Species Interactions and Community Ecology

A community is all the populations of different species living and interacting in the same place. The lions, hyenas, wildebeest, grasses, dung beetles and oxpeckers of the Serengeti form one community; the corals, fish, algae and sharks of a reef form another. Community ecology asks how species interact, how many can coexist, and why some places hold far more species than others.

What you will learn

• The main types of species interaction and their effects
• Competition, the competitive exclusion principle and resource partitioning
• Predation, herbivory and predator–prey cycles
• Parasitism, mutualism and commensalism
• How ecologists measure species diversity

Types of species interaction

Interactions are often summarised by their effect on each species: positive (+), negative (−) or neutral (0).

Competition

Competition occurs when two organisms need the same limited resource. It can be:

• Intraspecific: between members of the same species. This is usually the most intense, because individuals of the same species need exactly the same things.
• Interspecific: between different species.

Competition takes two forms. In interference competition, individuals fight directly or prevent others from reaching a resource, for example hyenas stealing a kill from cheetahs or birds defending nest holes. In exploitation competition, individuals simply use up a resource first, such as grazers eating the same grass.

The competitive exclusion principle

In the 1930s, the Russian ecologist Georgy Gause grew two species of Paramecium together in the laboratory on the same food. One always drove the other to extinction. This led to the competitive exclusion principle, also called Gause’s principle: two species competing for exactly the same limited resources cannot coexist indefinitely. One will eventually outcompete the other.

Resource partitioning

If competitors cannot share identical niches, how do so many similar species live together? Usually by dividing resources, a process called resource partitioning:

• On the Serengeti, zebras eat the tall, coarse tops of grasses, wildebeest follow and eat the leafy middle parts, and Thomson’s gazelles eat the short new shoots and herbs underneath. This sequence is known as a grazing succession.
• The ecologist Robert MacArthur showed in 1958 that five species of warbler in New England spruce forests feed in different parts of the same trees.
• Large African predators divide prey and time: lions take big prey mostly at night, cheetahs hunt smaller antelope by day, and leopards drag their kills into trees to keep them from lions and hyenas.

Over time, competition can lead to character displacement, in which competing species evolve differences. Darwin’s finches on islands with competitors often have more different beak sizes than on islands where they live alone.

Predation and herbivory

Predators and prey are locked in an evolutionary arms race. Prey evolve defences:

• Camouflage, like the stripes of a tiger in tall grass or the colour-changing skin of a chameleon.
• Warning colouration, like the bright colours of poison dart frogs and wasps.
• Mimicry: harmless species copying dangerous ones (Batesian mimicry, such as hoverflies resembling wasps), or dangerous species resembling each other (Müllerian mimicry, such as many stinging bees and wasps).
• Group living, which provides more eyes to spot danger and dilutes each individual’s risk.
• Speed, armour, spines and chemical defences.

Plants defend themselves against herbivores with thorns (acacias), toxic chemicals (milkweed, eucalyptus), tough leaves and even partnerships with ants that attack browsers.

Predator–prey cycles

The best-known example comes from fur trapping records kept by the Hudson’s Bay Company in Canada for nearly a century. The numbers of snowshoe hare and Canada lynx pelts rose and fell in a cycle of about 10 years, with lynx peaks lagging a year or two behind hare peaks.

In the 1920s, the mathematicians Alfred Lotka and Vito Volterra independently developed equations showing how such cycles can arise. When prey are abundant, predators thrive and multiply. More predators eat more prey, so prey decline. With less food, predators then decline, allowing prey to recover, and the cycle repeats. Explore the model below.

Real hare cycles are also driven by the hares’ food supply and by stress from predation risk, which reduces hare breeding. Field experiments in Yukon, Canada, that added food and excluded predators showed that both matter.

Parasitism

A parasite lives on or in a host and takes resources from it, usually without killing it quickly. Parasites are extraordinarily diverse; by some estimates, most species on Earth are parasites. Some manipulate their hosts’ behaviour: the parasite Toxoplasma gondii makes infected rodents less afraid of cats, helping it reach the cat, where it reproduces.

Brood parasites such as the common cuckoo lay their eggs in other birds’ nests. The cuckoo chick pushes the host’s eggs out, and the host parents raise it as their own.

Mutualism

In mutualism, both species benefit. Mutualisms underpin whole ecosystems:

• Pollination: around three-quarters of the world’s leading food crops benefit at least partly from animal pollinators.
• Mycorrhizal fungi connect with the roots of about 90 percent of land plants, trading nutrients for sugars.
• Corals and algae: reef-building corals host algae called zooxanthellae that provide most of their energy through photosynthesis. When stressed by heat, corals expel the algae and turn white, which is called coral bleaching.
• Oxpeckers eat ticks from the hides of large mammals, although they also sometimes peck at wounds, which blurs the line between mutualism and parasitism.
• Cleaner fish remove parasites from larger fish at “cleaning stations” on reefs.

Measuring diversity

Ecologists describe community diversity in two ways:

• Species richness: the number of species present.
• Species evenness: how equally individuals are spread among species.

A community with 10 species each making up 10 percent of individuals is more diverse than one with 10 species where one makes up 91 percent. Indices such as the Shannon index and Simpson’s index combine richness and evenness into one number.

Diversity is also described at three scales: alpha diversity (within a single site), beta diversity (the difference between sites) and gamma diversity (across a whole region).

Case study: the grazing succession of the Serengeti

The Serengeti holds millions of grazing animals that seem to compete for the same grass. In the 1970s, research by the ecologists Richard Bell and Sam McNaughton showed that they actually help each other. Zebras remove the tall, coarse grass stems, exposing the leafier parts that wildebeest prefer. Wildebeest grazing then stimulates new, protein-rich shoots that gazelles need. Each species prepares the grass for the next, so the migration moves across the plains in a predictable order. What looks like competition is partly facilitation.

Try it yourself

Choose five interactions you can observe near you, such as bees on flowers, birds at a feeder, or fungi on a log. Classify each using the table above. Which were hardest to classify, and why?

Common misconceptions

• “Competition always leads to extinction.” Species often coexist by partitioning resources.
• “Parasites are rare oddities.” Parasites are among the most diverse organisms and have major effects on ecosystems.
• “Mutualism means friendship.” Each partner acts in its own interest; mutualisms can break down into parasitism when conditions change.

Key terms

• Competitive exclusion principle: complete competitors cannot coexist indefinitely.
• Resource partitioning: dividing resources to reduce competition.
• Character displacement: evolution of differences between competing species.
• Mimicry: one species resembling another for protection.
• Mutualism: an interaction that benefits both species.
• Species richness and evenness: the number of species and how equally common they are.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Further reading

• MacArthur, R. H. (1958). “Population ecology of some warblers of northeastern coniferous forests.” Ecology.
• Krebs, C. J. et al. (2001). “What drives the 10-year cycle of snowshoe hares?” BioScience.