Biogeography is the study of patterns of
distribution of organisms in space with time. Many of the most
important physical factors of the land environment have very
distinct patterns of variations in different parts of the world.
This pattern we call climate. The climate of an area is the whole
range of weather conditions, temperature, rainfall, evaporation,
water, sunlight, and wind that it experiences through all seasons
of the year. Many factors are involved in the determination of the
climate of an area, particularly latitude, altitude and position in
relations to seas and land-masses. The climate in turn largely
determines the species of plants and animals that can live in an
area.
Each of these climate types and their major
sub-divisions has a number of characteristic plant and animal
communities that have evolved so that they are well-adapted to the
range of environmental factors in them.
The broad global distribution of different kinds
of flowering plants was used to define the six floral regions, or
realms, in the map.
Another approach is to divide the globe into
areas according to the types of plant communities found there. Such
characteristic communities are called biomes. There is no real
agreement about the number of biomes in the world because it is
often difficult to tell whether a particular type of vegetation is
really a distinct form, and also because many types of vegetation
have been modified by the impact of human populations.
Among the kinds of isolation that are chiefly
responsible for the origination of species, geographical isolation
is the most important, and involves physical barriers such as
oceans, mountain-ranges, or deserts which separate whole
populations. Geographical races are the chief raw materials from
which new species are formed, and it was the different finches on
the different Galapagos Islands which first suggested to Darwin
that evolution had occurred. Here, to various extents, geographical
isolation has assisted the origination of a number of
species.
A case in which geographical isolation may be
expected to produce its effects at almost any moment now is
provided by the gulls. These birds occupy a zone shaped like a ring
round the North Pole and form what B. Rensch has called a chain of
races. Starting with the British lesser- black-backed gull with its
dark mantle and yellow legs, this is found to grade into the
Scandinavian lesser-black-backed gull, and, continuing in an
easterly direction around the chain, this in turn grades into the
Siberian Vega gull with its lighter mantle and dull flesh-coloured
legs. The Siberian gull grades into the American herring-gull
which, in turn, grades into the British herring-gull with its light
mantle and pinkish legs. Although the British lesser-black- backed
gull may be regarded as belonging to the same species as all the
other gulls in the chain to the east of it, when it is compared
with the other end of the chain represented by the British
herring-gull, the two may almost be regarded as separate species.
Already they differ not only in colour but in habits, for the
latter nests on cliffs and is dispersive in winter whereas the
former breeds inland on moors and is migratory in winter. If at any
time the chain becomes severed by the erection of a sterility
barrier at any point, either through inability to breed, or through
a rupture of the chain by local extinction of the gull population,
the two British gulls will effectively have originated new
species.
The characteristic communities of plants and animals that are found
in different regions of the world are known as biomes. The
differences between biomes are not necessarily related to the
taxonomic classification of the organisms they contain, but rather
to the life-form (the form, structure, habits, and type of life-
history of the organism in response to its environment) of their
plants and animals. This concept of the life-form was first put
forward by the Danish botanist Christen Raunkaier in 1903. He
observed that the most common or dominant types of plants in a
climatic region had a form well suited to survive in prevailing
conditions. Thus in Arctic conditions, the most common plants are
dwarf shrubs and other low-growing plants; these have no extensive
above-ground growth that would be broken by heavy winter snowfalls,
and their buds are carried at or just below the surface of the soil
where they obtain the maximum protection from cold and wind in the
long winter. In warmer climates, the characteristic types of
vegetation are trees or tall shrubs that carry their buds and
reproductive structures well above the ground because they are
rarely exposed to severe weather conditions. Deserts usually
contain small plants, mostly quick- growing annuals, with little
above-ground growth, and buds and survival structures below the
soil surface, because of the risk of drought. Animals also show
distinct life-forms adapted to different climates, with
cold-resistant, seasonal, or hibernating forms in cold regions and
forms with drought-resistant skins or cuticles in deserts.
