Assessing species saturation: conceptual and methodological challenges

Abstract

Is there a maximum number of species that can coexist? Intuitively, we assume an upper limit to the number of species in a given assemblage, or that a lineage can produce, but defining and testing this limit has proven problematic. Herein, we first outline seven general challenges of studies on species saturation, most of which are independent of the actual method used to assess saturation. Among these are the challenge of defining saturation conceptually and operationally, the importance of setting an appropriate referential system, and the need to discriminate among patterns, processes and mechanisms. Second, we list and discuss the methodological approaches that have been used to study species saturation. These approaches vary in time and spatial scales, and in the variables and assumptions needed to assess saturation. We argue that assessing species saturation is possible, but that many studies conducted to date have conceptual and methodological flaws that prevent us from currently attaining a good idea of the occurrence of species saturation.

Global fern and lycophyte richness explained: How regional and local factors shape plot richness

“…However, it has been argued that the interactions between species and their environment can limit the number of locally co-occurring species numbers, leading to local saturation. If this is the case, an increase in regional species richness should not affect local diversity (see Olivares, Karger, & Kessler, 2018;Srivastava, 1999 for details and examples).…”

Disentangling the drivers of local species richness using probabilistic species pools

“…There are, however, several problems with interpreting our results as evidence for or against saturation. Saturation is always context dependent (Olivares, Karger, & Kessler, ), and in the case of the restios, we could argue that some relevés (the ones reaching the upper asymptote of Figure a), are saturated with respect to the LPSP. However, we cannot show that these relevés are also at equilibrium, or that other plant groups may affect the competitive relationships, which would be additionally necessary to confirm saturation (Olivares et al, ).…”

“…Saturation is always context dependent (Olivares, Karger, & Kessler, ), and in the case of the restios, we could argue that some relevés (the ones reaching the upper asymptote of Figure a), are saturated with respect to the LPSP. However, we cannot show that these relevés are also at equilibrium, or that other plant groups may affect the competitive relationships, which would be additionally necessary to confirm saturation (Olivares et al, ). In an ecosystem with remarkable turnover in species composition at relevé level, as shown for the Cape Point reserve (Slingsby et al, ), and with high habitat dynamics (e.g.…”

Alien species richness is currently unbounded in all but the most urbanized bird communities

“…-E. E. Dyer, Centre for Biodiversity and Environment Research, Dept of Genetics, Evolution and Environment, Univ. College London, London, UK. is unbounded, one typical approach is to assess whether an asymptotic response occurs when local species richness is correlated with species pool size (Olivares et al 2018). Studies often define species pool sizes at larger spatial scales (e.g.…”

“…For example, an asymptotic relationship indicates that sites located at the linear phase have an 'unbounded' alien species richness, while those at the asymptotic phase have an alien species richness 'capped' by other processes (Fig. 1) (Olivares et al 2018).…”