Ephemeral patches with unpredictable lifespans

Many environments, by their very nature, are not simply variable but ephemeral. Amongst the more obvious examples are decaying corpses (carrion), dung, rotting fruit and fungi, and temporary ponds. But note too that a leaf or an annual plant can be seen as an ephemeral patch, especially if it is palatable to its consumer for only a limited period. Often, these ephemeral patches have an unpredictable lifespan - a piece of fruit and its attendant insects, for instance, may be eaten at any time by a bird. In these cases, it is easy to imagine the coexistence of two species: a superior competitor and an inferior competitor that reproduces early.

One example concerns two species of pulmonate snail living in ponds in northeastern Indiana. Artificially altering the density of one or other species in the field showed that the fecundity of Physa gyrina was significantly reduced by interspecific competition from Lymnaea elodes, but the effect was not reciprocated. L. elodes was clearly the superior competitor when competition continued throughout the summer. Yet P. gyrina reproduced earlier and at a smaller size than L. elodes, and in the many ponds that dried up by early July it was often the only species to have produced resistant eggs in time. The species therefore coexisted in the area as a whole, in spite of P. gyrina's apparent inferiority (Brown, 1982). Among frogs and toads, on the other hand, the competitively superior tadpoles of Scaphiopus holbrooki are even more successful when ponds dry up because they have shorter larval periods than weaker competitors such as Hyla chrysoscelis (Wilbur, 1987).

. but not always

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