The Ecology Book
Page 6
An ultra-specialist
Giant pandas occupy a very specialized ecological niche, as their diet consists mainly of bamboo. Bamboo is a poor food source, low in protein and high in cellulose. Pandas can digest only a small proportion of what they eat, which means they have to eat a lot of bamboo—as much as 28 lb (12.5 kg) each day—and forage for up to 14 hours a day. It is unclear why pandas have become so dependent on bamboo, but some zoologists suggest it is because it is an abundant and reliable food source, and pandas are not skilled predators.
Pandas eat different parts of bamboo plants according to the seasons. In late spring, they prefer the first green shoots. They eat leaves at other times of the year, and stems in winter when little else is available. Pandas have evolved muscular jaws and a pseudothumb to manipulate bamboo stems. Their digestive tract is inefficient at processing large quantities of plant material because it remains similar to that of its carnivorous ancestors, although digestion is helped by the bacterial fauna in their gut.
See also: Competitive exclusion principle • Field experiments • Optimal foraging theory • Animal ecology • Niche construction
IN CONTEXT
KEY FIGURE
Georgy Gause (1910–86)
BEFORE
1925 Alfred James Lotka first uses equations to analyze variations in predator–prey populations, as does mathematician Vito Volterra, independently, a year later.
1927 Volterra enlarges and updates his 1926 study to include various ecological interactions within communities.
AFTER
1959 G. Evelyn Hutchinson extends Gause’s ideas and produces a ratio describing the limit of similarity between two competing species.
1967 Robert MacArthur and Richard Levins use probability theory and Lotka–Volterra equations to describe how coexisting species interact.
Competition is the driver of evolution; the need to be bigger, stronger, and better inevitably leads to adaptations that give a species an edge. When two species compete for identical resources, the one which has any advantage will outdo the other. As a result, the weaker of the two species either becomes extinct or adapts, so that it no longer competes. This proposition, known as the “competitive exclusion principle,” was set out by Russian microbiologist Georgy Gause and is also known as Gause’s Law.
Gause devised his principle from laboratory experiments, using cultures of microorganisms, rather than from observations in nature. In nature, he proposed, there were too many variables to draw conclusions about how ecological mechanisms work. He argued that little progress had been made since Darwin’s era in understanding how species compete for survival, whereas the experimental method had produced great advances in areas such as genetics. In fact, the competitive exclusion principle—although a useful theoretical model—is rarely seen in nature, simply because, in a bid to survive, a weaker competitor tends to quickly move on or adapt.
Five species of warblers are able to share the same tree, because each inhabits its own “niche.” Living in this way, without much overlap, the birds do not compete.
Avoiding competition
Most creatures can make the changes necessary for survival. A variety of birds can live in a garden during any one year because they all operate in different “niches.” They have contrasting beak shapes and sizes that allow them to eat different types of food—the robin preferring insects, the finch eating seeds. Their choice of habitat and feeding times might also vary; this is known as resource partitioning.
In 1957, Robert MacArthur noted this phenomenon in North American warblers. The five species he observed, each with distinctive, colorful markings, flitted in and out of coniferous trees, feeding on bugs and other insects. They could coexist in one habitat because they did not try to feed in the same part of the tree but at different heights and depths of the foliage. In this way they avoid competing with each other.
The red squirrel is smaller than the gray, and has a more restricted diet and habitat. Reds may also die from the squirrel parapoxvirus, which is carried by the grays but does not affect them.
“Let us make for this purpose an artificial microcosm… let us fill a test-tube with nutritive medium and introduce to it several species of protozoa consuming the same food or devouring each other.”
Georgy Gause
An invasive competitor
Problems often arise if an exotic species is suddenly introduced to an ecosystem. Britain’s red and gray squirrels provide a clear example. When the grey arrived from America in the 1870s, both squirrel species competed for the same food and habitat, which put the native red squirrel populations under pressure. The gray had the edge because it can adapt its diet; it is able, for instance, to eat green acorns, while the red can only digest mature acorns. Within the same area of forest, gray squirrels can decimate the food supply before red squirrels even have a nibble. Grays can also live more densely and in varied habitats, so have survived more easily when woodland has been destroyed. As a result, the red squirrel has come close to extinction in England.
Types of competition
The Competitive Exclusion Principle covers two main types of competition. Intraspecific competition is between individuals of the same species and ensures the survival of the fittest, so that only the healthiest individuals—or those best adapted to a particular environment—will breed. The second type is interspecific: competition between two different species that rely on the same resources. The most important of these will be the “limiting resource,” the one that both require in order to breed. Ecologists make a further two distinctions. Interference is when two organisms fight directly with each other over a limited resource, such as a mate or a preferred food. Exploitation is indirect competiton, such as stripping out a resource so there is none left for the competitor; this can be seen in plants, when a species’ uptake of nutrients or water is more efficient than that of its neighbors.
