Are diverse natural communities more stable or are stable natural communities more diverse?
Houlahan, Jeff*,1, on Data Sharing, NCEAS Working Group, 1 University of New Brunswick at Saint John, Saint John
ABSTRACT- A fundamental question in ecology is how changes in biodiversity affect critical functions of natural ecosystems. In particular, there is a great deal of interest in the relationship between diversity and ecosystem stability as represented by the variance in community abundance or biomass. Most recent empirical studies, primarily in small, experimental communities, have shown a positive relationship. However, even the direction of the causality is not understood, let alone the cause. The two leading hypotheses explaining how diversity can confer stability are: the statistical 'portfolio' effect, whereby averaging over multiple species inevitably results in lower community variance in more diverse communities, and/or the biological effect, that declines in one species are more likely to be offset by increases in another species in diverse communities, A rarely-discussed third explanation, that communities with relatively small population fluctuations will have higher species richness because the probability of stochastic extinctions will be smaller, implies that stability confers diversity. Our objectives were twofold: to estimate the relationship between diversity and stability in natural communities of diverse taxa, from diverse habitats at multiple scales and, to identify the principal mechanisms responsible for the relationships. We estimated the relationship between diversity and stability for 15 datasets (some of which had data for several decades and hundreds of sites), studying diverse taxa from phytoplankton to butterflies to birds. We found a positive diversity-stability relationship in most natural communities, although the strength of the relationship varied across taxa, habitats and scales. We then used a simple statistical technique, based on the mathematical relationships among species-richness, community-level variance, species-level variance, and covariance between species, to try and distinguish which mechanisms are causing the relationship between species richness and community variance. Preliminary results suggested that the 'portfolio' effect was ubiquitous, and that the relative importance of biological effects and population stability effects varied across taxa, habitats and scales.
Key words: diversity-stability, long-term, variance, covariance
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