Nevertheless, animal life-forms are usually far less easy to
recognize than are those of plants and, consequently, most biomes
are distinguished by the plants they contain and are named after
their dominant life-form.
There is no real agreement among biogeographers
about the number of biomes in the world. This is because it is
often difficult to tell whether a particular type of vegetation is
really a distinct form or is merely an early stage of development
of another, and also because many types of vegetation have been
much modified by the activities of man.
There are eight climatic biomes, a freshwater
one, a marine one. and several that are related to soils.
Tundra is found around the Arctic Circle, north of the
tree-line. Smaller areas occur in the Southern Hemisphere on sub-
Antarctic islands. Alpine tundra occurs above the tree-line on high
mountains, including those in the tropics. It is the most
continuous of biomes and the easiest to define. Winter temperatures
are ~57°c or lower: water melts at the soil surface in summer
(air temperature is rarely over I5°c) but there is always a
permanent layer of frozen soil underneath—the permafrost.
There is a very short growing season, and only cold- tolerant
plants can survive. Typical plants are mosses, lichens, sedges, and
dwarf trees. Large herbivores include reindeer, caribou, and musk
ox. Small herbivores include snow-shoe hares, lemmings, and voles.
Many birds migrate there from the south in summer, feeding on the
large insect populations in the tundra during that season.
Carnivores are Arctic fox, wolves, hawks, falcons, and owls.
Northern Coniferous Forest
(Taiga) forms an almost unbroken belt across the whole of
northern North America and Eurasia—and is one of the most
extensive biomes. Its northern margin with the tundra is sharp,
being the Arctic tree-line, but its southern limit is less definite
—taiga is also found on high mountains in lower latitudes,
such as the southern Rockies. In the northern forests, the winters
are long and cold, the summers short and often very warm. The soil
in winter is mostly frozen to a depth of about 2 metres, but thick
snow cover can keep soil temperatures as high as -7°c. Trees
are mostly evergreen conifers, able to photosynthesize all year and
to resist drought (a result of strong winds and extreme cold) with
their needle- shaped waxy leaves. They remain undamaged by
snowfalls because of their overall shape. Taiga usually contains
vast tracts of one or two tree species only, except along rivers.
The soil is podsol and invariably contains a layer of ash- white
sand, due to the leaching-out of bases and clays by humic acids
(organic acids produced by the decay of plant material). Animals in
this biome are limited by severe winters and the small number of
different habitats. The most important large herbivores are
deer—more species live here than in any other biome. Rodents
are plentiful and can burrow under snow and survive harsh winters.
Carnivores include wolves, lynxes, wolverines, weasels, mink, and
sable; omnivorous bears are also found. Birds either are adapted to
feeding in taiga, such as crossbill, or are summer migrants feeding
on the vast seasonal swarms of insects.
Temperate forest Vast tracts of the taiga are still in
the natural climax state, but little climax forest remains in the
temperate forest biome.
There are 4 basic types of temperate forest, (i)
Mixed forest of conifers and broad-leaf deciduous trees. This was
the original climax vegetation of much of north-central Europe,
eastern Asia, and north-east North America—little remains
today. (2) Mixed forests of conifers and broad- leaf evergreens.
This once covered much of the Mediterranean lands but very little
is left. It still occurs in the Southern Hemisphere, in Chile, New
Zealand, Tasmania, and South Africa. (3) Broad-leaf forests almost
entirely of deciduous trees. This formerly covered much of Europe,
northern Asia, and eastern North America, and is found in the
Southern Hemisphere only in Patagonia. (4) The rare broad-leaf
forest consisting almost entirely of evergreens. This occurs
throughout much of Florida, and also in north-east Mexico and in
Japan. In the Southern Hemisphere it occurs on the southern tip of
South Island, New Zealand. All these regions have very high
rainfall, and the dripping forests have been termed "temperate
rain-forests". In all temperate forests, there is frequently an
understorey of saplings, shrubs, and tall herbs, which is
particularly well- developed near the forest edge or where human
interference has occurred. Temperate forests have warm summers but
cold winters, except on western seaboards. Winter temperatures may
fall below freezing-point. The deciduous trees escape these cold
winters by losing their leaves; many plants have underground over-
wintering organs. The fauna includes bears, wild boar, badgers,
squirrels, woodchucks, many insectivores, and rodents. Predators
include wolves and wild cats (on the decline), red foxes, and owls.
