Community Ecology

If communities are not organisms, what decides which species share a place? The answer came through the idea of the niche. In 1957 G. Evelyn Hutchinson, a Yale limnologist with an enormous range of interests, defined it precisely. Take every condition and resource that matters to a species, such as temperature, humidity and the size of its food, and make each an axis. The niche is the region of that many-dimensional space where the species can persist. Two species cannot occupy the same niche indefinitely, as Gause's test tubes had shown in population ecology.

Field studies turned the idea into research. Hutchinson's student Robert MacArthur showed in 1958 that five species of warbler share the same spruce trees in New England by feeding in different parts of them. And in 1961 Joseph Connell moved the question outdoors with an experiment. On the Scottish island of Great Cumbrae, the small barnacle Chthamalus lives high on the shore and the larger Balanus below it. When Connell cleared Balanus from rocks lower down, Chthamalus survived there. Its realised niche was narrower than its fundamental niche because a competitor crushed and smothered it.

Competition is only one force. In 1963 Robert Paine began prising ochre starfish off a stretch of rocky shore in Washington State and throwing them into the sea. The starfish eat mussels, the best competitors for space on the rock. Without the starfish, mussels spread and smothered the barnacles, limpets and seaweeds, and the number of species in the plot fell from fifteen to eight. A single predator had been holding the community together. Paine called such species keystones, after the stone at the top of an arch.

Connell later generalised the point. In 1978 he argued that diversity is highest where disturbance, from storms, fires or predators, is neither too frequent nor too rare. With no disturbance the best competitors win. With too much, only the hardiest colonisers survive. This intermediate disturbance hypothesis was popular for decades, though critics have since argued that its logic is flawed and its evidence weak.

Lotka and Volterra wrote the logistic model for two competing species. Each species grows towards its own carrying capacity, $K_1$ or $K_2$, but each individual of the other species also uses up some of the room:

$$\frac{dN_1}{dt} = r_1 N_1 \left(1 - \frac{N_1 + \alpha_{12} N_2}{K_1}\right), \qquad \frac{dN_2}{dt} = r_2 N_2 \left(1 - \frac{N_2 + \alpha_{21} N_1}{K_2}\right).$$

The competition coefficient $\alpha_{12}$ says how much one individual of species 2 counts against species 1. If the two species had identical niches, each coefficient would be 1. The less their niches overlap, the smaller the coefficients.

Take $K_1 = 100$ and $K_2 = 80$, with $\alpha_{12} = 0.5$ and $\alpha_{21} = 0.6$. Setting both growth rates to zero gives the point where the species balance:

$$N_1^* = \frac{K_1 - \alpha_{12} K_2}{1 - \alpha_{12}\alpha_{21}} = \frac{100 - 40}{0.7} \approx 85.7, \qquad N_2^* = \frac{K_2 - \alpha_{21} K_1}{1 - \alpha_{12}\alpha_{21}} = \frac{80 - 60}{0.7} \approx 28.6 .$$

Both are positive, and a simulation started from ten of each settles there. The two species coexist, each held below its own carrying capacity by the other.

Now let the niches overlap more, so that $\alpha_{21}$ rises to 0.9. The formula gives $N_2^* = (80 - 90)/0.55 \approx -18$, which is impossible. There is no balance point with both species present. A simulation shows species 2 dwindling to nothing, while species 1 rises to its full carrying capacity of 100. This is Gause's competitive exclusion.

The general condition for stable coexistence is

$$\alpha_{12} < \frac{K_1}{K_2} \quad \text{and} \quad \alpha_{21} < \frac{K_2}{K_1} .$$

Here $K_2/K_1 = 0.8$, so the coefficient 0.6 allowed coexistence and 0.9 did not. In words, each species must limit itself more than it limits its rival. Differences in niche make that possible, which is why ecologists look for them wherever similar species live together.

The niche explained a great deal, but not everything. In a tropical forest, hundreds of tree species share the same light, water and soil nutrients. Stephen Hubbell, who from 1980 mapped every tree in a 50-hectare plot on Barro Colorado Island in Panama, proposed in 2001 a radical alternative. Suppose all species are equivalent, and which species occupies a gap left by a fallen tree is a matter of chance. Random drift, immigration and rare speciation then reproduce the proportions of common and rare species in real forests remarkably well. The neutral theory is a deliberate caricature, and a disputed one, but it forced ecologists to show where niches make a difference.

Communities are also parts of something larger. Energy from sunlight and nutrients from rock flow through them, and the whole system of living things and their physical surroundings became the subject of ecosystem ecology. The science of how species interact became, in turn, a basis for conservation biology.