The Distribution of Animals in Space (Chorology and Ecology)

The Point of View

An observant traveler going from home in any direction gradually leaves behind a familiar world of animals and plants and, if his travels are sufficiently extensive, arrives in a land of strange organisms for the most part quite unlike those he already knows. He discovers that no one kind is to be found everywhere, but that each kind has its own home territory beyond which it does not ordinarily venture. In imagination he might map out upon the globe the home patch, with all its irregular boundaries, which each of the 600,000 or more species of living animals occupies. Such a map would be exceedingly complex because the areas thus delimited would not only be very unequal in size but would also overlap each other in a great variety of ways like a gigantic palimpsest. A diagram to express this idea, in which the areas of only six instead of 600,000 species are involved, is shown in Figure 77.

Hypothetical limits of the distribution of six different species of animals, arranged in superimposed areas

Such a picture, moreover, if truly represented, would not be definite and fixed but would be a motion picture, presenting constant change like a rotating kaleidoscope, since the frontiers established by living things can never remain constant. It is well known that animals in the past occupied territory from which they are absent today, and that the contrary is equally true.

Evidence from fossils, for example, shows that at one time tropical parrots and, at another, arctic reindeer were natives of what is now temperate France; that elephants formerly roamed over the United States; and antarctic albatrosses flew over England. Just as history records a succession of civilizations, so plant and animal life, past, present, and future, presents a shifting scene.

The locality where any species of animal is found is as much a diagnostic characteristic of the kind of animal in question as its peculiarities of structure or behavior. It follows that any kind of fossil or living form loses much of its value for the scientist if the place it comes from is unknown.

Ecology and Chorology

The comparatively new biological science of Ecology (oikos, home; logos, discourse) deals with the intimate arrangement and behavior of organisms within their respective habitats. Ecology may be defined as “scientific natural history.”

The province of the more inclusive science of Chorology (choros, place; logos, discourse) is to determine the general distribution of animals and plants over the earth and to discover the why and wherefore of their occurrence. Chorology is a science for the traveler, while ecology is the science for the stay-at-home.


The immediate surroundings in which any animal is “at home” are called its habitat. In general animals are said to occupy either a land or a water habitat. Some of the more specific descriptive terms applied to habitats are: desert; forest; mountain; subterranean; prairie; meadow; marsh; pelagic; abyssal; pond; marine; fluviatile; and estuarial. This list of descriptive local areas may be almost indefinitely extended according to the minuteness with which the details are scrutinized.

The arrangement of these various kinds of habitats over the surface of the globe determines to a large extent the distribution of living forms. It is obvious, for instance, that arboreal animals are not to be expected in the open ocean, which constitutes about three fourths of the entire surface of the globe, and much less are fishes to be discovered in waterless deserts.

Animals found living successfully in any habitat must be measurably adapted for life conditions there, although there are many cases in nature of imperfect adaptation where a square peg is attempting to fill a round hole, and vice versa. The usual result in such a misfit is that the peg either explores until it finds its proper hole, or gradually changes to fit the hole that it is in. Both hole and peg are changeable things but the initiative of change belongs not to the hole but to the peg.

Any habitat is not occupied by all the animals and plants adapted to live in it. The prevalent idea, for example, that climate determines the distribution of organisms is largely erroneous. There are no grizzly bears in Switzerland, no birds-of-paradise in California, and no “snakes in Ireland,” although the climate in each case is suitable for the absentees. The equatorial forests of Africa and South America have practically the same climate, yet the former region is characterized by elephants, apes, leopards, giraffes, and guinea fowl, while the latter has none of these animals but does support tapirs, long-tailed monkeys, jaguars, and toucans, which are never found in Africa.

So long as mankind was satisfied with the naive supposition that the earth has been arbitrarily populated by independent acts of special creation, much as a person might arrange chessmen upon a board, there was no sense nor object in developing a science of chorology. There was nothing to explain. Leopards, for example, were in Africa and jaguars in South America because they were placed there, newly made, in the beginning. The two kinds of large cats were entirely independent in origin and without any relation to each other. When men developed the conception, culminating with Darwin, that all organisms are more or less related to each other as descendants of common ancestors, and that every species arose in the course of time by modification from some other species, then the manner of distribution over the earth became full of significance, seriously challenging the attention of thinking people.

The Laws of Distribution

Three laws governing the distribution of animals were formulated by Jordan and Kellogg in Animal Life. These laws may be stated as follows: Every species is found everywhere unless (1) it was unable to get there; (2) having “got there,” it was unable to stay; or (3) having arrived, it became modified into-another species. It will be profitable to consider these laws briefly.