Large herbivores are the deer. This biome is extremely rich in bird
species, especially woodpeckers, titmice, thrushes, warblers, and
finches.
Tropical rain-forestoccurs between the Tropics of Cancer and
Capricorn in areas where temperatures and light intensity are
always high and rainfall is greater than 200 cm a year (and is at
least 12 cm in the driest month). Because of this, there is a great
variety of trees: in some parts of the Brazilian rain-forests,
there are as many as 300 species of trees in 2 sq. km. The popular
image of the jungle—thick, steamy, and impenetrable—is
borne out only in those areas that man has at some time cleared,
especially along river margins; true climax tropical forest has
very little undergrowth. The canopy is extremely dense; the light
intensity below may be as low as i per cent of that above, and thus
only a few extremely shade- tolerant plants can survive there. Life
is concentrated in the canopy, where there is plenty of light. The
crowns of the trees are covered with epiphytes—plants that
use the trees only for support and are not parasites.
Lianas—vines rooted in the ground but with leaves and flowers
in the canopy—are also characteristic. Dead plants are
rapidly decomposed, so there is little undecayed plant matter on
the forest floor. The rate of turnover of nutrients is very high
and the tropical forest has a higher productivity than that of any
other terrestrial biome. The tropical rain forest biome contains
the greatest variety of animal life of any biome, because of the
richness of the food resources that it offers and the relative
constancy of the conditions of the environment through the year.
There is a great profusion of birds with many different
diets—seeds, fruit, buds, nectar or insects. Many of the
mammals are adapted to arboreal life (monkeys, sloths, ant-eaters,
many small carnivores) but there are also many ground-living forms,
including rodents, deer and peccaries. Amphibia, and reptiles,
especially snakes, are important as predators of small vertebrates
and invertebrates.
Temperate grassland occurs in regions where
rainfall is intermediate between those of desert and of temperate
forest, and where there is fairly long dry season. Temperate
grassland has many local names—the prairies of North America,
the steppes of Eurasia, the pampas of South America, and the veld
of South Africa—but the dominant plants in all of them are
the grasses, the most widespread and successful group of land
plants. The soil always contains a thick layer of humus, unlike
forest soils, but is more exposed than the latter, and therefore
more likely to dry out. The dominant animals are large grazing
mammals—on the North American prairies, vast herds of bison
and prong-horn (which man had virtually wiped out by the close of
the last century, but is now reintroducing); over the steppes of
Eurasia, the saiga antelope, wild horse, and wild ass once roamed
in herds; in the South American pampas, the natural grazer is the
guanaco; and in Australia, the kangaroos fill this role. All these
have been largely replaced by man with domestic grazing animals,
often with disastrous results, as we shall see in Chapter 8,
although grasses are adapted to withstand the effects of natural
grazing.
Tropical grassland or savannah is a term applied to any
tropical vegetation ranging from pure grassland to woodland with
much grass. It covers a wide belt on either side of the Equator
between the Tropics of Cancer and Capricorn. The climate is always
very warm and there is a long dry season, and thus the plants often
have drought-resisting features. The grass is much longer than that
of temperate grassland, growing to 3 metres. There is often a great
variety of trees, which also show drought-resisting features; a
typical group is the acacias. The dominant animals are large
grazing mammals, the African savannah having the greatest variety,
and burrowing rodents are also found. Large carnivores, such as
lions and hyenas, prey on the grazers.
