Thursday, December 09, 2004

Ecology of Natural Systems - Notes

Good Afternoon Fellow Environmentalists:

For those of you who were not able to receive the Natural Systems notes, here they are:

Ecology of Natural Systems Notes


Ecology is the study of the interactions between living things and their environments. Ecology comes from the Greek word oikos, which means home. The word ecosystem refers to the system of interactions between living and non-living things. Ecosystems are sometimes described in terms of the interactions and sometimes in terms of the area where interactions occur. This second definition is no longer as widely accepted as it once was. Newer ecosystem definitions emphasize the concept of interaction.

e·col·o·gy ( -k l -j )
(Biology, Ecology)
n. pl. e·col·o·gies
a. The science of the relationships between organisms and their environments. Also called bionomics.
b. The relationship between organisms and their environment.
2. The branch of sociology that is concerned with studying the relationships between human groups and their physical and social environments. Also called human ecology.
3. The study of the detrimental effects of modern civilization on the environment, with a view toward prevention or reversal through conservation. Also called human ecology.

[German Ökologie : Greek oikos, house; see weik-1 in Indo-European roots + German -logie, study (from Greek -logi , -logy).]
ec o·log i·cal ( k -l j -k l, k -), ec o·log ic (- k) adj.
ec o·log i·cal·ly adv.
e·col o·gist n.

Ecology is the scientific study of the processes influencing the distribution and abundance of organisms, the interactions among organisms, and the interactions between organisms and the transformation and flux of energy and matter.
The Institute of Ecology, (IES) definition emphasizes several things:
· The starting focus on organisms, aggregations of organisms, or systems incorporating organisms or their by-products.
· The bounding of ecology by both biological and physical sciences.
· The breadth of subject matters within ecology.
· The joint consideration of both biotic and abiotic aspects of the natural world.
· The focus can be on different proportions of biotic or abiotic aspects of nature depending on the ecological specialty.
· The relationships between organisms and the physical world can be bidirectional, although different specialties may emphasize the effect of the organisms (and systems containing them) on the physical world, or the effect of the physical world on the organisms.
· The boundary between the abiotic and the biotic aspects of ecology is blurry.
· The disciplinary focus is on "processes" "interactions" and "relations" rather than on the physical entities per se.
Ecology was originally defined in the mid-19th century, when biology was a vastly different discipline than it is today. The original definition is from Haeckel, who defined ecology as the study of the relationship of organisms with their environment. In the intervening century and a half, other definitions of ecology have been proposed to reflect growth of the discipline, to found new specialties, or to mark out disciplinary territory. This essay will consider some of the characteristics of these definitions, and will explore some conditions that suggest the need for a modern, inclusive definition of ecology.
There are three kinds of definition of ecology that are currently afoot. The first is basically the Haeckelian form -- the study of the relationship between organisms and environment. The second definition, which is perhaps the one that is heard most often, considers ecology to be the study of the distribution and abundance of organisms (Andrewartha and Birch 1954). The third definition is one that focuses ecology on the study of ecosystems (Odum 1971). The definition proposed by Elton (1927), that ecology is scientific natural history, will not be considered further, because it seems to be rather too quaint to underlie a modern science.
Desiderata of a Definition
A definition of ecology should have several characteristics. It should be brief, simple and direct, inclusive, reflective of the scope of the modern discipline, and allow for changes in the detail of the science. In addition, depending on the audience, the definition should indicate that ecology is a science rather than an ideology. Likewise, the discipline can emerge as concerned with the dynamics, or process of its subject, rather than only its objects and their statics. An important feature of a definition of ecology would be to point to both the abiotic and the biotic components of the natural world.
An Overview of the Three Kinds of Definition
The classical Haeckelian definition emphasizes both the living and the non-living components of the natural world. However, as a reflection of its vintage, it emphasizes that organisms are the relevant manifestation of the biotic world. The mid-19th century with its largely macroscopic view of the world neglected inconspicuous organisms, such as microbes, the chemical products of organisms in the environment, and ecological systems at larger scales or higher hierarchical levels than organisms.
Andrewartha and Birch (1954) reinforced the focus on the organism as the core of ecology. Their work clearly includes the abiotic environment as well as the biotic environment as factors influencing distribution and abundance. This is shown by their recognition of the importance of climatic fluctuations, for example. However, in its application, the definition of Andrewartha and Birch has often been associated with apredominately biotic focus. This definition has become somewhat of a rallying cry for community and population centered ecology. Clearly, this definition has not stimulated exploration of the frontier of ecology with the sciences of the physical environment.
The third definition of ecology emerges more from use than its original statement. Odum (1971) began with the Haeckelian definition, but his desire to establish a new kind of ecology -- ecosystem ecology -- led him further from that cornerstone than most. He provided several statements of the scope of ecology, including the difficult-to-interpret statement that ecology was simply environmental biology. Truest to his brand of ecosystem thinking was his definition of ecology as the study of the structure and function of nature. Although Odum's extreme reliance on emergent properties and resuscitation of superorganismic thinking have proven problematic to many ecologists, his loosening of the bonds of Haeckel's focus on the organism is useful.
The three kinds of definition each have their limits and advantages. The positive side of the first definition is that it is simple, and it emphasizes both biotic and abiotic aspects of nature, but on the negative side, perhaps it overemphasizes the organism as the focus. A positive point often overlooked, is that Haeckel's definition takes the relationship between the organisms and abiotic as the subject of ecology. Statements of the Haeckelian sort always should be cast as the study of relationships, rather than the study of organisms in relation to environment. The difference in emphasis may appear to be minor, but it indicates the deficiency of Haeckel's definition. The second definition emphasizes is positive in its emphasis on quantifiable and unambiguous parameters on which to focus, but it falls short because it omits many important ecological subjects. To the credit of the third definition is the recognition that ecology is about process and is not restricted to pattern or organisms alone. All of these definitions, of course, take as their starting point organisms. However, they are not in all cases explicit that ecology can consider all manner of systems (in the broadest sense) that include organisms and their products.
This brief overview emphasizes that the definitions have limits or connotations imposed by their vintage and history of use. Haeckel operated in a time when biology was dominated by focus on organisms as anatomical, physiological or taxonomic subjects. Many of the modern concerns of ecology, and indeed of biology, were far in the future when Haeckel wrote. Odum was concerned with the justification of ecosystem ecology as an academic specialty. He highlighted ways in which ecology differed from other university departments in the immediate post-World War II era.