First, it is not the suitability of a habitat so much as its accessibility from a place of origin, that determines the presence of an inhabitant. For instance, there are no hummingbirds in Africa, while there are over 450 species in South and Central America, not because Africa itself is unfavorable to hummingbird occupation, but because these tiny fairylike creatures have never been able to cross the wide oceans separating their ancestral American home from farawav Africa.

Second, there are many instances of animals and plants that have succeeded in invading new territory, but have been unable to hold their own there. Not all pioneers become settlers. At one time our federal government introduced a herd of camels after an adventurous sea voyage into the semi-arid region of the Southwest, and allowed them to run wild in the hope that they would multiply, spread, and eventually form a valuable addition to a region inhospitable to most large animals. The environment was very like that from which the animals came and the experiment might have proved successful but, as has been asserted, for the unfortunate fact that local cowboys, with little regard for consequences, had so much sport periodically rounding them up and putting them through their paces, that the strange incongruous beasts were literally worried to death.

Third, successful invaders may win out in occupying new territory at the expense of their own specific individuality. They are the adaptable round pegs thrown into new habitats of square holes, that nevertheless remain and square themselves to fit the new holes. They are the immigrants that have deserted the ways of their mother country and become naturalized in the land of their adoption. A classical illustration of cases of this kind, cited by Darwin in The Origin of Species, is that of animals upon the Galapagos Islands off the northwest coast of South America. Of 26 species of land birds found upon these islands, 23 species are similar to, but still specifically different from, those inhabiting continental land a few hundred miles away. The interpretation give by Darwin is that when the Galapagos group was separated from the mainland in recent geological times, a new habitat was formed in which various individuals of continental species were isolated. Each of these 23 species, changing gradually under the molding influence of isolation, grew to be sufficiently different from its original mainland ancestors and cousins to rank as a different and distinct species. These facts so impressed Darwin that he began to think about the origin of species, with the fortunate result that subsequently a great many other people were induced to think about the same subject.

To these three laws of distribution may be added a fourth, namely: Each species originated historically from some preceding species at some definite place, and its present distribution is the result of two opposing forces, Expansion and Repression.

Malthus' Law of Overpopulation

It would be as impossible for an unrestrained gas to remain in one place, as for any species of animals or plants to forego the attempt to occupy unoccupied territory to which it has access. The reason for this is the enormous possibilities of expansion inherent in the reproductive processes of all organisms, a condition formulated by Malthus (1766-1834) in his “Law of Overpopulation.” For example, when a single codfish produces 9,000,000 eggs in one season, it is obvious that infant mortality must come to the rescue, else in a few generations every available inch of space in the ocean would be preempted by codfish. Even slow-breeding animals like elephants, which produce perhaps six young in a lifetime of a hundred years, would, according to Darwin, require less than 800 years to produce from a single pair nearly 19,000,000 elephants. Allowing 20 feet of space for each elephant, this would make a continuous parade, which would have delighted Barnum, reaching nearly three times around the world at the equator. Elephants and codfish, however, do not multiply out of all bounds as the above theoretical figures suggest, for the expansive forces of reproduction are kept in control, year in and year out, by opposing repressive factors which maintain a balance in nature.

Factors Inducing Expansion

The Food Problem

Somewhere in one of his delightful essays, Dr. Samuel McChord Grothers presents the illuminating statement that “the haps and mishaps of the hungry make up natural history.” There is no doubt that the insistent need for food, as expressed by hunger, is a mainspring of animal activity that, like a centrifugal force, compels animals to go forth in the quest of what they may devour. Even among higher animals which exercise parental care, there comes a time when the young may expect no longer to share food with their parents but must seek fresh pastures. To illustrate with a botanical instance, it would be disastrous if the acorns produced by an oak tree all remained to grow up within the parental circle.

Not only is there competition for food and place among animals and plants of a kind, but there is also severe rivalry between different kinds of creatures for the same food supply. The miscellaneous company which at any time sits at Mother Nature’s table, does not always, or even often, observe the restrained table manners of polite society, so that there is every inducement to go elsewhere.

Change of Habitat

Another general factor that causes organisms to spread, is change in the habitat-occupied. Such a change may be temporary, like the drying up of ditches and streams that affects aquatic organisms, or it may be permanent, like deforestation at man’s hands, which leaves arboreal animals homeless and leads to flood conditions.

It may be sudden and catastrophic, like a prairie fire or a flood, forcing all sorts of animals to flee at once for their lives; it may be gradual like the change of seasons, when winter succeeds summer; or it may be so very slow that it extends over generations in time, like the relentless dawn of a glacial period.

