The negative effects of habitat fragmentation operate at the scale of dispersal

Abstract

Habitat loss is often considered the greatest near-term threat to biodiversity. Yet the impact of habitat fragmentation, or the change in habitat configuration for a given amount of habitat loss, has been intensely debated. We isolated effects of habitat loss from fragmentation on the demography, movement, and abundance of wild populations of a specialist herbivore, Chelinidea vittiger, by removing 2,088 patches across 15 landscapes. We compared fragmentation resulting from random loss, which is often considered in theory, to aggregated loss, which is often observed in the real world. When quantifying fragmentation caused by random vs. aggregated loss, aggregated loss led to less fragmented landscapes than random loss based on patch isolation, but more fragmented landscapes when based on isolation at a larger mesoscale scale defined by dispersal distances of C. vittiger. Overall, habitat loss decreased population size and demographic parameters, with thresholds occurring at approximately 70-80% patch loss. Synergistic effects also occurred, where an aggregated pattern of loss had negative effects at low, but not high, amounts of habitat loss. Effects on population size of C. vittiger were driven by reductions in movement and subsequent reproduction. The direction of habitat fragmentation effects from random and aggregated loss treatments, for a given habitat amount, was conflictingly positive or negative depending on the scale at which fragmentation was quantified. Fragmentation quantified at the scale of dispersal for this species best explained population size and highlighted that fragmentation had negative effects at a mesoscale. Our results emphasize the importance of quantifying habitat fragmentation at biologically appropriate scales.

Heterogeneity–diversity relationships in sessile organisms: a unified framework

“…2). We use the concept of fragmentation rather than spatial dependence or spatial autocorrelation because the former term is more prevalent in the ecological literature (Andr en 1994; Debinski & Holt 2000;Fischer & Lindenmayer 2007;Fahrig 2017;Fletcher et al 2018). Importantly, our definition of fragmentation refers to fragmentation per-se (Fahrig 2003) and is independent of the magnitude of compositional heterogeneity.…”

“…Our theoretical framework, as well as the predictions provided by our model, is highly relevant to biodiversity conservation and management. First, habitat loss and fragmentation are among the most studied aspects of human alternation of the environment, and their consequences for the persistence of individual species and whole communities have been examined in numerous theoretical and empirical studies (Andr en 1994; Debinski & Holt 2000;Fischer & Lindenmayer 2007;Matias et al 2013;Fahrig 2017;Fletcher et al 2018;Ramiadantsoa et al 2018). However, most of these studies have not explicitly tested for diversity responses to environmental heterogeneity.…”

Towards a unified framework for connectivity that disentangles movement and mortality in space and time

“…We evaluated the utility of the SAMC to predict observed movements of cactus bugs across 15 50 × 50 m landscapes. These landscapes were part of a larger experiment on the roles of habitat loss and fragmentation on population dynamics (Fletcher et al ). We briefly describe relevant aspects of this experiment for parameterising the SAMC (see Supporting Information and Fletcher et al for more).…”

“…These landscapes were part of a larger experiment on the roles of habitat loss and fragmentation on population dynamics (Fletcher et al ). We briefly describe relevant aspects of this experiment for parameterising the SAMC (see Supporting Information and Fletcher et al for more). In May–June 2014, we first removed all in situ C. vittiger and released 100 individuals (50 males, 50 females) in each landscape.…”

“…Finally, we calculated life expectancy of potential dispersers from cactus patches,. We relate this metric to variation in estimated population sizes, taken from Fletcher et al (), to interpret the role of disperser survival in driving population size across landscapes. See Supporting Information for more details.…”