Every sizable zone can be further separated into smaller habitat gradients, such as the soil's water content, and (14) these can be further divided into still smaller ones, until we reach individual animal species, which can then be demonstrated to contain gradients in internal conditions that support distinctive parasitic communities. (15) In this approach, the distinctions between groups can be categorized and examined in order to examine the underlying similarities between them. 1. An ecologist's first challenge is to deal with the reality that every type of environment has a unique collection of species that inhabit it. We refer to these animal associations—better yet, animal communities—because, as we will see—they are more than just collections of species coexisting; rather, they form intimately interwoven communities or societies that resemble our own. Animal ecologists have spent much of their time up to this point describing and classifying the diverse animal habitats and the species that inhabits them. Most developed nations have conducted preliminary biological surveys, with the exception of China and England, where animal ecology lags behind in an unusual fashion. We could specifically highlight the work of the American Bureau of Biological Survey, other institutions in that nation, and comparable surveys conducted in India at the behest of the late Dr. Annandale. The American Bureau of Biological Survey was originally established to investigate issues brought about by the introduction of the English sparrow into the United States. Before delving into the more complex issues of animal communities, it is evident that a list of the animals in various environments is a prerequisite. In the "Methods" chapter, we will address the subject of biological surveys once more. 2. Different classification schemes for animal communities have been proposed; some are rather practical, while others are utterly ridiculous. But as the latter are never used, they just provide as good instances of what should be avoided: creating an excessive number of definitions and a plethora of superfluous technical jargon. It is important to always bear in mind that the professional ecologist must and will always rely, for a large portion of his data, on the observations of individuals such as local naturalists, gamekeepers, fishers, and other non-professional scientists. The distribution, habits, and style of life of animals are frequently so unpredictable and hard to determine that it is always vital to draw upon the special expertise of individuals who have been studying animals in one area for a considerable amount of time. Making an initial biological survey and compiling lists of the animals in various communities is a relatively easy task. Naturally, a great deal of effort, persistence, and skill in understanding animal behavior are needed for this preliminary work; however, the true scope of the task and the challenge of gathering the appropriate class of data become apparent when one delves into the more complex issues of animal life and tries to construct the food cycles, which will be covered later. Thus, it is important to emphasize how important it is to stay in touch with all sensible men who live near wild animals for a significant portion of their lives. In order to achieve this effectively, ecology should not appear to be any more complex or difficult than it actually is, nor should it be possible to claim that "ecology consists in saying what everyone knows in language that nobody can understand." The writer has gained a great deal of invaluable knowledge about the social organization of animals from the writings of men like W. H. Hudson, as opposed to professional zoologists. Even if this is the case, it is also true that the Old Testament and Shakespeare's plays contain more ecology than the majority of zoological textbooks ever published! There is merit to the anonymous writer's opinion expressed in Nature: "The notion that the truth can be sought in books is still widely prevalent, and the present dearth of illiterate men constitutes a serious menace to the advancement of knowledge." Given all of this, it would seem pointless to create intricate, scholarly categorization of animal populations. After all, what is to be said for a scientist who calls the community of animals living in ponds the “tiphic association” or refers to the art of gardening as “chronic hemerecology"?? 3. It is important, however, to get some general idea of the variety and distribution of the different animal communities found in the world. The existence of such a rich variety of communities is to be attributed to two factors. In the first place, no one animal is sufficiently elastic in its organization to withstand the wide range of environmental conditions that exist in the world, and secondly, nearly all animals tend, during the course of evolution, to become more or less specialized for life in a narrow range of environmental conditions, for by being so specialized, they can be more efficient. This tendency towards specialization is abundantly shown throughout the fossil record and is reflected in the numerous and varied animal communities of the present day. Primarily, there is specialization to meet particular climatic and other physical and chemical factors, and secondarily, animals become adapted to a special set of biotic conditions—food, enemies, etc. The effect of the various environmental factors on animals is a subject that will be followed up on separately in the next chapter. 4. One way of giving some idea of the range of different animal habitats and of the communities living in them is to take some of the big gradients in the environment and show how the communities change as we pass from one end to the other. The biggest of these is the gradient in temperature and light intensity between the poles and the equator, which owes its existence to the globular shape of the world. At one extreme are the regions of the polar ice pack, with their peculiar animal communities living in continuous daylight during the summer and continuous darkness in the winter, and with corresponding abrupt seasonal changes in temperature. At the other end of the scale, we have equatorial rain forest, with a totally different set of animals adapted to life in a continuously hot climate and, in many cases, in continuous semi-darkness, in the shade of the tropical trees. An animal like Bosman’s Potto sees less light throughout the year than the Arctic fox. In between these extremes, we have animal communities accustomed to a moderate amount of heat and light. 