by DK
Extinction events
In extreme cases, a trophic cascade can lead to species extinction—as in the case of Steller’s sea cows, marine mammals that once lived in the Bering Strait but were declared extinct in 1768. It has recently been argued that this extinction was caused by a calamitous trophic cascade, triggered by the hunting to virtual extinction of sea otters for the fur trade. The over-exploitation of sea otters allowed the population of sea urchins, their usual prey, to rise above a critical threshold. Sea urchins eat kelp, so the growth of their population led to a collapse in the extent of kelp forests—the sea cows’ food source. Although the sea cows themselves were not being hunted, they soon became extinct. Understanding how such interventions, and the introduction of nonindigenous species, can damage trophic cascades is vital in shaping conservation measures today.
Steller’s sea cow was a giant sirenian discovered by the naturalist Georg Steller in 1741. Its extinction is the cause of debate: was it hunted to death, or did its food source disappear?
“Herbivores are usually expected to be well fed and carnivores are usually expected to be hungry.”
Lawrence Slobodkin
Early humans and megafauna
Cave paintings in Altamira, Spain, show the importance of bison to early humans. The wild population became extinct in 1927, but captive herds have since been reintroduced.
In the last 60,000 years, which includes the end of the last ice age, about 51 genera of large mammals became extinct in North America. Most were herbivores, including ground sloths, mastodons, and large armadillos, but many were carnivores, such as American lions and cheetahs, scimitar cats, and short-faced bears.
Many of the extinctions occurred between 11,500 and 10,000 years ago, shortly after the arrival and spread of the Clovis people, who were hunters. One of the most convincing theories about these developments is the “second-order predation hypothesis,” which suggests that the humans triggered a trophic cascade. The people killed the large carnivores, which competed with them for prey. As a result, predator numbers were reduced and prey populations rose disproportionately, resulting in overgrazing. The vegetation could no longer support the herbivores, with the result that many herbivores starved.
See also: Predator–prey equations • The food chain • The ecosystem • Energy flow through ecosystems • Evolutionarily stable state • Biodiversity and ecosystem function
IN CONTEXT
KEY FIGURES
Robert H. MacArthur (1930–72), Edward O. Wilson (1929–)
BEFORE
1948 Canadian lepidopterist Eugene Munroe suggests a correlation between island size and butterfly diversity in the Caribbean.
AFTER
1971–78 In the US, biologist James H. Brown studies mammal and bird species variety on forest “islands” in the Great Basin of California and Utah.
2006 Canadian biologists Attila Kalmar and David Currie study bird populations on 346 oceanic islands and discover that species variety depends on climate as well as area and isolation.
Island, or insular, biogeography examines the factors that affect the species richness of isolated natural communities. Charles Darwin, Alfred Russel Wallace, and other naturalists had written about island flora and fauna in the 19th century. Their studies were conducted on actual islands in the ocean, but the same methods can be used to look at any patch of suitable habitat surrounded by unfavorable environment that limits the dispersal of individuals. Examples include oases in the desert, cave systems, city parks in an urban environment, freshwater pools within a dry landscape, or fragments of mountain forest between nonforested valleys.
In the mid-20th century, ecologists began more intensive studies into species distribution on different islands, and how and why they varied. In the US, biologists Edward Wilson and Robert MacArthur constructed the first mathematical model of the factors at play in island ecosystems and, in 1967, they outlined a new theory of island biogeography.
Their theory proposed that each island reflected a balance between the rate of new species arriving there and the rate at which existing species become extinct. For example, a habitable but relatively empty island would have a low extinction rate since there are fewer species to become extinct. When more species arrive, competition for limited resources increases. At a certain point, smaller populations will be outcompeted, and the rate of species extinction will rise. An equilibrium point occurs when the species immigration rate and the rate of those becoming extinct are equal; this may remain constant until a change occurs in either rate.
The theory also proposes that the rate of immigration depends on the distance from the mainland, or another island, and declines with increased distance. The area of an island is a further factor. The larger it is, the lower its rate of extinction, because if native species are pushed out of prime habitat by new immigrants, they have a better chance of finding an alternative, albeit imperfect (“suboptimal”) habitat. Larger islands are also likely to have a greater variety of habitats or microhabitats in which to accommodate new immigrants. A combination of variety and lower rates of extinction produces a greater species mix than on a small island—the “species-area effect.” The actual species in the mix will change over time, as a result of colonization and extinction, but will remain relatively diverse.
Mangrove-fringed islands in the Florida Keys—now protected for their diverse range of marine and terrestrial life—were the focus of research to test the island biogeography theory.
“Unless we preserve the rest of life, as a sacred duty, we will be endangering ourselves by destroying the home in which we evolved.”
