An ecological definition of 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’ (AUSR) 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 AUSR has previously been estimated for patchy habitat types
(islands, habitat islands and fragments), providing a benchmark to compare
this threshold of small fragment size between studies. However, if AUSR
can be readily determined within individual study systems, it would also
provide an objective threshold to separate small and large fragments under
the AUSR 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
AUSR 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, AUSR can provide useful insights on the relative importance
of narrow- vs wide-ranging species for studies of patch-size dependence in
ecological phenomena.