Abstract
Land‐use changes, which cause loss, degradation, and fragmentation of natural habitats, are important anthropogenic drivers of biodiversity change. However, there is an ongoing debate about how fragmentation per se affects biodiversity in a given amount of habitat. Here, we illustrate why it is important to distinguish two different aspects of fragmentation to resolve this debate: (a) geometric fragmentation effects, which exclusively arise from the spatial distributions of species and habitat fragments, and (b) demographic fragmentation effects due to reduced fragment sizes, and/or changes in fragment isolation, edge effects, or species interactions. While most empirical studies are primarily interested in quantifying demographic fragmentation effects, geometric effects are typically invoked as post hoc explanations of biodiversity responses to fragmentation per se. Here, we present an approach to quantify geometric fragmentation effects on species survival and extinction probabilities. We illustrate this approach using spatial simulations where we systematically varied the initial abundances and distribution patterns (i.e., random, aggregated, or regular) of species as well as habitat amount and fragmentation per se. As expected, we found no geometric fragmentation effects when species were randomly distributed. However, when species were aggregated, we found positive effects of fragmentation per se on survival probability for a large range of scenarios. For regular species distributions, we found weakly negative geometric effects. These findings are independent of the ecological mechanisms which generate nonrandom species distributions. Our study helps to reconcile seemingly contradictory results of previous fragmentation studies. Since intraspecific aggregation is a ubiquitous pattern in nature, our findings imply widespread positive geometric fragmentation effects. This expectation is supported by many studies that find positive effects of fragmentation per se on species occurrences and diversity after controlling for habitat amount. We outline how to disentangle geometric and demographic fragmentation effects, which is critical for predicting the response of biodiversity to landscape change.
Highlights
Anthropogenic land‐use changes cause the loss, the fragmentation, and the degradation of natural and seminatural habitats (Fischer & Lindenmayer, 2007; Harrison & Bruna, 1999) and are considered as one of the most important drivers of past, current, and future biodi‐ versity change (Millennium Ecosystem Assessment, 2005; Newbold et al, 2015; Pereira et al, 2010; Pimm et al, 2014)
We present an approach to quantify geometric fragmentation effects on species survival and extinction proba‐ bilities
To understand and resolve the contrasting results observed in empirical studies, we suggest distinguishing two different aspects of fragmentation: (a) geometric fragmentation effects, which arise solely from the spatial arrangement of habitat fragments relative to species distributions in continuous landscapes and specify if individuals are located in habitat fragments or in the surrounding matrix, and (b) demographic fragmentation effects, which
Summary
Anthropogenic land‐use changes cause the loss, the fragmentation, and the degradation of natural and seminatural habitats (Fischer & Lindenmayer, 2007; Harrison & Bruna, 1999) and are considered as one of the most important drivers of past, current, and future biodi‐ versity change (Millennium Ecosystem Assessment, 2005; Newbold et al, 2015; Pereira et al, 2010; Pimm et al, 2014). Chisholm et al (2018) presented an approach for the quantification of geometric effects, called “short‐term species loss” in their study Their approach focuses on species distributions predicted by neu‐ tral models, which by definition exclude regular distributions and assume that all species share the same dispersal parameters and show similar spatial distributions (Hubbell, 2001). We address these gaps by presenting a more generic approach to quantify geometric fragmentation effects on species abundances and their survival. We argue that it is essential to understand the con‐ sequences of both geometric and demographic fragmentation ef‐ fects in order to reconcile the mixed results of previous research and to advance the debate about the consequences of fragmenta‐ tion for biodiversity
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