Ecosphere - In ecology: the Earth, all of the organisms living on it, and all of the environmental factors that act on the organisms.

Ecosphere = Biosphere + Geosphere

Biosphere - n] the regions of the surface and atmosphere of the Earth (or other planet) where living organisms exist.

Geosphere - The non-living portion of the earth, excluding the atmosphere, hydrosphere and biosphere.

Summary of Major Influences on Global Climate
This first glance at the whole Earth System and how it determines the state of the global climate should give you a good sense its complexity and some of the important interconnections. You might also begin to see that the operation of the system involves several cycles (carbon cycle, water cycle, etc.), which are systems that are more or less closed. In these cycles, material passes from place to place in a cyclical fashion. If you track a particular water molecule over millions of years, you may see it move from the atmosphere to the land to the oceans to the deep interior, then up through a volcano to the atmosphere. Many of these cycles represent important subsystems of the global climate system, and we will explore these cycles throughout this book.


Ecosystem -- A discrete ecological unit consisting of all of its constituent organisms and its total environment.
1. The Earth is composed of both living and nonliving parts. The living parts include animals, plants, fungi and microscopic organisms; the nonliving parts include the sun, water, air/gases, rocks and land forms.
2. An ecosystem is a group of living and nonliving parts within an environment that interact with each other.
An ecosystem is the complex of living organisms, their physical environment, and all their interrelationships in a particular unit of space
An ecosystem is a higher level of organization the community plus its physical environment. Ecosystems include both the biological and physical components affecting the community/ecosystem. We can study ecosystems from a structural view of population distribution or from a functional view of energy flow and other processes.
An ecotone is a sensitive transition area between two adjacent ecological communities.

ec•o•tonePronunciation: (ek'u-tOn", E'ku-), [key] —n. Ecol. the transition zone between two different plant communities, as that between forest and prairie.

An ecotone is a zone where two ecosystems overlap. It is in this zone of overlap that much of the action that biologists enjoy takes place. Many biologists call this action in an ecotone the "edge effect."
Traditionally, an ecotone is defined as the place where two plant communities come together. Here we will use a more expanded definition and include all organisms (plants and animals) as well as non-living substances like rocks and soil.

Ecotones often have a diversity of wildlife because animals common to both overlapping ecosystems are brought together. Even though you may not see the animals themselves, they leave behind a lot of indirect evidence. Footprints, scat (droppings), and feathers are all common evidence. Ecotones where healthy ecosystems overlap usually show diversity, and diversity means more forms of wildlife. The absence of diversity in ecotones is often a clue that there are problems in the ecosystems that overlap.
It was 1935. British ecologist Sir Arthur Tansley formed it as a contraction of "ecological system" and used it in a scientific paper he wrote to clarify the meaning of other ecological terms that were in use at the time ( Tansley 1935).
Tansley coined the word to emphasize his view that a science of ecological systems would not emerge from the study of vegetation by extension of vegetation concepts and terms, but rather through the study of " ecosystem".
The term ecosystem has been defined in a number of ways. Among them are definitions by:
R. E. Linderman (1942)
"...all those physical, chemical, and biological processes in a space-time continuum of any magnitude."
E. P. Odum (1971)
"Living organisms and their nonliving (abiotic) environment are inseparably interrelated and interact upon each other. Any unit that includes all of the organisms (i.e., the "community") in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e., exchange of materials between living and nonliving parts) within the system is an ecological system or ecosystem."
D. R. Johnson and J. K. Agee (1988)
" ecosystem is any part of the universe chosen as an area of interest, with the line around that area being the ecosystem boundary and anything crossing the line being input or output."

