by DK
Each ecosystem is located in a particular area, with characteristics unique to its environment, and behaves as a self-contained and self-regulating system. Together, the patchwork of ecosystems across the globe form what Austrian scientist Eduard Suess called the biosphere—the sum total of all ecosystems.
In this ecosystem, plants use the Sun’s energy for photosynthesis. As shown by the pale arrows, this energy is passed on—to herbivores, who eat plants; to predators, who eat herbivores; and to saphrophytes, who take energy from decomposing remains and transfer nutrients to the soil. At each stage, some energy is lost as heat.
External factors
Various external factors, such as climate and the geological makeup of the surrounding environment, can affect an ecosystem. One constant external force that affects all ecosystems is the Sun. The supply of energy that it provides enables photosynthesis and the capture of CO2 from the atmosphere; some of this energy is distributed through the ecosystem and through the food chain. In the process, some of this energy dissipates as heat.
Other external factors, however, can arise unexpectedly to create pressures on ecosystems. All ecosystems are subjected to external disturbances from time to time, and must then go through a process of recovery. These disturbances include storms, earthquakes, floods, droughts, and other natural phenomena, but are increasingly the result of human activity—through the destruction of natural habitats by deforestation, urbanization, pollution, and the cumulative effects of anthropogenic (human-induced) climate change. Humans can also be responsible for the introduction of invasive species. Without these external factors, an ecosystem would maintain its state of equilibrium, and retain a stable identity.
A small glacial lake, or tarn, in the English Lake District. Each tarn has an ecosystem that varies according to many factors, including the degree of nutrient enrichment in the water.
“There is no waste in functioning … ecosystems. All organisms, dead or alive, are potential sources of food for other organisms.”
G. Tyler Miller
Science writer
Resistance and resilience
Ecosystems are often strong enough to withstand some natural external disturbances and retain their equilibrium. Some are more resistant to disturbance than others, and have adapted to the particular disturbances normally associated with their environment. In some forest ecosystems, for example, the periodic fires caused by electrical storms cause only a minor imbalance in the ecosystem.
Even when severely disrupted by external disturbances, some ecosystems have a resilience that enables them to recover. However, other ecosystems are more fragile, and if they are disturbed may never be able to regain their equilibrium.
The resistance and resilience of an ecosystem is generally thought to be related to its biodiversity. If, for example, there is only one species of plant performing a particular function in the system, and that species is not frost-resistant, an abnormally severe winter could deplete the species enough to have a major impact on the system as a whole. In contrast, if there are several species with that role in the system, it is more likely that one will be resistant to the disturbance.
The human factor
Some disturbances can be severe enough to be catastrophic for an ecosystem, damaging it beyond the point of recovery and so causing a permanent change in its identity, or even its demise. The fear is that much of the disturbance caused by human activity has the potential to cause such permanent damage, particularly when it involves the wholesale destruction of habitat and the consequent depletion of its biodiversity. In addition, some have suggested that human influence has created a new category of ecological systems, dubbed “techno-ecosystems.” For example, “cooling ponds” are manmade ponds, built to cool down nuclear power plants, but they have become ecosystems for aquatic organisms.
The relationship between humans and natural ecosystems is not all negative. In recent years, scientific data has fueled public awareness of the benefits that ecosystems afford humankind, including the provision of food, water, nutrients, and clean air, as well as the management of disease and even climate. There is now a growing commitment from many governments across the world to use these benefits both responsibly and sustainably.
The Eden Project, in Cornwall, UK, simulates a rain forest ecosystem in one of its giant dome greenhouses. The domes’ panels are slanted to absorb plenty of light and thermal energy.
ARTHUR G. TANSLEY
A free-thinking Fabian socialist and atheist, Arthur Tansley was one of the most influential ecologists of the 20th century. Born in London in 1871, he studied biology at University College London, where he later taught. In 1902, he founded the journal New Phytologist and he later established the British Ecological Society, becoming founding editor of its Journal of Ecology. In 1923, he took a break from teaching to study psychology with Sigmund Freud in Vienna. He was later Sherardian Professor of Botany at the University of Oxford. He retired in 1937, but maintained a special interest in conservation. Tansley was appointed the first Chairman of the UK’s Nature Conservancy in 1950, five years before his death.
Key works
1922 Types of British Vegetation
1922 Elements of Plant Ecology
1923 Practical Plant Ecology
1935 “The use and abuse of vegetational terms and concepts,” Ecology
1939 The British Islands and Their Vegetation
See also: Animal ecology • The food chain • Energy flow through ecosystems • The biosphere • The Gaia hypothesis • Environmental feedback loops • Ecosystem services
IN CONTEXT
KEY FIGURE
Raymond Lindeman (1915–42)
BEFORE
1913 American zoologist Victor Shelford produces the first illustrated food webs.
1920 Frederic Clements describes how groups of plant species are associated in communities.
1926 Russian geochemist Vladimir Vernadsky sees that chemicals are recycled between living and nonliving things.
1935 Arthur Tansley develops the concept of the ecosystem.
AFTER
1957 American ecologist Eugene Odum uses radioactive elements to map food chains.
1962 Rachel Carson draws attention to the accumulation of pesticides in food chains, in her book Silent Spring.
In 1941, Raymond Lindeman submitted the final chapter of his Ph.D. thesis for publication in the prestigious journal Ecology. Titled “The Trophic-dynamic Aspect of Ecology,” it was about the relationship between food chains and the changes over time in a community of species.
