Recent analyses suggest that positive patch area effects on biodiversity occur ubiquitously when comparing equal-effort samples from remnant habitat patches. The mechanisms underlying the emergence of this so-called “ecosystem decay” remain poorly understood despite conservation relevance.
We leverage spatially-explicit information on the occurrence of plant species across the Swiss Alps (415 plots, 668 species) to test two mechanisms compatible with the emergence of patch-scale ecosystem decay: (i) plots sampled within small patches might have lower biodiversity than plot sampled within large patches (plot-scale decay hypothesis), and (ii) plots sampled within large patches might share a lower proportion of species (turnover hypothesis). We found evidence that patch-scale ecosystem decay occurs in our system. While plots sampled in large patches tended to be more dissimilar, supporting the turnover hypothesis, we did not find support for the plot-scale decay hypothesis. Additionally, distance between plots and elevational changes explain turnover between plots better than patch area effects.
Taken together, our results indicate that applications of ecosystem decay in biodiversity conservation require a better understanding of the mechanisms that potentially underlie this pattern. Patch area effects might be less important than previously assumed when assessing landscape-scale biodiversity, because such effects can be confused with the effects of distance-decay in community similarity, environmental heterogeneity, and sampling effort. More broadly, these findings align with mounting evidence that protecting as much habitat as possible – regardless of whether such habitat exists continuous or fragmented – might be the most effective means to sustain biodiversity across human-dominated landscapes.