Fragmentation, grazing, and the species-area relationship

Background/Question/Methods

Habitat loss is one of the greatest threats to species persistence. Gauging the scale of this problem requires quantitative methods that can predict the number of extinctions resulting from habitat loss. For the past three decades, the Species-Area Relationship, an empirical relationship between the number of species present in an area and the size of that area, has been this tool. However, it fails to incorporate threats to species aside from habitat loss and the heterogeneous distribution of these threats across habitats. Recent studies have improved species-area predictions by incorporating not only direct effects of area on richness, but also indirect effects of area (through area-mediated predator abundance), on prey species richness. We extend this work to test the hypotheses that the indirect effects of multiple threats of grazing and trampling in addition to fragmentation will amplify the effect of area on species richness and that this effect will be greatest in zones closest to the fragment edge. We test our hypotheses with a field study of land snail richness in fragments with and without the additional threats of grazing and trampling. 

Results/Conclusions

Our study supports the hypotheses that fragments with multiple threats in addition to habitat loss harbor fewer species than fragments without these threats, and that this effect is non-uniform across fragments, populations, and species.  Recent studies have highlighted the need to think of threats in concert, but in practice, this is frequently still not done.  Either a sole threat is examined in isolation, or the assumption is made that multiple threats are additive.  Reducing habitat area reduces the number of species present, and the addition of grazing to already fragmented habitat reduces the population sizes of remaining species. The resulting risk is therefore a potential extinction debt due to the inherent issues of small population sizes, and therefore, a greater loss of species than would have been expected without accounting for the additional impacts of grazing. Because of this, we assert that the consideration of the effects of multiple threats on area and the heterogeneity of these effects within and across fragments, species, and populations is crucial to conservation efforts and planning.