Evol Ecol Res 10: 667-698 (2008) Full PDF if your library subscribes.
Evaluating neutrality and the escalation hypothesis in brachiopod communities from shallow, high-productivity habitats
Adam Tomašových
Geological Institute, Slovak Academy of Sciences, Dúbravská 9, 84005 Bratislava, Slovakia
Correspondence: A. Tomašových, Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA.
e-mail: tomasovych@uchicago.eduABSTRACT
Background: Two hypotheses that can explain restricted distribution of brachiopods in present-day environments are: (1) escalation – more competent predators and competitors limit brachiopods to refuges; and (2) brachiopods are negatively affected by drift driven by demographic and environmental stochasticity. Brachiopods, once dominant invertebrates of the benthos, tend to be found in low-productivity habitats at present. But in the San Juan Islands, large-sized brachiopods occur in shallow and productive habitats with relatively high consumer pressure and space-limitation, suggesting that the escalation hypothesis may need to be modified.
Questions: Does ecological equivalence explain brachiopod abundance, co-occurrence, and community-structure patterns in the San Juan Islands? Is brachiopod abundance highest in patches with the lowest intensity of biotic interactions and is their niche breadth smaller than in other groups, in accord with the escalation hypothesis?
Methods: To evaluate the neutrality of hard-bottom benthic communities with brachiopods, I compare the observed patterns of the variation in community composition (measured by Bray-Curtis similarities) and co-occurrence patterns with those predicted by (1) randomized null models and (2) simulations of the neutral model of ecological equivalence. I analyse overall niche separation based on the relationship between environmental variables and abundances (percent covers) with the multivariate analyses at two spatial scales: patch (metre to decimetre) and site (decametre). To test the prediction of the escalation hypothesis, I analyse brachiopod niche positions and breadth by computing weighted mean and weighted standard deviation of patch and site scores occupied by brachiopods.
Results: Variation in community composition among patches is generally higher than the neutral model of ecological equivalence predicts. Benthic groups co-occur less commonly than predicted by randomized null models in sites with high variation among patches. They show significant niche separation with respect to disturbance and substrate complexity at patch scales. Neither of two null models thus accounts for the microhabitat differences between brachiopods and other invertebrates. Brachiopods have unique niche positions in patches with the lowest disturbance, highest complexity, and highest percent cover, and have a smaller niche breadth, relative to other groups, along a disturbance gradient and with respect to substrate complexity. At the site scale, environmental variance in complexity and disturbance is lower than at patch scales, overall niche separation is weak, compositional variation among sites is lower than among patches, and co-occurrence patterns are random.
Conclusion: The inability of models assuming ecological equivalence to produce the observed patterns in community structure implies that random assembly does not explain abundance patterns at patch scales. Although brachiopods are most common on patches with the lowest disturbance and have narrow niche breadth, they are not space limited and dominate on topographically complex substrates at patch scales. This contradicts the idea that brachiopods are inferior competitors for space. At site scales, brachiopods co-occur with other sessile epifauna and grazers in highly heterogeneous sites with varying complexity and disturbance.
Keywords: Brachiopoda, co-occurrence, escalation, marine community ecology, neutral theory, niche.
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