Space size relative to prey width (Sp/Py) influences macrofaunal colonization of artificial structures

Abstract

We tested an index of habitat complexity, Sp/Py (inter-structural space size/prey body width), by deploying artificial structures on a coral reef near Dibba, United Arab Emirates. The structures were polyethylene tube bundles with tube diameters of 3, 12 and 24 mm and a mixed treatment composed of all 3 tube sizes. We counted and identified mobile macrofauna that colonized the interior of the tubes and divided them into 3 body width categories: small (< 3 mm), medium (3-12 mm) and large (>12 mm). Medium and large fauna were excluded from 3 mm tubes, and large fauna were excluded from 12 mm tubes. Small fauna were significantly more abundant in 24 mm tubes compared with 3 mm tubes, with intermediate abundance in 12 mm tubes. A second experiment determined that small fauna probably responded positively to tube interior volume, or responded to differences in hydrodynamic properties between the treatments. Medium fauna were more likely to be found in 12 mm tubes that matched their own body width than in 24 mm tubes. Species richness was significantly lower in the 3 mm tubes compared with the other treatments, in part because errant polychaetes, gastropods, caprellid amphipods and crabs were less likely to be found in the 3 mm tubes. These results indicate that habitats with very narrow inter-structural space sizes may be 'too complex' and may exclude certain fauna, resulting in decreased species richness. Also, some fauna may prefer habitats with intermediate space sizes that match their own body width.

Effect of habitat complexity attributes on species richness

“…More compartmentalized, less strongly connected architecture would in turn increase community persistence and allow the co-existence of a greater number of species (Thébault andFontaine 2010, Stouffer andBascompte 2011). However, some of the previous studies did not observe a significant relationship between space-size heterogeneity and richness (Bartholomew 2002, Kelaher 2003, Bartholomew and Shine 2008. For example, variation in vertical heterogeneity due to different frond lengths of coralline algae did not affect gastropod diversity (Kelaher 2003), and increased heterogeneity in artificial seagrass plots did not result in increased faunal richness independent of faunal abundance (Bartholomew 2002).…”

“…Therefore, species richness may actually decline after the space sizes are no longer biologically meaningful for most organisms. This has been demonstrated in an experiment with reef macrofauna colonization of artificial tube structures of three different sizes (Bartholomew and Shine 2008). Kelaher (2003) demonstrated that greater density of fronds of coralline algae was negatively correlated with gastropod diversity and abundance, and proposed that there may be an upper threshold of complexity, beyond which negative effects of overly high density of structures are observed.…”

Importance of habitat structure as a determinant of the taxonomic and functional composition of lentic macroinvertebrate assemblages

“…Since we did not measure body size, it is unknown if this was a relevant trait in our experiment. Related to this, suitability of a habitat is scale dependent; a small organism in proportion to the structure it lives on perceives the complexity of a habitat differently compared to a larger organism (Bartholomew and Shine 2008;Warfe et al 2008).…”

Space size relative to prey width and total cover in an area both influence the habitat choices of freshwater angelfishPterophyllum scalarein mesocosms

“…Bartholomew and Shine (2008) measured Sp/Py and examined prey habitat use of small artificial structures. They found that, with one exception, small coral reef-dwelling prey were excluded from spaces smaller than their body widths (Sp/Py less than 1) and that some prey organisms seemed to prefer spaces that closely matched their body widths (Sp/Py slightly greater than 1) compared with spaces that were much wider than their body widths.…”