Pattern without bias: local–regional richness relationship revisited

Abstract

For decades, ecologists have been testing for species saturation by using regression analysis to determine the relationship between local and regional species richness. The cumulative result of scores of studies and meta-analyses has led to a general consensus that evidence of species saturation is relatively uncommon. However, the bias induced on the regression by the arbitrary choice of local and regional area has threatened to undermine this consensus and has even led to the proposal to abandon the regression method entirely. Nonetheless, the use of regression analysis to determine local-regional richness relationships continues. We performed a meta-analysis of almost 100 data sets using a log-ratio-based regression model that is free of the scale-induced bias inherent in more commonly used models. This reanalysis contradicts the general consensus: saturated relationships are at least as common as unsaturated patterns. However, a discouragingly large proportion of studies produce no discernable patterns at all. An explanation for such a plethora of species saturation patterns poses a challenge to our understanding of the processes that govern community assembly.

Competition, facilitation and environmental severity shape the relationship between local and regional species richness in plant communities

“…Thus, with the log‐ratio method proposed by Szava‐Kovats et al (), Goncalves‐Souza et al (2013) and Szava‐Kovats et al () found that saturated relationships were at least as common as unsaturated patterns. Szava‐Kovats et al () also found that large proportion of studies produce no discernable patterns. Additionally, Gonçalves‐Souza et al () argued that instead of focusing solely on local/regional richness plots to infer processes from patterns, additional approaches should be used to really understand the drivers of local community structure.…”

“…Although positive interactions such as mutualism have been early thought as local processes potentially contributing to local species richness (Ricklefs ), to our knowledge facilitation has been rarely proposed as a potential mechanism for shaping the LSR/RSR relationship (but see Szava‐Kovats et al ). This is quite surprising since facilitation has been included in mainstream ecological theory for two decades (Bertness and Callaway , Callaway , Bruno et al , Lortie et al , Brooker et al , , Schöb et al , McIntire and Fajardo , Soliveres et al ) and has even been proposed as an important mechanism enlarging species niches (or more accurately species' habitats), suggesting its important role for local species richness (Hacker and Gaines , Bruno et al , Michalet et al , Xiao et al , Le Bagousse‐Pinguet et al ).…”

“…Gonçalves‐Souza et al () and Szava‐Kovats et al () applied this new method and found via independent meta‐analyses consistent contradictions to the general consensus detected using the traditional approach. They both found that more than a third of the communities were misclassified in original studies and that saturated relationships were at least as common as unsaturated patterns, although Szava‐Kovats et al () also found intermediate and indeterminate curves. The primary implication is that this new method should replace the traditional approach in subsequent contrasts of local versus regional drivers.…”

Measuring size and composition of species pools: a comparison of dark diversity estimates

“…; Cornell and Harrison ), thereby highlighting the processes causing community saturation (Szava‐Kovats et al. ). Finally, the absence of species can reflect dispersal limitations and local extinctions, which are of concern to nature conservationists, but also relevant from a restoration and invasion point of view (Pärtel et al.…”

“…For example, the absolute values of species richness, particularly when estimated at one spatial scale, are of a limited value for comparing biodiversity across ecosystems, regions, or taxonomic groups if not relativized to its potential values (Gotelli and Colwell 2001;P€ artel et al 2011). Also, understanding the mechanisms regulating biodiversity in assemblages at a particular site requires identifying, which and how many species have been excluded during the assembling process (de Bello et al 2012;Cornell and Harrison 2014), thereby highlighting the processes causing community saturation (Szava-Kovats et al 2013). Finally, the absence of species can reflect dispersal limitations and local extinctions, which are of concern to nature conservationists, but also relevant from a restoration and invasion point of view (P€ artel et al 2011;Kalusov a et al 2014).…”