In all cases, however, when an environment becomes unfavorable, there are at least four alternatives open to the inhabitants: (1) organisms may simply succumb to the environmental change, completing their normal life cycles before the unfavorable conditions befall, as in the case of annual plants and most insects; (2) they may retire from active life and mark time while temporary unfavorable conditions last, as do hibernating animals and encysting protozoans, or trees that shed their leaves in winter and ‘"hold their breath” until spring; (3) they may remain plastic enough to change themselves as the environment changes, thus keeping pace by adaptation to new conditions; or (4) they may forsake unlivable surroundings and seek a more favorable place to carry on, like migrating birds, grasshoppers, and emigrants of all kinds. This latter alternative of migration, brought about by change in the habitat, plays an important role in the distribution of animals and plants.

Means of Dispersal

The ways and means, direct and indirect, that are employed by organisms for dispersal, furnish a fascinating chapter in natural history. Only a few of the most common agents may be mentioned here.

Among plants wind is an important agent. In many instances seeds are rigged with ballooning or parachuting devices, or are so light as to be easily borne upon currents of air. The tiny dust-like seeds of certain orchids, for instance, have been known to float in air from Holland across the North Sea, while molds testify to the efficiency of air movements in scattering spores of these ubiquitous organisms everywhere.

Over 60 species of North American birds have been reported, which have reached Europe and become established there by being borne out of their migratory routes by winds, while flying insects like grasshoppers ate frequently assisted 111 their widespread movements by air currents.

Water furnishes another highway for travel. The uneasy tides keep the congested inhabitants of the seashore constantly stirred up and on the move, while flowing streams and ocean currents act continually as agents in the involuntary transfer of all sorts of organisms from one place to another. Even floating icebergs are precarious rafts upon which stray arctic animals are sometimes borne some distance into new regions.

Animals themselves assist each other in dispersal in a multitude of ways, as stowaways, kidnappers, and “thumbies.” The larval glochidia of certain sluggish fresh-water clams of the genera Unio and Anodonta, fasten themselves to the gills of swiftly moving fishes, thus stealing a ride to some distant point in the stream where they detach themselves and set up their semi-stationary housekeeping in a new place. Parasites naturally go wherever their hosts go, and so are introduced into the society of new hosts. Animals are particularly useful agents in scattering the seeds of plants. “Sticktights” and burrs of all sorts are makeshifts on the part of plants to steal a ride by attaching to passing animals. Seeds of various kinds too are embedded in attractive fruits with the result that they are eaten by animals and so deposited in some new locality after passing unscathed through the digestive tube of the traveling host. Thus cherry bushes are planted beneath a wood thrush’s nest, and fences along the waysides are draped with poison ivy by feathered conservationists.

The mistletoe, which grows parasitically as an air plant attached to the bark of trees, presents an extreme instance of distribution through animal agency. Doves eat the seeds of the mistletoe, because they are encased in alluring sticky berries. Frequently it happens, much as when the traditional small boy emerges from the jam closet, that remains of the feast adhere around the margin of the mouth. The dove flies away to another tree where it performs its toilet by wiping a sticky beak, with the adhering seeds, upon a branch. The seeds are wiped off in this way and stuck to a fresh branch in the exact location favorable for the growth of a new epiphytic plant in a new place.

Of all animals man, however, has probably done more than any other in furthering the spread of organisms. In many instances this has been done intelligently and to the ultimate benefit of man himself, as in the case of cultivated plants and domesticated animals. The biological landscape has been changed almost everywhere by the transforming hand of man. Crops of various kinds dot the surface of the globe where wilderness once flourished, while introduced flocks and herds roam in safety oyer territory once the possession and battleground of native wild animals.

Frequently man has made serious mistakes from the human standpoint in meddling with the balance of nature. The introduction of that over-successful “avian rat,” the English sparrow, into the society of American birds has been many times regretted by man on account of injury to native birds with which it comes into competition. The bloodthirsty mongoose, that was brought to Jamaica and also to Hawaii to kill rats infesting sugar cane fields, proved to be an efficient rat exterminator, but it went further and destroyed other animals, particularly chickens. As a result poultry-raising in these islands has been seriously interfered with, and now a price has been set on the head of every mongoose.

