Coexistence of competing species of seaweed flies: the role of temperature

Abstract. Competition is shown to be occurring within and between the congeners Coelopa frigida and C.pilipes; it is noted that the two species frequently coexist despite ongoing competition. Observations on natural wrack‐beds indicate that there is a marked difference in the distributions of the larvae: C.frigida larvae aggregate in cooler parts of the bed, C.pilipes in warmer parts. This difference in microdistribution reflects a broader‐scale difference in the geographical distributions of the species ‐ C.pilipes being the more southerly of the two. The larval distributions are shown to be caused primarily by the behaviour of the larvae themselves ‐ not by choices made by ovipositing females, nor (at least to any great extent) by differential survival. The different micro‐distributions within beds constitute a form of niche difference which will cause competitive abilities to be frequency‐dependent and hence have a stabilizing effect. It is possible that this effect may be supplemented by others; and, in particular, effects operating at the level of the‘linear meta‐population’may be worthy of further investigation both in Coelopa and in other coastal species.