Biogeography of the Earth

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The distribution of plants and animals (or flora and fauna) is related to the variation in climate, soils, and topography on Earth. Over time, plants and animals have occupied and adapted to the particular environmental conditions in which they live. The giant saguaro cactus, for example, stores water in fleshy stems to nourish itself in the hot desert, while the heavy, shaggy coat of the musk oxen helps protect it from the cold arctic wind. In this chapter you will become familiar with what affects the geographic distribution of plant and animal species1.

We describe areas with an abundance of vegetation as net primary productivity (see short animation hereor Productive regions are dark green, less productive light brown. The most productive regions appear through the tropics, while the subtropics show very low productivity. The region of high net productivity shifts north and south through the year1. Why does this spatial and temporal pattern in net primary productivity occur and what are the implications? Will this pattern change in the future?


Biogeography is the study of the geographical patterns of plant and animal species. To understand the distribution of plant and animal species on Earth, a fundamental knowledge of ecology and ecosystem dynamics is required. Ecology is the study of the interactions among organisms. An ecosystem is a functioning entity of all the organisms in a biological system generally in equilibrium with the inputs and outputs of energy and materials in a particular environment. It is the basic ecological unit of study. There are two kinds of ecosystems, aquatic and terrestrial. An ecosystem is comprised of habitats, biological communities, and ecotones1

A biome is often referred to as a global-scale community of plants and animals and is the largest subdivision of the biosphere. A biome may contain many different kinds of smaller ecosystems. Biomes are typically distinguished on the basis of the characteristics of their vegetation because it makes up the largest portion of biomass. Biomes are subdivided by formation class, vegetation units of a dominant species1.

habitat is the natural environment in which an organism lives. Most African elephants live on savannas and in dry woodlands.  Bass prefer a habitat of warm, calm, clear water and are usually found in slow-moving streams, ponds, lakes, and reservoirs. Habitats can be identified at different spatial scales. Macrohabitats are delineated by climate and subdivided on the basis of their vegetation1

Microhabitats are smaller in size, such as the habitat along a stream channel or a layer within the canopy of a rain forest. Each species has specific habitat parameters (temperature, moisture, and nutrient availability)1

Within a habitat, organisms “occupy” a niche. A niche is the function or occupation, of a life-form within a community. An organism’s niche incorporates the physical (habitat), chemical, and biological factors that maintains the health and vitality of the organism. An organism’s interaction with the abiotic factors of its environment (heat and moisture) defines its niche. The food requirements, and those that prey on it, are part of the organism’s niche. A niche, therefore, is the sum of an organism’s physiological adaptation to, and interaction with, its physical environment1. The image below depicts an example of how different birds help support a forest ecosystem.

The variation of life determines the biodiversity of an ecosystem. The biodiversity of an ecosystem reflects the variety and abundance of plant and animal species within it. It includes the variety of habitat types with a landscape that support life1.

The effectiveness of an organism to occupy a habitat depends in part on its means of transportation. Animals must use their own locomotion, while plants disperse by wind, running water, ocean currents, and animals. Thus, climate and topographic barriers are more of an impediment to animals than plants. For either, continental drift poses a significant barrier to diffusion. The separation of continents has isolated plants and animals in the past thus preventing their complete occupation of a suitable habitat. Continental collisions have opened land bridges for habitat occupation. Sea level changes have similarly affected plant and animal distributions. Lowered sea level, as what occurred during the last ice ages, resulted in chains of islands being connected opening migration routes for animal and plant species. Sea level rise during post-glacial times isolated habitats. Isolation thus prevented plant and animal migration1. Presently, trends in global warming are affecting the distribution of parasites carrying infectious diseases (more on this when we discuss climate change at the end of the semester).

