Resolving the SLOSS dilemma for biodiversity conservation: a research agenda

Abstract

In biodiversity conservation, the “SL > SS principle” that a single (or few) large habitat patches (SL) conserve more species than several small patches (SS) is used to prioritize protection of large patches while down-weighting small ones. However, empirical support for this principle is lacking; most studies find SS > SL. We propose a research agenda to resolve this dilemma by asking, “are there consistent, empirically-demonstrated conditions leading to SL > SS?” We develop a hypothesis to answer this question, the “SLOSS cube hypothesis,” which predicts SL > SS only when all three of the following are true: between-patch movement is low, population dynamics are not influenced by spreading-of-risk, and large-scale across-habitat heterogeneity is low. We then propose methods to test this prediction. Many tests are needed, comparing gamma diversity across multiple landscapes varying in number and sizes of patches. If the prediction is not generally supported across tests, then either the mechanisms leading to SL > SS are extremely rare in nature, or they are outweighed by countervailing mechanisms leading to SS > SL (e.g. lower competition or higher immigration in SS), or both. In that case, the SL > SS principle should be abandoned.

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

“…38.2% SES values within the range of −1.96 and 1.96, and 61.5% SES values below −1.96), birds (87.5% SES values within the range of −1.96 and 1.96, and 12.0% SES values below −1.96) and spiders (87.3% SES values within the range of −1.96 and 1.96, and 12.6% SES values below −1.96) (Figure 2), indicating a random or anti‐nestedness species distribution across islands (Matthews et al, 2015). Because isolation is not an important factor structuring the assembly of woody plants, birds, and spiders in the study archipelago (Hu et al, 2021; Wilson et al, 2016), the higher immigration/colonization rate of species among islands may lead to a weaker nestedness of species composition along the island size gradient (Fahrig et al, 2022; Matthews et al, 2015). In the future studies, the relationship between the degree of nestedness and SLOSS pattern can be further analysed in systems with different nestedness patterns.…”

“…In a previous study addressing islands of this archipelago, Liu et al (2018) found that accumulation of both environmental heterogeneity and species richness was greater for sets of small islands than for sets of large islands with equivalent cumulative area. Given a positive relationship between environmental and habitat heterogeneity, several smaller islands are likely to contain a greater diversity of micro‐habitats compared with fewer or single larger islands (Fahrig et al, 2022; Gavish et al, 2012; Liu et al, 2018; MacDonald et al, 2018a; Tscharntke et al, 2012). Additionally, even though species richness of individual small island may be lower than the species richness of an equal‐sized sampling plot in a large island (Phillips et al, 2017), several small heterogeneous islands may harbour more species than a single large island if species turnover among small islands is substantial (Deane et al, 2020; Liu et al, 2018).…”

The disproportionately high value of small patches for biodiversity conservation

“…These and other explanations for why several small patches often contain more species than few large ones (SS > SL) are reviewed in Fahrig et al. (2022). In fact, cases of SL > SS are very rare in empirical datasets, while the reverse pattern (SS > SL) is very common (Quinn & Harrison, 1988; Fahrig, 2020).…”

“…Furthermore, species richness across sets of small patches can be higher than across few large ones when disturbances are less likely to spread across many small patches (den Boer, 1968;Diamond, 1976;Simberloff & Abele, 1976) or when species interactions are stabilized across many small patches (Huffaker, 1958;Diamond, 1976). These and other explanations for why several small patches often contain more species than few large ones (SS > SL) are reviewed in Fahrig et al (2022). In fact, cases of SL > SS are very rare in empirical datasets, while the reverse pattern (SS > SL) is very common (Quinn & Harrison, 1988;Fahrig, 2020).…”

“…Prioritizing large patches of habitat over small patches is a tenet in biodiversity conservation worldwide (e.g., the Ontario Wetland Evaluation System in Canada, the New South Wales Biodiversity Offsets Policy in Australia, or the Programa Especial Selva Lacandona in Mexico; other examples in Fahrig et al., 2022; Wintle et al., 2019). Nevertheless, data often indicate that the conservation value of small patches is disproportionately high (Bennet & Arcese, 2013; Deane & He, 2019; Fahrig, 2020; Tulloch et al., 2016; Wintle et al., 2019).…”