See also: Evolution by natural selection • Ecological niches • Animal ecology • The ecosystem • The ecological guild • Niche construction • Invasive species
IN CONTEXT
KEY FIGURE
Joseph Connell (1923–)
BEFORE
1856 British scientists John Lawes and Joseph Gilbert start the Park Grass Experiment at Rothamsted, to test how different fertilizers affect the yield of hay meadows.
1938 Harry Hatton, a French ecologist, conducts one of the first marine ecology field experiments, on barnacles on the Brittany coast.
AFTER
1966 American ecologist Robert Paine removes the starfish Pisaster ochraceus from tide pools in a Pacific coast ecosystem, to test the effect of its absence on other species.
1968 The Experimental Lakes Area, comprising 58 freshwater lakes, is established in Ontario, Canada, to study the effects of nutrient enrichment (eutrophication).
Experimentation is crucial in ecology. Without it, our ideas about why organisms behave the way they do would be largely speculative. Rigorous observation is also essential, but, much of the time, experimentation is needed for a full understanding of those observations.
Three main types of ecological experiments are used to test theories: mathematical models, laboratory experiments, and field experiments. Each method has its merits, but it is only recently that the benefits of field experiments have been recognized. Before the 1960s, experiments outside a laboratory were a rarity.
A laboratory, however, is an artificial environment, where organisms may not behave as they do in their natural habitat. For example, bats leaving a roost at dusk may follow different routes to their foraging areas in spring and late summer. The potential reasons for the switch—changes in prey distribution and predator threats; seasonal differences in tree cover; or human disturbance and light pollution—cannot be established in a laboratory. Mathematical modeling might help predict patterns, but would be less effective at identifying the causes of change. To understand the bats’ behavior, a study of their natural environment is crucial, and this is achieved on
ly through research in the field.
Field experiments allow different factors to be manipulated to test their relevance. In the bat example, street lights could be switched off to evaluate the impact of light pollution on their behavior change.
Rain forest ecosystems are some of the most species-rich environments on Earth. This makes them especially valuable sites for ecologists to conduct experiments in the field.
Scottish barnacles
In 1961, American ecologist Joseph Connell published the results of his research on barnacles on the Scottish coast. Since free-swimming barnacle larvae can settle anywhere, Connell had tested why the lower part of the intertidal zone was colonized by Balanus balanoides barnacles and the upper part by Chthamalus stellatus. He wanted to know if this was due to competition, predation, or environmental factors.
Connell manipulated the local environment, and monitored it for over a year. In one area, he removed the Chthamalus barnacles. They were not replaced by Balanus, which suggested that Balanus could not tolerate the desiccation that occurred in the upper zone at low tide. Connell then removed the Balanus population from the lower zone, and found that Chthamalus barnacles did replace them. Both species could live in the lower zone, but only one could survive higher up. This suggested that Chthamalus was better able to deal with the harsh conditions of the upper zone, but was outcompeted by Balanus lower down. The “fundamental niche” of Chthamalus (where the species would normally be able to survive) encompassed both zones, but its “realized niche” (the actual area it inhabits) was more restricted.
This experiment showed that Balanus could live only in the lower intertidal zone, while Chthamalus could live in both the upper and lower zones, but was outcompeted by Balanus in the lower zone.
“[Connell’s] studies … have improved our understanding of the mechanisms that shape population and community dynamics, diversity, and demography.”
Stephen Schroeter
Marine scientist
Diversity experiments
In the early 1970s, Connell and American ecologist Daniel Janzen published an explanation of the degree of tree diversity in tropical forests: the Janzen–Connell hypothesis. Connell mapped trees in two rain forests in North Queensland, Australia, and found that seedlings tended to be less successful when their nearest neighbor was of the same species. Each species is targeted by specific herbivores and pathogens, which will also eat or attack smaller, weaker individuals of the species nearby. This prevents “clumping” of one tree species.
In 1978, Connell proposed the intermediate disturbance hypothesis (IDH). This states that both high and low levels of disturbance reduce species diversity in an ecosystem, so the greatest range of species can be expected between those extremes. Several studies support IDH. One, carried out in waters off Western Australia, examined the effects of wave disturbance on diversity. Species diversity was found to be low both at exposed offshore sites and at sheltered sites.
See also: Ecological niches • A modern view of diversity • Animal behavior • The ecosystem • Niche construction
IN CONTEXT
KEY FIGURE
Dan Janzen (1939–)
BEFORE
1862 Charles Darwin proposes that an African orchid with a long nectar receptacle must be pollinated by a moth with an equally long proboscis.
1873 Belgian zoologist Pierre-Joseph van Beneden first uses the term “mutualism” in a biological context.
1964 The term “coevolution” is first used by American biologists Paul Ehrlich and Peter Raven to describe the mutualistic relations between butterflies and their food plants.
