A useful index? The most common biodiversity measure (alpha diversity) represents a snapshot in space and time. As such it suffers from inconsistencies in sampling and interpretation. Importantly, its meaning remains unclear because species turnover will be higher at small as opposed to large scales, which may result in a cumulative richness assuming values significantly different from alpha. The question is how these metrics relate to one another. To answer it, we used 49 rock pool invertebrate communities sampled annually over 20 years (n=14). We classify species as transient, cyclic (recurrent), lost, added, and permanent. These classifications themselves are scale-dependent in a complex way as a species may be transient at one spatial scale, permanent at another, and cyclic at a longer temporal scale. We then ask whether alpha is a reliable indicator of local species diversity and, if not, how turnover contributes to its quality or failure.
Results/Conclusions
Surprisingly, a decrease in turnover in a pool was associated with a high cumulative richness (r2=0.44). This indicates that species permanence and addition but not turnover were the driving force behind the biodiversity buildup over longer time scales. The reason for this is a positive correlation between turnover and the number of major disturbances a pool experienced (drying up events). Notably, at sites where turnover was low (and cumulative richness was high), cyclic species appeared more frequently than transient species (r2=0.39). This underscores an important role of external species source in maintenance of high diversity over time (cyclic species constitute on average 53% of all species). However, the picture remains incomplete without taking the mean alpha diversity of a site (individual pool) into consideration: Pools with high turnover tended to have higher cumulative richness relative to their mean richness (r2=0.56) indicating that their long-term richness would be much underestimated if recorded at short time scale as compared to pools with high alpha richness. Take home message: At a given spatial scale alpha diversity and cumulative diversity capture different aspects of local species dynamics and are differently modulated by compositional change rate (turnover). Thus, research on and management of biodiversity is likely to incur a range of unknown but significant biases unless all the components are considered.