Chaparral occurs where there are mild wet winters and
pronounced summer droughts (known as Mediterranean climate), and in
areas with less rain than grasslands. The vegetation is
sclerophyllous (hard-leaf) scrub of low-growing woody plants,
mainly evergreen, with hard, thick, waxy leaves—adaptations
to drought. In the Northern Hemisphere it occurs mainly in
countries fringing the Mediterranean basin, but also in north-west
Mexico and California. Formerly this biome had a varied flora and
fauna, with many herbivores such as ground squirrels, deer, and
elk, and mountain lions and wolves as their predators, but this has
been greatly reduced by man. In the Southern Hemisphere there are
small areas of chaparral in southern Australia, southern Chile, and
South Africa.
Deserts are areas experiencing
extreme drought. A good definition is those areas where rainfall is
less than 25 cm per year, or—if higher—is mostly lost
immediately by evaporation. Deserts can be divided into hot deserts
(such as the Sahara) with very high daytime temperatures, often
over 5O°c, and low night-time temperatures below 2O°c
with relatively mild winters, and cold deserts (such as the Gobi
Desert in Mongolia) with very severe winters and long periods of
extreme cold. Typical desert has large areas of barren rock or sand
and very sparse vegetation. Desert plants are adapted to drought in
various ways: some have drought-resistant seeds; others have small
thick leaves that are shed in dry periods; yet others, such as the
New World cacti, are succulents, storing water in their stems.
Desert animals are mostly small enough to hide under stones or in
burrows during the intense daytime heat in hot deserts. Certain
rodents are well adapted to desert life—they live in cool
burrows, are largely nocturnal, and waste very little water in
their urine. Insects and reptiles lose little water, having
waterproof skins and excreting almost dry, crystalline urine.
Deserts spread when wind carries the top sand away, or when man
encourages his domestic animals to overgraze their edges.
Freshwater biomes are far less
self-contained than those of the surrounding land or the open sea.
They receive a continual supply of nutrients from the land, but
much of this is washed downstream in the rivers and there is an
overall loss of organic material. Thus they are generally far less
rich in nutrients than oceans, and usually less productive than
either sea or land environments. They are more changeable than
ocean or land biomes; rivers gradually wear away the land through
which they pass and thus the river biome itself gradually changes,
and many small ponds are seasonal, drying up in summer. There is a
wide range of freshwater environments ranging from small ponds and
streams to vast lakes and wide rivers. At the lower end of the
scale, they are often better considered merely as a wet extension
of the surrounding terrestrial biome.
The dominant plants of larger lakes and slow
rivers are phytoplankton, but larger floating and rooted plants
cover considerable areas. Many of the animals are restricted to the
freshwater habitat; amphibians, though living on land, need fresh
water in which to breed; land animals use fresh water for drinking
and bathing; and many birds are adapted to the freshwater habitats.
Animal communities in large lakes correspond to planktonic,
nektonic, and benthic communities of the oceanic biome (see below).
Some large lakes have well-defined shores, constituting sub-biomes:
examples are the Great Lakes with their dune systems, or Lake
Victoria with its muddy shores. Marshes (salt marshes and
freshwater marshes) are best considered as intermediate between
marine or freshwater biomes and the surrounding terrestrial biome,
and estuaries are transitional both between freshwater and marine
biomes and also between the water and the land. They have a very
complex structure and are highly productive, with a great variety
of plant and animal life. Freshwater habitats have suffered greatly
from pollution by man— toxic industrial wastes, detergents,
and vast quantities of sewage are dumped into rivers and lakes, and
cause the extinction of all but a few resistant forms of
life.
Marine biomes Land covers only 29 per
cent of the earth's surface, whereas the oceans take up 71 per
cent, with an average depth of 3900 metres. It is impossible to
distinguish regional biomes in the seas, because of the uniformity
of the marine environment and of the ease of distribution of its
inhabitants. On the land, animals and plants of different latitudes
have different life-forms; in the sea, animals do have distinctive
forms, but these vary according to the depth at which they live,
rather than according to latitude—for example, deep-sea
animals are of a life- form especially adapted to cope with high
pressures and total darkness. Water has a higher specific heat than
soil or rock, and even the warmest oceans never reach the high
temperatures of tropical forests or hot deserts. Similarly, the
coldest seas are never as cold as the tundra or northern forests.
