Seawalls do not sustain viable populations of limpets

Increasing urban development in coastal areas is leading to reduction and fragmentation of natural habitats and a proliferation of artificial structures, such as seawalls. These new structures provide a novel habitat for several organisms, but recent studies have shown differences between the composition of populations in assemblages inhabiting them and those present on natural rocky shores. Preliminary observations also suggested that populations of limpets may differ in size and reproductive output between seawalls and natural rocky shores in Sydney Harbour (Australia). To test hypotheses about the influence of artificial habitat on populations of marine invertebrates, we examined the densities, size and reproductive output of the pulmonate limpetSiphonaria denticulataat midshore intertidal levels on several seawalls and natural rocky shores in Sydney Harbour. Populations on seawalls were mainly composed of juveniles and small adults; large adults were generally found only on rocky shores. There were also significantly fewer and smaller egg masses on seawalls. Experimental manipulations suggested that differences in reproductive output are related to the maximal size that adults attain in each habitat. Populations on seawalls make a smaller contribution to the total reproductive output in any given area, and, thus, it can be predicted that replacement of natural habitats with seawalls may have negative effects on the maintenance of local populations. Populations of organisms living in different habitats in similar densities may show important differences in biological performance. Thus, differences in reproductive output between habitats, even at the same densities, will potentially influence dynamics of populations of limpets.