Several years ago a gentleman in Medford, Massachusetts, who conceived the idea that some more hardy insect than the silkworm might be found to spin silk, and at the same time feed upon less restricted food than mulberry leaves, brought back from Europe a few gipsy moths, Porthetria dispar, to use in experiments. The box in which they were contained, so the story goes, was accidentally knocked out of an open window and some of the moths escaped, but for the time the incident was forgotten. This was in 1869. By 1889 the descendants of these chance immigrants had prospered to so great an extent that the people around Medford became alarmed and a town meeting was held at which $300 was appropriated to fight the pests. “In that summer,” the record shows, “the numbers were so enormous that the trees were completely stripped of their leaves, the crawling caterpillars covered the sidewalks, the trunks of the shade trees, the fences, and the sides of the houses, getting into the food and into the beds.” Dr. Lutz in his Field Book of Insects published in 1921 wrote: “Millions of dollars have been spent in an effort, so far unsuccessful, to free us from the invader, and the most that has been done has been to confine it to New England.”

The white cabbage butterfly, Pieris rapae, first came to America from Holland in a sloop load of wormy cabbages landed at Quebec in 1861. Twenty years later, according to the Department of Agriculture at Washington, it had colonized America on the Atlantic Coast from Hudson’s Bay to Florida. In 1886 it had arrived at Denver, and in 1900 had reached the Pacific Coast, having accomplished the conquest of the entire United States and a part of Canada in less than thirty years.

Shipworms, Teredo, on the outside of the hulls of vessels, and rats on the inside have spread themselves the world over wherever shipping has gone. In 1827 mosquitoes, traveling as “wigglers” in the bilge water of a sailing vessel, arrived at the Hawaiian Islands. They have prospered since then and made the Islands their own.

Several years ago a marble statue, made by the sculptor Thorwaldsen in Italy, was set up in Copenhagen. It is said that, as an accidental result, twenty-five species of Italian weeds, the seeds of which were in the straw packing around the statue, made their appearance in the immediate vicinity.

Similar instances of the effects of the interference of meddlesome man with the natural arrangement of organisms in space could be indefinitely multiplied.

Factors of Repression

Among various factors which hinder world conquest on the part of any single species of animals or plants, are: (1) inadequate means of dispersal; (2) non-adaptability to new conditions; and (3) barriers of different kinds.

Inadequate Means of Dispersal

The difficulties of “getting there” are not especially apparent in the case of free-moving animals like birds and insects. They become very real, however, as well as serious for many organisms whose structure is not particularly adapted for locomotion over considerable distances, yet the race is by no means always to the swift. The story of the tortoise and the hare finds plenty of parallels in nature.

When Gould, with his searching eye for mollusks of all kinds, wrote A Report on the Invertebrata of Massachusetts in 1841, he made no mention of Littorina iitorea, a small familiar periwinkle that at present is one of the most abundant species of snails along the Atlantic Coast. In 1855 Morse found a few of these animals in the Bay of Chaleur at the mouth of the Saint Lawrence river, which had been accidentally brought over in ballast from their original home in Europe. By 1875 Verrill reported two as a rare find at Woods Hole, Massachusetts, several hundred miles to the southward, and in 1880 Smith found the first one to be noted as far south as New Haven, Connecticut.

Another example of the surprising spread of an animal handicapped by poor methods of locomotion is that of Sagartia luciae, a small semi-transparent sea-anemone that lives attached to stones in the tidal zone. It was discovered at New Haven by Verrill in 1892, who named it luciae in honor of his daughter Lucy. In 1895 it was reported at Newport, R. I.; in 1898 at Woods Hole, Mass.; in 1899 at Nahant, Mass.; in 1901 at Salem, Mass.; and in 1903 at Cold Spring Harbor, Long Island, N. Y., where it is now abundant. Thus this “stationary” animal spread itself over several miles of coast line within a single decade.

Non-adaptability to New Conditions

The non-adaptability of organisms to new habitats, which they may have invaded, is doubtless much greater than appears on the surface, for it surely, acts as a deterrent to their spread. Successful invaders that gain a new foothold and retain it catch the eye and claim attention, while unsuccessful ones which reach the Promised Land but are unable to establish themselves there, escape attention and pass unnoticed.

Many plants that thrive under cultivation, like maize or Indian corn, appear to be unable to maintain themselves in nature when by chance they are allowed to run wild.

The yellow fever mosquito, Stegomyia jasciata, fortunately does not succeed north of a certain dead-line, although no doubt it has repeatedly crossed this invisible limit, like the English ivy that clothes the walls of southern buildings in luxuriance, but fails to grow well in more northern situations, in spite of being repeatedly planted and nurtured there.


Barriers which check or stop organisms on all sides are at least three in kind: Physical, Geographical, and Biological.

