The habitat amount hypothesis predicts that fragmentation poses a threat to biodiversity: A reply to Fahrig

Abstract

AbstractIn Saura (Journal of Biogeography, 48, 11–22, 2021), I showed that the habitat amount hypothesis (HAH) has been considerably misinterpreted in several ways. In her response to my findings, Fahrig (Journal of Biogeography, 2021) admits that some predictions that were previously attributed to the HAH do not logically derive from it. She has however one main objection to my conclusions: that there are some cases where the HAH predicts higher site‐level species richness with more fragmentation in a region. I here explain why this is a partial and questionable observation that distracts and potentially misleads our understanding of the HAH predictions. It does not appropriately represent the fundamental and overwhelming negative effects of fragmentation on site‐level species richness that are predicted by the HAH. The HAH predicts that the highest site‐level species richness in a region will happen when all habitat is found in a single and compact habitat patch. Any departure from this zero‐fragmentation case, as well as any additional increase in the number, isolation, elongation or perforation of patches per se, will always have negative effects on site‐level species richness according to the HAH. I more briefly discuss a relatively minor comment by Fahrig on the possible slopes of the species–area relationship when the HAH holds. I conclude that the views and interpretations that have prevailed since the HAH was proposed—that the HAH negates the importance of fragmentation—can no longer be maintained. The HAH predicts that habitat configuration matters for conservation and that fragmentation is a threat to biodiversity.