Biodiversity may wax or wane depending on metrics or taxa

Biodiversity changes have proven surprisingly complex to estimate and understand. While there are negative trends at a global scale such as the substantial losses of vertebrate species (1), changes at local scales may show large variation, with no clear overall trend (2, 3). Because assessing and improving the status of biodiversity are at the core of international agreements such as the Convention on Biological Diversity and the associated Aichi Biodiversity Targets for 2020 (4), we need to know when trends in biodiversity may differ and the causes of such differences. In PNAS, Magurran et al. (5) report that different components of biodiversity do not have the same trends over time in tropical freshwater ecosystems, and that these trends differ among taxonomic groups (fishes, invertebrates, and diatoms). Magurran et al. (5) quantify biodiversity changes at 16 river sites in Trinidad over 19 time points covering the dry and wet seasons of 5 y. They collected over 670,000 individuals, which were identified at different resolutions in fishes (species), invertebrates (family), and diatoms (morphospecies) because taxonomy is still poorly known for many groups in the tropics. They focus on two aspects of biodiversity changes: temporal α diversity, measured using the number of species and functions of their relative abundance, and temporal β diversity, which represents change in assemblage composition over time and is measured as turnover in species identities and relative abundance (Fig. 1). Different diversity measures emphasize different characteristics of assemblages (6). Magurran et al. (5) use 11 metrics, ranging from the number of observed or estimated species at a site and metrics emphasizing evenness or dominance to a range of (dis)similarity measures to evaluate compositional differences between assemblages. These are based on either presence/absence or abundance data and emphasize turnover in species identities (species replacement) and nestedness [associated with richness change (7)]. In this tropical ecosystem, Magurran et al. (5) regard situations where α diversity declines or where compositional dissimilarity increases (change in temporal β diversity) as unfavorable. Most time series did not show evidence for a systematic change in α diversity, whereas trends in temporal β diversity were more variable and differed among taxonomic groups. The numbers of sites exhibiting statistically significant change in α diversity were two, one, and zero, respectively, for fish, invertebrates, and diatoms, whereas for β diversity, the numbers were two, two, and five sites, respectively. The study by Magurran et al. (5) highlights that while species turnover may be high and change, the number of species or the relative species dominance within an assemblage may not show significant changes. Temporal variability of α diversity may be represented as bounded fluctuations around some equilibrium value, as when one defines population regulation as fluctuations in numbers of individuals around so-called “carrying capacity. ” Evidence for such regulation of one diversity dimension exists for a wide set of plant and animal assemblages, given, of course, that major disturbances such as land use transformation are excluded (8). As for population regulation, this implies a negative covariation among species fates (considering species extinction and colonization as analogous to individual death and fecundity). Studies of single species or single groups may therefore lead to biased understanding or prediction of diversity changes, since other species or groups may show opposite responses. Take some of the models predicting biodiversity changes, which are based on singlespecies distributions and use climate covariates as predictors. Species distribution models can be stacked; that is, species are modeled independently and predictions added to provide predictions for diversity. However, if diversity is indeed regulated, one should try to constrain such models by including species dependencies, and how to achieve this is an active research topic (9). Studies like that of Magurran et al. (5), by quantifying how much local diversity fluctuations are bounded, are important contributions to this debate.