5. These examples serve to introduce a very important idea, namely, the effect of vegetation on the habitats and distribution of animals. Although a tropical rain forest partly owes its existence to the intense sunlight of the tropics, inside the forest it is quite dark, and it is clear that plants have the effect of translating one climate into another and that an animal living in or under a plant community and dependent on it may be living under totally different climatic conditions from those existing outside. Each plant association therefore carries with it, or rather in it, a special local "climate that is peculiar to itself. Since they block out radiant energy sources like light and heat as well as rain, plants generally have a blanketing effect. Therefore, their overall impact is to lessen the intensity of any given natural climate. In addition, they lessen the quantity of evaporation from the soil's surface, which dampens the air more than they otherwise would. Taking a comprehensive view of the situation, very little vegetation—which occasionally does not cover the ground at all—can be found in the far north. The forest gets denser and taller as we move south, eventually reaching an area with sporadic trees. Because the soil is too shallow (frozen below a certain depth) to support adequate root development, unless by considerable sideways growth, these are spaced widely apart. Then there are actual forests, though they are still not very lush. And lastly, there are the vast equatorial belt rain forests. The gradation basically consists of roots gradually filling the soil and vegetation covering the surface after that. 6. Animals are impacted by plant ecosystems in another significant way. Typically, we observe that the boundaries between two plant communities are relatively distinct when we observe them coexisting. The greenery that grows on a mountainside or around a lake are two examples of this. There is a straightforward explanation for this distinct division across plant communities. Since plants typically compete with one another for light, if one plant in a community outgrows the others, it can effectively shut off most of the light to the others, cementing its dominance. The majority of temperate plant groups have this characteristic. Common heather (Calluna), beech trees in a forest, and rushes (Funcus) in a marsh are a few examples. The ultimate struggle is typically for light, although the process of competition is not always this straightforward. A variety of intricate elements may influence the relative growth of the competing species. Sometimes the dominant species eliminates its rivals by flooding the smaller plants below with an abundance of dead leaves, or by using some other strategy, rather than by shading them out. Dominance as a main phenomenon is still present. There is currently a struggle for light at the intersections of two plant communities, which primarily resolves into a conflict between the two dominating species. Similar to the conflict between two strong groups within a community, a fight for light cannot be won by compromise. If a single plant triumphs, it does so totally. Every plant now has a specific set of ideal conditions for growth, and as these parameters deviate from the ideal, the efficiency of growth decreases. There is always a point in an environmental gradient where one dominant, and thus one community, transitions very suddenly into another, because each dominant has different optimal circumstances. A regular and gradual gradient in the soil's water content, for example, from a lake's edge to a dry moor, may have once existed. However, due to plant dominance, this gradient has been transformed into a series of distinct vegetation zones that partially obscure the original gradient and sometimes even change the surrounding conditions to create a step-like series. 7. Since each plant community has a unique set of "climatic" conditions for the animals that live there, it is evident that the boundaries of these communities tend to be fairly sharply defined. As a result, the abrupt changes in conditions at the edges of the plant communities will be reflected in the animals. Green plants rely on sunlight to feed them. This implies that rather than graded series exhibiting no abrupt changes, animal species would typically be separated into distinct ones adapted to various plant zones. Additionally, it indicates that animal groups are far more different from one another than they would be if they were all dwelling in a series of open plant associations like the arctic fjaeldmark (stony desert) or in a single continuous gradient of circumstances. If this were not the case, we would not have those easy classifications of the entire fauna into communities that are so helpful for practical reasons, making the study of animal connections much more difficult. In talks concerning the formation of species, it is commonly claimed that gradients are the only sort of environmental variables that effect creatures. It's evident that this isn't always the case. 8. As previously indicated, vertical zones of vegetation on the sides of mountains exhibit abrupt changes between plant groups, with small-scale vegetation correlating to large-scale latitude zones. These are known as "life zones" in America, where the presence of large mountain ranges is one of the factors contributing to the increased focus on ecology in that nation. The stunning spectacle of a huge number of vegetation zones, which has so greatly drawn the attention of American ecologists, is not seen in England, since the mountain ranges are located in the north. In the tropics, where one may eat wild strawberries in the mountains and wild bananas at sea level in the same day, this vertical zonation is most noticeable. A provides one of the most insightful explanations of this occurrence. The work of botanists has given us fairly clear ideas about the distribution of vegetation zones, but we still know very little about the precise distribution and boundaries of the animal communities in these life zones, as well as their relationships to the plants (R. Wallace, account of his travels through Java). Many American researchers have focused on specific animal