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.

Methods
The data are collated from two different databases. The first of these was a subset (78/202) of the database collated from the literature on discrete metacommunities (islands, habitat islands and fragments), with the origins and primary sources for the data described in Deane (2022) and Deane et al. (2024).

To increase sample size, we added 60 datasets from the FragSAD database (Chase, et al. 2019), available from the Dryad data repository (https://doi.org/10.5061/dryad.595718c, August 2019 version, accessed 8 December 2020).

All datasets included metadata on broad taxonomic group (birds, invertebrates, non-avian vertebrates and plants), fragment type (‘forest’, ‘grassland’, or ‘island’, respectively forest or woodland fragments within a terrestrial matrix, grass/shrub-dominated fragments within a terrestrial matrix, and forest habitat fragments isolated by water due to reservoir creation), and a four-level categorical indicator of survey effort (Appendix S1; Deane 2022). In total, 138 datasets were available for modelling individual landscapes and for meta-regression across habitats.