Lindeman had spent five years studying the life forms in an aging lake at Cedar Creek Bog, Minnesota, and was especially interested in the changes in the lake as, year by year, aquatic habitat gradually gave way to land. He received his Ph.D., but his paper was initially rejected by Ecology, for being too theoretical.
Lindeman had painstakingly sampled everything in the lake, from the aquatic plants and microscopic algal plankton to the worms, insects, crustaceans, and fish that fed upon one another and depended on each other for their existence. He stressed that the community of organisms could not be properly understood on its own; instead, it must be examined in the context of its wider surroundings. The living (biotic) organisms and the nonliving (abiotic) components (air, water, soil minerals) were linked together by nutrient cycles and energy flows. This entire system—the ecosystem—was the central ecological unit.
Producers and consumers
Lindeman’s research showed how an ecosystem is powered by a stream of energy from one organism to another. The organisms can be grouped into discrete “trophic levels” (feeding levels)—from producers (plants and algae), which absorb energy in the form of sunlight to make food, to consumers (animals). “Primary consumers” are the herbivores that eat the plants; “secondary consumers” are animals that eat the herbivores. Each trophic level depends on the preceding one for its survival. At the same time, dead material accumulating from each stage is b
roken down by decomposers, such as bacteria and fungi, and materials in the form of nutrients are recycled back to feed plants and algae.
Lindeman also demonstrated how some of the energy at each trophic level is lost as waste, or converted into heat when organisms respire. By combining the results of his own study with data from a wide range of other sources, he was able to build a picture of this system as it worked in Cedar Creek Bog.
British ecologist G. Evelyn Hutchinson, considered to be one of the founding fathers of modern ecology, was Lindeman’s mentor at Yale University. He recognized the importance of his student’s work to the future development of ecology, and he lobbied for Lindeman’s paper to be accepted. Lindeman, who had always suffered from ill health, died in 1942 from cirrhosis of the liver at the tragically young age of 27, just four months before his trophic-dynamic paper—now seen as a classic in its field—was finally published.
Boneworms are deep-sea creatures that feed on the remains of animals such as whales. They grow “roots” to break down the bones, thereby recycling nutrients from the dead material.
“… biological communities could be expressed as networks or channels through which energy is flowing and being dissipated…”
G. Evelyn Hutchinson
Measuring productivity
This thermal image of an elephant shows how some of the animal’s heat is lost. Both its body temperature and its manure are warmer than the surroundings.
Lindeman’s trophic-dynamic theory helped to clarify the idea of ecosystem productivity, which ecologists had previously defined in rather vague terms. The productivity of a plant or animal is measured by its growth in organic material, or biomass. This is never equal to the organism’s energy input, because the conversion of solar energy into leaf in the case of plants, or the conversion of food into flesh in the case of an animal, is never 100 percent efficient. Some energy is released as heat, most of which is lost via respiration—an essential aspect of metabolism in all living things.
Warm-blooded animals lose a lot of heat when their body temperature is much higher than that of their surroundings. All animals also lose energy when they excrete urine. In addition, not all the material in an animal’s food can be digested in its gut, and the material that is expelled as feces represents unused chemical energy.
See also: Ecological niches • Nonconsumptive effects of predators on their prey • The food chain • The ecosystem
IN CONTEXT
KEY FIGURE
Nelson Hairston (1917–2008)
BEFORE
1949 Aldo Leopold publishes A Sand County Almanac, drawing attention to the ecological impact of hunting wolves on mountain plant life.
AFTER
1961 Lawrence Slobodkin, an American marine ecologist, publishes The Growth and Regulation of Animal Populations, a key ecology textbook.
1980 Robert Paine describes the “trophic cascade effect,” when predators are removed from an intertidal ecosystem.
1995 The reintroduction of gray wolves to Yellowstone National Park sets in motion a series of ecosystem changes.
Soon after the end of World War II, Aldo Leopold, an ecologist and one of the top wildlife management experts in the United States, challenged the view that wolves should be eradicated because they threatened livestock. In A Sand County Almanac, he wrote of the destructive effect that removing this top predator would have on the rest of the ecosystem. In particular, he said, it would lead to overgrazing of mountainsides by deer. Leopold’s view was an early expression of the idea of trophic cascades, although he himself did not use that term.
