by DK
Spruce budworm caterpillars have devastated balsam fir forests in eastern North America six times since the 18th century. Holling described this process as having two very different states: one with young, fast-growing trees and few budworms; and one with mature trees and very large numbers of budworms.
Between outbreaks of budworms, young balsam fir grow alongside spruce and white birch trees. Eventually, the fir becomes dominant. A combination of this dominance and a sequence of very dry years stimulates a huge increase in the budworm population. The mature fir is destroyed, giving the spruce and birch an opportunity to regenerate. By keeping the balsam fir in check, the budworm also maintains the spruce and birch. Without it, the fir trees would crowd out the others. So the system is unstable but at the same time resilient.
See also: The food chain • The ecosystem • Energy flow through ecosystems • Trophic cascades
IN CONTEXT
KEY FIGURES
Hal Caswell (1949–), Stephen P. Hubbell (1942–)
BEFORE
1920 Frederic Clements describes how plant species are associated with each other in communities.
1926 Henry Gleason proposes that ecological communities are organized more randomly.
1967 Richard Root introduces the concept of the ecological guild—a group of species exploiting resources in similar ways.
AFTER
2018 A review headed by Dutch ecologist Marten Scheffer suggests that, although species that use the same resources may be competitively equivalent, they may also differ according to their response to stress-inducing factors, such as drought or disease.
Biodiversity is shaped globally by new species appearing and others becoming extinct. Community ecology has traditionally held that interactions between species play a vital role in determining this process. If two species compete for similar resources, for example, either the stronger pushes the weaker to extinction, or each is driven into a narrower niche of specialism.
In 1976, however, American ecologist Hal Caswell proposed a “neutral” theory of biodiversity. It maintained that ecologically similar species are competitively equal, and whether species become common or rare is down to chance processes.
The “null” model
In the early 2000s, American ecologist Stephen P. Hubbell developed a mathematical model known as the “null” hypothesis, published in The Unified Theory of Biodiversity and Geography (2001), that supported Caswell’s theory. He tested his model by studying real communities.
Neutral theories of biodiversity have dominated community ecology in recent years. However, an Australian study of coral reefs, published in 2014, focusing on once-dominant species that have been almost lost to overfishing, did not support the theory. According to Hubbell, species are interchangeable, so others should have increased to take their place. The fact that this did not happen in this case suggests that the neutral theory is flawed. The question of what maintains diversity remains an open one.
“Caswell made a bold attempt to create a neutral theory of community organization.”
Stephen P. Hubbell
See also: Human activity and biodiversity • Island biogeography • Climax community • Open community theory
IN CONTEXT
KEY ORGANIZATION
National Science Foundation (created 1950)
BEFORE
1926 Russian geochemist and mineralogist Vladimir Vernadsky formulates the theory of the biosphere in which everything on Earth lives.
1935 Pioneering British ecologist Arthur Tansley defines an ecosystem as encompassing all the interactions between a group of living creatures and their environment.
AFTER
1992 At the Earth Summit in Rio de Janeiro, there is international consensus on the importance of protecting the biosphere.
1997 The Kyoto Protocol to reduce greenhouse gas emissions is signed by 192 countries.
An in-depth understanding of ecosystems requires long-term study. In 1980, the US National Science Foundation set up six Long Term Ecological Research (LTER) sites to study long-term, large-scale ecological phenomena. There are currently 28 sites, five of which have been running since 1980. Ecologists are amassing datasets that will enable in-depth knowledge to be shared.
A forest ecosystem
One of the six original research sites is Andrews Forest in Oregon. It provides a good example of a temperate rain forest, enjoying mild, wet winters and cool, dry summers. With 40 percent being old-growth conifer forest, there is a high degree of biodiversity across its forest, stream, and meadow ecosystems. Ecologists have recorded thousands of species of insects, 83 bird species, 19 conifer species, and 9 species of fish. Projects aim to observe how land-use (such as forestry) and natural phenomena (fires, floods, climate) affect hydrology, biodiversity, and carbon dynamics—the way carbon and nutrients move through the ecosystem. There are many other long-term research sites worldwide with researchers logging data on ecosystems. With free access to the information, the research can be easily disseminated globally.
Log decomposition is being studied over a 200-year period at six old-growth forest sites in Andrews Forest, Oregon. The experiment began in 1985.
See also: The ecosystem • The biosphere • Sustainable Biosphere Initiative • Ecosystem services
IN CONTEXT
KEY FIGURE
John Maynard Smith (1920–2004)
BEFORE
1944 Mathematician John von Neumann and economist Oskar Morgenstern use a theory of games of strategy to devise a mathematical theory of economic and social organization.
