Controlling sampling bias
SLOSS comparisons are unbiased only when sampling effort is constant on
a per-area basis, i.e., when sets of patches that add to the same total
area also add to the same total sampling effort (Gavish et al.2012; Fahrig 2020). However, most ecological datasets – including many
collected in FragSAD – are sampled more intensively in small patches
than in large patches, on a per area basis. For example, patches
differing in size are often sampled with the same number of plots,
traps, or transects. To remove this bias in favor of small patches, we
implemented a novel resampling (rarefaction) procedure that generated
100 simulated metacommunities from each dataset, where the number of
individuals ”sampled” in each patch was proportional to patch size, and
individuals were resampled randomly without replacement. The number of
individuals sampled in each patch was determined by the lowest density
of individuals per unit area observed across all the patches in the
given metacommunity (usually the density of individuals in the largest
patch), multiplied by the area of each patch, and adjusted for effects
of patch size on the density of individuals (Chase et al. 2020a)
(Appendix). This procedure resulted in species lists across sets of
patches derived from a constant sampling effort per area, and accounted
for effects of patch size on the density of individuals.