A recent question that has bedeviled landscape ecology is: what is the role of the area of habitat versus the role of habitat fragmentation?
All agree that the loss of habitat has been the major cause of species extinction and the major threat to biodiversity. Whereas conventional wisdom, supported by decades of research and conservation, has supported the effects of habitat loss AND fragmentation in reducing biodiversity, some scientists have staked out the polar extreme that only habitat amount affects biodiversity. As evidence in the academic literature supports the role of both habitat loss and fragmentation in reducing biodiversity, an important question remains: when is habitat fragmentation of importance in biodiversity conservation?
A new study provides an answer. Rybicki and colleagues created a model of spatial dynamics in fragmented landscapes. The scientific literature is replete with such models at the population level. The novel aspect of this research was to add a dimension of real world landscapes: different species that interact to affect each others’ abundances are affected differently by habitat loss and fragmentation. These species interactions then affect the persistence of individual species and ultimately biodiversity across landscapes.
The authors focus on the competitive interactions between species. This focus has a long-standing tradition in ecology, most famously in the demonstration of a time lag between fragmentation and extinction (known as the extinction debt). Landscapes varied in habitat amount and fragmentation. Spatial variation was imposed through variation in resources that then affect species survival and reproduction. Species also varied in their traits, especially dispersal.
This variability produced results whereby fragmentation could have positive or negative effects on biodiversity. Of clearest relevance for conservation, fragmentation could have positive effects on biodiversity if the total area of habitat in the landscape was high. Conversely, negative effects appeared when total area of habitat in the landscape was low. The magnitude of the effect of fragmentation was affected particularly by species’ dispersal abilities.
By addressing the mechanisms underlying fragmentation’s effects, this work overcomes a recognized limitation of the habitat amount hypothesis that is devoid of mechanism. The work is consistent with empirical studies that also show species loss is particular affected by habitat fragmentation when habitat area declines below thresholds.
Rybicki and colleagues best summed up their paper’s relevance to the ecological and conservation communities: “[W]e conclude that it may be time to move on from debating whether fragmentation matters or not, onto developing a comprehensive and fine‐grained understanding of when and how fragmentation matters.”
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