The principles underlying the study of ecosystems are based on the view that all the elements of a life-supporting environment of any size, whether natural or man-made, are parts of an integral network in which each element interacts directly or indirectly with all others and affects the function of the whole. All ecosystems are contained within the largest of them, the ecosphere, which encompasses the entire physical Earth (geosphere) and all of its biological components (biosphere).
An ecosystem can be categorized into its abiotic constituents, including minerals, climate, soil, water, sunlight, and all other nonliving elements, and its biotic constituents, consisting of all its living members. Linking these constituents together are two major forces: the flow of energy through the ecosystem, and the cycling of nutrients within the ecosystem.
The fundamental source of energy in almost all ecosystems is radiant energy from the sun. The energy of sunlight is used by the ecosystem's autotrophic, or self-sustaining, organisms. Consisting largely of green vegetation, these organisms are capable of photosynthesis--i.e., they can use the energy of sunlight to convert carbon dioxide and water into simple, energy-rich carbohydrates. The autotrophs use the energy stored within the simple carbohydrates to produce the more complex organic compounds, such as proteins, lipids, and starches, that maintain the organisms' life processes. The autotrophic segment of the ecosystem is commonly referred to as the producer level.
Organic matter generated by autotrophs directly or indirectly sustains heterotrophic organisms. Heterotrophs are the
Organic matter generated by autotrophs directly or indirectly sustains heterotrophic organisms. Heterotrophs are the consumers of the ecosystem; they cannot make their own food. They use, rearrange, and ultimately decompose the complex organic materials built up by the autotrophs. All animals and fungi are heterotrophs, as are most bacteria and many other microorganisms.
Together, the autotrophs and heterotrophs form various trophic (feeding) levels in the ecosystem: the producer level, composed of those organisms that make their own food; the primary-consumer level, composed of those organisms that feed on producers; the secondary-consumer level, composed of those organisms that feed on primary consumers; and so on. The movement of organic matter and energy from the producer level through various consumer levels makes up a food chain. For example, a typical food chain in a grassland might be grass (producer) mouse (primary consumer) snake (secondary consumer) hawk (tertiary consumer). Actually, in many cases the food chains of the ecosystem overlap and interconnect, forming what ecologists call a food web. The final link in all food chains is made up of decomposers, those heterotrophs that break down dead organisms and organic wastes. A food chain in which the primary consumer feeds on living plants is called a grazing pathway; that in which the primary consumer feeds on dead plant matter is known as a detritus pathway. Both pathways are important in accounting for the energy budget of the ecosystem.
As energy moves through the ecosystem, much of it is lost at each trophic level. For example, only about 10 percent of the energy stored in grass is incorporated into the body of a mouse that eats the grass. The remaining 90 percent is stored in compounds that cannot be broken down by the mouse or is lost as heat during the mouse's metabolic processes. Energy losses of similar magnitude occur at every level of the food chain; consequently, few food chains extend beyond five members (from producer through decomposer), because the energy available at higher trophic levels is too small to support further consumers.
The flow of energy through the ecosystem drives the movement of nutrients within the ecosystem. Nutrients are chemical elements and compounds necessary to living organisms. Unlike energy, which is continuously lost from the ecosystem, nutrients are cycled through the ecosystem, oscillating between the biotic and abiotic components in what are called biogeochemical cycles. Major biogeochemical cycles include the water cycle, carbon cycle, oxygen cycle, nitrogen cycle, phosphorus cycle, sulfur cycle, and calcium cycle. Decomposers play a key role in many of these cycles, returning nutrients to the soil, water, or air, where they can again be used by the biotic constituents of the ecosystem.
The orderly replacement of one ecosystem by another is a process known as ecosystem development, or ecological
The orderly replacement of one ecosystem by another is a process known as ecosystem development, or ecological succession. Succession occurs when a sterile area, such as barren rock or a lava flow, is first colonized by living things or when an existing ecosystem is disrupted, as when a forest is destroyed by a fire. The succession of ecosystems generally occurs in two phases. The early, or growth, phase is characterized by ecosystems that have few species and short food chains. These ecosystems are relatively unstable but highly productive, in the sense that they build up organic matter faster than they break it down. The ecosystems of the later, or mature, phase are more complex, more diversified, and more stable. The final, or climax, ecosystem is characterized by a great diversity of species, complex food webs, and high stability. The major energy flow has shifted from production to maintenance.
Human interference in the development of ecosystems is widespread. Farming, for example, is the deliberate maintenance of an immature ecosystem--one that is highly productive but relatively unstable. Sound management of ecosystems for optimal food production should seek a compromise between the characteristics of young and mature ecosystems, and should consider factors that affect the interaction of natural cycles. Short-term production can be maximized by adding energy to the ecosystem in the form of cultivation and fertilization. Such efforts, however, can hinder efficient energy use in the long run by producing an imbalance of nutrients, an increase in pollutants, or a heightened susceptibility to plant diseases as a consequence of intensive inbreeding of crops.
Although an awareness of the interdependence between human society and its environment was already prominent in ancient philosophy and religion, the formulation of the basic principles of systems ecology as a scientific discipline began in the late 19th century. During the second half of the 20th century, the study of ecosystems has become increasingly sophisticated and is now instrumental in the assessment and control of the effects of agricultural development and industrialization on the environment. On farms, for instance, it has shown that optimal long-term production of pasturage requires a moderate grazing schedule in order to ensure a steady renewal of the moisture and nutrient content of the soil and has emphasized the need for multiple-use strategies in the cultivation of arable lands. Systems ecology has been concerned with the consequences of accumulated insecticides and has provided a way of monitoring the climatic effects of atmospheric dust and carbon dioxide released by the burning of fossil fuels (e.g., coal, oil, and natural gas). It has helped to determine regional population capacities and has furthered the development of recycling techniques that may become essential in humanity's future interaction with the environment.
Excerpt from the Encyclopedia Britannica without permission.
Classification of Communities

