Fair tests of the habitat amount hypothesis require appropriate metrics of patch isolation: An example with small mammals in the Brazilian Atlantic Forest

Habitat amount drives the functional diversity and nestedness of anuran communities in an Atlantic Forest fragmented landscape

“…As stated by Fahrig (2013), the correct definition of habitat for the species group under evaluation is an essential step to understand how the reduction in the amount of habitat affects the number of species in local landscapes. This approach has been used in other studies, considering only a subgroup of species to conduct analyses for communities (Melo et al, 2017;Pardini et al, 2010;Vieira et al, 2018). (De Coster et al, 2015), and the replacement of large-fruited trees by early-successional, small-seeded species at fragment edges (Magnago et al, 2014).…”

“…However, the relationship between habitat amount and species richness requires appropriate definition of habitat and selection of species group to make sure that only species that use predominantly the same habitat type are included in the species richness estimate. For example, Vieira, Almeida‐Gomes, Delciellos, Cerqueira and Crouzeilles () used forest cover as a proxy of habitat amount and then selected only forest species of non‐volant small mammals to test the habitat amount hypothesis. Indeed, there is evidence of a positive relationship between species diversity and total amount of habitat in fragmented landscapes (Andrén, ; Melo, Sponchiado, Cáceres & Fahrig, ; Pardini, Bueno, Gardner, Prado & Metzger, ; Püttker, Bueno, Barros, Sommer & Pardini, ; Smith, Fahrig & Francis, ), but also of the existence of fragmentation thresholds or regime shifts in the relationship between the amount of habitat and the number and composition of species (Andrén, ; Banks‐Leite et al., ; Pardini et al., ).…”

Habitat fragmentation and species diversity in competitive communities

“…The habitat amount hypothesis has been criticised for lacking an underlying mechanistic explanation of how species interactions and community dynamics affect species richness, and which would predict the appropriate spatial scale at which the hypothesis holds (Hanski 2015;Fletcher et al 2018; but see Jackson & Fahrig 2012;Fahrig 2013). Recent empirical results (MacDonald et al 2018;Vieira et al 2018) suggest that response of species diversity to habitat loss and fragmentation is best explained by connecting the habitat amount hypothesis to the theory of island biogeography (MacArthur & Wilson 1967) and habitat diversity hypothesis (Williams 1964). Indeed, it is important to take into account the scale of the landscape, as the relative importance of habitat diversity and amount varies with island or fragment area (Lomolino & Weiser 2001;Sfenthourakis & Triantis 2009), which may explain why the habitat amount hypothesis remains so controversial.…”

Support for the habitat amount hypothesis from a global synthesis of species density studies

“…Early tests of the HAH have been inconsistent, with some studies supporting the hypothesis (Melo et al ), some refuting it (Haddad et al ; Saldhana Bueno & Peres ), and others providing partial support (Martin ; Viera et al ). The strongest tests of the HAH will (1) be conducted at the plot rather than patch scale, to control for sampling intensity and (2) if conducted at the patch scale, they will include many patches ranging in size from large to small, to avoid idiosyncrasies of the species–area relationship when patches are small (Lomolino & Weiser ), few patches are sampled (Triantis et al ), or the range in patch sizes is low (Watling & Donnelly ).…”

“…Early tests of the HAH have been inconsistent, with some studies supporting the hypothesis (Melo et al 2017), some refuting it (Haddad et al 2017;Saldhana Bueno & Peres 2019), and others providing partial support (Martin 2018;Viera et al 2018). The strongest tests of the HAH will (1) be conducted at the plot rather than patch scale, to control for sampling intensity and (2) if conducted at the patch scale, they will include many patches ranging in size from large to small, to avoid idiosyncrasies of the species-area relationship when patches are small (Lomolino & Weiser 2001), few patches are sampled (Triantis et al 2012), or the range in Figure 1 The habitat amount hypothesis predicts that species richness in equally sized sample plots (species density; black squares) is determined by the total habitat area (all green polygons) in a local landscape (large circles), rather than the size of the local patch in which plots are located (dark green polygons).…”