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The effects of environmental stability on benthic community structure were examined at eleven sites (ten streams and a wind-swept lake shore) in the CassCraigieburn region, New Zealand. Physicochemical conditions, apart from stability, were similar at all sites. Epilithic biomass was considerably higher at the more stable sites, but the composition of periphyton communities, and amounts of benthic organic matter present were more strongly influenced by the nature of the riparian vegetation than by stream stability. Invertebrate species richness and density were markedly higher at the more stable sites, but species evenness peaked at sites of intermediate stability. Sites of high and low stability had species-abundance distributions that were modelled best by the log series distribution, whereas sites of intermediate stability were modelled best by the log normal distribution. Communities were dominated by a common core of taxa at all sites, although their relative abundances changed markedly between sites. Differences appeared to be related to a combination of environmental stability and site location (e.g., in forest or grassland). Persistence of the dominant taxa was high at all sites, but persistence of the entire fauna was higher at the stable sites. Communities at the more unstable sites appeared to be less complex and were expected to have higher resilience (i.e., ability to recover from disturbances) than those at more stable sites. Analysis of the local stability of community matrices indicated that matrices were unstable at all sites, although those at the less stable sites had eigenvalues closer to the stability criterion. These sites also had higher theoretical resilience if eigenvalues beyond the 'criterion for stability were ignored. An experimental study of recovery rates in four streams of different stability did not provide any support for higher resilience at less stable sites, all communities recovered at a similar rate. The composition of invertebrate communities at several of the less stable sites could be attributed to simple random colonisation processes; but community structure at the stable sites could not, although the reason for this remains unclear. Finally, food web structure was strongly influenced by environmental stability, with shorter food chains, higher connectance and an overall more variable structure in the less stable streams. This may be a direct response to stability per se or mediated by it indirectly through its effect on the food base of the communities. |
