Beyond the Fragmentation Threshold Hypothesis: Regime Shifts in Biodiversity Across Fragmented Landscapes

Abstract

Ecological systems are vulnerable to irreversible change when key system properties are pushed over thresholds, resulting in the loss of resilience and the precipitation of a regime shift. Perhaps the most important of such properties in human-modified landscapes is the total amount of remnant native vegetation. In a seminal study Andrén proposed the existence of a fragmentation threshold in the total amount of remnant vegetation, below which landscape-scale connectivity is eroded and local species richness and abundance become dependent on patch size. Despite the fact that species patch-area effects have been a mainstay of conservation science there has yet to be a robust empirical evaluation of this hypothesis. Here we present and test a new conceptual model describing the mechanisms and consequences of biodiversity change in fragmented landscapes, identifying the fragmentation threshold as a first step in a positive feedback mechanism that has the capacity to impair ecological resilience, and drive a regime shift in biodiversity. The model considers that local extinction risk is defined by patch size, and immigration rates by landscape vegetation cover, and that the recovery from local species losses depends upon the landscape species pool. Using a unique dataset on the distribution of non-volant small mammals across replicate landscapes in the Atlantic forest of Brazil, we found strong evidence for our model predictions - that patch-area effects are evident only at intermediate levels of total forest cover, where landscape diversity is still high and opportunities for enhancing biodiversity through local management are greatest. Furthermore, high levels of forest loss can push native biota through an extinction filter, and result in the abrupt, landscape-wide loss of forest-specialist taxa, ecological resilience and management effectiveness. The proposed model links hitherto distinct theoretical approaches within a single framework, providing a powerful tool for analysing the potential effectiveness of management interventions.

Species–fragmented area relationship

“…Even forest fragments greater than 5,000 ha had only 7.2 species on average out of the original 18 species. Another study comparing the occurrence of 27 forest specialist small mammal species in three landscapes of 100 km 2 in the Atlantic forest in Brazil found that about half of the species persisted in the landscape with 30% forest cover, but only one species persisted in the landscape with 10% forest cover (43,44). This result is consistent with data for forest-inhabiting birds in Fig.…”

Correlation of Borrelia burgdorferi Sensu Lato Prevalence in Questing Ixodes ricinus Ticks with Specific Abiotic Traits in the Western Palearctic

“…Infection rates in ticks are largely "diluted" because the high host densities buffer drastic changes in B. burgdorferi sensu lato prevalence. A low connectivity rate for habitat patches is not always synonymous with low prevalence in ticks (32). The classification map of Europe must be regarded as the best explanation of the tick prevalence rates as an output of abiotic variables, and not as a map of risk for humans.…”

Fragment edge and isolation affect the food web: effects on the strength of interactions among trophic guilds

“…To resolve these doubts, we recommend a landscape-scale approach in future research, with the inclusion of matrix structure and habitat amount in the analysis. Over larger spatial scales (Pardini et al 2010), the absence of patch size effects can also be related to the high level of deforestation in our study area (3% forest cover). To support this explanation, future studies should examine this relationship between patch size and trophic interactions along the regional gradient of landscape degradation.…”