Finding niches in the lumps and gaps of community body-size distributions: A multi-method approach

Background/Question/Methods

The distribution of organisms along a body-size axis can reveal whether neutral or niche processes lie behind community structure: the neutral hypothesis for example predicts that such a distribution should be continuous, whereas organisms grouped in lumps can be indicative of the presence of niche-based processes. Recent studies of both aquatic and terrestrial communities have revealed discontinuous body-size distributions, indicating niche processes drive community structure. Identifying patterns in community-structure and their drivers is an important step towards distinguishing resilient from vulnerable communities, as changes in community structure could serve as indicators of an impending regime shift. However, while some of these studies conclude that these discontinuities are exogenously driven through discontinuities in the physical environment, other studies point towards endogenous drivers of community structure, arguing that competition alone can explain the observed body-size distributions.  The methods used, their assumptions and the levels of data aggregation that lead to these contrasting conclusions differ considerably, making it difficult, if possible at all, to understand where exogenous or endogenous processes act or to make cross-comparisons between studied communities.

Results/Conclusions

We here compare four commonly used methods – the body-mass difference index, a kernel density estimator, the DIP-test and a latent class analysis – to produce a systematic framework to study discontinuities in community body-size distributions. We apply these models to two 20-year phytoplankton time-series from the Baltic Sea and analyze the results obtained in the context of each model’s assumptions and the levels of data aggregation. We find that niche processes drive the phytoplankton community structure in the Baltic Sea, that the effect of endogenous versus exogenous effects on structure are related to the scale of observation and that the patterns observed are not constant over time. We relate changes in community structure to environmental changes in the Baltic Sea, opening the door to further studies on indicators of system resilience.