Abstract
The relationship between an island’s 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 that drive it — random sampling, disproportionate effects, and heterogeneity. 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 disentangling the possible ecological mechanisms underlying the ISAR using parameters derived from individual-based rarefaction curves estimated across spatial scales. 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 random sampling. However, if the derived metrics change with island area, we can reject random sampling as the only operating mechanism and infer that effects beyond sampling (i.e., disproportionate effects and/or heterogeneity) are also operating. Finally, if parameters indicative of within-island spatial variation in species composition (i.e., β-diversity) increase with island area, we can conclude that 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.
Highlights
The relationship between the area sampled and the number of species in that area —the species– area relationship (SAR)— is one of the oldest laws in ecology (e.g., Arrhenius 1921, Lawton 1999, Lomolino 2000, Drakare et al 2006)
We propose the use of within-island species richness relationships (Type II or Type III curves from Scheiner 2003, Scheiner et al 2011) to evaluate the mechanisms underlying among-island island species–area relationship (ISAR) relationships (Type IV curves from Scheiner 2003, Scheiner et al 2011)
The island species–area relationship (ISAR) —depicting how the numbers of species increase with the size of the island or habitat patch— is one of the most well-known patterns in biogeography
Summary
The relationship between the area sampled and the number of species in that area —the species– area relationship (SAR)— is one of the oldest laws in ecology (e.g., Arrhenius 1921, Lawton 1999, Lomolino 2000, Drakare et al 2006). If the ISAR is primarily driven by heterogeneity, we would expect there to be no relationship between αSn and island size but a strong relationship with γSn, giving us a significant βSn relationship with island size (Fig. 3b) Such a pattern was observed by Sfenthourakis and Panitsa (2012) for plants on Greek islands in the Aegean Sea. In Fig. 3b, we have illustrated a case where heterogeneity influences rare as well as common species, indicating an effect on both βSn and βSPIE (not shown, but implied because the slope at the base of the curve [i.e., PIE] is influenced). It could be that local population dynamics do not depend on the numbers of individuals and types of species in local neighborhoods, at least during the time scale in which habitat fragmentation has taken place
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