MEPS 354:147-160 (2008)  -  DOI: https://doi.org/10.3354/meps07205

ABSTRACT: Quantifying spatial variation of populations of animals and plants is fundamental to understanding the relative importance of different ecological processes that influence them. Small-scale patterns in marine invertebrate densities are often thought to be determined by organisms’ responses to very localized cues, e.g. quality of habitat or food. In contrast, large-scale patterns have often been attributed to much larger-scale processes, such as availability of larvae, wave-action or upwelling. However, if features of habitat (which primarily influence populations at a small scale) vary over larger spatial scales, population variation over these larger scales may be due entirely to local responses to habitat occurring at a small scale. This study used artificial units of habitat (AUHs) to standardize features of habitat, in order to measure rates of colonization of subtidal gastropods at scales of 20 cm to 4 km. Patterns were compared for 8 common and 6 rare species, in addition to the assemblage of gastropods. Many of the species that colonized these AUHs were found in similar densities to those in natural habitats. Despite the standardisation of habitat, all species showed small-scale differentiation among AUHs only 20 cm apart, again mimicking patterns in natural patches of habitat. This was most probably due to choices of whether to colonize or not, rather than passive colonization forced by local environmental factors, because different species showed different patterns of colonization among AUHs. There was little additional variation in numbers across scales from 1 m to 4 km. The rare species showed greater small-scale and less large-scale variation than did the common species, but species in each group did not show similar patterns among individual AUHs. Where there are infrequent sample units that have large numbers of individuals (or species), they affect means at larger scales, causing larger-scale variation. Understanding small-scale responses to habitat must underpin studies of broad-scale patterns, so that changes in local processes having influences over large scales can be differentiated from broad-scale processes. This also has large implications for assessments of environmental impacts and monitoring of biodiversity, two of the most important tasks for ecologists in the immediate future.

KEY WORDS: Artificial habitat · Gastropoda · Patchiness · Spatial scales · Variation