A meta-analysis of the ecological significance of density in tree invasions

Species richness, resource availability, and disturbance are the primary factors considered in assessing the invasibility of plant communities. Nonetheless, the density of individuals in a community is a common and easy trait to measure. The ecological significance of the density of both native and invasive tree species was assessed using a systematic review and formal meta-analysis. The densities of recipient communities and invasive exotic tree species in novel ranges were identified in the published literature. In addition, we compared by means of a meta-analysis: (i) densities of invasive versus native species in invaded communities; (ii) densities of native species in invaded versus uninvaded communities; and (iii) densities of invasive species along distance gradients from initial locus of invasion. Invasive trees were found at higher densities than native species in recipient communities. Invasions by woody species were also recorded in communities with relatively low densities of natives suggesting that (i) low density forests may be more susceptible to invasion and/or (ii) density of the recipient community may be reduced during the invasion process. In addition, comparison of native species densities between invaded and uninvaded stands from the same community suggests that invasive trees negatively affect density of native trees once established. Therefore, the widely reported low density and often richness of native plants in invaded communities cannot be directly linked to ecosystem susceptibility to invasion without considering concomitant impacts. These findings suggest that density is a key preliminary determinant or factor which should be considered when assessing tree invasion dynamics. (Levine and D’Antonio 1999) or fluctuating resources (Davis et al. 2000). Including environmental and ecological impacts of exotics on recipient communities in the definition of invasive species is still debated (Valéry et al. 2008) since measurement of impacts is difficult. Therefore, recent studies have focused on exotic tree impacts on recipient communities using changes in community diversity via exclusion of natives (e.g. Fang 2005), changes in the associated understorey diversity (e.g. Avalos et al. 2006), or changes in forest functions (e.g. Gómez-Aparicio et al. 2008; Martin et al. 2009). Mason and French (2008) suggested that the impact of an exotic woody species in natural systems is dependent on characteristics of the invaded community (vegetation structural density, richness of both native and exotic growth forms and community variability). However, exotic species do not automatically pose a threat to native biota (Goodenough 2010) and little is known about the impact of invasive trees on the density of native species from the recipient community. Comparing the density of native species for invaded and non-invaded patches from the same community in the novel range provides a relevant assessment of the impact of the invasive tree since more invaders should at some level at least lead to less native trees (e.g. Gareca et al. 2007). Given the increasing rate of invasion of exotic plants and the potential for density measurements of native and invasive species to explain invasion patterns, an enhanced understanding of the general trends associated with density and invasion by trees will not only increase the scope of generality of the theories explaining invasion, but further our capacity to predict and manage invasion in forests. As the number of studies published on this topic increases, synthetic treatment can help assess whether trends across studies are emerging and help refine studies of particular processes (Adams et al. 1997). Although narrative reviews are useful as summaries of the knowledge within a discipline, a quantitative and direct comparison of effect sizes between studies via meta-analysis provides the means to assess the evidence for a particular hypothesis (Gates 2002). Our primary purpose is thus to test the conceptual viability of density in understanding invasibility and impacts both of which are often blended and tested in various ways using the more specific hypotheses associated with the invasion literature. We used a comprehensive review of the literature to identify the density ranges of recipient communities and invasive exotic tree species in novel ranges and a formal meta-analysis to compare densities of invasive versus native species in invaded communities; densities of native species in invaded versus uninvaded stands from the same community; and densities of invasive species along distance of study site from initial locus of invasion. Specifically, we addressed the following questions: (1) Are papers studying biological invasion always focused on species clearly defined as invasive, i.e. with a high density relative to native species in the recipient community or is the definition based on impact? (2) Do invaded communities have low density of native species? (3) Is the density of native species lower in invaded than in non-invaded patches from the same community? (4) Are densities measured along the colonization front relevant to infer extent of invasion?