Species face multiple global changes that can lead to decline and extinction through one of two routes: habitat destruction, such as occurs locally with land-use change, and habitat degradation, which often occurs through larger-scale changes such as climate change.
While most populations are naturally patchy, both degradation and destruction can also force initially contiguous local populations into smaller and more isolated units. Poor understanding of how metapopulation declines differ under degradation and destruction poses a growing problem for conserving increasingly fragmented and declining populations.
We use metapopulation theory and simulations to model the relative impact of habitat destruction and degradation on metapopulation abundance and extinction thresholds to ask: 1) Which will have a greater negative impact on metapopulation viability and size, habitat degradation or destruction of individual patches? 2) How does dispersal ability alter the relative impact of these two processes? 3) How do the relative impacts of destruction and degradation change with the spatial distribution of patches in a landscape?
We compared the relative impacts of habitat destruction and degradation by creating a standardised amount of habitat loss, with this loss occurring through patch removal (destruction) or an equivalent percent reduction in carrying capacity of all patches (degradation).
Results/Conclusions
We identified two thresholds that determine the severity and importance of global changes for metapopulations. First, the impacts of habitat destruction shift from minor to drastic beyond a threshold. This nonlinear impact of habitat destruction is exacerbated by habitat clustering and limited dispersal, which make metapopulations increasingly susceptible to further destruction as fewer critical patches remain.
The second threshold delineates when the negative impacts of habitat destruction surpass those of habitat degradation. Habitat degradation is more harmful than destruction in metapopulations that are close to intact, but the non-linear effects of habitat destruction cause it to become more harmful as habitat loss increases. This threshold occurs at variable levels of habitat loss, with poor dispersers in clustered landscapes experiencing the threshold at close to 50% habitat loss, whereas less dispersal-limited species experience it at close to complete habitat loss. The second threshold also determines how rarity influences species responses to global changes, with rare species mainly impacted by degradation unless the second threshold is very low.
Our results warn against the sole use of patch occupancy in species viability assessments, and create general predictions for how rarity and dispersal differences among species influence the relative impact of habitat destruction and degradation.