1964 British biologist W.D. Hamilton applies game theory to the evolution of social behavior in animals.
1965 Hamilton uses game theory to describe the ecological consequences of natural selection.
1976 Richard Dawkins popularizes the idea of evolutionarily stable strategies.
AFTER
1982 John Maynard Smith applies the theory to evolution, sexual biology, and life cycles.
The field of behavioral ecology seeks to explain how the behavior of animals—what they eat, how they socialize, and so on—has evolved to suit their particular environment. The driving force is natural selection because the environment favors individuals with certain genes—some genes are “better” for certain situations and not for others—which are then passed on to offspring. Because the behavior of animals is influenced by genes, behavior must be influenced by natural selection as well.
Adaptive behavior
In 1972, British evolutionary biologist John Maynard Smith introduced a theory known as the evolutionarily stable strategy (ESS), that helped explain how behavioral strategies appear by natural selection. Just as factors such as food and temperature can affect animals, so can the behavior of other species. Maynard Smith suggested that an ESS adapts to the behavior of other animals, and cannot be beaten by competing strategies, thus giving animals the best chance to pass on their genes. He argued that only natural selection could upset this balance—hence why an ESS is “stable”—and that these behavior patterns are genetically preprogrammed.
ESS has its roots in game theory: a mathematical way of working out the best strategy in a game. Many examples of how animals behave emerge as being evolutionarily stable strategies, such as territorial behavior and hierarchies. For example, the genetically pre-programmed “rules” of “if resident, fight and defend” or “if visiting, give in and retreat,” which would help animals retain territory, combine to make territorial behavior an ESS.
Behavior arising from conflicts over space and territory might emerge as evolutionarily stable strategies. Fruit bats jostle for the best spots in the trees, with alpha males driving weaker bats down to lower branches.
Balancing strategies
The payoff that an individual animal gains—or the price it risks paying—by displaying a particular behavior can be quantified, so biologists can work out which strategies are likely to be most stable by using mathematical models (see box). If th
e model does not match the behavior of animals in the real world, then it suggests that stability has not evolved.
In real rather than hypothetical ecosystems, it is not a single strategy that is stable, but the balance between two or more strategies within the system as a whole. The overall balance is therefore better called an evolutionarily stable state, and not a strategy. Such a balance emerges when all individuals have equal fitness: they pass on their genes to the same extent. The state remains stable, even when there are minor changes in the animal’s environment.
The hawk-dove “game”
The simplest demonstration of John Maynard Smith’s evolutionarily stable strategy (ESS) concerns a hypothetical response to aggression known as the hawk-dove “game.” In this, individuals can either be hawkish and fight until badly injured, or dovish and posture, but then retreat. Hawks will outmatch doves, but could be seriously harmed in a fight with another hawk. Doves routinely escape injury, but waste time in posturing. Which strategy would be better for passing on genes? Maynard Smith and his collaborators devised a mathematical model to provide the answer, and—in this instance—being more hawkish than dovish emerged as the ESS. It predicts a ratio of seven hawks for every five doves, which is equivalent to any one individual being hawkish seven-twelfths of the time, and dovish five-twelfths of the time.
See also: Evolution by natural selection • The selfish gene • Predator–prey equations • Ecological niches • Trophic cascades • Biodiversity and ecosystem function
IN CONTEXT
KEY FIGURE
Michel Loreau (1954–)
BEFORE
1949 At the California Institute of Technology in the US, the first phytotron (research greenhouse) is built to study how an artificial ecosystem can be manipulated.
1991 In the UK, an Ecotron, a set of experimental ecosystems in computer-controlled units, is created at Imperial College, London.
AFTER
2014 Leading ecologists in the US say that the effect of diversity loss on ecosystems is at least as great as—or even greater than—that of fire, drought, or other drivers of environmental change.
2015 A paper published in Nature provides evidence that biodiversity increases an ecosystem’s resilience in a broad range of climate events.
In an age when human activities are rapidly eroding the complex mix of species in different habitats, ecologists have increasingly focused on how biodiversity loss affects the way ecosystems work. If species are replaced or lost altogether, can an ecosystem remain intact—or does this damage ecosystem function?
Such questions were the focus of the Biodiversity and Ecosystem Function (BEF) conference held in Paris in 2000. More than 60 leading international ecologists, including Michel Loreau, director of the Centre for Biodiversity Theory and Modeling in Moulis, France, outlined diverse research; some looked more closely at species, others at what makes an ecosystem work. Loreau maintains that a new unified ecological theory is necessary to combat extreme environmental challenges. That, he says, requires the integration of community ecology (the study of how species interact in ecosystems) with ecosystem ecology (research into the physical, chemical, and biological processes that connect organisms and their environment).
A phytotron built in 1968 in North Carolina, US, now includes 60 growth chambers, four greenhouses, and a controlled-environment facility for studying plant diseases and insects.
“Biodiversity loss… is likely to decrease the ability of ecosystems to resist the effects of climate change.”
