Fish species richness and incidence patterns in isolated and connected stream pools: effects of pool volume and spatial position |
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Authors: | Christopher M Taylor |
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Institution: | (1) Department of Biological Sciences, Mississippi State University, Mississippi State, MS 39762-5759, USA fax: 601-325-7939, e-mail: ctaylor@ra.msstate.edu, US |
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Abstract: | I tested the effects of pool size and spatial position (upstream or downstream) on fish assemblage attributes in isolated
and connected pools in an upland Oklahoma stream, United States. I hypothesized that there would be fundamental differences
between assemblages in these two pool types due to the presence or absence of colonization opportunities. Analyses were carried
out at three ecological scales: (1) the species richness of pool assemblages, (2) the species composition of pool assemblages,
and (3) the responses of individual species. There were significant species-volume relationships for isolated and connected
pools. However, the relationship was weaker and there were fewer species, on average, in isolated pools. For both pool types,
species incidences were significantly nested such that species-poor pools tended to be subsets of species-rich pools, a common
pattern that ultimately results from species-specific differences in colonization ability and/or extinction susceptibility.
To examine the potential importance of these two processes in nestedness patterns in both pool types, I made the following
two assumptions: (1) probability of extinction should decline with increasing pool size, and (2) probability of immigration
should decline in an upstream direction (increasing isolation). When ordered by pool volume, only isolated pools were significantly
nested suggesting that these assemblages were extinction-driven. When ordered by spatial position, only connected pools were
significantly nested (more species downstream) suggesting that differences in species-specific dispersal abilities were important
in structuring these assemblages. At the individual-species level, volume was a significant predictor of occurrence for three
species in isolated pools. In connected pools, two species showed significant position effects, one species showed a pool
volume effect, and one species showed pool volume and position effects. These results demonstrate that pool size and position
within a watershed are important determinants of fish species assemblage structure, but their importance varies with the colonization
potential of the pools. Isolated pool assemblages are similar to the presumed relaxed faunas of montane forest fragments and
land bridge islands, but at much smaller space and time scales.
Received: 6 December 1996 / Accepted: 10 December 1996 |
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Keywords: | Colonization Extinction Nestedness Sp ecies-volume relationship Stream fishes |
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