Edward O. Wilson
Monitoring mangroves
In 1969, Wilson and his student Daniel Simberloff conducted a field experiment that tested the theory on six small mangrove islands in the Florida Keys in the US. They recorded the species living there, then fumigated the mangroves to remove all the arthropods, such as insects, spiders, and crustaceans. In each of the next two years, they counted returning species to observe their recolonization. The Florida Keys experiment showed that distance did indeed play an important role: the farther an island was from the mainland, the fewer invertebrates returned to recolonize the area.
New waves of immigration can, however, save even faraway island species from extinction. This is more likely to happen with certain bird species—which can travel long distances quickly—than with, for example, small mammals. There is also the so-called target effect, where some islands are more favored destinations because of the habitat they provide. Given the choice of a treeless island and one with woodland, a tree-nesting bird will naturally opt for trees.
An island’s size and distance from the mainland both affect its species richness. Islands closer to the mainland will receive more random dispersion of organisms; the larger island gets the most, the island furthest away gets the least.
ROBERT H. MACARTHUR
Born in Toronto, Canada, in 1930, and later relocating to Vermont in the US, Robert MacArthur originally studied mathematics. In 1957, he received his Ph.D. from Yale University for his thesis exploring ecological niches occupied by warbler species in conifer forests. MacArthur’s emphasis on the importance of testing hypotheses helped transform ecology from an exclusively observational field to one that employed experimental models as well. This methodology is reflected in The Theory of Island Biogeography, which he coauthored with Edward O. Wilson. MacArthur received awards throughout his career, and was elected to the National Academy of Sciences in 1969. In 1972, he died of renal cancer. The Ecological Society of America awards a biennial prize in his name.
Key works
1967 The Theory of Island Biogeography
1971 Geographical Ecology: Patterns in the Distribution of Species
Human impact
The key factors influencing the species mix on an oceanic island are its degree of isolation, how long it has been isolated, its size, the suitability of its habitat, its location relative to oce
an currents, and chance arrivals (for example, organisms washed up on mats of floating vegetation). Most of these factors apply to any similar, isolated habitats, not just actual islands.
The impact of humans—who probably began visiting isolated islands in the Pacific at least 3,000 years ago—has sometimes been dramatic. In recent centuries, people took dogs, cats, goats, and pigs with them when they colonized islands in the Pacific and elsewhere; inadvertently, they also carried rats on their boats. On many islands, rats ate the eggs of breeding seabirds and the seeds of endemic plants, some of which grew nowhere else. On the Galapagos Islands, dogs ate tortoise eggs, native iguanas, and even penguins. Goats competed with Galapagos tortoises for food and wiped out up to five species of plant on the Santiago Islands.
The arrival of humans, however, has not always reduced species richness on islands. Researchers discovered the important role of ship-assisted colonization of islands in the Caribbean. Despite its relatively small size, Trinidad, for instance, has more species of anole lizards than the much larger island of Cuba, because economic sanctions since the 1960s have meant that fewer boats (and their lizard stowaways) dock in Cuba.
“Island” habitats
In the early 1970s, American biologist James H. Brown applied the Wilson–MacArthur model to “islands” of coniferous forest on 19 mountain ridges in the Great Basin of California and Utah. The ridges are separated from each other by a vast sagebrush desert. Brown discovered that the diversity and distribution of small mammals (excluding bats) in the isolated forests could not be explained in terms of an equilibrium between colonization and extinction. Some species had become extinct, but no new species had arrived for millions of years, so Brown dubbed the mammals “relicts.” A few years later, his analysis of resident bird populations on the ridges revealed that new bird species had arrived from larger, similar forests in the Rocky Mountains to the east and in the Sierra Nevada to the west. Brown concluded that certain species groups—especially those that fly—are more likely to be successful immigrants than others.
Ecologists also studied the diversity of beetles and flies in nine parks of different sizes in Cincinnati, Ohio. Area was the best predictor of species richness, but when the ecologists coupled their findings with data on population sizes, they calculated that an increased size of parkland acts primarily to reduce extinction rates rather than to provide habitats for new species.
Central Park in Manhattan, New York City, is an “island” in an urban setting. Its checklist includes 134 bird species, 197 insect, 9 mammal, 5 reptile, 59 fungi, and 441 plant species.
“Destroying rainforest for economic gain is like burning a Renaissance painting to cook a meal.”
Edward O. Wilson
Conservation practices
Soon after the island biogeography theory was developed, ecologists began to apply it to conservation. Nature reserves and national parks were seen as “islands” in landscapes altered by human activity. When first creating protected areas, ecologists debated the optimum size: was one big reserve better than several smaller ones? As the island theory shows, biodiversity depends on a number of factors, and different species benefit in different settings. A sizable mammal will not survive in a small reserve, but many small organisms will thrive there. In places under pressure from human activity, the island theory has also encouraged the creation of wildlife corridors. These link areas of suitable habitat, which helps maintain ecological processes—for example, allowing animal movement and enabling viable populations to survive—without requiring a great expansion of protected areas.
