Sampling effects drive the species-area relationship in lake zooplankton

The Island Species-Area relationship (ISAR) describes how the numbers of species increases with increasing size of an island (or island-like habitat, such as lakes), and is one of the oldest laws in ecology. Despite its conceptual importance, there remains a great deal of ambiguity regarding the ISAR and its underlying processes. We compiled data from sampled zooplankton assemblages from several hundred lakes in North America and Europe to examine the influence of the three main hypothesized mechanisms leading to ISARs - passive sampling, disproportionate effects, and habitat heterogeneity. We compiled data on lake zooplankton assemblages that reported sample-level and lake level species richness estimates, as well as relative abundance data. In both North American and European lakes, we found a consistent and strong increase in total species richness with increasing lake area. However, when we compared the number of species standardized by number of individuals, there was no relationship between lake area and sample-level species richness or an estimate of species relative abundances, calculated as the Probability of Interspecific Encounter (PIE; a measure of evenness). This was true even when multiple samples were taken across lakes and combined, reducing the likelihood that habitat heterogeneity was driving the results. Overall, our results suggest that the ISAR of zooplankton in these lakes was most likely determined by sampling effects rather than disproportionate effects or habitat heterogeneity leading to more species in larger lakes. Understanding the mechanisms driving ISAR results such as ours can also help us develop predictions for biodiversity change when the area of these habitats changes. 

Methods
We searched the literature for studies on freshwater zooplankton having abundance and/or richness data in European and North American lakes using Google Scholar and a variety of open access online databases (e.g., Dryad, FigShare, LTER) with the following keywords: “zooplankton”, “abundance”, “richness”, “species”, “freshwater”, and “lakes”. The list of datasets used as well as their references is included in the file "dataset_references". The main file "all_data_diversity_metrics" contains the different biodiversity metrics calculated (i.e., rarefied richness, Sn and a measure of evenness, SPIE) as well as the corresponding lake areas (given in hectares, ha).