Competition, Niches and Resource Partitioning

Interactions shape populations

So far we have mostly treated one population on its own. But populations live in communities, and their interactions (competition, predation and symbiosis) are among the most important limiting factors. The next three lessons look at each type. A handy notation:

• + the interaction helps the species,
• − it harms the species,
• 0 it has no effect.

Competition is −/−: both competitors are worse off than they would be alone.

Kinds of competition

• Intraspecific (same species): seedlings of one maple shading each other, male moose competing for mates. Because it intensifies as density rises, it is a key density-dependent factor and the main source of the logistic brake.
• Interspecific (different species): red squirrels and grey squirrels competing for seeds, or purple loosestrife crowding out native cattails in an Ontario wetland.

Competition can also be classified by how it works:

• Exploitation (indirect): whoever uses the resource first wins. Two fish species eating the same zooplankton never need to meet.
• Interference (direct): one individual actively blocks another. A red-winged blackbird defends its marsh territory; some plants release chemicals that inhibit neighbours (allelopathy).

Gause and competitive exclusion

In the 1930s, Russian ecologist G. F. Gause grew two species of Paramecium (single-celled protists that eat bacteria) in test tubes.

• Alone, each species grew logistically to its own carrying capacity.
• Together, with the same food, P. aurelia kept growing while P. caudatum declined and died out.

From experiments like these came the competitive exclusion principle: two species that compete for exactly the same limiting resources cannot coexist indefinitely. The slightly better competitor eventually excludes the other.

Gause also found that P. caudatum could coexist with a different species, P. bursaria. P. bursaria fed mainly in a different part of the tube (near the bottom, where there was less oxygen; it carries algae inside its cell that make oxygen), so the two used different parts of the resources.

Niches: fundamental and realized

A niche is a species' whole "way of life": what it eats, where and when it is active, the temperatures it tolerates, what eats it. In Grade 11 you defined a niche as a species' role; now we split it in two.

• Fundamental niche: everything a species could use with no competitors.
• Realized niche: what it actually uses when competitors are present. It is usually smaller.

Connell's barnacles. In field experiments published in 1961, Joseph Connell studied two barnacle species on a rocky shore in Scotland. Chthamalus lived high on the rocks, Balanus lower down. When Connell removed Balanus, Chthamalus thrived much lower on the rocks, so its fundamental niche extended down there. But where Balanus was present, the faster-growing Balanus smothered, crushed and undercut Chthamalus. Balanus could not invade the upper zone, because it dries out at low tide. So:

• Chthamalus's realized niche (upper shore) is much smaller than its fundamental niche (upper and lower shore), because of competition.
• Balanus's upper limit is set by a physical factor (drying out), its law-of-tolerance limit.

How similar species coexist

If competitive exclusion is real, why do so many similar species live together? Because they divide resources.

Resource partitioning. In 1958, Robert MacArthur studied five species of warblers (Cape May, yellow-rumped, black-throated green, Blackburnian and bay-breasted) feeding on insects in the same spruce trees in the northeastern United States. Each species fed mostly in a different part of the tree (the top, outer branches, inner branches near the trunk, lower branches) and some differed in nesting time and hunting behaviour. Small differences in where and how they fed reduced competition enough for all five to coexist.

Other examples: owls and hawks hunt the same small mammals at different times (night and day); diving ducks and dabbling ducks feed at different depths.

Character displacement. Where two competing species live together, natural selection can make them more different than where they live apart. On the Galápagos, two species of ground finches have similar beak sizes on islands where each lives alone, but different beak sizes on an island where they live together, so they eat seeds of different sizes.