Abstract
We investigated 1) the role of area per se in explaining anuran species richness on reservoir forest islands, after controlling for several confounding factors. We also assessed 2) how sampling design affects the inferential power of island species–area relationships (ISARs) aiming to 3) provide guidelines to yield reliable estimates of area‐induced species losses in patchy systems. We surveyed anurans with autonomous recording units at 151 plots located on 74 islands and four continuous forest sites at the Balbina Hydroelectric Reservoir landscape, central Brazilian Amazonia. We applied semi‐log ISAR models to assess the effect of sampling design on the fit and slope of species–area curves. To do so, we subsampled our surveyed islands following both a 1) stratified and 2) non‐stratified random selection of 5, 10, 15, 20 and 25 islands covering 1) the full range in island size (0.45–1699 ha) and 2) only islands smaller than 100 ha, respectively. We also compiled 25 datasets from the literature to assess the generality of our findings. Island size explained ca half of the variation in species richness. The fit and slope of species–area curves were affected mainly by the range in island size considered, and to a very small extent by the number of islands surveyed. In our literature review, all datasets covering a range of patch sizes larger than 300 ha yielded a positive ISAR, whereas the number of patches alone did not affect the detection of ISARs. We conclude that 1) area per se plays a major role in explaining anuran species richness on forest islands within an Amazonian anthropogenic archipelago; 2) the inferential power of island species–area relationships is severely degraded by sub‐optimal sampling designs; 3) at least 10 habitat patches spanning three orders of magnitude in size should be surveyed to yield reliable species–area estimates in patchy systems.
Highlights
The species–area relationship (SAR) is the earliest and best-documented pattern in spatial ecology (Rosenzweig 1995, Tjørve et al 2018)
We assessed 2) how sampling design affects the inferential power of island species–area relationships aiming to 3) provide guidelines to yield reliable estimates of areainduced species losses in patchy systems
We investigated the role of island-scale replication and the range in island size in detecting a positive island species–area relationships (ISARs) for anurans at the Balbina Hydroelectric Reservoir (BHR) landscape
Summary
The species–area relationship (SAR) is the earliest and best-documented pattern in spatial ecology (Rosenzweig 1995, Tjørve et al 2018). The ‘obvious fact that the larger the area taken the greater the number of species’ (Arrhenius 1921) has led to a number of. Larger areas are more species-rich because they have more individuals and contain a wider spectrum of habitats (Rosenzweig 1995). Given a random abundance distribution, the larger number of individuals recorded over larger areas should imply more species (i.e. sampling effect; Hill et al 1994). The greater variety of habitats encompassed by larger areas supports species restricted to specific habitats and those requiring a combination thereof (i.e. habitat diversity effect; Connor and McCoy 2001)
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