There are two basic categories of communities: terrestrial (land) and aquatic (water). These two basic types of community contain eight smaller units known as biomes. A biome is a large-scale category containing many communities of a similar nature, whose distribution is largely controlled by climate
Terrestrial Biomes: tundra, grassland, desert, taiga, temperate forest, tropical forest.
Aquatic Biomes: marine, freshwater.
Major terrestrial biomes. Image from Purves et al.,
Terrestrial Biomes
Tundra and Desert
The tundra and desert biomes occupy the most extreme environments, with little or no moisture and extremes of temperature acting as harsh selective agents on organisms that occupy these areas. These two biomes have the fewest numbers of species due to the stringent environmental conditions. In other words, not everyone can live there due to the specialized adaptations required by the environment.
Tropical Rain Forests
Tropical rain forests occur in regions near the equator. The climate is always warm (between 20° and 25° C) with plenty of rainfall (at least 190 cm/year). The rain forest is probably the richest biome, both in diversity and in total biomass. The tropical rain forest has a complex structure, with many levels of life. More than half of all terrestrial species live in this biome. While diversity is high, dominance by a particular species is low.
While some animals live on the ground, most rain forest animals live in the trees. Many of these animals spend their entire life in the forest canopy. Insects are so abundant in tropical rain forests that the majority have not yet been identified. Charles Darwin noted the number of species found on a single tree, and suggested the richness of the rain forest would stagger the future systematist with the size of the catalogue of animal species found there. Termites are critical in the decomposition and nutrient cycling of wood. Birds tend to be brightly colored, often making them sought after as exotic pets. Amphibians and reptiles are well represented. Lemurs, sloths, and monkeys feed on fruits in tropical rain forest trees. The largest carnivores are the cats (jaguars in South America and leopards in Africa and Asia). Encroachment and destruction of habitat put all these animals and plants at risk.
Epiphytes are plants that grow on other plants. These epiphytes have their own roots to absorb moisture and minerals, and use the other plant more as an aid to grow taller. Some tropical forests in India, Southeast Asia, West Africa, Central and South American are seasonal and have trees that shed leaves in dry season. The warm, moist climate supports high productivity as well as rapid decomposition of detritus.
With its yearlong growing season, tropical forests have a rapid cycling of nutrients. Soils in tropical rain forests tend to have very little organic matter since most of the organic carbon is tied up in the standing biomass of the plants. These tropical soils, termed laterites, make poor agricultural soils after the forest has been cleared.
About 17 million hectares of rain forest are destroyed each year (an area equal in size to Washington state). Estimates indicate the forests will be destroyed (along with a great part of the Earth's diversity) within 100 years. Rainfall and climate patterns could change as a result.
Costa Rican cloud forest. Image from the Botanical Society of America website,
Temperate Forests
The temperate forest biome occurs south of the taiga in eastern North America, eastern Asia, and much of Europe. Rainfall is abundant (30-80 inches/year; 75-150 cm) and there is a well-defined growing season of between 140 and 300 days. The eastern United States and Canada are covered (or rather were once covered) by this biome's natural vegetation, the eastern deciduous forest. Dominant plants include beech, maple, oak; and other deciduous hardwood trees. Trees of a deciduous forest have broad leaves, which they lose in the fall and grow again in the spring.
Fall color in the eastern deciduous forest. Note the presence of a few evergreens among the hardwoods. Image from the Botanical Society of America website,
Sufficient sunlight penetrates the canopy to support a well-developed understory composed of shrubs, a layer of herbaceous plants, and then often a ground cover of mosses and ferns. This stratification beneath the canopy provides a numerous habitats for a variety of insects and birds. The deciduous forest also contains many members of the rodent family, which serve as a food source for bobcats, wolves, and foxes. This area also is a home for deer and black bears. Winters are not as cold as in the taiga, so many amphibian and reptiles are able to survive.
Shrubland (Chaparral)
The shrubland biome is dominated by shrubs with small but thick evergreen leaves that are often coated with a thick, waxy cuticle, and with thick underground stems that survive the dry summers and frequent fires. Shrublands occur in parts of South America, western Australia, central Chile, and around the Mediterranean Sea. Dense shrubland in California, where the summers are hot and very dry, is known as chaparral. This Mediterranean-type shrubland lacks an understory and ground litter, and is also highly flammable. The seeds of many species require the heat and scarring action of fire to induce germination.
Chaparral vegetation (predominantly Adenostema) in California. Image from the Botanical Society of America website,
Grasslands occur in temperate and tropical areas with reduced rainfall (10-30 inches per year) or prolonged dry seasons. Grasslands occur in the Americas, Africa, Asia, and Australia. Soils in this region are deep and rich and are excellent for agriculture. Grasslands are almost entirely devoid of trees, and can support large herds of grazing animals. Natural grasslands once covered over 40 percent of the earth's land surface. In temperate areas where rainfall is between 10 and 30 inches a year, grassland is the climax community because it is too wet for desert and too dry for forests.
Most grasslands have now been utilized to grow crops, especially wheat and corn. Grasses are the dominant plants, while grazing and burrowing species are the dominant animals. The extensive root systems of grasses allows them to recover quickly from grazing, flooding, drought, and sometimes fire.