Sometimes humans aid in the dispersal of plants and animals. Humans have intentionally or unintentionally introduced species into habitats that they would otherwise not have been able to on their own, sometimes with disastrous effects. The inadvertent introduction of the African Honey Bee in South America is a notable example. Imported to spur production of honey by mating with other native species, this aggressive bee was accidentally released. With few natural predators, populations exploded and has migrated to the southern United States. People have been attacked by swarms of these “killer bees” when disturbing them. Hawaii’s biota evolved in relative isolation. But after its discovery by white culture, the inadvertent, and the sometimes purposeful introduction of alien plant and animal species, have endangered Hawaii’s native organisms1.

Principal of Limiting Factors

The plants and animals that succeed in occupying a particular niche are those that can easily adapt to the unique environmental conditions of a site. Each plant and animal in the community has a specific range of tolerance for particular environmental conditions. Climate factors are the most important influence over the successful establishment of plant and animal communities. Two climatic factors are important, sunlight and moisture1.

Not only is the amount of sunlight available important but the duration and quality of light are important too. For instance, at high altitudes the intense ultra violet light may inhibit the growth of particular plants. The intensity of light affects photosynthesis and rate of primary productivity. The duration of sunlight affects the flowering of plants and the activity patterns of animals. The availability of water is important for the survival of most life forms. But plants require water for a number of life processes like germination, growth and reproduction too. The principle of limiting factorssays that the maximum obtainable rate of photosynthesis is limited by whichever basic resource of plant growth is in least supply. The availability of energy and moisture varies geographically. For example, at high latitudes the limiting factor is generally energy availability while in low latitudes moisture is the limiting factor to growth1.

Plants of a particular region have adapted to the temperature and moisture conditions in which they live. Most gardeners are familiar with plant hardiness (growing) zone mapsor The zones are based on the minimum temperature experienced and thus tolerated by different species of plants. There have been recent signs that these zones are starting to shift due to global warming1.

Ecology of Vegetation and Plant Succession

Plants evolve a variety of adaptations to the light and moisture availability within a particular environment in order to flourish. Plants adaptations include those of leaf form and canopy structure (the roof of foliage formed by the crowns of trees). For instance, a hard, needle leaf structure is an adaptation to extreme temperatures and low moisture status in winter. The leaves of some rain forest trees have a special joint at the bottom of their stalk that enables them to twist and turn to follow the light as the sun passes from east to west overhead. Deciduous trees drop their leaves to cut transpiration loss during dry periods and when temperatures are very cold1. We have deciduous plants here in the Bay Area, but the east coast in particular is known for spectacular fall seasons as the foliage changes color and the leaves drop (see image on the next page).

Conifer needles are an important adaptation to the extreme conditions present in the climate of the boreal forest. Pine needles contain very little sap, so freezing is not much of a problem. Conifer needles have a unique structure which limits the loss of water, a precious commodity in this environment. Pine needles have fewer stomata than broadleaf tree leaves. The stomata are recessed into pits on the needle and aligned in a groove on its underside. The groove in the needle creates a small layer of still air which slows the loss of water vapor by diffusion. Water loss is further reduced by the thick waxy coating common to pine needles. Water is “shut off” from the tree when the ground completely freezes. Under these circumstances the stomata close-up to prevent loss of water from the tree1.

Fleshy “leaves”, like those of desert succulents or thick skin like that of the giant Saguaro cactus helps retain moisture. The Baobab tree (see image on the next page), found in the wet/dry tropical (savanna) climate stores water in its trunk to combat the long drought period experienced in that climate.

Plants have adapted particular root structures to live in arid regions. Deep tap roots draw moisture hidden deep below the surface while extensive near – surface root systems catch moisture as it infiltrates into soil. The Havard Oak (see below) is a shrub found in the semi-arid southwestern United States. It is well adapted to the dry conditions having an extensive root system and tap roots that extend 15 to 20 feet deep. Tap roots “equal to a man’s thigh” are not uncommon. Above ground, thick waxy leaves reduce water loss through transpiration. Some desert grasses have rolled surfaces to reduce water loss from the inner surface and hairs which reduce air movement1.

