Community‐based processes behind species richness gradients: contrasting abundance–extinction dynamics and sampling effects in areas of low and high productivity

Abstract

Aim To consider the role of local colonization and extinction rates in explaining the generation and maintenance of species richness gradients at the regional scale.Location A Mediterranean biome (oak forests, deciduous forests, shrublands, pinewoods, firwoods, alpine heathlands, crops) in Catalonia, Spain. MethodsWe analysed the relative importance of direct and indirect effects of community size in explaining species richness gradients. Direct sampling effects of community size on species richness are predicted by Hubbell's neutral theory of biodiversity and biogeography. The greater the number of individuals in a locality, the greater the number of species expected by random direct sampling effects. Indirect effects are predicted by the abundance-extinction hypothesis, which states that in more productive sites increased population densities and reduced extinction rates may lead to high species richness. The study system was an altitudinal gradient of forest bird species richness. ResultsWe found significant support for the existence of both direct and indirect effects of community size in species richness. Thus, both the neutral and the abundance-extinction hypotheses were supported for the altitudinal species richness gradient of forest birds in Catalonia. However, these mechanisms seem to drive variation in species richness only in low-productivity areas; in high-productivity areas, species richness was uncorrelated with community size and productivity measures. Main conclusionsOur results support the existence of a geographical mosaic of community-based processes behind species richness gradients, with contrasting abundance-extinction dynamics and sampling effects in areas of low and high productivity.

Local richness along gradients in the Siskiyou herb flora: R. H. Whittaker revisited

“…1, pathway A), both because Whittaker (1960) observed this and because numerous other studies have found such an effect (e.g., Stevens and Carson 1999, Gotelli and Colwell 2001, Carnicer et al 2007, Honkanen et al 2010. Considerable evidence supports the energy-based random sampling mechanism, in which energy limits the total number of individuals in a community, and species are random assemblages from a regional pool (Evans et al 2005, Carnicer et al 2007, Honkanen et al 2010. Considerable evidence supports the energy-based random sampling mechanism, in which energy limits the total number of individuals in a community, and species are random assemblages from a regional pool (Evans et al 2005, Carnicer et al 2007, Honkanen et al 2010.…”

“…1, pathway A), both because Whittaker (1960) observed this and because numerous other studies have found such an effect (e.g., Stevens and Carson 1999, Gotelli and Colwell 2001, Carnicer et al 2007, Honkanen et al 2010. 1, pathway A), both because Whittaker (1960) observed this and because numerous other studies have found such an effect (e.g., Stevens and Carson 1999, Gotelli and Colwell 2001, Carnicer et al 2007, Honkanen et al 2010.…”

“…Perhaps the most applicable here is a passive sampling effect, in which numbers of individuals are limited by energy availability, and local assemblages are random draws from a regional species pool (Evans et al 2005). More complex proposed mechanisms for abundance-richness relationships generally operate at larger spatial scales than those we studied, and testing among them would require additional information that we lack (Evans et al 2005, Carnicer et al 2007. Such an interpretation is consistent with the local-regional richness relationship we found, as well as with the unsaturated behavior observed in many local plant communities (Stohlgren et al 2008).…”

“…Variation in numbers of individuals, or ''community size,'' has been shown to be a fundamental aspect of almost any attempt to explain such patterns (Gotelli and Colwell 2001). Community size may vary along productivity gradients due to self-thinning or for other reasons, and may affect species richness via a passive sampling effect or more complex mechanisms (Oksanen 1996, Stevens and Carson 1999, Gotelli and Colwell 2001, Evans et al 2005, Carnicer et al 2007, Honkanen et al 2010; see also Multivariate model structure). Competitive exclusion is another central aspect of most explanations for variation richness along gradients; species richness may be limited at high productivity because competition is stronger (Grime 1973), there is less spatial heterogeneity in resources to enable coexistence (Tilman and Pacala 1993), or selfthinning reduces richness via a reduced number of individuals (Stevens and Carson 1999).…”

More individuals but fewer species: testing the ‘more individuals hypothesis’ in a diverse tropical fauna

“…These experiments provide insight into the plausibility of the MIH, but are from species poor, low productivity systems. Richness -density relationships and their underlying mechanistic links might be different in higher productivity, more diverse systems (Carnicer et al 2007). Here, we consider how resource manipulations and disturbance affect the number of individuals and species of litter ants within one of the most diverse sites for ants worldwide (Longino et al 2002).…”