Persistence of Stream Invertebrate Communities in Relation to Environmental Variability

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Abstract
The persistence of community structure through time was examined for benthic invertebrates at twenty-seven stream sites in southern England [UK] by comparing surveys in 1976 and 1984. Persistence in species composition was assessed as Jaccard''s Index (JACC) and as the number of the fifteen most abundant species in 1976 still present in 1984 (NUMB). Persistence in species ranking was assessed by Spearman''s correlation coefficients at each site between species abundances in 1976 and 1984; both for total species (SPTO) and for the fifteen most abundant species in 1976 (SP15). Such methods were able to reveal genuine change. There were marked changes in 8 years both in species composition and ranking. JACC lay between 0.29 and 0.58 (mode < 0.5) and NUMB between 4 and 15 (mode 11). Of the twenty-seven values of SPTO and SP15, 21 and 24, respectively, were positive with only 7 and 5, respectively, being significantly different from zero. Ordination and classification of the communities gave similar results for 1976 and 1984. Axis 1 of the ordination in both years was highly correlated with pH and Axis 2 with distance from the stream source. Samples from the same site but in different years separated out together at the first division of the classification but not at subsequent division. A ''press'' perturbation experiment was performed when populations of the benthic fish Cottus gobio L. were established at two previously fishless sites. Persistence in community structure did not differ from unperturbed sites. Overall, persistence was greatest at low discharge, upstream sites with cool summer temperature regimes and low, stable pH. We speculate upon the relative importance of biotic and abiotic influences in the structuring of these communities.