“I will argue that every scrap of biological diversity is priceless …”
Edward O. Wilson
The rebirth of Krakatau
Krakatau’s deadly eruption sent up an ash cloud 50 miles (80 km) high that altered global weather patterns and caused a temperature drop of 2.2°F (1.2°C) for five years.
In 1883, volcanic eruptions devastated the Indonesian island of Krakatau, wiping out flora and fauna on the island and nearby Sertung and Panjang. By 1886, mosses, algae, flowering plants, and ferns had returned to Krakatau, borne either on the wind or as seeds on the surf. The first young trees emerged in 1887; various insect species, and a single lizard, were discovered in 1889. Recent research shows that the level of immigration to Krakatau and its neighbors peaked during the period of forest formation, from 1908 to 1921, but extinctions were at their height when the dense tree canopy prevented sunlight from reaching the forest floor, between 1921–33. Although the immigration of land birds and reptiles has almost stopped, new species of land mollusk and many insect groups are still arriving from Sumatra and Java, both just under 28 miles (45 km) away.
See also: Evolution by natural selection • Predator–prey equations • Field experiments • The ecosystem
IN CONTEXT
KEY FIGURE
Crawford Stanley Holling (1930–)
BEFORE
1859 Charles Darwin describes the interdependence between species as an “entangled bank.”
1955 In the US, Robert MacArthur proposes a measure of ecosystem stability that increases as the number of interactions between species multiplies.
1972 In contrast with MacArthur, Australian ecologist Robert May argues that more diverse communities with more complex relationships may be less able to maintain a stable balance between species.
AFTER
2003 Australian ecologist Brian Walker works with Crawford Holling to refine the definition of resilience.
The capacity for ecosystems to recover following a disturbance—such as a large fire, flood, hurricane, severe pollution, deforestation, or the introduction of an “exotic” new species—is known as ecological resilience. Any of these impacts can upset food webs, often dramatically, and human activity is responsible for an increasing number of them.
Staying resilient
Canadian ecologist Crawford Stanley Holling first proposed the idea of ecological resilience to describe the persistence of natural systems in the face of disruptive changes. Holling argued that natural systems require stability and resilience, but—contrary to what previous ecologists had assumed—these are not always the same qualities.
A stable system resists change in order to maintain the status quo, but resilience involves innovation and adaptation. Holling wrote that natural, undisturbed systems are likely to be continually in a transient state, with populations of some species increasing and others decreasing. However, these population changes are not as important as whether the whole system is being fundamentally altered. The resilience of a system can be described either by the time it takes to return to equilibrium after a big shock or by its capacity to absorb disturbance.
One example that Holling studied was the fisheries of the Great Lakes in North America. A large tonnage of sturgeon, herring, and other fish was harvested in the early decades of the 20th century, but overfishing dramatically reduced the catches. Despite subsequent controls on fishing, populations in the Great Lakes did not recover. Holling suggested that the intense fishing had progressively reduced the resilience of the ecosystem.
Holling argued that ecological resilience is not always positive. If a freshwater lake experiences a large input of nutrients from agricultural fertilizers, for example, it will become eutrophic: algae will thrive, depleting the lake’s oxygen and making it unsuitable for fish. Such a lake may be resilient, but it will become less biodiverse. Holling claimed that three critical factors determine resilience: the most a system can be changed before crossing a threshold that makes total recovery impossible; the ease or difficulty in making a big change to the system; and how close to the threshold a system is currently.
“Ecosystems are dynamic—constantly changing and inherently uncertain, with potential multiple futures …”
Crawford Stanley Holling
A thick green scum of algae covers parts of Lonar Lake, in Maharashtra, India. Algae thrive in high-nutrient conditions, but decomposing algae consume oxy
gen, and depleted levels of oxygen lead to fewer fish surviving.
Changing states
According to Holling’s view, resilience at the ecosystem level is enhanced by its populations not being too rigid—meaning that the components of the ecosystem can change. One example is the disappearance of most American chestnuts from forests in eastern North America, which was largely compensated for by the expansion of oaks and hickories. For Holling, this counted as resilience, because although the exact mix of tree species had changed, broad-leaved forest still remained.
Ecologists now understand that ecosystems can have more than one stable state. In Australia, for example, woodlands dominated by mulga trees can exist in a grass-rich environment that supports sheep-farming, or in a shrub-dominated environment that is totally unsuitable for sheep.
The role of budworm
Spruce budworm larvae in Quebec, Canada, feed voraciously on fir and spruce before they pupate. Moths emerge about a month later, ready to mate.