The portfolio effect is a well-known mechanism by which the overall productivity of an ecological community can be stabilized by the inclusion of a variety of species, such that the impact of a sudden collapse (or super-abundance) of one species is mitigated by the independent fluctuations of many others.
Existing measures of diversity such as Shannon’s index quantify how the community is dominated by a subset of species, in principle reducing stability. We show how a tendency for species to fluctuate in a positively correlated way (community synchrony) or negative correlations between abundances (compensation) further affect total variability. Investigating these relationships allows us to define an ‘equivalent richness’ of the community, which affects its overall stability.
We examine multi-decade annual abundance time series of species from a range of communities, including plant communities in grasslands, freshwater zooplankton communities and fish communities in lakes, and corals in marine reef communities. We apply wavelet-based methods to dis-aggregate the fluctuations in these time-series by timescale, and examine how the coefficient of variation of the total community abundance depends on the ‘equivalent richness’ of its fluctuating constituents.
Results/Conclusions
We show that the portfolio effect is widespread throughout a range of different communities. Individual communities demonstrate different levels of richness, diversity and synchrony, translating to different ‘equivalent richness’ values, and resulting in different levels of community stability.
Community synchrony, perhaps due to the tendency of individual species to respond in the same way to an environmental driver, results in lower ‘equivalent richness’. The variability and synchrony of the species’ abundances can be examined on a timescale-specific basis using wavelet-transform-based techniques, and the ‘equivalent richness’ is found to depend on the timescale of fluctuations being examined.