Predators help keep a balance in a food web by regulating the populations of other animals. When they attack and eat prey, they affect the number and behavior of that prey—since prey move away when predators are present. The impact of a predator can extend down to the next feeding level (trophic level), affecting the population of the prey’s own food source. In essence, by controlling the population density and behavior of their prey, predators indirectly benefit and increase the abundance of their prey’s prey.
Indirect interaction that occurs across feeding levels is described by ecologists as a trophic cascade. By definition, trophic cascades must cross at least three feeding levels. Four- and five-level trophic cascades are also known, although these are less common.
Ocher starfish prey on sea creatures such as mussels and limpets. In a famous experiment, Robert Paine took them out of their rock pools to observe the effect on the rest of the food web.
Controlling factors
In 1960, the American ecologist Nelson Hairston and his colleagues Frederick Smith and Lawrence Slobodkin published a key paper entitled “Community Structure, Population Control, and Competition,” which examined the factors that control populations of animals on different trophic levels. They concluded that populations of producers, carnivores, and decomposers are limited by their respective resources. Competition occurs between species on each of these three trophic levels. They also found that herbivore populations are seldom limited by the supply of plants, but are limited by predators, so they are unlikely to compete with other herbivores for common resources. The paper highlighted the important role of top-down forces (predation) in ecosystems, and bottom-up forces (food supply).
American ecologist Robert Paine was the first to use the term “trophic cascade” when, in 1980, he described changes in food webs that were brought about by the experimental removal of predatory starfish from the intertidal zone in Washington State. The concept of trophic cascades is now generally accepted, although debate continues as to how widespread they are.
Top-down cascades
This type of cascade is clearly demonstrated when a food chain is interrupted by the removal of a top predator. The ecosystem may continue to function despite the shift in species composition; alternatively, the removal of one species may lead to the ecosystem’s breakdown. In the US, a trophic cascade on the coast of southern New England is believed to be responsible for the die-off of saltmarsh habitat. Recreational anglers have reduced the number of predatory fish to such an extent that the number of herbivorous crabs has expanded dramatically. The resulting increase in the consumption of marsh vegetation has had a knock-on effect on other species that depend on it.
Trophic cascades can also be disturbed by the introduction and spread of a nonnative species, as happened when the omnivorous mud crab indigenous to waters on the east coast of North America and Mexico became common in the Baltic Sea in the 1990s. Crabs, which are keystone species in many coastal food webs, feed on benthic (seafloor) communities—bivalves, gastropods, and other small invertebrates—with devastating efficiency, creating a strong top-down cascade. The increase in the number of mud crabs in the Baltic, where there are no equivalent predators, resulted in a dramatic decline in the mix of benthic invertebrate species. This in turn led to an increase in floating nutrients, which ultimately boosted phytoplankton rather than species on the sea floor. The net effect of the crabs’ arrival was to transfer nutrients from the sea floor to the water column—the water between the sediment and the surface—and to degrade the ecosystem.
Californian yellow bush lupines are fast growing and invasive. The plant can upset the ecosystem by causing elevated nitrogen levels that attract nonnative species.
Bottom-up cascades
If a plant—a primary producer—is removed from an ecosystem, a bottom-up cascade may result. For example, if a fungal disease causes grass to die-off, a rabbit population that depends on it will crash. In turn, the predators that eat rabbits will starve or be forced to move away, and the entire ecosystem could break down. Conversely, if planting or conservation efforts boost the mix of plant life, more herbivores (including the pollinators that help plants reproduce and spread) will be attracted, and with them more predators.
In the bottom-up model, the responses of herbivores and their predators to increased plant variety follow in the same direction: more plants support more herbivores and more
predators. This is in contrast to top-down cascades, in which more predators lead to fewer herbivorous prey and a greater mix of plants.
“Just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.”
Aldo Leopold
Beetles, ants, and moths
Investigating trophic cascades in four-tier systems is more difficult, because predators at the top feeding level may eat predators at the level below and also herbivores below them, so the relationships become very complex. In 1999, researchers studying trophic cascades in tropical rain forest in Costa Rica got around this problem by studying a system of three trophic levels of invertebrates, in which the top predator—a clerid beetle—ate the predatory ants in the level below it, but not the herbivores in the level below that. When the number of predatory beetles in the study area was increased, the population of predatory ants fell dramatically. This reduced the pressure on dozens of species of herbivorous invertebrates, which therefore ate more vegetation. The leaf area of the plants in the study was consequently reduced by half.
Not all the “players” in trophic cascades are obvious or visible. Some are tiny and live underground. For example, yellow bush lupines—plants that live on the Californian coast—are consumed by the caterpillars of ghost moths, which eat the lupines’ roots. In turn, nematodes—wormlike invertebrates—parasitize the caterpillars. If these nematodes are in the soil, they will limit the population of caterpillars, and fewer of the lupines’ roots will be affected.