Temperate grasslands include the Russian steppes, the South American pampas, and North American prairies. A tall-grass prairie occurs where moisture is not quite sufficient to support trees. A short-grass-prairie survives on less moisture and occurs between a tall-grass prairie and desert.
Short grass prairie, Nebraska. Image from the Botanical Society of America website,
Animal life includes mice, prairie dogs, rabbits, and animals that feed on them (hawks and snakes). Prairies once contained large herds of buffalo and pronghorn antelope, but with human activity these once great herds ahve dwindled.
The savanna is a tropical grassland that contains some trees. The savanna contains the greatest variety and numbers of herbivores (antelopes, zebras, and wildebeests, among others). This environment supports a large population of carnivores (lions, cheetahs, hyenas, and leopards). Any plant litter not consumed by grazers is attacked by termites and other decomposers. Once again, human activities are threatening this biome, reducing the range for herbivores and carnivores. Will extinction of the great cats be a result?
Deserts are characterized by dry conditions (usually less than 10 inches per year; 25 cm) and a wide temperature range. The dry air leads to wide daily temperature fluctuations from freezing at night to over 120 degrees during the day. Most deserts occur at latitudes of 30o N or S where descending air masses are dry. Some deserts occur in the rainshadow of tall mountain ranges or in coastal areas near cold offshore currents. Plants in this biome have developed a series of adaptations (such as succulent stems, and small, spiny, or absent leaves) to conserve water and deal with these temperature extremes. Photosynthetic modifications (CAM) are another strategy to life in the drylands.
The Sahara and a few other deserts have almost no vegetation. Most deserts, however, are home to a variety of plants, all adapted to heat and lack of abundant water (succulents and cacti). Animal life includes arthropods (especially insects and spiders), reptiles (lizards and snakes), running birds (the roadrunner of the American southwest and Warner Brothers cartoon fame), rodents (kangaroo rat and pack rat), and a few larger birds and mammals (hawks, owls, and coyotes).
Saguaro and cholla cacti in association with palo verde trees in the Sonoran desert, AZ. Note the lack of a canopy and the scarcity of ground cover. Image from the Botanical Society of America website,
Taiga (Boreal Forest)
The taiga (pronounced "tie-guh") is a coniferous forest extending across most of the northern area of northern Eurasia and North America. This forest belt also occurs in a few other areas, where it has different names: the montane coniferous forest when near mountain tops; and the temperate rain forest along the Pacific Coast as far south as California. The taiga receives between 10 and 40 inches of rain per year and has a short growing season. Winters are cold and short, while summers tend to be cool. The taiga is noted for its great stands of spruce, fir, hemlock, and pine. These trees have thick protective leaves and bark, as well as needlelike (evergreen) leaves can withstand the weight of accumulated snow. Taiga forests have a limited understory of plants, and a forest floor covered by low-lying mosses and lichens. Conifers, alders, birch and willow are common plants; wolves, grizzly bears, moose, and caribou are common animals. Dominance of a few species is pronounced, but diversity is low when compared to temperate and tropical biomes.
Image of a Larix-dominated area of the taiga biome. Image from the Botanical Society of America website,
Temperate rain forest, Washington. Note the dense understory of ferns and herbaceous plants. Image from the Botanical Society of America website,
The tundra covers the northernmost regions of North America and Eurasia, about 20% of the Earth's land area. This biome receives about 20 cm (8-10 inches) of rainfall annually. Snow melt makes water plentiful during summer months. Winters are long and dark, followed by very short summers. Water is frozen most of the time, producing frozen soil, permafrost. Vegetation includes no trees, but rather patches of grass and shrubs; grazing musk ox, reindeer, and caribou exist along with wolves, lynx, and rodents. A few animals highly adapted to cold live in the tundra year-round (lemming, ptarmigan). During the summer the tundra hosts numerous insects and migratory animals. The ground is nearly completely covered with sedges and short grasses during the short summer. There are also plenty of patches of lichens and mosses. Dwarf woody shrubs flower and produce seeds quickly during the short growing season. The alpine tundra occurs above the timberline on mountain ranges, and may contain many of the same plants as the arctic tundra.
View of the tundra, locality unknown. Image from
Caribou, an animal characteristic of the tundra. Image from
Climate, Altitude and Terrestrial Biomes
Climate controls biome distribution by an altitudinal gradient and a latitudinal gradient. With increases of either altitude or latitude, cooler and drier conditions occur. Cooler conditions can cause aridity since cooler air can hold less water vapor than can warmer air.
Effect of temperature on precipitation. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
Deserts can occur in warm areas due to a blockage of air circulation patterns that form a rain shadow.
Air circulation patterns and the global distribution of wet and dry areas. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
Rainshadows and deserts. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
Aquatic Biomes
Conditions in water are generally less harsh than those on land. Aquatic organisms are buoyed by water support, and do not usually have to deal with desiccation. Despite covering 71% of the Earth's surface, areas of the open ocean are a vast aquatic desert containing few nutrients and very little life. Clearcut biome distinctions in water, like those on land, are difficult to make. Dissolved nutrients controls many local aquatic distributions. Aquatic communities are classified into: freshwater (inland) communities and marine (saltwater or oceanic) communities.
