Aggregation of taxa and scale: Potential concerns for functional analysis of stability

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Jurek Kolasa, Brooke Northey, Abdul M Shaikh and Matthew P. Hammond, Biology, McMaster University, Hamilton, ON, Canada

Background/Question/Methods
Comparing ecological patterns across scales helps uncover mechanisms that otherwise could not be reliably inferred from data analyzed at one scale. A growing interest in functionally oriented analysis of communities requires some data aggregation relative to taxon-based studies.
For example, metacommunity framework can parse regional stability of multispecies aggregate (abundance, biomass) into finer grain component populations. Here, as species temporal population data are aggregated, degree of their asynchronous variation affect regional stability. We thus hypothesize that aggregating species populations into pairs (or more species), based on some functional criterion, should change levels of asynchrony and hence our conclusions as to which mechanisms dominate contributions to metacommunity (metaecosystem) stability. To test this general idea, we examined zooplankton data from North Temperate Lakes LTER and Jamaican rock pool invertebrates. We focused on how population variability stabilizes aggregated abundances at larger scales. For each data set, we performed two parallel analyses, one for taxon- and one for function-based population numbers. Taxon analysis used population density series at the species level. For the functional analysis we assigned species to pairs by their size similarity and combined their abundance into ‘functional species’ (a functional criterion applicable across different systems). As the total regional asynchrony, ω, is in principle constant, a comparison among systems reveals impact of aggregation. We evaluated three contributors to regional stability: asynchrony among local populations, δ; among metapopulations, βmp; and among populations of distinct species at separate locations (cross-community asynchrony, βcc).

Results/Conclusions

While in both systems cross-community variation, βcc, was the greatest stabilizing factor, the effect of functional aggregation varied across metrics, with differences varied in size and sign across ecosystem types. Metapopulations of functional species, βmp, were more stabilizing in lakes, and less stabilizing in rock pools when compared to taxonomic species in lakes, but this was opposite for βcc where functional species played larger stabilizing role. These differences may suggest system-dependent interpretations of insurance hypothesis to emphasize either size or taxon diversity.
To summarize, analyses based on taxonomy paint a different picture than those based on species attributes (size similarity here). We conclude that one analysis cannot substitute for the other. Taken together, they may offer guidance under which scenarios of species loss a regional stability of aggregate organism abundance/biomass may be most affected. While we used a generalized ‘functional species’, the finding highlights potential volatility of emerging patterns and of prospective inferences.