Invasibility and Species Richness of a Community: A Neutral Model and a Survey of Published Data

Abstract

A widespread but controversial idea in ecology states that the number of invaders of a species assemblage depends on its species richness. Both negative and positive relationships have been reported. We examined whether a simple neutral model where assemblages are generated by drawing individuals from two pools of identical species (native and alien) can predict this relationship. We performed a meta-analysis of published data on this relationship.The neutral model showed that in communities with low and fixed numbers of individuals, the relationship between the number of aliens and native species is strong and negative. This becomes weaker as the proportion of species from both pools already present in the community increases. The relationship between alien and native species richness becomes positive when the number of individuals is allowed to vary, because the richness depends on number of individuals and/or area sampled. The meta-analysis showed that scale-dependence of the relationship between alien and native species richness is universal and compatible with the neutral model. Unless more evidence is available to refute it, the relationship between native and alien species richness should be considered a result of a neutral processes due to constraints on the number of individuals in the community.

A Two‐Part Measure of Degree of Invasion for Cross‐Community Comparisons

“…We define invasibility as the degree to which an existing community is susceptible to new species' becoming established and persisting in it. There is no evidence that any community is saturated with species, especially at larger scales (Loreau 2000), or that the richness of exotic species in any community is at equilibrium (Davis et al 2000;Herben et al 2004). Thus, all communities are invasible to some degree (Williamson 1996) and a reasonable comparison of their invasibility may be made only when communities are under similar propagule pressure.…”

“…Nevertheless, owing to the lack of a standard definition, invasibility has been measured in many ways (Lonsdale 1999;Cleland et al 2004), and to date results of studies show great variation in the relationships between invasibility and various biotic and abiotic variables across scales, habitats, and geographic regions. Most researchers studying habitat invasibility have used the richness of exotic species present as the measure of community invasibility (e.g., Lonsdale 1999;Byers & Noonburg 2003;Herben et al 2004;Davies et al 2005), although some have used measures such as survivorship, establishment, density, size, and biomass (or cover) of individual or all exotic species in the community (e.g., Robinson et al 1995;Kennedy et al 2002;Von Holle & Simberloff 2005). Although these studies shed light on biodiversity patterns and species assembly rules, the variety of definitions and inconsistent measures that exist make comparisons among communities and studies difficult.…”

Biological invasions and the neutral theory

“…The null expectation for randomly assembled communities is, thus, the observed negative relationship between native and exotic species richness (Fridley et al, 2004). At a larger scale, and solving the paradox, null models produce a positive correlation between these variables (see Fridley et al, 2004;Stark et al, 2006) and provide a strong support to neutral theory predictions (Stohlgren et al, 2003;Herben et al, 2004but see Rejmánek, 2003.…”

“…The negative relationship between native biodiversity and invasion at local spatial scales apparently show that communities with high species richness are less susceptible to invasion and, thus, already invoked as important evidence against the neutral theory (see Tilman, 2004;Chase, 2005). Using null models, however, Herben et al (2004) and Fridley et al (2004) showed that the observed negative correlation is mathematically inevitable for communities with a fixed and small number of species (as species are either 'exotics' or 'natives'). The null expectation for randomly assembled communities is, thus, the observed negative relationship between native and exotic species richness (Fridley et al, 2004).…”

“…However, the invasion of natural communities by alien species, when considered as a large-scale experiment, can be used to contrast some predictions of neutral theory. Some data have been interpreted as evidence of incompatibilities between neutral theory and invasion ecology (Tilman, 2004;Chase, 2005), but the same data were also used to support the theory (Davis, 2003;Herben et al, 2004). Therefore, we explicitly developed neutral predictions and contrasted them with widespread ecological patterns shown in studies on biological invasions.…”