Inferring the role of habitat heterogeneity in SLOSS (single large or several small) for beetles, spiders, and birds in forest reserves

Global conservation strategies aim to increase the area of nature reserves. To implement this goal, especially in fragmented landscapes like Central Europe, we must understand whether a Single Large (SL) reserve Or Several Small (SS) reserves has higher species richness (SLOSS), and why. To date, most studies find more species in SS than SL (SS>SL). The most commonly invoked explanation is higher habitat heterogeneity across SS than SL. We assessed SLOSS for beetles, spiders and birds in 44 forest reserves of three forest types in Central Europe, and tested several predictions based on this heterogeneity hypothesis. We assessed SLOSS in two ways: Quinn-Harrison-curves, and a new approach, the ‘SLOSS ratio’ of species richness standardized by sample coverage, in SL vs. SS. As habitat heterogeneity is challenging to measure directly, we indirectly tested the heterogeneity hypothesis through the following predictions: the SS>SL pattern should be stronger (1) for taxa with finer space use, (2) when there are more SS reserves, spread over more area, (3) when the reserves have been established for a longer time, allowing divergence among patches, (4) when the SS reserves include multiple forest types, and (5) for forest types with a lower frequency of stand-replacing disturbances. We found SS>SL for all taxa, and we found support for three of the predictions based on the heterogeneity hypothesis: (1), (2), and (4). We infer that a set of many small forest reserves is an appropriate objective for conservation planning and can make a strong contribution to global conservation goals.