The surface temperature is never greater than 3o°c and rarely
falls below o°c. Marine organisms obviously have no problems
in obtaining sufficient water, but light is a limiting factor. The
tiny photosynthetic plants (phytoplanktori) are restricted to the
upper photic zone (the uppermost 200 metres); virtually no light
penetrates below 500 metres. Atmospheric oxygen and carbon dioxide
are plentiful at the surface and these gases are also dissolved in
the water. Pressure is an important factor limiting the downward
movement of shallow-water species. Sea-water is much richer in
nutrients than fresh water, and these are recycled to the photic
zone by upwellings of deep currents. In some other areas, surface
waters converge and descend. Where these are already exhausted of
nutrients, the area of descent forms a "desert", such as the
Sargasso Sea in the southern North Atlantic. Such areas are the
only virtually unproductive parts of the surface waters.
There are three principal marine biomes. (i) The
oceanic biome of open water, away from the immediate influence of
the shore. This is further divided into the planktonic sub-biome
containing free-floating plankton (mostly microscopic organisms
with buoyancy mechanisms); the nektonic sub-biome of active
swimmers, including fish, squids, turtles, and marine mammals; and
the benthic sub-biome, whose fauna is especially adapted for life
on the sea floor. (2) The rocky shore biome, dominated by large
algae that show zonation up the shore (see Fig. 21). Life here is
in constant danger of desiccation when uncovered by the water. (3)
The muddy or sandy shore biome where mud and sand is constantly
being washed ashore by the sea, and the animals are often in danger
of being buried. The main plants are thin green algae growing in
flat sheets on the shore, such as the sea-lettuce (Ulva). Animals
include burrowing worms, and also wading birds, which are important
predators of the invertebrate fauna.
Soil The distribution of soils is
another important aspect of bio- geography. Terrestrial plants are
mostly rooted in the soil and obtain from it water and their
nutrients, such as nitrates and phosphates. Because all animals are
ultimately dependent on plants for food, they are also in turn
dependent on the soil. Many factors influence the ability of plants
to root and take up nutrients from the soil. Most important are the
structure of the soil and its texture (the size of the particles of
which it is composed), the amount of nutrients actually present,
and the quantity of water with gases dissolved in it, and air
spaces, that it contains. These are different in different types of
soil. It is not surprising, therefore that soil has a strong
influence on the distribution of living things. However, the
relationship between organisms and soils is complex because the
soil is largely produced by interaction between the organisms that
live in it and the local climate.
Habitats and microhabitats
In the British Isles, the badger is most common
in southern and western England and rarest in East Anglia and parts
of Ireland and Scotland. Where they occur, badgers most frequently
make their burrows orsetts in woods and copses,
especially where these give easy access to the pastureland where
badgers often feed, and where the soil is well-drained and suitable
for digging. The biologist calls these places where the badger
lives its habitat. Nearly all animal and plant
species seem to have such recognizable habitats where they are
found more often than in other places.
For many organisms, especially larger ones,
distribution can be conveniently considered in terms of such units
of habitat as "woodland," "grassland," or "seashore." But most
species have specific distributions even within such units of the
environment as the habitat. The woodland habitat, for instance,
consists of a host of smaller microhabitats—the
humus and leaf-litter layer of the soil, rotting logs, the ground
flora zone, the various levels of the tree canopy, tree trunks, and
beneath the bark of living trees. Certain characteristic species of
animals and plants are found in each of these microhabitats, and so
the distribution of these species in woodland coincides more or
less closely with that of their microhabitats. Some species are
found in more than one of these areas, but generally each species
has a particular microhabitat that may be termed its
"headquarters," in which it occurs most frequently and in the
highest numbers. Even within habitats that are simpler in structure
than woodland—such as grassland—many different
microhabitats occur and, as will be explained later, the number of
microhabitats present is an important factor determining the number
of species that may live in a habitat.
