Competition, parasitism and partnerships

Competition happens when the fitness (the ability to grow and reproduce, r) of one species is lowered by another. It happens most between closely related species, but unrelated species can compete too: in some South American lakes, flamingoes and fishes compete for the same food, zooplankton.

• On the Galapagos Islands, the Abingdon tortoise died out within ten years after goats were brought to the island, because the goats were far better at grazing.
• On the rocky coast of Scotland, the larger barnacle Balanus pushes the smaller Chathamalus out of the zone they both could use, as Connell showed.

Gause's competitive exclusion principle says that two closely related species competing for the same limited resources cannot live together for long: the weaker one is eventually eliminated. But species can avoid it by dividing up the resource: resource partitioning. MacArthur showed that five species of warblers could live on the same tree because each fed in a different part of it, at different times.

Parasitism. A parasite lives on or in a host, feeding on it, without killing it quickly. Many parasites are host-specific, and host and parasite co-evolve: if the host evolves a defence, the parasite evolves a way round it.

• Ectoparasites feed on the outside of the host: lice on humans, ticks on dogs, and the plant Cuscuta, which grows on hedge plants and has lost its chlorophyll and leaves, taking all its food from the host.
• Endoparasites live inside. Many have complex life cycles with more than one host: the human liver fluke passes through a snail and a fish before reaching humans.
• Brood parasitism: the koel (cuckoo) lays its eggs in a crow's nest, and the crow raises them. The koel's eggs have evolved to look like the crow's, so the crow does not detect them.

Commensalism (+, 0):

• An orchid growing on a mango branch gains support; the mango is unaffected.
• Barnacles on the back of a whale get a free ride.
• Cattle egrets follow grazing cattle, which stir up insects from the grass for them to catch.
• Clown fish live among the stinging tentacles of a sea anemone, which protect them from predators.

Mutualism (+, +):

• Lichens: a fungus and an alga (or cyanobacterium) live together; the alga makes food, the fungus gives shelter and water.
• Mycorrhizae: fungi and plant roots (see chapter 8).
• Fig trees and wasps: each fig species is pollinated by only one species of wasp, which also lays its eggs in the fig fruit.
• The Mediterranean orchid Ophrys uses 'sexual deceit': one of its petals looks like a female bee. Male bees try to mate with it (pseudocopulation), and so carry its pollen to other flowers.