AFTER
2014 Researchers discover an unusual yet beneficial three-way mutualism involving sloths, algae, and moths.
In biology, there are several kinds of interaction between organisms. One species in an ecosystem may lose out to another when competing for the same resources. A prey species may be eaten by a predator. There are also symbiotic relationships, in which one species benefits but not at the expense of the other, or where one organism does not benefit but still survives. In the relationship known as “mutualism,” both organisms benefit from the relationship.
A tree and its ants
In the mid-1960s, Daniel Janzen, a young American ecologist, became fascinated by the amazing mutualistic relationship between acacia trees and ants in eastern Mexico. His research was one of the first in-depth studies of such an interaction. The two partners were the swollen-thorn acacia and the acacia ant, which lives in the bullhorn-shaped thorns of the tree. He found that queen ants sought out unoccupied shoots, cut a hole in one of the swollen thorns, and laid their eggs, sometimes leaving the thorn to forage on the tree’s nectar. Larvae hatching from the eggs then fed on the acacia’s leaf-tips, with their rich supplies of sugars and proteins. The larvae later metamorphosed into worker ants. In time, all the tree’s thorns became occupied, with up to 30,000 ants living in a colony.
Janzen showed that, unless the acacia ants were present to defend it, the swollen-thorn acacia lost the ability to withstand damage caused by insects that ate its leaves, stems, flowers, and roots. Without the ants, a tree would be stripped of its leaves and die within six months or a year. Because it could not sustain growth, it was also likely to be shaded out by competing trees. Janzen clipped thorns and cut or burned shoots to remove ants from trees, and found that the ants moved back in when new thorns started to grow.
In return for food and shelter, the ants provided two services for the tree: they defended its foliage from leaf-eating insects and ate potentially competitive tree seedlings growing close by. Janzen described the acacias and their ants as “obligate mutualists”, meaning that one species would die out without the other. If the ants were removed, the swollen-thorn acacia would have no means of defending itself. And if the acacia trees were removed, the ants would have no home.
Ants and their larvae shelter inside the swollen thorn of an East African whistling thorn acacia tree. In return the ants swarm from their nests to protect the tree from herbivores.
Benefits for all
There are two fundamental types of mutualism—service-resource and service-service relationships. They are defined by the nature of the relationship between the partner organisms, whether it is the provision of a service or the supply of a resource—both are usually key to survival. Service-resource relationships are common in nature, with the fertilization, or pollination, of flowers by butterflies, moths, bees, flies, wasps, beetles, bats, or birds the most widespread example. The resource (pollen) is provided by the flower, and the service (pollination) is provided by the animal. It is estimated that nearly three-quarters of flowering plants (some 170,000 species) are pollinated by 200,000 animal species. Typically, a pollinating insect is attracted to a flower by its colors or scent to drink nectar or collect pollen, and pollen attaches to part of the insect’s body to be carried to the next flower, where it is deposited. The flower and its pollinator have evolved to make this mechanism work effectively.
Some plants have also evolved a service-resource relationship in which birds and mammals disperse their seeds, spores, or fruit. Seeds may become attached to the fur of a mammal browsing the plant’s leaves; when the mammal wanders away, it disperses the seed. The vile odor of stinkhorn fungi attracts flies, which lick the fungi’s slime and thence disperse their spores. When a bird swallows a fruit, it carries the seeds with it as it flies away; the indigestible seeds may be excreted in faeces far from where they were eaten. In all these situations, the plants provide a resource (food) and the mammals, flies, and birds provide a service (transport).
However, not all mutualistic relationships involve plants. In Africa, birds named oxpeckers and herbivorous mammals such as impalas and zebras practise another kind of service-resource mutualism. The oxpeckers pick ticks from the mammals’ fur, removing irritation and a source of disease, while at the same time having a meal. Oxpeckers also make loud calls when they sense danger, alerting the ma
mmal host as well as other oxpeckers.
In the insect world, some ants and aphids carry out a different form of service-resource mutualism. While the aphids feed on plants, the ants protect the aphids. Subsequently, the ants consume the honeydew that the aphids release, using a “milking” process on their smaller partners, by stroking them with their antennae.
Service-service mutualisms, in which both organisms offer each other protection, are far less common. One unusual relationship takes place in the western Pacific Ocean, between around 30 species of clownfish and 10 species of venomous sea anemones. The sea anemones’ stinging, toxin-filled nematocysts, or capsules, on their tentacles kill most small fish that come close, but not the clownfish. Its thick layer of protective mucus provides immunity against the anemone’s sting, allowing the fish to live within the tentacles. In return for the protection offered by the sea anemones’ venomous tentacles, the clownfish deters predatory butterfly fish, removes parasites from its host, and also provides nutrients from its faeces.
The fig wasp and the fig share a complex service-resource mutualism, in which the wasp provides the service of pollination and the fig plants provide a secure environment for the wasp eggs to develop.
“There is mutual aid in many species.”
Pierre-Joseph van Beneden
Belgian zoologist
Cooperative evolution
Relationships between service and resource providers have developed over millions of years in a process called “coevolution”—the evolution of two or more species that affect each other reciprocally.