Ecological interactions in the provision of habitat by urban development: whelks and engineering by oysters on artificial seawalls

Abstract

Increases in human population cause increased urbanization of most habitats, including the shoreline. This has many consequences for coastal environments, in particular the trend for artificial structures, such as seawalls, to replace natural habitats. Seawalls and natural shores support many of the common intertidal species, but others important on rocky shores are absent from or rare on many seawalls. The whelk Morula marginalba Blainville is an abundant and important predator on rocky shores of south-eastern Australia, but is infrequently recorded on artificial substrata. In Sydney Harbour, where the Sydney rock oyster (Saccostrea glomerata Gould) was locally abundant, densities of M. marginalba on some seawalls appeared to be similar to those on rocky shores and to be larger than where there were few oysters.We sampled densities and sizes of whelks in four habitats, predicting and corroborating that: (i) on seawalls with many oysters, there would be more whelks than on seawalls with few oysters; (ii) where there are many oysters, densities of whelks would be similar on seawalls and rocky shores; and (iii) whelks would be larger where oysters were abundant. Growth and survival of whelks were measured to test hypotheses from the observed differences in size and density. Survival was greater in habitats with many oysters, which could explain differences in density, but size-specific differences in survival could not explain differences in size among habitats. On seawalls but not on rocky shores, slower growth could explain the smaller size of whelks where there were few oysters. Seawalls provide important habitat for M. marginalba, but only via their indirect effects, mediated by oysters. These interactions cannot be predicted from those on natural rocky shores. Predicting how developed areas provide suitable habitat, either in management of conservation or in assessments of potential impacts clearly depends on understanding the roles of biogenic habitats.

Engineering novel habitats on urban infrastructure to increase intertidal biodiversity

“…(Opisthobranchia). These species are typically absent from, or rare on, seawalls (Chapman 2003(Chapman , 2006Jackson et al 2008). Limpets of the species Siphonaria denticulata were extremely common (>7,000 adults counted).…”

“…Interrelationships among species may also change. Jackson et al (2008) showed that biogenic habitat created by oysters increased densities of the whelk Morula marginalba on seawalls, although, on natural shores, whelks are not necessarily associated with oysters (Fairweather 1988).…”

Differential patterns of distribution of limpets on intertidal seawalls: experimental investigation of the roles of recruitment, survival and competition

“…Common species on rocky shores may be found at unnatural densities or are absent from seawalls (Chapman 2003, Jackson et al 2008). The most abundant grazing animals on seawalls are limpets, which are major components of assemblages on natural intertidal shores, structuring algal and faunal assemblages (Branch 1981, Hawkins & Hartnoll 1983, Underwood et al 1983.…”

Does richness of Oligochaeta (Annelida) follows a linear distribution with habitat structural heterogeneity in aquatic sediments?

“…In aquatic habitats the structural heterogeneity of sediments is related to a variety of factors such as variability of particle size (Brown, 2003) and availability of different food resources, as organic matter (Schwindt et al, 2001;Da Rocha et al, 2006). Variations in both particle size and food resource are often linked to presence of other organisms associated with these habitats (Jackson et al, 2008;Buschbaum et al, 2009). Habitat heterogeneity is a key factor enhancing the stability of communities, such as macroinvertebrates in environments exposed to great disturbances (Mykra et al, 2011).…”