Species diversity and salt concentration. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
The Marine Biome
The marine biome contains more dissolved minerals than the freshwater biome. Over 70% of the Earth's surface is covered in water, by far the vast majority of that being saltwater. There are two basic categories to this biome: benthic and pelagic. Benthic communities (bottom dwellers) are subdivided by depth: the shore/shelf and deep sea. Pelagic communities (swimmers or floaters suspended in the water column) include planktonic (floating) and nektonic (swimming) organisms. The upper 200 meters of the water column is the euphotic zone to which light can penetrate.
Coastal Communities
Estuaries are bays where rivers empty into the sea. Erosion brings down nutrients and tides wash in salt water; forms nutrient trap. Estuaries have high production for organisms that can tolerate changing salinity. Called "nurseries of the sea" because many young marine fish develop in this protected environment.
Rocky shorelines offer anchorage for sessile organisms. Seaweeds are main photosynthesizers and use holdfasts to anchor. Barnacles glue themselves to stone. Oysters and mussels attach themselves by threads. Limpets and periwinkles either hide in crevices or fasten flat to rocks.
Sandy beaches and shores are shifting strata. Permanent residents therefore burrow underground. Worms live permanently in tubes. Amphipods and ghost crabs burrow above high tide and feed at night.
Coral Reefs
Areas of biological abundance in shallow, warm tropical waters. Stony corals have calcium carbonate exoskeleton and may include algae. Most form colonies; may associate with zooxanthellae dinoflagellates. Reef is densely populated with animal life. The Great Barrier Reef of Australia suffers from heavy predation by crown-of-thorns sea star, perhaps because humans have harvested its predator, the giant triton.
Oceans cover about three-quarters of the Earth's surface. Oceanic organisms are placed in either pelagic (open water) or benthic (ocean floor) categories. Pelagic division is divided into neritic and three levels of pelagic provinces. Neritic province has greater concentration of organisms because sunlight penetrates; nutrients are found here. Epipelagic zone is brightly lit, has much photosynthetic phytoplankton, that support zooplankton that are food for fish, squid, dolphins, and whales. Mesopelagic zone is semi-dark and contains carnivores; adapted organisms tend to be translucent, red colored, or luminescent; for example: shrimps, squids, lantern and hatchet fishes. Bathypelagic zone is completely dark and largest in size; it has strange-looking fish. Benthic division includes organisms on continental shelf (sublittoral), continental slope (bathyal), and the abyssal plain.
Sublittoral zone harbors seaweed that becomes sparse where deeper; most dependent on slow rain of plankton and detritus from sunlit water above. Bathyal zone continues with thinning of sublittoral organisms. Abyssal zone is mainly animals at soil-water interface of dark abyssal plain; in spite of high pressure, darkness and coldness, many invertebrates thrive here among sea urchins and tubeworms.
Thermal vents along oceanic ridges form a very unique community. Molten magma heats seawater to 350oC, reacting with sulfate to form hydrogen sulfide (H2S). Chemosynthetic bacteria obtain energy by oxidizing hydrogen sulfide. The resulting food chain supports a community of tubeworms and clams.
Zones within the marine biome. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
The Freshwater Biome
The freshwater biome is subdivided into two zones: running waters and standing waters. Larger bodies of freshwater are less prone to stratification (where oxygen decreases with depth). The upper layers have abundant oxygen, the lowermost layers are oxygen-poor. Mixing between upper and lower layers in a pond or lake occurs during seasonal changes known as spring and fall overturn.
Lakes are larger than ponds, and are stratified in summer and winter. The epilimnion is the upper surface layer. It is warm in summer. The hypolimnion is the cold lower layer. A sudden drop in temperature occurs at the middle of the thermocline. Layering prevents mixing between the lower hypolimnion (rich in nutrients) and the upper epilimnion (which has oxygen absorbed from its surface). The epilimnion warms in spring and cools in fall, causing a temporary mixing. As a consequence, phytoplankton become more abundant due to the increased amounts of nutrients.
Lake overturn. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
Life zones also exist in lakes and ponds. The littoral zone is closest to shore. The limnetic zone is the sunlit body of the lake. Below the level of sunlight penetration is the dark profundal zone. At the soil-water interface we find the benthic zone. The term benthos is applied to animals that live on the bottom.
Rapidly flowing, bubbling streams have insects and fish adapted to oxygen-laden water. Slow moving streams have aquatic life more similar to lake and pond life.
Community Density and Stability Back to Top
Communities are made up of species adapted to the conditions of that community. Diversity and stability help define a community and are important in environmental studies. Species diversity decreases as we move away from the tropics. Species diversity is a measure of the different types of organisms in a community (also referred to as species richness). Latitudinal diversity gradient refers to species richness decreasing steadily going away from the equator. A hectare of tropical rain forest contains 40-100 tree species, while a hectare of temperate zone forest contains 10-30 tree species. In marked contrast, a hectare of taiga contains only a paltry 1-5 species. Habitat destruction in tropical countries will cause many more extinctions per hectare than it would in higher latitudes.
Environmental stability is greater in tropical areas, where a relatively stable/constant environment allows more different kinds of species to thrive. Equatorial communities are older because they have been less disturbed by glaciers and other climate changes, allowing time for new species to evolve. Equatorial areas also have a longer growing season.
