Abstract
AbstractThe habitat amount hypothesis (HAH) predicts that species richness in a habitat site increases with the amount of habitat in the ‘local landscape’ defined by an appropriate distance around the site, with no distinct effects of the size of the habitat patch in which the site is located. It has been stated that a consequence of the HAH, if supported, would be that it is unnecessary to consider habitat configuration to predict or manage biodiversity patterns, and that conservation strategies should focus on habitat amount regardless of fragmentation. Here, I assume that the HAH holds and apply the HAH predictions to all habitat sites over entire landscapes that have the same amount of habitat but differ in habitat configuration. By doing so, I show that the HAH actually implies clearly negative effects of habitat fragmentation, and of other spatial configuration changes, on species richness in all or many of the habitat sites in the landscape, and that these habitat configuration effects are distinct from those of habitat amount in the landscape. I further show that, contrary to current interpretations, the HAH is compatible with a steeper slope of the species–area relationship for fragmented than for continuous habitat, and with higher species richness for a single large patch than for several small patches with the same total area (SLOSS). This suggests the need to revise the ways in which the HAH has been interpreted and can be actually tested. The misinterpretation of the HAH has arisen from confounding and overlooking the differences in the spatial scales involved: the individual habitat site at which the HAH gives predictions, the local landscape around an individual site and the landscapes or regions (with multiple habitat sites and different local landscapes) that need to be analysed and managed. The HAH has been erroneously viewed as negating or diminishing the relevance of fragmentation effects, while it actually supports the importance of habitat configuration for biodiversity. I conclude that, even in the cases where the HAH holds, habitat fragmentation and configuration are important for understanding and managing species distributions in the landscape.
Highlights
Understanding how habitat amount and configuration affect species richness, occurrence or abundance has been one of the major focuses of research in ecology and biogeography, given its central importance for conservation planning and for landscape management
The Habitat Amount Hypothesis (HAH) states that species richness, occurrence or abundance in a given habitat site should increase with the amount of habitat in the circular ‘local landscape’ defined by a certain distance D surrounding that site, with no additional effects of the area of the habitat patch in which the habitat site is located
The prevailing view and interpretation of the HAH has been that, if or where it holds, it negates the existence of fragmentation effects and implies that conservation strategies should focus on retaining the maximum overall amount of habitat regardless of its configuration (Fahrig, 2013; Haddad et al, 2017; Melo et al, 2017; Torrenta & Villard, 2017; MacDonald et al, 2018; Bueno & Peres, 2019), as summarized in the Introduction
Summary
A key specification of the Habitat Amount Hypothesis (HAH) is the distance D that defines the circular ‘local landscape’ (scale of effect) over which the amount of habitat determines the response variable (usually species richness, abundance or occurrence) in a site (Fahrig, 2013). Habitat fragmentation (breaking apart of the habitat into a larger number of smaller patches) in the landscape, while holding constant the amount of habitat, negatively affects, according to the HAH predictions, species richness, abundance or occurrence in all or many of the habitat sites in the landscape (Figures 2 and 4). As I have shown above, the HAH does predict, when applied over entire landscapes made up of multiple habitat sites, negative effects of habitat fragmentation, elongation or inter-patch distance on species richness, abundance or occurrence in all or many of the habitat sites in the landscape This holds both when considering landscapes with the same extent as a local landscape (Figure 4) and when considering larger landscapes (Figures 2 and S4.2), as well as when considering patches either smaller or larger than the extent of a local landscape (Figures 2, S4.2 and 4)
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