An Ecological Definition and Objective Threshold for Differentiating Small Fragments

In an increasingly fragmented natural world, understanding how different ecological phenomena vary with patch size has many motivations. Examples include the assembly of biodiversity, ecosystem service provision and the suitability of fragments for habitat specialist species. A common approach to such questions divides fragments into small and large size classes for separate analysis. However, lack of an objective definition and means to differentiate 'small' from 'large' patches limits our ability to compare findings across studies, arguably impeding progress toward any unified views. Because larger and smaller fragments tend, on average, to respectively over-represent narrow- and wide-range species, an 'area for unbiased species representation' (A_USR) can be defined at some intermediate fragment size predicted to contain species at incidence frequencies approximating that of the overall landscape. A central tendency for A_USR has previously been estimated for patchy habitats (islands, habitat islands and fragments), providing a benchmark to compare this threshold of small fragment size between studies. However, if A_USR can be readily determined within individual study systems, it would also provide an objective threshold to separate small and large fragments under the A_USR definition. Here we assess this potential for 138 published datasets from various fragmented landscapes using an index comparing species incidence frequencies in each fragment with that of the overall landscape. Regressing this index on fragment area yielded an estimate for A_USR in over 90% of cases, suggesting broad applicability as an objective way to separate fragments into two size classes. Regression slopes provide further information on the relative representation of narrow- vs. wide-range species, with ~80% being numerically consistent with the overall negative trend. Requiring only the same data as the island species-area relationship, A_USR can provide useful insights on the relative importance of narrow- vs. wide-ranging species for studies of patch-size dependence in ecological phenomena.