The depth diversity gradient is found in aquatic communities. Increasing species richness with increasing water depth. This gradient is established by environmental stability and the increasing availability of nutrients.
Community stability refers to the ability of communities to remain unchanged over time. During the 1950s and 1960s, stability was equated to diversity: diverse communities were also stable communities. Mathematical modeling during the 1970s showed that increased diversity can actually increase interdependence among species and lead to a cascade effect when a keystone species is removed. Thus, the relation is more complex than previously thought.
Change in Communities Over Time Back to Top
Biological communities, like the organisms that comprise them, can and do change over time. Ecological time focuses on community events that occur over decades or centuries. Geological time focuses on events lasting thousands of years or more.
Community succession is the sequential replacement of species by immigration of new species and local extinction of older ones following a disturbance that creates unoccupied habitats for colonization. The initial rapid colonizer species are the pioneer community. Eventually a climax community of more or less stable but slower growing species eventually develops.
During succession productivity declines and diversity increases. These trends tend to increase the biomass (total weight of living tissue) in a community. Succession occurs because each community stage prepares the environment for the stage following it.
Primary succession begins with bare rock and takes a very long time to occur. Weathering by wind and rain plus the actions of pioneer species such as lichens and mosses begin the buildup of soil. Herbaceous plants, including the grasses, grow on deeper soil and shade out shorter pioneer species. Pine trees or deciduous trees eventually take root and in most biomes will form a climax community of plants that are stabile in the environment. The young produced by climax species can live in that environment, unlike the young produced by successional species.
Secondary succession occurs when an environment has been disturbed, such as by fire, geological activity, or human intervention (farming or deforestation in most cases). This form of succession often begins in an abandoned field with soil layers already in place. Compared to primary succession, which must take long periods of time to build or accumulate soil, secondary succession occurs rapidly. The herbaceous pioneering plants give way to pines, which in turn may give way to a hardwood deciduous forest (in the classical old field succession models developed in the eastern deciduous forest biome).
Early researchers assumed climax communities were determined for each environment. Today we recognize the outcome of competition among whatever species are present as establishing the climax community.
Climax communities tend to be more stable than successional communities. Early stages of succession show most growth and are most productive. Pioneer communities lack diversity, make poor use of inputs, and lose heat and nutrients. As succession proceeds, species variety increases and nutrients are recycled more. Climax communities make fuller use of inputs and maintain themselves, thus, they are more stable. Human activity (such as clearing a climax forest community to establish a farm field consisting of a cultivated pioneering species, say corn or wheat) replaces climax communities with simpler communities.
Communities are composed of species that evolve, so the community must also evolve. Comparing marine communities of 500 million years ago with modern communities shows modern communities composed of quite different organisms. Modern communities also tend to be more complex, although this may be a reflection of the nature of the fossil record as well as differences between biological and fossil species.
Disturbance of a Community
The basic effect of human activity on communities is community simplification, an overall reduction of species diversity. Agriculture is a purposeful human intervention in which we create a monoculture of a single favored (crop) species such as corn. Most of the agricultural species are derived from pioneering communities.
Inadvertent human intervention can simplify communities and produce stressed communities that have fewer species as well as a superabundance of some species. Disturbances favor early successional (pioneer) species that can grow and reproduce rapidly.
Ecosystems and Communities Back to Top
Ecosystems include both living and nonliving components. These living, or biotic, components include habitats and niches occupied by organisms. Nonliving, or abiotic, components include soil, water, light, inorganic nutrients, and weather. An organism's place of residence, where it can be found, is its habitat. A niche is is often viewed as the role of that organism in the community, factors limiting its life, and how it acquires food.
Producers, a major niche in all ecosystems, are autotrophic, usually photosynthetic, organisms. In terrestrial ecosystems, producers are usually green plants. Freshwater and marine ecosystems frequently have algae as the dominant producers.
Consumers are heterotrophic organisms that eat food produced by another organism. Herbivores are a type of consumer that feeds directly on green plants (or another type of autotroph). Since herbivores take their food directly from the producer level, we refer to them as primary consumers. Carnivores feed on other animals (or another type of consumer) and are secondary or tertiary consumers. Omnivores, the feeding method used by humans, feed on both plants and animals. Decomposers are organisms, mostly bacteria and fungi that recycle nutrients from decaying organic material. Decomposers break down detritus, nonliving organic matter, into inorganic matter. Small soil organisms are critical in helping bacteria and fungi shred leaf litter and form rich soil.
Even if communities do differ in structure, they have some common uniting processes such as energy flow and matter cycling. Energy flows move through feeding relationships. The term ecological niche refers to how an organism functions in an ecosystem. Food webs, food chains, and food pyramids are three ways of representing energy flow.
