by DK
The term coevolution was coined by American biologists Paul Ehrlich and Peter Raven in 1964, but a century before the word existed, the naturalists Charles Darwin and Alfred Russel Wallace were already aware of the concept, not least through their observation of orchids. Like many other flowering plants, orchids rely on insects to pollinate them. Some have extraordinary structures in which to hold nectar and pollen. To lure the insect pollinators, the plants offer them a drink of energy-giving nectar. This fascinated Darwin, who was given a specimen of the Madagascar orchid in 1862. The flower stores its nectar in a hollow spur nearly 30 cm (12 in) long. Darwin and Wallace speculated that only a large moth could have a proboscis long enough to reach the nectar—a theory eventually proven in 1997. If the orchid’s spur were shorter, a moth could drink without picking up pollen and so would not pollinate the flower. If the spur were longer, a moth would not visit.
The clownfish and sea anemone could both survive without the other’s protection, but their coevolved mutual relationship gives them a much higher chance of survival.
Yuccas and their moths
In the hot, arid regions of the Americas, there is a remarkable mutualistic relationship between yucca shrubs and yucca moths. No other insects pollinate these plants, and no other plants host yucca moth caterpillars. A female yucca moth collects pollen from the flower of one yucca plant and deposits it in the flower of another yucca, fertilizing the plant as it does so. The moth then cuts a hole in the flower’s ovary and lays an egg; she may lay several in the same flower. When the eggs hatch, the caterpillars feed on the seeds developing in the flower but do not eat them all, leaving enough for the plant to propagate. If too many eggs are laid in one flower, the plant sheds it before the caterpillars hatch – leaving those insects to starve. Without these moths, the yuccas would not pollinate and would soon die out. Without the yuccas, the moths would have nowhere to lay and nurture their eggs, and they too would not survive.
See also: Evolution by natural selection • Ecological niches • Competitive exclusion principle • Animal ecology
IN CONTEXT
KEY FIGURE
Robert Paine (1933–2016)
BEFORE
1950s In Kenya, farmer and conservationist David Sheldrick introduces elephants to Tsavo East National Park, and discovers this results in a major increase in biodiversity.
1961 Fieldwork by American ecologist Joseph Connell on Scotland’s rocky shores shows that removing predatory whelks alters the distribution of their barnacle prey.
AFTER
1994 In the US, a group of ecologists led by Brian Miller publishes a paper explaining the valuable role prairie dogs play as a keystone species.
2016 Fieldwork leads marine ecologist Sarah Gravem to conclude that organisms can be keystone species in some places but not in others.
A keystone species plays a crucial role in the way an ecosystem functions, even though it is often a small part of the overall biomass of the ecosystem. Because it exerts a disproportionately large effect on the environment relative to its biomass, if a keystone species disappears from an ecosystem, that ecosystem will change dramatically. The importance of keystone species was brought to light by the American biologist Robert Paine—who derived the term from the central “keystone” at the top of an arch that stops it from collapsing—in his 1969 article “A Note on Trophic Complexity and Community Stability.”
“Do you want an auto mechanic who… can name, list, and count all of the parts of your engine, or one who really understands how each part interacts with the others to make a working engine?”
Robert Paine
The keystone concept
In the 1960s, Paine spent several years studying the animals of the intertidal zone of Tatoosh Island on the Pacific coast of Washington State. He removed the ocher starfish and watched its key prey, a mussel whose numbers had been kept in check by the starfish, dominate the zone, replacing other subordinate species. The removal of a single, keystone species had a clear impact on many others. Paine developed the idea to include the concept of “trophic cascades”—the strong, top-down effects that ripple through an ecosystem and its organisms. Since Paine’s work with starfish, several studies have demonstrated that there are many other keystone organisms, and they each fulfill their role in different ways.
ROBERT PAINE
Born in 1933, in Cambridge, Massachusetts, Robert Paine studied at Harvard. After a stint in the US Army, where he was the battalion gardener, Paine focused his research on marine invertebrates. His study of the relationship between starfish and mussels on the Paciic coast led him to propose the concept of keystone species—the disproportionate impact that a single species can have on its ecosystem.
Paine worked for most of his career at the University of Washington, where he popularized field manipulation experiments, or “kick-it-and-see” ecology. He was awarded the International Cosmos Award by the National Academy of Sciences in 2013, and died in 2016.
Key works
1966 “Food Web Complexity and Species Diversity,” American Naturalist
1969 “A Note on Trophic Complexity and Community Stability,” American Naturalist
1994 Marine Rocky Shores and Community Ecology: An
Experimentalist’s Perspective
Ecological engineers
Prairie dogs in the American Midwest are a good example of a keystone species whose impact is the result of their “engineering” activities. Huge colonies of these small mammals dig networks of tunnels beneath the prairie grasslands. They sleep and raise their young in these extensive burrows, converting the grassland into a suitable habitat.
