Succession in Stream Biofilms is an Environmentally Driven Gradient of Stress Tolerance |
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Authors: | Sophia I Passy Chad A Larson |
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Institution: | (1) Department of Biology, University of Texas at Arlington, Box 19498, Arlington, TX 76019-0498, USA |
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Abstract: | The century-long research on succession has bestowed us with a number of theories, but little agreement on what causes species
replacements through time. The majority of studies has explored the temporal trends of individual species in plant and much
less so in microbial communities, arguing that interspecific interactions, especially competition, play a key role in community
organization throughout succession. In this experimental investigation of periphytic succession in re-circulating laboratory
streams, we examined the density and the relative abundance of diatoms and soft algae for 35 days across gradients of low
to high nutrient supply (nitrogen + phosphorus) and low to intermediate current velocity (10 vs. 30 cm·s−1). All algal species were classified into trophic groups and morphological guilds, both of which responded more strongly to
nutrient than current velocity manipulations, as shown by regression analyses. We concluded that within the manipulated environmental
ranges: (1) Succession was a gradient of stress tolerance, driven primarily by nutrient supply and secondarily, by current
velocity. Nutrient supply had a qualitative effect in determining whether the contribution of species tolerant vs. sensitive
to nutrient limitation would increase through time, while current velocity had a quantitative influence and affected only
the rate of this increase. (2) The mechanism of algal succession at a functional level was a neutral coexistence, whereby
the tolerant low profile guild maintained high density when overgrown by sensitive species, while sensitive species, constituting
mostly the motile and high profile guilds, were neither facilitated nor inhibited by tolerant species but controlled by the
environment. It is suggested that the mechanism of succession may depend on the level of biological organization with interspecific
interactions giving way to neutral coexistence along the hierarchy from species to functional groups. Considering that the
functional makeup is strictly environmentally defined, while species composition reflects local and regional species pools
that may exhibit substantial geographic variability, succession is deterministic at a functional level but stochastic at a
species level. |
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