Unpredictable gaps the preemption of space

When two species compete on equal terms, the result is usually predictable. But in the colonization of unoccupied space, competition is rarely even handed. Individuals of one species are likely to arrive, or germinate from the seed bank, in advance of individuals of another species. This, in itself, may be enough to tip the competitive balance in favor of the first species. If space is preempted by different species in different gaps, then this may allow coexistence, even though one species would always exclude the other if they competed 'on equal terms'.

For instance, Figure 8.11 shows the results of a competition experiment between the annual grasses Bromus madritensis and B. rigidus, which occur together in Californian rangelands (Harper, 1961). When they were sown simultaneously in an equiproportional mixture, B. rigidus contributed overwhelmingly to the biomass of the mixed population. But, by delaying the introduction of B. rigidus fugitive annuals and competitive perennials coexistence of a competitive mussel and a fugitive sea palm

first come, first served

Bromus Rigidus

Figure 8.11 The effect of timing on competition. Bromus rigidus makes an overwhelming contribution to the total dry weight per pot after 126 days growth when sown at the same time as B. madritensis. But, as the introduction of B. rigidus is delayed, its contribution declines. Total yield per pot was unaffected by delaying the introduction of B. rigidus. (After Harper, 1961.)

Figure 8.11 The effect of timing on competition. Bromus rigidus makes an overwhelming contribution to the total dry weight per pot after 126 days growth when sown at the same time as B. madritensis. But, as the introduction of B. rigidus is delayed, its contribution declines. Total yield per pot was unaffected by delaying the introduction of B. rigidus. (After Harper, 1961.)

into the mixtures, the balance was tipped decisively in favour of B. madritensis. It is therefore quite wrong to think of the outcome of competition as being always determined by the inherent competitive abilities of the competing species. Even an 'inferior' competitor can exclude its superior if it has enough of a head start. This can foster coexistence when repeated colonization occurs in a changing or unpredictable environment.

if the environment typically changes long before the equilibrium can be reached.

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