The prairie dogs’ constant digging dramatically increases soil turnover and allows nutrients and water from rain and snow to penetrate deeper than would otherwise be the case. The damp, nutrient-rich soil encourages a diversity of plants, and birds such as Mountain Plovers feed and nest in the short grass. Predators like Ferruginous Hawks and black-footed ferrets are attracted to the area to hunt for prey, and the ferrets and tiger salamanders use the burrows for shelter. Almost 150 species of plant and animal are known to benefit from prairie dog colonies. Although there are “losers”—notably vertebrates that favor tall vegetation—the prairie dogs’ presence increases overall biodiversity. When colonies die out, scrubby patches of mesquite vegetation replace short grasses, plovers abandon the area, and predator numbers decline.
Black-tailed prairie dogs look out from their burrow in a field in Wyoming. Study of this species has revealed its key role in fostering diversity in its native habitat.
Coral cleaners
The princess parrotfish in the Caribbean is another keystone species, this time because of the consequences of its feeding. The fish lives around coral reefs, where corals fight each other for light, nutrients, and space. The parrotfish scrapes the surfaces of the corals to remove layers of algal seaweed to eat. If the parrotfish did not do this, clumps of seaweed would grow on the corals, smothering as well as chemically damaging the reef. If the parrotfish was overfished or died out from disease, the health of the reefs would rapidly deteriorate.
Each pack of wolves in the Yellowstone National Park has its own territory. Many of the territories overlap, and numbers fluctuate from year to year, with 108 wolves recorded in 2016.
Landscape managers
On African grasslands, elephants smash down small and medium-sized trees for food, helping maintain savanna as grassland and opening up new areas that were formerly woodland. This destructive behavior helps maintain the feeding habitat for grazing animals such as zebras, antelope, and wildebeest. It also indirectly helps the predators that hunt the grazers—including lions, cheetahs, and hyenas—and the smaller mammals that burrow in grassland soils. Without the elephants, these animals would soon disappear. Elephants are also very important seed dispersers; undigested seeds pass through their gut, are then defecated, and later germinate. Up to one-third of all West African tree species depend on elephants for th
eir seed dispersal. Elephants also dig and maintain waterholes, which benefit many other species.
Forest-dwelling Asian elephants have a similar role. In southeast Asia, they smash through gaps and clearings in woodland, opening up holes in the canopy. The new plants that grow in these unshaded areas add to the forest’s plant and animal diversity and also help a broader range of animals to thrive there.
“Every species in the coastal zone is influenced in one way or another by the ecological effects of sea otters.”
James Estes
American marine biologist
Keystone predators
The sea otter is a marine mammal that lives in the Pacific coastal waters of North America. In the 18th and 19th centuries, they were hunted extensively for their fur. By the early 20th century, they had been wiped out in many areas, and their total population was thought to be fewer than 2,000 individuals. Since 1911, legal protection has led to a slow increase in numbers.
Sea otters are important because they eat large numbers of sea urchins. These seafloor-dwelling invertebrates graze on the lower stems of kelp that grow up from the seabed, causing it to drift away and die. If the kelp disappears, however, so do the many other marine invertebrates that graze on it. “Forests” of kelp also absorb large amounts of atmospheric carbon dioxide and, by slowing water currents, help protect coastlines from storm surges. The protection that sea otters offer kelp along stretches of open coast is therefore particularly significant.
Unlike the sea otter, some keystone species are also “apex” predators at the top of the food chain, such as the gray wolf. Before 1995, there had been no gray wolves in Yellowstone National Park for at least 70 years. American elk were common in the park, but there was just a single colony of beavers. That year, 31 wolves were introduced to the park and by 2001 their numbers had increased to more than 100, largely due to the abundance of elk for food.
The presence of wolves in the park forced the elk to become more mobile. Rather than over-grazing willow, aspen, and cottonwood trees in favored locations, the elk moved on, allowing plants to regenerate and provide a food source for other herbivores, such as beavers. Within 10 years, the number of beaver colonies increased from one to nine. Beaver dams helped revive wetlands, and wetland wildlife flourished. The increase in elk carcasses also benefited carrion-eaters—especially coyotes, red foxes, grizzly bears, Golden Eagles, Ravens, and Blackbilled Magpies—as well as several smaller scavengers.
Jaguars are apex predators in South and Central American forests, preying on more than 85 species. Although there are very few jaguars in any given area, their impact on the numbers of other predators—such as caimans, snakes, large fish, and large birds—as well as herbivores, such as capybaras and deer, has a significant ripple-down effect on their ecosystem. Left unchecked, the herbivores could devour most of the plants and destroy the habitat on which so many other species depend.
“By protecting a keystone species such as the prairie dog, the public could be educated about the value of ecosystem conservation.”
Brian Miller
American ecologist
Keystone plants
Not all keystone species are animals. One example is the fig tree, of which there are about 750 species, mostly found in tropical forests. In this habitat, most fleshy-fruited plant species share one or two peaks of ripening each year. Fig trees bear fruit throughout the year, supporting many animals when other trees are fruitless. More than 10 percent of the world’s bird species and 6 percent of mammals (a total of 1,274 species) are known to eat figs, as do a small number of reptiles and even fish. Fig trees therefore provide a vital support mechanism for fruit-eating species. Without them, fruit bats, birds, and other creatures would decline or disappear.
