Habitat complexity and species diversity in rivers
Platt, Jacqueline Berengaria
1. Rivers are highly structured ecosystems in which habitat complexity and heterogeneity are postulated to affect species diversity and abundance. However, the exact links between habitat structure and organisms such as macro-invertebrates remain poorly understood.\ud \ud 2. Field surveys at the patch and reach scale were combined with a field experiment in the Rivers Wye and Usk, Wales, to: i) quantify variations in habitat complexity and heterogeneity among habitats and river sections, ii) assess their influence on macro-invertebrates, iii) separate the ecological effects of confounding physical factors and iv) determine whether habitat type and heterogeneity promote nestedness of assemblages.\ud \ud 3. At the patch scale, surface complexity but not habitat heterogeneity increased macro-invertebrate diversity, richness and abundance independently of surface area, but only when habitat type was ignored. Surface complexity and heterogeneity were minor determinants of variations in macro-invertebrates among habitat types.\ud \ud 4. A field experiment involving baskets containing cobbles, pebbles, gravel or 50:50 mixtures revealed that flow type explained significant variations in macro-invertebrate richness, abundance and composition, and appeared to filter organisms based on their body size. Surface complexity and interstitial volume had no ecological effect, implying that differences in assemblages among mineral habitats may result from flow-related effects.\ud \ud 5. Macro-invertebrate assemblages occurring in some mineral habitats, typically with lower macro-invertebrate diversity, richness, abundance and trait diversity, appeared to be nested sub-sets of those occurring in some organic habitats. Nested assemblages had reduced trait diversity and altered trait representation. River sections containing fewer habitats supported assemblages of lower abundance, which appeared to be weakly nested sub-sets of those in heterogeneous sections.\ud \ud 6. Habitat complexity and heterogeneity are concluded to have consistently weak effects on macro-invertebrates at the scales studied. In contrast, habitat type and flow type affected macro-invertebrate assemblage composition, structure, and traits, with important implications for river management and conservation.
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