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

Abstract

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.

Species accumulation in small–large vs large–small order: more species but not all species?

“…Archipelagos were more likely to show large patch dependence than either fragments of formerly continuous habitat or naturally occurring habitat islands, consistent with patch-scale evidence (Chase et al 2020 ; Gooriah et al 2021 ). Post hoc analysis of null model results showed an interesting numerical trend consistent with island biogeography (MacArthur and Wilson 1967 ), where increasingly isolated archipelagos had increasing large patch dependence (Δ RD for reservoir or freshwater lake islands = − 0.035, continental archipelagos = − 0.050, oceanic archipelagos = − 0.078).…”

“…In contrast with patch type, effect size did not differ between taxonomic groups. This might be in part due to the coarse nature of the classifications used but is not an unusual result (Chase et al 2020 ; Gooriah et al 2021 ). There was, however, weak evidence invertebrates and birds had a higher frequency of small-patch dependence than non-avian vertebrates.…”

Passive sampling hypothesis did not shape microbial species–area relationships in open microcosm systems

“…The species–area relationships (SAR), whereby species richness increases with the area, is one of the few laws in ecology (Adler & Lauenroth, 2003 ; MacArthur et al, 1965 ; Rosindell & Chisholm, 2021 ). Although SAR research has become a hot topic in ecology today, some of the hypotheses used to explain its formation are still unclear (Gooriah & Chase, 2020 ; Gooriah et al, 2021 ; Ohyama et al, 2021 ; Tjørve et al, 2017 ).…”

“…The passive sampling hypothesis is one of the main theories used to explain how SARs emerge (Burns et al, 2009 ; Cam et al, 2002 ; Gooriah et al, 2021 ; Yamaura et al, 2016 ). This hypothesis outlines an island species–area relationships (ISAR), which is an independent systematic sampling used to forecast the colonists on islands by stochastic dispersal from the mainland.…”

How does variation in total and relative abundance contribute to gradients of species diversity?

“…The interplay of these mutually dependent components determines the shape of the regional species-area relationship and ultimately the diversity of local samples at any spatial scale (Tjørve et al, 2008). Therefore, analyzing diversity in terms of these components can provide deeper insights into the nature of multidimensional biodiversity patterns than analyses of species richness alone (Blowes et al, 2017;Chase et al, 2018), and in turn, this may allow for a better understanding of the processes that shape and maintain diversity gradients at a given scale (Blowes et al, 2020;Gooriah et al, 2021).…”