Reintroducing beavers to the UK
Beavers were wiped out in the UK 400 years ago, but the beneficial role of this keystone mammal is now better understood. Beavers are ecological engineers, building dams and canals, and their presence increases biodiversity.
In 2009, 11 beavers were reintroduced to Knapdale Forest, Scotland, and in 2011, the Devon Wildlife Trust introduced a pair to a fenced enclosure. Both projects have been monitored to test their impact on the environment. In Knapdale Forest, the beavers’ dams changed the water level of a loch, and Devon’s beavers built several dams on the headwaters of the Tamar River, creating 13 new freshwater pools and making surrounding areas wetter.
In Devon, the damp areas created by beavers led to an increase in the number of bryophyte species (mosses and liverworts), and the range of aquatic invertebrates has risen from 14 to 41 species. Increased numbers of flying insects have also improved bat diversity, with two nationally rare bat species drawn into the area. More beaver reintroduction projects are now planned in the UK.
See also: Predator–prey equations • Mutualisms • Animal ecology • Trophic cascades • Evolutionarily stable state
IN CONTEXT
KEY FIGURES
Ronald Pulliam (1945–), Graham Pyke (1948–), and Eric Charnov (1947–)
BEFORE
1966 John Merritt Emlen, Robert MacArthur, and Eric Pianka outline the concept of optimal foraging in two articles published in the American Naturalist magazine.
AFTER
1984 Argentinian–British zoologist Alejandro Kacelnik researches the foraging behavior of starlings to illustrate the marginal value theorem (MVT).
1986 Belgian ecologist Patrick Meire investigates prey selection by oystercatchers.
1989 Swiss environmental scientists T. J. Wolfe and Paul Schmid-Hempel examine how the weight of nectar carried by bees has an effect on the bees’ foraging behavior.
Every plant and animal on Earth needs resources to survive. Plants obtain their nutrients and water from soil, and sunlight provides the energy for photosynthesis. Animals generally have to work harder to find their food—they have to move, and this uses extra resources. Optimal foraging theory (OFT) proposes that animals try to gather resources in the most efficient way to avoid using additional energy. Searching for and capturing food takes energy and time. The animal needs to gain maximum benefit for minimal effort in order to achieve optimal fitness. OFT helps predict the best strategy that an animal can use to achieve this goal.
“Diets should be broad when prey are scarce, but narrow if food is abundant.”
Eric Pianka
Foraging theories
The first theory of foraging by wild animals did not emerge until the mid-1960s, when Americans Robert MacArthur and Eric Pianka examined the question of why, when a range of food was available to them, animals often restricted themselves to a few preferred types of prey. They argued that natural selection favored animals whose behavior maximized their net energy intake per unit of time spent foraging. An animal’s foraging time includes searching for prey and the killing and eating of the food (handling time).
These ideas were developed by American ecologists Ronald Pulliam and Eric Charnov and Australian ecologist Graham Pyke. It seems that OFT works best for mobile foragers seeking immobile prey, and some researchers believe it is less relevant when prey are mobile.
Key choices
Animals must choose which types of food to eat, which is rarely straightforward. For example, American ecologists Howard Richardson and Nicolaas Verbeek studied Northwestern Crows feeding on clams in the intertidal zone of British Columbia. The crows put lots of effort into digging clams out of the mud, opening the shells, and feeding on the animal inside. The ecologists noticed that smaller clams went unopened and concluded that the crows had to make an energy trade-off between handling time and edible food. The time and energy needed to open up small clams was better spent digging for another, larger clam. A similar study with oystercatchers and mussels found that the largest mussels were left—they had thicker, barnacle-clad shells, so opening them was more difficult. The oystercatchers benefited more by looking for thin-shelled mussels, despite their smaller size.
Animals also have to m
ake choices about where and when to feed. The longer a starling spends in one patch of suitable grassland, for example, the harder it will become to find prey, so it has to decide when to abandon that patch and move to another—an example of what is known as the “marginal value theorem.” Foraging animals also need to consider a range of other factors such as the presence of predators, the number of animals competing for the same food, and the impact of human activity.
Oystercatchers, despite their name, are reliant on cockles and mussels as their primary food source. Without these shellfish, they are forced to forage farther inland.
“The expected behavior of animals with respect to available resources can be used to predict … the biotic structure … of communities.”
Ronald Pulliam
Echolocating bats
Technological advances have greatly helped research into the hunting strategies of animals. Insectivorous bats (also known as microbats) use echolocation in the dark to locate and pursue flying insect prey, such as moths and midges. A team of Japanese scientists set out to study the bats’ feeding behavior using microphone array measurements and mathematical modeling analysis. The researchers recorded the echolocation calls and flight paths of the bats and discovered that they often directed their sonar not just at their immediate prey but at the next target they were lining up as well.