.Ecosystems and Biomes

.Ecological Niches

.Plant and Animal Dispersal

The miracle of our planet is its life mysterious in origin and wondrously diverse-inhabiting every portion of the surface of the lands and the soils below, the surface waters of the continents, the oceans, and the lower atmosphere.

All of these are components of the biosphere: the rea;m of living organism on our planet. The organism inhabiting the biosphere are incredibley diversified, consisting of more than 50 primary subdivisions known as phyla. The system for classifying organisms descends from phyla through classes, orders, families, and genera, to the species level.

A species is composed of related inviduals that closely resemble one another and are able to breed only among themselves. About one and a half million species have been named and described in some manner. Two-thirds of these are animals-most of then insects. It is estimated that as many as 8 or 9 million unnamed and undescribed additional species exist, as many as a million species of mites alone!

Although we often speake of the planet and animal kingdoms, this is an oversimplification; some authorities recognize four or five kingdoms at the broadest level of classification. Vast numbers of unicellular bacteria, algae, slime molds, fungi, and related life forms are neither plant nor animal according to usual definitions, and must be assigned to seperate catgories.

The criteria on which such broad classifications are based include cell characteristics, mode of nutrition (absorption, photosynthesis, or ingestion), degree and mechanism of motility (movement), mode of reproduction, level of organization and degree of tissue differentiation, and development of sensory-neuro-motor system and muscle fibers.

All forms of life require energy and liquid water. The ultimate energy source for all but certain chemolithotrophic bacteria is solar radiation. On planets beyond the earth's orbit the lower level of slar energy received causes any water present to be frozen and therefore unable to react chemically, thus precluding biological activity. The two planets within the earth's orbit and nearer to the sun (Mercury and Venus) receive so much energy that their water has been vaporized and is present only in the upper atmosphere of Venus (Mercury has too little gravity to have retained an atmosphere).

The earth's nearest planetary neighbors, Venus (nearer to the sun) and Mars (farther from the sun) differ enormously. The surface temperature of Venus is high enough to mely lead. This is a result of the planet's proximity to the sun and of a " runaway " greenhouse effect produced by an atmosphere rich in Co2. By contrast, Mars is a cold and nearly airless desert, swept by dust storms, with eidence of a permanently frozen layer below the land surface.

Neither planet has any trace of liquid water. Thus it is not surprising that instrument packages landed on their surfaces hane failed to sense any indication of life. On the fortunately situated earth, where liquid water is uniquely abundant, solar energy is converted to chemical energy by the earth's green plants through the process of photosynthesis. The chemical energy is stored in plant tissue; it causes plant growth, and because plants are ingested as food by animals, it also sustains all animal life, including humans.

Even the fossil fuels, coal, petroleum, and natural gas have chemical energy derived from solar energy and concentrated in former plants and microscopic marine animals.

Physical geographers are concerned with the precesses of the biosphere, including the ways in which solar energy and precipitation control biotic productivity, the resultant interactions among various life forms, how these interactions vary geographically, and how life forms have spread over the earth's surface and adapted to various habits. These are all extensive topics, and we can give only the briefest introduction to them here. They are subject matter of the subfield of biogeography.

.Energy Flow Through Ecosystems .Food Chains and Food Webs .Energy Flow Pyramids

PLANT AND ANIMAL INTERACTIONS

The precding information haver shown that the atmospheric circulation patterns can be thought of as a global delivery system for energy and moisture. In the following we shall relate the patterns of natural vegetation types on the earth to associate climatic characteristics.

We shall also see that plants and animals live in associations, or communities, that offer mutual benefits to the individual members. Animals rely directly or indirectly on plants for food (chemical energy), but they also help to propagate and spread plants. Squirrels not only eat nuts and acorns; they also bury them for future use, thereby planting future trees.

Animals that eat fruit distribute the indigestible seed in their fecal droppings. In the wet tropics even fish eat fallen fruit, convey the seeds, and thereby help to propagate forest trees. Flowers, with their perfumes and bright colors, could have evolved only to attract animals to the plants.

Insects that seek flower nectar inadvertently transport the pollen that is essential for plant reproduction. Without such insects many kinds of plants could not survive. Clearly, plants have evolved mechanisms to take advantage of animal activity, and animals have evolved processes to utilize plant resources.

Ecosystems and Biomes

The interactions of plants and animals, and their relationship with their physical environment, give the communities definite patterns of organization. However, the task of tracing and understanding all of these interactions isextraordinarily difficult. For example, a typical midlatitude forest may contain fifty or more spicies of plants and thousands of species of animals, primarily insects. Each seems to play a role in the functioning of the community as a whole.

The study of the interactions among organisms with their particular habitat or environment is known as ecology (from the Greek oikos: house). One way to organize the character of these interactions is to focus on the basic ecological unit, which is known as an ecosystem. An ecosystem is a community of plants and animals generally in equilibrium with the inputs of energy and materials in a particular environment. Thus an ecosystem has both biotic (living) and abiotic (non-living) components.

