Does functional redundancy stabilize fish communities?

Functional redundancy is a community property thought to be contributing to ecosystem resilience. It is argued that trophic (or other) functional groups with more species have more linkages and opportunities to buffer variation in abundance of individual species. We explored this concept with a 30year time series of data on 83 species sampled in the IBTS trawl survey. Our results were consistent with the hypothesis that functional redundancy leads to more stable (and by inference more resilient) communities. Over the time series trophic groups (assigned by diet, size (Lmax) group, or both factors) with more species had lower CVs in abundance and biomass than did trophic groups with fewer species. These findings are also consistent with Bernoulli’s Law of Large Numbers, a rule that does not require complex ecological and evolutionary processes to produce the observed patterns. Through iterative randomizations of the species’ time series into groupings of the same sizes as the functional groups, we developed expected pdfs of CVs in abundances and biomasses, assuming only the Law of Large Numbers was at work. The observed CVs of all groupings were not significantly different from these simulated distributions. These results do not prove the absence of ecological processes contributing the greater stability of functional groups with more redundancy, however they do not justify invoking any such processes. The results support management approaches that maintain species richness, but do not require management to try to protect complex (and poorly understood) ecological processes.