groups (mostly birds and mammals). One particularly excellent study of the Yosemite region of the Sierra Nevada is Grinnell and Storer's (49), which includes a wealth of fascinating information on the animal ecology and precise data on vertebrate distribution in relation to life zones. g. Regarding the distribution and makeup of animal communities, there is one more crucial factor. A certain proportion of the animal population, say, should be restricted to the Canadian zone if we consider the animal community there. In fact, the dispersal of certain species is rigidly governed by the type of flora. We refer to these species as "exclusive" to that particular community. An excellent illustration of this are the game birds found in Great Britain. Lower down on the heather moors, the red grouse (Lagopus scoticus) takes the place of the ptarmigan (Lagopus mutus), which inhabits the alpine zone of vegetation. The pheasant (Phasianus colchicus) is primarily found in deciduous woods, but another bird, the capercaillie (Tetrao urogallus), inhabits coniferous woodlands. Perdix perdix, the common partridge, originates from cultivated environments that include grasslands. Here are some examples of birds that are restricted to particular plant associations. It's also possible to observe that these birds all lead similar lives in terms of diet and routines. Though the precise species varies depending on the affiliation, there is always some sort of large vegetarian bird there. Another well-known example is the common grass mouse (Microtus agrestis), which is primarily found living underground in grassland and feeding on the grass's roots. It only "boils over" into nearby habitats at particularly bountiful times. One species of plant attracts a large number of vegetarian insects, and if that plant is restricted to one association, the animal is similarly constrained. The oak (Quercus robur) is home to hundreds of unique insects, and that number rises significantly when parasites are taken into account. 10. As we continue our investigation of one zone, we ought to discover that, while not completely restricted, a few species are very abundant there. These species are referred to as "characteristic." The long-tailed mouse (Apodemus sylvaticus), which lives in woods but is not restricted to them, is a good example of this kind. Data from trapping in a single region close to Oxford revealed that 82%. 18% of specimens were caught in wooded areas. transpired outside on newly established plantations and sometimes even in the open. Although the animal is not as strictly restricted to a single environment as, say, Microtus, we can nevertheless term it a wood mouse in this district with great justification. Some of the unique and distinctive birds found in Britain have been described by Thorpe 12 using plant associations. It is found that these two kinds of animals—the exclusives and the characteristics—may frequently represent only a relatively small part of the entire community in the majority of cases in which we have any complete knowledge (and they are scarce). Many animal species are able to roam freely throughout multiple vegetation zones, either due to their ability to survive a wider range of environments than their counterparts or because they are not constrained by the direct or indirect impacts of the vegetation. The common bank vole (Evotomys glareolus), which, unlike the Apodemus described above, may be found in woodlands as well as wood margins, shrub communities, and young plantations, is an example of this kind of distribution. In actuality, 47 percent of the numbers are to be compared with Apodemus. of the specimens found in forests, and 53%. outdoors, mostly in areas with shrubs or young trees. While doing fieldwork, it should always be kept in mind that many common and significant species originate from multiple zones. It may actually be rather arbitrary to divide the animals living on, say, a mountainside into communities that correspond to the exclusive and characteristic species of each life zone. After studying the animals of an English heath for several years, Richards states: **"The most common animals in a plant community are often those most common elsewhere."** However, it is also likely true that animals that live in different vegetation zones exhibit a noticeable tendency for their distribution boundaries to coincide with the plant zone boundaries. This makes sense only when you consider how the plant communities have created a step-like gradient in the surrounding area. 11. The sea and freshwater lakes exhibit a significant vertical gradient as well, which is brought on by a decrease in light penetration with increasing depth. Both the communities of free-living organisms (plankton) and the bottom-dwelling organisms (benthos) exhibit this gradient. The absence of light causes the plants to grow scarcer as we descend, until there are no plants left at all. The animals that live in these environments must rely on each other or the dead bodies of organisms that fall from the above, well-lit zone to survive. The plants have the same tendency to form zones as they do on land, and one of the most fascinating aspects of marine communities is the competition—and sometimes total dominance—that certain animals that have adapted to a sedentary lifestyle have with the plants (seaweeds of various kinds). This is because, unlike on land, where an animal must go seek its food, in the sea and, to a lesser extent, in freshwater, an animal can sit motionless and have it provided to it. Among land animals, web-spinning spiders are virtually unique in that they have mastered the ability to remain stationary while receiving prey flown aloft. Certain large spiders can really trap little birds in their webs in the tropics. Many animals in the sea sit motionless in one spot, allowing their food to almost fall into their jaws. In fact, food is most likely not a limiting factor for these animals; instead, competition is for available sitting spots. As a result, we observe that these animals have outwardly plant-like behaviors. In large regions of the tropical ocean, corals nearly entirely replace seaweeds in the shoreline and shallow waters. Like other animals, corals feed on plankton organisms or tiny organic particles in the water. This phenomenon is likely caused by certain temperature conditions in the sea or the plankton that lives in these