Michel Loreau
Complex cycles
Scientists of both disciplines firmly believe that biodiversity, especially species and genetic diversity, is an important driver of ecosystem functioning. Ecosystems are powered by an input of energy and recycling of nutrients: plants and animals grow, die, and decompose, returning nutrients to the soil and restarting the cycle. These processes depend on the species within the ecosystems, which in turn depend upon one another as they interact—as predators and prey, for example. Many ecologists argue that a large variety of complementary species are needed to keep an ecosystem working and make it resilient to change. Others say that a few key species may be more important to stop ecosystems from collapsing.
When researching such issues, ecologists have tended to use both traditional observational fieldwork and also sophisticated mathematical models. More recently, research has begun to incorporate the manipulation of ecosystems in a more controlled way, on plots of land, for example, or within closed systems housed in giant greenhouselike facilities called phytotrons. The experiments help to establish what factors—such as numbers of species, or species type and dominance—affect ecosystems in the long term. Their findings show that the effects of biodiversity on ecosystem functions are complex. While the most diverse ecosystems tend to be the most productive, their success also depends on climate and soil fertility.
There is more to be learned about how plant diversity affects soil processes, the role of microbe biodiversity in the soil, and the effects of mutualistic species, such as flowering plants and pollinating insects. Much has been achieved, but questions remain, and the unifying theory that Loreau is seeking has still to be devised.
“One of the distinctive and fascinating features of ecological systems is their extraordinary complexity.”
Michel Loreau
Habitat fragmentation
Barro Colorado Island in the Panama Canal of Central America was formed in 1914, when tropical rain forest was flooded by damming, creating an isolated fragment of forest surrounded by water. Since 1946, the area has been studied in detail by biologists of the Smithsonian Institution and elsewhere to determine the effects of this habitat fragmentation: species diversity on the island has declined, and top predators are among the most vulnerable species. In the US, studies of habitat fragmentation and its effects on diversity in the Florida Keys led to Robert MacArthur and E.O. Wilson’s seminal Theory of Island Biogeography (1967).
From such environments, planners have learned important lessons about how to conserve species in isolated patches of habitat—sometimes in the midst of cities—that are set aside as reserves. Barro Colorado, and places like it, have also provided vital opportunities for study, where ecologists can explore how changing species diversity affects the functioning of an ecosystem at every level.
See also: Mutualisms • Keystone species • The ecosystem • Organisms and their environment • Invasive species
INTRODUCTION
The distribution of organisms through space and time is a fundamental interest of ecology. Early in the 19th century, Prussian explorer Alexander von Humboldt, a founding father of ecology, made detailed studies of plant geography in Latin America. Philip Sclater described the global distribution of bird species, and Alfred Russel Wallace did the same for other vertebrates, proposing six zoogeographic regions that are largely still in use today.
Communities
Early fieldwork concentrated on the distribution and abundance of organisms, but later in the 19th century scientists increasingly recognized that survey data could also throw light on interactions between species. In a sense, this represented the true birth of the field of ecology. Pioneers included American naturalist Stephen A. Forbes, who studied wild fish populations in the 1880s, and Danish botanist Johannes Warming, who examined the interaction between plants and their environment and introduced the idea of plant communities.
The link between climate and a region’s dominant vegetation type was set out by German botanist Andreas Schimper, who produced a worldwide classification of vegetation zones in 1898. In the early years of the 20th century, ecologists devoted more attention to the interrelatedness of all organisms within an ecosystem, exemplified by Russian scientist Vladimir Vernadsky’s concept of the biosphere.
While studying the vegetation growing on sand dunes along the shore of Lake Michigan in the 1890s, American botanist Henry Chandler Cowles realized that there was a succession of plant spe
cies, with “pioneer” plants being replaced by others, which were in turn themselves supplanted. Fellow American Frederic Clements used the term “climax community” to describe the endpoint of this succession. In 1916, he proposed that global vegetation patterns could be thought of as “formations,” or large communities of plants—and the organisms that depended on them—which reflected the regional climate. In relatively wet, temperate regions, for example, deciduous forest may dominate, but grassland tends to dominate in drier, more temperate areas. Clements argued that these climax communities were bound together and could be thought of as single, complex organisms.
Clements was soon challenged by American botanist Henry Gleason, who agreed that plant communities could be mapped, but argued that since individual plant species have no common purpose, the idea of integrated communities was invalid. His view found support in the 1950s, in the field studies of Robert Whittaker and the numerical research of John Curtis.
In 1967, American ecologist Richard Root proposed the idea of the “guild,” a group of organisms—closely related or otherwise—that exploit the same resources. Later, ecologists James MacMahon and Charles Hawkins refined the definition of a guild to species that “exploit the same class of environmental resources,” regardless of how they do it.
New ideas
Many new ideas enriched the study of ecology in the late 20th and early 21st centuries. The metapopulation concept was advanced by the Finn Ilkka Hanski, who argued that a population of a species is made up of differing, dynamic components. One part of a population may become extinct, while another thrives. The thriving element may subsequently help reestablish the population that has died out.