Temperature is a widespread physical barrier. The exclusion of “cold-blooded” animals, such as amphibians and reptiles, from the occupation of lands of prevailingly low temperatures, is quite evident. In general temperature zones extend not only in latitude north and south from the equator, but also in altitude in a parallel succession from tropical sea level to high mountain peaks (Fig. 78).

Diagram of the general parallel sequence of organisms in altitude and longitude

In the ocean, pressure acts as a barrier that stratifies the inhabitants within certain limits to which they have become specifically adapted. Deep-sea fishes cannot pass freely from abysses to surface waters, nor can pelagic forms sink far below and survive. Similarly in the air there is an upper altitude limit beyond which flying birds cannot rise.

Humidity sets up a barrier which, according to its degree, is largely impassable to exploring organisms, dependent upon a certain optimum of moisture.

Light, another physical barrier, halts the traffic of nocturnal darkness lovers, although it usually has more of an ecological than chorological bearing. Green plants, on the other hand, do not live in the abyssal regions of the ocean because photosynthesis cannot be carried on there in the absence of light.

Geographical barriers are such features of the earth’s surface as oceans, land masses, rivers, mountains, waterfalls, deserts, forests, and the like. A barrier to one organism, however, may be a highway to another. Thus, a desert would form an impassable barrier to a squirrel but not to a camel, while a forest in which a squirrel would revel would prove an effectual barrier to a camel.

Biological barriers are bound up in the first place with the eternal food problem, since absence of food of a particular kind in a region may prevent the advance of invading animals, while poverty of soil discourages occupation by plants dependent upon the missing soil constituents.

Secondly, biological barriers often take the form of other animals, by habit predaceous or parasitic, which hinder or forbid advance in certain directions.

Thirdly, the greatest biological barrier of all is man, since he is able to control the forces of nature far more than any of his animal allies. It should also be pointed out that limiting biological barriers may exist within animals themselves in the form of scanty wits, lack of initiative or adaptability, causing failure to enter into new areas even though the door of opportunity swings wide open.

If the factors of expansion and repression were equal in all directions, the area occupied by each species would remain constant as a perfect circle, but such a condition is unknown. The irregular shapes and boundaries of claims actually staked out in nature by various organisms proclaim the complex interaction of the fundamental opposing forces that determine distribution.

Classification of Life Realms

An attempt has been made by chorologists to divide the land masses of the world into life realms, according to the distribution of animals and plants. Such realms in no way coincide with the familiar political boundaries that separate nations from each other, being much more indefinite in their limits.

It is evident that life realms must vary according to the kind of animal or plant inhabitants selected to serve as determinants. Perhaps the first serious attempt to divide the earth into zoological realms was made in 1851 by Sclater, who based his conclusions upon the distribution of birds. There are, however, very apparent objections to utilizing vagrant and barrier-defying creatures like birds for this purpose. Accordingly Murray in 1866, and more in detail Alfred Russell Wallace in 1876, divided the surface of the earth into zoological regions based chiefly on the distribution of mammals. Reptiles, amphibians, fresh-water fishes, insects, and spiders have each in turn been used as the foundation for zoological map making, as well as various combinations of animals, but mammals undoubtedly present the most advantages for this purpose. The reason for this lies in the fact that mammals are warm-blooded, capable of occupying a great range of habitats, and being the most recently evolved large group of animals on the earth, have not had as much time as other types of animals to radiate from their centers of origin, with consequent confusion as to which species are native or endemic, and which introduced.

Number of mammalian families represented in each of wallaces subregions

Wallace’s classification consists of six large regions, each divided into four subregions, as indicated in Table I. This table shows also the number of different families of mammals represented in each of the 24 subregions. It will be seen that bats (Chiroptera) are most generally represented, there being no subregion that does not have at least one of the five families of bats within its borders, while whales (Cetacea) do not appear at all, because they are not definitely associated with any land masses.

The richest subregion so far as numbers of mammalian families goes is the South African, although the East African, West African, Mediterranean, and Indo-Chinese are likewise conspicuously populous. The poorest is the New Zealand subregion that has no native mammals with the exception of two families of bats.

The mercator map of the world (Fig. 79) shows roughly the extent of each of Wallace’s regions and subregions.

Mercator map of the world, divided into zoological regions and sub-regions

The study of chorology helps to solve such puzzles as (1) why related species, like the edentates in South America, are found in the same continental areas; (2) why areas physically alike, as Africa and South America, have different kinds of inhabitants; (3) why an original center for a species, like Wyoming for lemurs, may have no living representatives today; and (4) why regions near together, for example, Florida and the Bahamas, may have quite diverse faunas and floras, while regions remote from each other, like North America and Eurasia may have many similar forms.