Producers absorb solar energy and convert it to chemical bonds from inorganic nutrients taken from environment. Energy content of organic food passes up food chain; eventually all energy is lost as heat, therefore requiring continual input. Original inorganic elements are mostly returned to soil and producers; can be used again by producers and no new input is required.
The flow of energy through an ecosystem. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
Energy flow in ecosystems, as with all other energy, must follow the two laws of thermodynamics. Recall that the first law states that energy is neither created nor destroyed, but instead changes from one form to another (potential to kinetic). The second law mandates that when energy is transformed from one form to another, some usable energy is lost as heat. Thus, in any food chain, some energy must be lost as we move up the chain.
The ultimate source of energy for nearly all life is the Sun. Recently, scientists discovered an exception to this once unchallenged truism: communities of organisms around ocean vents where food chain begins with chemosynthetic bacteria that oxidize hydrogen sulfide generated by inorganic chemical reactions inside the Earth's crust. In this special case, the source of energy is the internal heat engine of the Earth.
Food chains indicate who eats whom in an ecosystem. Represent one path of energy flow through an ecosystem. Natural ecosystems have numerous interconnected food chains. Each level of producer and consumers is a trophic level. Some primary consumers feed on plants and make grazing food chains; others feed on detritus.
The population size in an undisturbed ecosystem is limited by the food supply, competition, predation, and parasitism. Food webs help determine consequences of perturbations: if titmice and vireos fed on beetles and earthworms, insecticides that killed beetles would increase competition between birds and probably increase predation of earthworms, etc.
The trophic structure of an ecosystem forms an ecological pyramid. The base of this pyramid represents the producer trophic level. At the apex is the highest level consumer, the top predator. Other pyramids can be recognized in an ecosystem. A pyramid of numbers is based on how many organisms occupy each trophic level. The pyramid of biomass is calculated by multiplying the average weight for organisms times the number of organisms at each trophic level. An energy pyramid illustrates the amounts of energy available at each successive trophic level. The energy pyramid always shows a decrease moving up trophic levels because:
Only a certain amount of food is captured and eaten by organisms on the next trophic level.
Some of food that is eaten cannot be digested and exits digestive tract as undigested waste.
Only a portion of digested food becomes part of the organism's body; rest is used as source of energy.
Substantial portion of food energy goes to build up temporary ATP in mitochondria that is then used to synthesize proteins, lipids, carbohydrates, fuel contraction of muscles, nerve conduction, and other functions.
Only about 10% of the energy available at a particular trophic level is incorporated into tissues at the next level. Thus, a larger population can be sustained by eating grain than by eating grain-fed animals since 100 kg of grain would result in 10 human kg but if fed to cattle, the result, by the time that reaches the human is a paltry 1 human kg!
A food chain is a series of organisms each feeding on the one preceding it. There are two types of food chain: decomposer and grazer. Grazer food chains begin with algae and plants and end in a carnivore. Decomposer chains are composed of waste and decomposing organisms such as fungi and bacteria.
Energy flow and the relative porportions of various levels in the food chain. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.
Food chains are simplifications of complex relationships. A food web is a more realistic and accurate depiction of energy flow. Food webs are networks of feeding interactions among species.
The food pyramid provides a detailed view of energy flow in an ecosystem. The first level consists of the producers (usually plants). All higher levels are consumers. The shorter the food chain the more energy is available to organisms.
Most humans occupy a top carnivore role, about 2% of all calories available from producers ever reach the tissues of top carnivores. Leakage of energy occurs between each feeding level. Most natural ecosystems therefore do not have more than five levels to their food pyramids. Large carnivores are rare because there is so little energy available to them atop the pyramid.
Food generation by producers varies greatly between ecosystems. Net primary productivity (NPP) is the rate at which producer biomass is formed. Tropical forests and swamps are the most productive terrestrial ecosystems. Reefs and estuaries are the most productive aquatic ecosystems. All of these productive areas are in danger from human activity. Humans redirect nearly 40% of the net primary productivity and directly or indirectly use nearly 40% of all the land food pyramid. This energy is not available to natural populations.
Links Back to Top
The Rain Forest Report Card Maps, images, morphed movies showing the effects of deforestation, and more make this a site to see for further information about the rain forests and their plight.
Manu: Peru's Hidden Rain Forest PBS documentary, part of the Living Edens series. Links to animals, plants, and people of this area. Quite a nice resource, as are many of the PBS websites!
Population Ecology This site, maintained by Alexi Sharov of the Department of Entomology at Virginia Tech provides a great start to the study of population ecology. Links to people, organizations, online lectures, and other items of interest are provided.
Planet Earth - a suite of interactive learning activities on ecology Aimed at high school students and teachers this site offers a series of great activities that will allow application of the concepts learned to real world problems, such as the Wolves of Yellowstone.
All text contents ©1995, 2000, 2001, by M.J. Farabee. Use for educational purposes is encouraged.
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