A framework for disentangling ecological mechanisms underlying the island species–area relationship

The relationship between an island9s size and the number of species on that island (the island species-area relationship (ISAR)), is one of the most well-known patterns in biogeography, and forms the basis for understanding biodiversity loss in response to habitat loss and fragmentation. Nevertheless, there is contention about exactly how to estimate the ISAR, and the influence of the three primary ecological mechanisms, sampling, area per se, and heterogeneity, that drive it. Key to this contention is that estimates of the ISAR are often confounded by sampling and estimates of measures (i.e., island-level species richness) that are not diagnostic of potential mechanisms. Here, we advocate a sampling-explicit approach for dissecting the possible ecological mechanisms underlying the ISAR using individual-based rarefaction curves estimated across spatial scales. Specifically, we show how ISAR mechanisms can be inferred by comparing the relationship between island area and diversity indices derived from regional (gamma-) and local (alpha-) individual-based rarefaction curves. If the parameters derived from rarefaction curves at each spatial scale show no relationship with island area, we cannot reject the hypothesis that ISARs result only from sampling effects. However, if the parameters derived from the rarefactions change with island area, effects beyond sampling (i.e., area per se or heterogeneity) are operating. Additionally, if parameters indicative of within-island spatial variation in species composition (i.e., beta-diversity) increase with island area, intra-island compositional heterogeneity plays a role in driving the ISAR. We illustrate this approach using representative case studies, including oceanic islands, natural island-like patches, and habitat fragments from formerly continuous habitat, illustrating several combinations of underlying mechanisms. This approach will offer insight into the role of sampling and other processes that underpin the ISAR, providing a more complete understanding of how, and some indication of why, patterns of biodiversity respond to gradients in island area.