Habitat structure, resources and diversity: the separate effects of surface roughness and macroalgae on stream invertebrates

Habitat structure has pervasive effects on community composition and diversity, with physically complex habitats often containing more species than physically simple ones. What factors or mechanism drive this pattern is little understood, but a complicating problem is that different sources of habitat structure can be confounded in both surveys and experiments. In this study, we carried out an experiment in which two sources of habitat structure, attached macroalgae and substrate surface texture, were separately manipulated to discern their joint and separate effects: upon the diversity and composition of colonizing macroinvertebrates in a stony, upland stream. Because stream algae vary markedly in abundance in both space and time, we also sampled the epilithon of stream stones at two spatial scales on eight dates over 2 years to gain some preliminary data on how stream algae vary between individual substrata over time. Experimental substrata had either a smooth (siltstones, sandstones, crystal-poor felsic volcanics, plain paving bricks) or rough (granodiorites, crystal-rich felsic volcanics, sand-blasted paving bricks) surface. We allowed these substrata to be colonized naturally by macroalgae, mostly the filamentous red alga Audouinella hermannii. Half of each of the rough and smooth substrata were selected at random and the macroalgae gently sheared off. All substrata were defaunated with a household insecticide with little field persistence, set out randomly through the study riffle, and invertebrates allowed to colonize them for 14 days. Some substrata were sampled immediately to check the efficacy of faunal and algal removals, which proved to be successful. Experimental results showed that both surface texture and macroalgae increase species richness independently of each other. Surface texture had no effect on densities, while macroalgae increased colonization densities, but rarefaction showed that both sources of habitat structure increased species richness above values expected simply on the basis of the numbers of colonists. However, reference stones with high macroalgal cover had the same species richness as those with low cover, suggesting that the effects of macroalgae on species richness are transient relative to those associated with surface texture. Epilithon samples taken at different times suggest that the magnitude of spatial variation in plant growth alters with time. If plants generally recolonize rough surfaces more quickly than smooth, then the effects of habitat structure on macroinvertebrates ought to be strongest after major disturbances during growing seasons of plants.