Many quite large and active animals show a
tendency to confine themselves to certain parts of a large habitat.
The spider monkeys (Ateles') of the lowland
forests of central and northern South America are active climbers,
able to jump long distances, and family groups move about the
forest a great deal. If the monkeys are observed over a period of
time, however, they can be seen to spend most of their time in the
lower parts of the high canopy of the forest, and especially on the
smaller peripheral branches of the trees. The reason for this is
clear; over 90 per cent of the diet of spider monkeys consists of
fruit and nuts, and these food resources will naturally be found
most abundantly on the smaller, fast-growing parts of the trees. It
is probably best to call areas like these, preferred by a largish,
active animal, minor habitats, rather than
microhabitats, and to keep the latter term for subdivisions with
more clearly defined boundaries.
Limits of distribution
Surrounding the areas of a species' distribution,
whether this is considered on a geographical, habitat, or
microhabitat scale, are areas where the species cannot maintain a
population because physical conditions or lack of food resources
are too extreme to permit survival. These areas can be viewed
as barriers that must be crossed by the species if
it is to disperse to other favourable., but as yet uncolonised,
places—much as the European settlers had to cross ocean
barriers to colonize North America or Australia. Any climatic or
topographic factor, or combination of factors, may provide a
barrier to the distribution of an organism. For example, the
problems of locomotion or of obtaining oxygen and food are quite
different in water and air. As a result, organisms which are
adapted for life on land are unable to cross oceans: their eventual
death will be due, in varying proportions, to drowning, to
starvation, to exhaustion and to lack of fresh water to drink.
Similarly, land is a barrier to organisms which are adapted to life
in sea or fresh water, because they require supplies of oxygen
dissolved in water rather than as an atmospheric gas, and because
they desiccate rapidly in air. Mountain ranges, too, form effective
barriers to dispersal because they present extremes of cold too
great for many organisms. The amount of rainfall, the rate of
evaporation of water from the soil surface, and light intensity are
all critical factors limiting the distribution of most green rooted
plants. But in all these cases, and in most others, the ultimate
barriers are not the hostile factors of the environment but the
species' own physiology, which has become adapted to a limited
range of environmental conditions. In its distribution a species is
therefore the prisoner of its own evolutionary history.
At the habitat level, the microhabitats of
organisms are surrounded by areas of small-scale variation of
physical conditions, or microclimates —similar,
but on a much smaller scale, to geographical variations in
climate—and of food distribution. These form barriers
restricting species to their microhabitats. The insects that live
in rotting logs, for instance, are adapted by their evolution to a
microhabitat with a high water content, and relatively constant
temperatures. The logs provide the soft woody materials and
micro-organisms they need for food, and also give good protection
from predators. Around the logs are areas with fewer, or none, of
these desirable qualities, and attempts by the animals to leave
their microhabitat would result for many of them in death by
desiccation, starvation, or predation.
Overcoming the barriers
A few inhabitants of rotting logs do occasionally make the
dangerous journey from one log to another, and this shows that very
few environmental factors are absolute barriers to the dispersal of
organisms and that they vary greatly in their effectiveness. Most
habitats and microhabitats have only limited resources, and the
organisms living in them must have mechanisms enabling them to find
new habitats and resources when the old ones become exhausted.
These mechanisms often take the form of seeds, resistant stages,
or—as in the case of the insects of the rotting-log
microhabitat—flying adults with a fairly high resistance to
desiccation.
Relatively new volcanic islands, such as the
Galapagos have been eventually colonised as a result of dispersion
by air and sea. Human transport of species across the world
either deliberately or inadvertantly has overcome many former
continental barriers to species.