- Although rivers are highly structured physically, generalisations about the consequences for macroinvertebrates remain elusive. In part, this reflects the difficulty of quantifying structure per se as well as differentiating the effects on organisms of complexity (i.e. the total abundance of structural features), heterogeneity (i.e. the composition and spatial arrangement of different structural features) and surface area.
- Three hypotheses about habitat structure were tested at the patch scale (<0.1 m2) in tributaries of the Rivers Wye and Usk, mid-Wales (U.K.): (i) greater habitat heterogeneity and surface complexity alter macroinvertebrate assemblage composition and increase diversity, richness and abundance, (ii) the effects of complexity on macroinvertebrates are distinct from those of increased surface area, and (iii) habitat structure (heterogeneity and complexity) is a major determinant of variations in macroinvertebrates among habitat types (bedrock, silt, sand, gravel, pebbles and cobbles).
- Surface complexity, described using fractal dimension, and surface area were estimated from ten riverbed profiles at 0.25 × 0.25 m locations sampled for macroinvertebrates. Habitat heterogeneity was determined within 1 m radii and described by principal components reflecting the patchiness of habitat mosaics (PC1) and the evenness of patch sizes (PC2). Relationships between these heterogeneity PCs, fractal dimension, surface area, habitat type and macroinvertebrates were analysed using models excluding and including habitat type. We also used hierarchical partitioning to investigate the variance explained independently and jointly by each of the variables.
- Bedrock was the least complex habitat, whilst pebbles were the most complex. Habitat mosaics surrounding cobbles had the most variable patch sizes, whilst those around gravel or bedrock were the most even.
- Complexity (but not heterogeneity) increased macroinvertebrate diversity and abundance independently of surface area, but independently accounted for <5% of the variation in macroinvertebrates. Complexity and surface area also independently increased taxonomic richness, but rarefaction showed that this was an artefact of increased abundances. Habitat type explained more of the variation (up to 21%) and rendered complexity and surface area redundant in our models. Bedrock, silt and sand typically had reduced diversity, richness and abundance of (mostly) Ephemeroptera, Plecoptera and Trichoptera, whilst silt had abundant Chironomidae.
- These results support the first two hypotheses, but only in part. Habitat heterogeneity did not affect macroinvertebrates, whilst the effects of complexity were weak. The major implication is that habitat type affects macroinvertebrates through factors other than structure alone, and we advocate a wider reappraisal of the processes involved.