1. 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.2. 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.3. 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. 4.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 metapopulation' may be worthy of further investigation both in Coelopa and in other coastal species.

Recciwd 2 Lkcembt-r 1994, acccptedfor fibluation 24 F e h t y 1995In spite of abundant evidence that intra-and inter-specific competition occurs in natural communities, there is surprisingly little to suggest it is a major force promoting genetic change. This report assesses the genetic effects of competition in two species of seaweed fly, Coclopajigidu and C. pi&a. In laboratory cultures of C.fngrda the relative survival of heterozygotes at the A& locus, which was being used as a marker for the large ap chromosomal inversion, was greater than that of homozygotes. In monocultures of C.fng2da this competitive superiority was dependent on larval density. At low densities facilitation was seen, whereas at high larval densities there was competition. In mixed cultures of the two species, interspecific competition contributed to the differential mortality of C.&&z, and observations of natural populations suggested that competition may have similar effects to those described in laboratory culture. A possible mechanism involving the supply of nutritive microorganisms is proposed to underly both intra-and inter-specific competition. In seaweed flies, competition and the consequent differential mortality appear to be forces maintaining rather than reducing genetic variation.

Recciwd 2 Lkcembt-r 1994, acccptedfor fibluation 24 F e h t y 1995In spite of abundant evidence that intra-and inter-specific competition occurs in natural communities, there is surprisingly little to suggest it is a major force promoting genetic change. This report assesses the genetic effects of competition in two species of seaweed fly, Coclopajigidu and C. pi&a. In laboratory cultures of C.fngrda the relative survival of heterozygotes at the A& locus, which was being used as a marker for the large ap chromosomal inversion, was greater than that of homozygotes. In monocultures of C.fng2da this competitive superiority was dependent on larval density. At low densities facilitation was seen, whereas at high larval densities there was competition. In mixed cultures of the two species, interspecific competition contributed to the differential mortality of C.&&z, and observations of natural populations suggested that competition may have similar effects to those described in laboratory culture. A possible mechanism involving the supply of nutritive microorganisms is proposed to underly both intra-and inter-specific competition. In seaweed flies, competition and the consequent differential mortality appear to be forces maintaining rather than reducing genetic variation.