Interrelationships among organisms, and competition

1Why does a farmer pull up healthy seedlings?

In a maize field in Musanze, a farmer drops two or three seeds into each planting hole. A few weeks later she pulls out all but one or two seedlings in each hole and throws away plants that look perfectly healthy. She is not wasting them. Seedlings that stay crowded share the same light, water and nitrate, and each one ends up small with a poor cob. This is competition, and it is only one of the ways in which organisms affect each other. In Senior 3 you met food webs and named these relationships. In this unit you analyse them as an ecologist does: what each partner gains or loses, what evidence shows it, and how it controls the size and distribution of populations.

2Classifying interactions by their effects

Ecologists describe each interaction by its effect on the growth, survival or reproduction of each partner: + (benefits), − (is harmed) or 0 (is not significantly affected).

Saprophytism is the odd one out: a saprotroph such as a mushroom feeds on dead organic matter, so there is no living partner to be harmed. It is still a form of dependence, because the saprotroph needs the bodies and wastes of other organisms, and they in turn need the minerals it releases.

3The effects of interrelationships

These interactions do more than decide who eats whom. They:

• regulate population size: competition, predation and parasitism become stronger as a population grows denser, so they act as density-dependent factors that keep numbers near the carrying capacity;
• set where species live: a species is often missing from a habitat it could survive in, because a competitor or predator excludes it;
• drive adaptation: predators and prey, and parasites and hosts, evolve in response to each other (coevolution);
• cycle matter and energy: saprotrophs and mutualists such as Rhizobium return carbon, nitrogen and phosphorus to the soil and air.

4Intraspecific competition

Members of one species need exactly the same resources, so intraspecific competition is usually the most intense competition of all. As density rises, each individual's share falls. The results are:

• slower growth and smaller adults, as in the crowded maize;
• higher death rates: in a dense stand of young eucalyptus trees, many saplings die from shading as the stand grows (self-thinning);
• lower birth rates, because poorly fed females produce fewer young;
• territories and dominance: during the breeding season a male impala defends a patch of grassland and drives other males away, so only some males mate;
• emigration of young animals to new areas.

Because its strength rises with density, intraspecific competition is the main reason a population levels off at the carrying capacity.

5Interspecific competition and the niche

Two species compete only if they share a resource that is in short supply. How strongly they compete depends on how much their ways of life overlap, which ecologists describe with the idea of a niche.

Evidence 1: Gause's Paramecium cultures

In the early 1930s the Russian ecologist G. F. Gause grew two ciliates, Paramecium aurelia and Paramecium caudatum, in tubes of nutrient liquid with bacteria as food. Grown in separate tubes, each species increased and then levelled off. Grown together in one tube, P. aurelia persisted but P. caudatum died out within about 16 days: P. aurelia took the bacteria faster. When Gause instead paired P. caudatum with P. bursaria, both survived, because the two species fed in different parts of the tube. Their niches did not overlap completely.

Evidence 2: Connell's barnacles

On rocky shores in Scotland, J. H. Connell (published 1961) studied two barnacles. Small Chthamalus lives high on the shore; larger Balanus lives lower down. When Connell scraped Balanus off some rocks, young Chthamalus survived in the lower zone too. Where Balanus was left, it grew over, crushed or undercut the Chthamalus. So the lower limit of Chthamalus is set by competition, and its upper limit by drying out at low tide, which Balanus tolerates less well.

How competitors coexist

Many similar species do live together. They avoid exclusion by resource partitioning: dividing a resource so that their realised niches differ. In a classic study, R. H. MacArthur (1958) found five species of warbler feeding in the same spruce trees in North America, each searching mainly in a different part of the tree. On the savanna, zebras crop the tall, coarse grass, while wildebeest and Thomson's gazelles graze the shorter, leafier grass beneath it, a pattern well studied in the Serengeti.