waters. Similar to plants, corals on a reef typically form zones, each dominated by one or more species. It appears that gradients in elements like the quantity of silt in the water, wave activity, etc., control the zonation. Numerous calcareous algae that strongly resemble the corals in appearance grow among them. Although plants are comparatively more common along ocean coasts the further one travels from the equator, some animal species, such as hydroids, can create zones between seaweed-dominated zones even in the Arctic. There is another distinct and universally occurring vertical gradient in circumstances. This is the difference in salinity between the water in mountainous areas and the sea. All kinds of materials are continuously washed out of the rocks and soil by the action of rain. These enter rivers and streams and momentarily gather at different depths in lakes and ponds. However, we find that generally speaking, the purer the water, the higher we go since the salts are constantly being rinsed down. This rule must be applied with exceptions in regions like Central Asia, which have high plateaus where numerous salt lakes form, or in mountain ranges where the uppermost portions are built of highly soluble rocks. Overall, however, we may identify an alpine or upland zone of fluids with minimal salts and frequently somewhat acidic reactions. Lower down, in areas like the Norfolk Broads or the meres of Lancashire and Cheshire, the rivers and standing waters have a significantly higher salinity content. The gradient then abruptly becomes steeper as it passes through estuaries and lagoons with brackish water before reaching the ocean. The sea is significantly more salted than interior lakes and ponds because it has been there for a longer period of time, yet it is actually just one end of a gradient that began high in the mountains. In each of these categories of water, there are distinct animal relationships that are clearly visible. Of course, other factors—temperature in particular—are significant, but the salt content alone is unquestionably crucial as a regulating factor because it affects the water's hydrogen ion concentration in addition to acting directly. 13. There are several smaller gradients in the external circumstances inside each of the large zones, which are due to significant climatic variances, and these variations give rise to a variety of more or less distinct plant and animal relationships. These gradients are brought about by biotic factors like animal grazing or by regional differences in soil and climate. The gradient in the soil's water content is one clear example. The animal community of a dry heather moor can be found at one end, while the plankton of a lake is made up of species that can float and swim freely. There would be areas of marsh, reed swamp, and so on, each with its own unique population of species, in between these two extremes. "These various zones are due to the fact that at one end of the gradient there is much soil and practically no water (at any rate in summer), while at the other there is much water and very little soil, the proportion of soil and water gradually changing in between. | 14. We can carry the subdivision of animal communities further and split up one ordinary plant association, like an oak wood, into several animal habitats, e.g. tree-tops, tree-trunks, lower vegetation, ground surface, and underground, and we should find that each of these habitats contained an animal community which could be treated to some extent at least as a self-contained unit. Again, each species of plant has a number of animals dependent upon it, and one way of studying the ecology of the animals would be to take each plant separately and work out its fauna. Finally, each animal may contain within its own body a small fauna of parasites, and these again can be split up into associations according to the part of the body which they inhabit. If we examine the parasites of a mouse, for instance, we find that the upper part of the in testine, the lower part, the caecum, the skin, the ears, each have their peculiar fauna. It is obviously impossible to enumerate all the different gradients in the environment and all the different communities of animals which inhabit them. One habitat alone, the edge of a pond, or the ears of mammals, would require a whole book if it were to be treated in an adequate way. The aim of the foregoing sketch of the whole subject is to show that the term ‘‘ animal community ” is really a very elastic one, since we can use it to describe on the one hand the fauna of equatorial forest, and on the other hand the fauna of a mouse’s czecum. For general descriptions of the animal communities of the more important habitats, the reader may be referred to a book on animal geography by Hesse,’!! and to a more recent book by Haviland.” 15. The attention of ecologists has been directed hitherto mainly towards describing the differences between animal communities rather than to the fundamental similarity between them all. The study of these differences forms a kind of animal ethnology, while the study of the resemblances may be compared to human sociology (soon to become social science). As a matter of fact, although a very large body of facts of the first type has been accumulated, few important generalisations have as yet been made from it. So much is this the case that many biologists view with despair the prospect of trying to learn anything about ecology, since the subject appears to them at first sight as a mass of uncoordinated and _ indigestible facts. It is quite certain that some powerful digestive juice is required which will aid in the assimilation of this mass of interesting but unrelated facts. We have to face the fact that while ecological work is fascinating to do, it is unbearably dull to read about, and this must be because there are so many separate interesting facts and tiny problems in the lives of animals, but few ideas to link the facts together. It seems certain that the key to the situation lies in the study of animal communities from the sociological point of view. This branch of ecology is treated in Chapter V., but first of all it is necessary to say something about the subject of ecological succession— an important phenomenon discovered by botanists, since it enables us to get a fuller understanding of the distribution and relations of animal and plant communities.