An ecosystem can be as small as a tidal pool or a single tree, or as large as a lake or a forest. Its boundaries may be either sharply defined or gradational, and it may include several different biotic communities. The concept is a flexible one, defined by function rather than scale.

An ecological system on the largest scale is known as a biome. Biomes are highly generalized ecosystem types recognized on a global scale, such as deserts, temperate grasslands, or tropical rainforests, ( such as we have in the Commonwealth of Dominica), with their faunas and associated physical characteristics. The concept of biomes includes broad climatically influenced associations of plants and animals, that can be portrayed on a world map.

Every biome is a synthesis of many seperate ecosystems, unified by some common characteristic such as the general morphology of the vegetation (i.e., forest, grass, or shrubs), and associated animal types. Biomes are designated in various ways, and may be divided into subtypes.

. Ecological Niches

Associated with the concept of the ecosystem is the idea of the ecological niche, which focuses on the specific ways an organism actually functions in its particular habitat. The ecological niche of a species is the combinationation of environmental factors under which the spesies can exploit a source of energy sufficiently to be able to survive, reproduce, and successfully colonize other similar habitats. The scale of an ecological niche varies with the organism under conisderation.

For bacteria it may be a microscopic pit on a rock surface. A single grizzly bear's niche might be the full extent of a sprawling mountain range and the surrounding lowlands. It is very interesting to note that organisms do not occupy all of the ecological niches to which they are well adapted. Example: there are no polar bears in the Antarctic region, despite the availability of appropriate climatic conditions, and abundant food supplies in Antarctic waters.

Conversely, penguins are not found in the northern hemisphere, though they should do well in the Arctic. By the same token, the large apes (chimpanzees and gorillas) of the equatorial forest of Africa are not found in similar environments in Central and South America, which contains plentiful resources that these animals normally exploit. The African giraffes would be at home in the Australian bushes and the tree-studded llanos of Venezuela, however, they are not there, and were never there. These are sure glaring large scale examples of ecological niches that is underutilized.

Plant and Animal Dispersal

Biogeographers are interested in knowing why some ecological niches are completely exploited and others are not. The explanations involve the geographical locations of the centers of evolution of the organism adapted to the niche, their mode of dispersal, competition from other organisms, and geographical barriers to their diffusion. These barriers include impassable to-pography, large water bodies, and unsuitable climates.

A high quantity of ecological niches are more effectively occupied by plants than by animals. Animals do colonize new areas rapidly, nonethelsee nust do so by their own movement (locomotion) while plant dispersal takes place by means of running water, the wind, animals, and the ocean currents. Surely, cold-adapted polar bears and penguins could not cross the low latitude deserts, and humid equatorial region to go from one hemisphere to another, though water barriers could not present any difficulty to them.

And in order for gorillas to migrate outward from the African forests, they would have to swim the oceans or cross the deserts, thus lacking water, and suitable food resources. However, had they evolved about two hunderd (200) million years ago in Triassic times rather than within past nine (9) million years, gorillas could have walked overland into the Americas, which for many millions of years were connected to the African continent. And so, one can imagine that indeed, the dinosaurs of the Triassic period were common to both the Old and New Worlds.

The mysteries of plant and distributions can be explained by the horizontal movements of the continents, and can be discussed at legnth. The continents appear to have been in motion on the earth's surface since their formation: colliding and separating, opening up migration routes and then sundering them, permitting multitudes of organism to spill outward from centers of evolution at certain times, and locking them in isolation at other times.

Some continental areas, i.e., Australia, long isolated from other land masses, has the most primitive and unusual of all existing faunas, including the pouched marsupial animals epitomized by the kangaroo and duck-billed platypus. The marsupials evolved more than 0ne hundred (100) million years ago in South America, spread to Australia, perhaps by the way of an ice-free Antarctica. Nonetheless, the highly diversified marsupials remaining in South America were unable to compete with the large numbers of more modern placental mammals entering from North America after the link at Panama was established only 2 or 3 million years ago. In isolated Australia the marsupials experienced no such competition, and have lingered on to the present as a boilogical curiosity.

Modification of sea level also affect animal dispersal. The classic example of faunal discontinuities resulting from sea level fluctuations is the area of Malaysia, Indonesia, and Australia. Some scientific observers recognize various Biogeographical Realms, groupings which reflect the evolutionary centers and patterns of dispersal of specific plant or animal associations. These realms reveal the close proximity of the northern hemisphere land masses to each other over a long period of time, the barrier imposed by the subtropical deserts, and the isolation of certain landmasses, and oceanic islands.

Biological realms contain related species. They are different from the biomes resulting from the convergent evolution of different species toward similar forms suited to particular habitats. Thus the very similar appearing plants and animals of the desert biome in various parts of the world are actually genetically unrelated species that have evolved virtually identical strategies to minimize heat stress and water loss. Such adaptations give the flora and fauna within the various geographic components of a global biome very similar appearances, though entirely different evolutionary tracks and Biogeographical Realms are represented.