Quantifying assemblage distinctness with time: an example using temperate epibenthos
Artificial substrata are an often-used tool in assessing community development; here we quantify the changing presence of functional groups of benthos with replicate, depth, site, and time in order to explain differences in the structure of the surrounding mature communities. We placed replicate, machined slate panels (15 x 15 x 1 cm) in the intertidal, 6 and 12 m at two sites of differing flow rates at Lough Hyne, SW Ireland. They were placed at the same time of year and removed after 2, 6, 12, and 24 months from 1999 to 2001, to compare with a similar experiment run from 1997 to 1999. Furthermore, to examine trends of local assemblages older than 24 months, we examined boulders from each depth/site combination. Percentage cover of 13 functional groups, selected in relation to their competitive abilities, was measured using point intercept analysis of high-resolution digital images of the panels. These data were then quantified using our derived measure of distinctness. Replicates became more similar, and sites and depths both became more distinct with time from 2 to 24 months, although there was less distinctness at the high flow site throughout all stages of the study. Principal components analysis (PCA) of data suggested that all site/depth panel combinations only became truly distinct at the 24-month stage of the study. From 2 to 24 months, 'good' competitors increased in space, 'intermediate' competitors peaked after approximate to 12 months, and 'poor' competitors reduced in their space occupation with increasing time. Both in similarity and space occupation of functional groups, the 1997-1999 and 1999-2001 data showed high levels of pattern convergence. That there was little difference between assemblages at 12 and 24 months validated our upper time limit of period of study, but at 24 months, assemblages still differed from natural substrata and we suggest that many years would have been necessary to eliminate this difference.