Canopy structures reflect the environmental conditions vegetation grows in. The conical canopies of conifers help shed snow and catch low angle sun rays during the long winters where they grow. The rain forest displays a multi-layered canopy. Each layer possesses organisms adapted to the environmental conditions found in it. A canopy can be so thick and dense, like that found in the rain forest, that little light penetrates to the surface. The lack of light for understory growth creates an open forest structure that you can see into for some distance. Where canopy density is low, more light filters to the surface creating a thick ground cover and a closed forest structure. Standing on the floor of a closed forest, it’s nearly impossible to see more than a few meters into it1.

The image above provides a view of a canopy structure in a tropical rainforest. Emergents are those trees that have grown above the main canopy tree line. Lianas are vines that grow typically on tree trunks.

Plant Communities

Rarely is any location dominated by a single species of plant. A plant community refers to the associated plant species that form the natural vegetation of any place. For instance, a midlatitude forest is comprised of a community of trees, shrubs, ferns, grasses, and flowering herbs. Plant communities provide a habitat for animals and significantly modify the local environment. Plant communities affect soil type when organic material decomposes into the soil altering soil moisture retention, infiltration capacity, soil structure and soil chemistry. Trees shade the forest floor, reducing incident solar radiation and lowering temperatures of both the soil and the air. Reduced incident light decreases evaporation keeping soils moister beneath the forest canopy. These impacts affect animal habitats and the diversity of animal species which are associated with these plant communities1.

An ecotone (see image on the following page) is a plant community in a distinct zone of transition between other more extensive communities. Ecotones vary in scale, from local (between forest and field) to global (savannas). Within an ecotone plants of different environmental tolerances often intermingle. For instance, grasses adapted to low moisture conditions intermingle with deciduous trees within a prairie – forest ecotone1.

Plant Succession

Natural vegetation of a particular location evolves in a sequence of steps involving different plant communities. The evolutionary process is known as plant succession. Plant succession usually begins with a fairly simple community known as a pioneer community. The pioneer community, and each successive community alters the environment in such a way to permit new communities to occupy a site. These alterations of the environment include changes in site microclimate and soil conditions1.

climax community is the result of a long period of plant succession. Climax communities usually exhibit a good deal of species diversity and thus are relatively stable systems. Disturbance renews a successional sequence. Plant succession was renewed after the explosion of Mt. St. Helens with the subsequent disruption of biotic communities that inhabited the region. Human disturbance related to tropical deforestation has renewed the successional sequence of plant communities in the tropical rain forest1. Here(or

is a nice, short video depicting various climax communities throughout the world.

Tropic Levels and Food Chains

The biotic elements that comprise an ecosystem fall into one of several trophic levels. The trophic level of an organism is its position in a food chain, the sequence of consumption and energy transfer through the environment. For example, a simple grazing food chain is comprised of a producer (i.e. vegetation that can photosynthesize and produce simple sugars for consumption), a primary consumer (i.e. an herbivore than consumes a producer), a secondary, and possibly tertiary consumer (i.e. an organism that consumer a primary consumer), an apex predator (i.e. an organism that is at the top of the food chain and is not consumed by anything other than decomposers), and lastly decomposers (i.e. an organism such as bacteria or fungus or mold that breaks down and consumes organisms). Note that producers, especially those thriving in soil, will use those nutrients produced by the decomposers and the whole food chain will start over again.

Two laws of physics are important in the study of energy flow through ecosystems. The first law of thermodynamics states that energy cannot be created or destroyed; it can only be changed from one form to another. Energy for the functioning of an ecosystem comes from the Sun. Solar energy is absorbed by plants where in it is converted to stored chemical energy1.

The second law of thermodynamics states that whenever energy is transformed, there is a loss energy through the release of heat. This occurs when energy is transferred between trophic levels as illustrated in a food web. When one animal feeds off another, there is a loss of heat (energy) in the process. Additional loss of energy occurs during respiration and movement. Hence, more and more energy is lost as one moves up through trophic levels. This fact lends more credence to the advantages of a vegetarian diet. For example, 1350 kilograms of corn and soybeans is capable of supporting one person if converted to beef. However, 1350 kilograms of soybeans and corn utilized directly without converting to beef will support 22 people!1