SLOSS‐based inferences in a fragmented landscape depend on fragment area and species–area slope

Abstract

Aims Whether a Single Large Or Several Small (SLOSS) habitat patches contain more species is central to the debate about how habitat fragmentation threatens species diversity. However, the geographical and biogeographical variables that affect emergent SLOSS patterns remain poorly understood. Here, we quantified SLOSS‐based diversity patterns of woody plant, bird and spider assemblages in a subtropical archipelago of land bridge islands. Location Thousand Island Lake, Zhejiang Province, China. Taxon Woody plants, birds and spiders. Methods We analysed species accumulation curves and species–area relationships (SARs) to quantify SLOSS‐based patterns of all, common and rare species for each taxon across different groupings of islands. Differences in the number of species between a single large island and sets of several small islands were measured in these analyses using a Saturation index (SI) and SLOSS index. Generalized additive models were used to assess the relationships between SI and SLOSS index values and the maximum area of islands included in the analysis, the slope of SARs (i.e. z‐score) and the degree of nestedness. Results SI values increased with the maximum area of islands for three taxa, while SLOSS index values only increased with the maximum island area for woody plants. SI values increased as z‐scores of woody plants decreased, and showed a quadratic relationship for birds and spiders. SLOSS index values decreased as z‐scores of three taxa increased. The degree of nestedness and SI values or SLOSS index values were weakly correlated with all, rare and common species in the three taxa. Main conclusions SLOSS‐based inferences of fragmentation effects are contingent on variation in the maximum area of patches included in analyses and the slope of SARs in fragmentated landscapes. Whether conservation efforts should prioritize a single large or several small patches depends on the geographical (e.g. maximum fragment area) and biogeographical (e.g. slope of SAR) attributes of a fragmentated landscape.

Species accumulation in small–large vs large–small order: more species but not all species?

“…Moreover, the outcome depends on the island species–area relationship (ISAR), which describes how species richness changes with increasing patch area in independent draws from the regional species pool (Scheiner et al 2011 ) not how species accumulate as patches are combined (Matthews et al 2016 ). A relationship between SLOSS analysis and the ISAR was recently noted for multiple taxa in island archipelagos (Liu et al 2022 ); this study explains why it is a general result. This relationship with the ISAR also explains how SLOSS analysis constrained only to specialist or endangered species might still show small patch preference (e.g., Fahrig 2020 ; Richardson et al 2015 ; Riva and Fahrig 2022 ; Tscharntke et al 2002 ); any taxon with an island species–area curve power law exponent less than ~ 0.3 is likely to result in QH curves where the small–large curve lies always above the large–small curve.…”

Macrophyte islands as an experimental model: new perspectives and a step-by-step protocol to access assumptions of Island Biogeography theory and habitat fragmentation

“…Given the great resemblance that islands' systems have to natural reserves surrounded by altered habitats, the ideas generated by the island biogeography theory are also of great impact on conservation policies (see Diamond 1975). From this conservation approach emerged the SLOSS debate (Single Large or Several Small) in which is discussed what is more desirable to keep unaltered, many small fragments or fewer of larger sizes (Liu et al, 2022).…”