Testing species-stone area and species-bryophyte cover relationships in riverine macroinvertebrates at small scales
2008
HEINO, JANI | KORSU, KAI
1. The species-area relationship is considered amongst the few genuine laws in ecology. Although positive species richness-stone area relationships have been found previously in stream systems, very few studies have simultaneously examined species-individuals, individuals-area, species-bryophyte biomass and individuals-bryophyte biomass relationships. We examined these relationships based on temporally replicated assessments of macroinvertebrates on stones at two river sites. 2. We found only one significant species-area relationship out of six relationship tested, and two significant individuals-area relationships. Even these significant relationships were weak, however. By contrast, we detected significant and rather strong relationships between species richness and the number of individuals at both river sites on all three sampling dates. We also found significant relationships of both species richness and the number of individuals with bryophyte biomass at both river sites on all sampling occasions. One of the river sites was disturbed by a bulldozer, and the species-bryophyte biomass relationships were somewhat stronger after the disturbance event. 3. Our findings are quite surprising, given that there were very weak species-area relationships on stream stones. By contrast, our results suggest a pivotal role for bryophyte biomass in determining the species richness and the number of individuals of stream macroinvertebrates at this small scale. The most probably origin of these relationships begins with bryophyte cover, which determines the number of individuals, and subsequently passively affects species richness. Thus, there is not necessarily a direct mechanism that determines the variability of species richness on stream stones. 4. Experimental studies are needed to disentangle the various mechanisms (e.g. passive sampling, provision of more food, more niche space, flood disturbance refugia) by which bryophyte biomass affects stream macroinvertebrates.
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