Synthesis reveals that island species–area relationships emerge from processes beyond passive sampling

Aim: The island species–area relationship (ISAR) quantifies how the number of species increases as the area of an island or island-like habitat gets larger and is one of the most general patterns in ecology. However, studies that measure the ISAR often confound variation in sampling methodology and analyses, precluding appropriate syntheses of its underlying mechanisms. Most ISAR studies use only presence–absence data at the whole-island scale, whereas we planned to use a framework that applies individual-based rarefaction to synthesize whether and how the ISAR differs from the null expectation of the passive sampling hypothesis.

Location: Five hundred and five islands from 34 different archipelagos across the world, including oceanic islands, lake islands and forest islands.

Major taxa studied: Local assemblages of plants, invertebrates, herpetofauna, birds and mammals.

Methods: We collated local-scale species abundance data from multiple archipelagos (median of 12 islands per study) and used a rarefaction-based approach to synthesize the relationship between island size and (1) sample effort-controlled rarefied species richness, or (2) an effective number of species derived from the probability of interspecific encounter (an index of community evenness).

Results: When we applied rarefaction to control for sampling effort, the numbers of species and their relative abundances across all studies differed from the passive sampling hypothesis. Our measure of evenness also increased with island size, suggesting that the disproportionate effects we observed influenced both rarer and more common species. We found few associations between the slope of this effect and island type or taxon, but we did find that island archipelagos with greater elevational heterogeneity also deviated more from the null expectation than those with less heterogeneity.

Main conclusions: Using a synthetic approach across island archipelagos, we reject the null expectation that passive sampling causes the ISAR and instead suggest that ecological mechanisms leading to disproportionate (non-random) effects on larger relative to smaller islands are predominant.