Edge effects and patch size in seagrass landscapes: an experimental test using fish

Abstract

Edge effects and bed size are 2 main landscape-scale parameters that may affect fish in seagrass. We tested their influence on the species richness per unit area and density of fish in 6 Zostera capricorni seagrass beds ranging in size from 2300 to 211200 m2, in the Pittwater estuary, NSW, Australia. The effect of edge interacted with bed size. The species richness was lower in edge than inner regions of the larger beds, but did not differ between regions in smaller beds, in which the species richness was similar to the inner regions of large beds. The density of fish (all species combined) varied with bed size in 1 of the 2 seasons sampled, but not with region. The densities of some individual species were lower in the larger beds compared to smaller beds (5 species), while others were greater (3 species), although for some species this was inconsistent between seasons. We then tested whether species richness and abundance were responding to patch area, perimeter, or perimeter: area ratio (P:A ratio) using artificial seagrass units (ASUs) of different shapes and sizes. All ASU designs had similar total abundances of fish species (per ASU), yet 1 design was smaller in area, so these smaller ASUs had greater species richness per unit area. The ASUs were smaller than the natural seagrass beds, and it seems that, although edge effects appeared in large natural seagrass beds, in smaller patches the total area rather than amount of edge influenced the species richness.