Patterns of plant invasion along an environmental stress gradient

Abstract

Question: Do stressful environments facilitate plant invasion by providing refuges from intense above-ground competition associated with productive areas, or prevent it by favouring locally adapted native species? Location: An invaded and fragmented oak savanna ecosystem structured along a landscape-level stress gradient associated with soil depth, elevation, and canopy openness. Methods: Vegetation and environmental data were collected from 184 plots in seven savanna remnants along the gradient. Using multivariate (CCA) and post-hoc regression analyses, we determined the relationship between environment and the richness and abundance of invasives. Results: 46 of 119 species were naturalized exotics. CCA indicated the importance of environmental variation (mostly soil depth) for community structure but not for invasion; invasive species richness was similar in all areas. However, the abundance of invasives and their impacts on native diversity appear to increase significantly in less stressful habitats. Deeper soils had lower evenness and significantly fewer native species. This result was associated with dominance by exotic perennial grasses and large increases in vegetation height, suggesting strong above-ground competition. Conclusions: Low-stress environments were not more invasible per se but appear to be more susceptible to invasion by species with strong competitive impacts. The causes of decreasing exotic impact with decreasing soil depth may reflect shifts in competitive intensity or an increased importance of stress tolerance, both of which may favour natives. Alternatively, this ecosystem may simply lack high-impact invaders capable of dominating shallow soils. Conservation challenges are twofold for this endangered plant community: controlling invasives that currently dominate deeper-soils and accounting for a diverse pool of invaders that proliferate when the current dominants are removed.

New Arrivals: An Indicator for Non-indigenous Species Introductions at Different Geographical Scales

“…However, in contrast to most indicators of human impacts, the Cumulative number of NIS fails to show a direct correlation with environmental degradation gradients (MacDougall et al, 2006). Whether, or not, NIS become established is only partly related to the environmental status of an area; and it also depends on biological traits of the species (e.g., Cardeccia et al, in press), integrity of native ecosystems (Didham et al, 2005) and availability of resources (Davis, 2009).The "...Cumulative number of NIS..." as such, is of lesser importance for management than the "...Number of species transferred by a vector(s)..., " which aids any practical prioritization of preventive measures.…”

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

“…Nevertheless, owing to the ethically unpalatable possibility of introducing potentially harmful species into ecosystems when conducting field experiments, DI is the best indicator of invasibility available for most systems. Indeed, it is the only indicator available for observational studies investigating patterns across varying environmental conditions and it has been used extensively (e.g., Lonsdale 1999;MacDougall et al 2006). …”

“…For example, in a fragmented oak savanna ecosystem, the species richness of exotics does not differ across a gradient of growing conditions, but the abundance of exotics increases significantly with decreasing stress (e.g., higher soil moisture; MacDougall et al 2006). In riparian vegetation in France, covers of native and exotic species are negatively correlated, but their richness values are positively correlated (Tabacchi & PlantyTabacchi 2005).…”

Life history variation and demography of the invasive plant Eschscholzia californica Cham. (Papaveraceae), in two altitudinal extremes, Central Chile

“…For native plants, it has been documented that later stages of life cycle (juveniles-adults) are sensitive at higher altitude because they are largely exposed to physical disturbances, water stress and/or the snow covering (Cavieres & Arroyo 1999), thus decreasing plant performance and reproduction (Körner 2003, Johnston & Pickering 2004, Becker et al 2005, MacDougall et al 2006. The demography of E. californica, suggests that this is not the case, and the fate of seeds within the seed bank seems to be critical for the population outcome.…”