Distinguishing effects of area per se and isolation from the sample‐area effect for true islands and habitat fragments

Abstract

The island species area relationship (ISAR) is an important tool for measuring variation in species diversity in variety of insular systems, from true‐island archipelagoes to fragmented terrestrial landscapes. However, it suffers from several limitations. For example, due to the sample‐area effect, positive relationships between species and area cannot be directly interpreted as evidence for deterministic effects of area per se. Additionally, richness‐based analyses may obscure species‐level responses to area and isolation that may better inform conservation practice. Here, we use random placement models to control for variation in abundance, occupancy and richness associated with the sample‐area effect, allowing deterministic effects of area and isolation, and how they vary with species' functional traits, to be resolved using linear mixed effects models. We demonstrate the utility of this approach using a butterfly assemblage persisting on a naturally fragmented landscape of lake islands. The ISAR did not significantly deviate from random placement in relation to island area, isolation or habitat diversity, supporting stochastic assembly consistent with the sample‐area effect. Such inferences support the habitat amount hypothesis, which prioritizes preserving the maximum amount of habitat irrespective of its degree of fragmentation. However, species‐level analyses demonstrated that species' abundances were significantly lower on both smaller and more isolated islands than what is predicted by the sample‐area effect. Moreover, effects of area per se were significantly greater for smaller, less mobile and rare species. Species' occurrences also significantly deviated from predictions of the sample‐area effect in relation to island isolation. Thus, our approach illustrates that richness‐based analyses not only result in incorrect inferences on mechanisms underlying ISARs, but also obscure important effects of area per se and isolation on individual species that vary with functional traits. We therefore suggest that these effects should not be solely inferred from richness‐based analyses, but rather evaluated on a species‐by‐species basis.

The importance of patch shape at threshold occupancy: functional patch size within total habitat amount

“…Although the hypothesis has generated debate, mostly about its mechanisms and consequences (e.g., Hanski 2015 ; Fletcher et al 2018 ; Deane 2022 ), it has garnered considerable empirical support (reviewed by Watling et al 2020 ). Additionally, few analyses of the HAH to date have found evidence for an effect of patch size alone on richness or occurrence, a finding that would be inconsistent with the HAH (e.g., Haddad et al 2017 ; MacDonald et al 2021 ).…”

“…8), should be correctly defined for the species or species group included in the species richness estimate. This suggests identifying more restrictively defined species subgroups associated with more homogeneous patch types (MacDonald et al 2021 ). This reduces the complicating influence on species richness of increasing environmental heterogeneity inherent in more broadly defined plant communities or other biotopes (Wiens 1989 ; Ricklefs and Lovette 1999 ; Tews et al 2004 ).…”

Testing the Resource Hypothesis of Species–Area Relationships: Extinction Cannot Work Alone

“…First, it is difficult to distinguish the role of resources from the role of area per se. SAR is shaped by both resources and area via extinction [ 15 , 16 ], and an increase in natural habitat area typically entails a rise in resources. Second, the quantity of resources in vast habitats may entice more species to settle there, influencing SAR [ 17 ].…”

Island biogeography of soundscapes: Island area shapes spatial patterns of avian acoustic diversity

“…With reduced colonization rates, species diversity declines as islands become more isolated (MacArthur & Wilson, 1967). Therefore, the lack of major acoustic contributors may impoverish the soundscapes (MacDonald et al, 2021; Morand, 2000). In addition, only species with high dispersal abilities could reach remote islands (Diamond, 1975; Gillespie & Baldwin, 2010), causing remote islands to possess more similar species composition.…”

“…similar vocal fauna) on these remote islands (Burivalova et al, 2019; Rappaport et al, 2022). However, for islands surrounded by many other islands far from the mainland, the turnover process of soniferous populations migrating from one island to another may be beneficial for maintaining soundscape diversity (Blumstein & Daniel, 2005; MacArthur & Wilson, 1967; MacDonald et al, 2021). Hence, it is possible for these islands to produce a high diversity of soundscapes because of the high connections between surrounding islands.…”