Implications for biodiversity conservation in fragmented
landscapes
Our results are consistent with the landscape planning strategy proposed
by Arroyo-RodrÃguez et al. (2020) for forest biodiversity protection in
human-dominated naturally forested regions. They suggested that 40% of
the landscape should be dedicated to forest, and that three quarters of
that forest should be in small patches while the remaining quarter
should be in a single large patch (or a few large patches). Retaining a
large patch would support the persistence of herptile populations, while
small patches would maximize the diversity and persistence of other
taxa.
Most notably, patch-size thresholds used to prioritize habitat for
biodiversity protection are orders of magnitude larger than the sizes of
the small patches that we found to have disproportionately high
biodiversity value on a per-area basis (Fig. 4, vertical lines; reviewed
in Fahrig et al. (2022)). These thresholds may be imposed for
practical reasons (Armsworth et al. 2018), but they are not
supported by biological evidence. In addition, while some species
requiring large amounts of habitat likely have already disappeared from
human-dominated regions, there is no evidence that these species are
generally negatively affected by fragmentation per se (independent of
habitat loss, Fahrig (2017); Fig. 1, left panel). If anything, the
opposite is confirmed in previous analyses suggesting that many small
patches harbor both more specialist species and more species of
conservation concern than few large patches (Fahrig 2020; Riva & Fahrig
2022). Therefore, protecting a very large number of small patches that
sum to a high total habitat density across a landscape appears to be an
effective solution for sustaining populations of most species. This
seems to be the case for carnivores across Europe, a group expected to
rely strongly on large patches, but in fact showing widespread
population recoveries across patchy anthropogenic landscapes (Chapronet al. 2014).
We caution that our results apply in regions with patches similar in
size to those analyzed here, which were generally centered around 10-ha
(Fig. 4). It is possible that analyses using much smaller patches would
not show the predominant SS > SL pattern. However, as the
small patches in the datasets were generally much smaller than those
normally considered for protection, our conclusion that the default
priority for habitat protection should shift to several small patches
seems robust. Additionally, we lack detailed information about the
spatial extents of the studies, which could conceivably affect the SLOSS
outcomes. A recent meta-analysis suggests that the extent of most
studies in ecology is < 100,000 ha (Estes et al. 2018).
The datasets we used here from Chase et al. (2019) appear to be
consistent with this pattern based on inspection of maps in the papers
describing each dataset. Further work is needed to determine whether the
generality of our results is affected by the spatial extent of the study
area or by the spatial arrangement of small and large patches.