The argument that the threat posed by introduced species is overblown is often buttressed by the observation that native species sometimes also become invasive. An examination of the literature on plant invasions in the United States shows that six times more nonnative species have been termed invasive than native species, and that a member of the naturalized nonnative pool is 40 times more likely than a native species to be perceived as invasive. In the great majority of instances in which a native plant species is seen as invasive, the invasion is associated with an anthropogenic disturbance, especially changed fire or hydrological regime, livestock grazing, and changes wrought by an introduced species. These results suggest that natives are significantly less likely than nonnatives to be problematic for local ecosystems.

BackgroundIn the emerging field of community and ecosystem genetics, genetic variation and diversity in dominant plant species have been shown to play fundamental roles in maintaining biodiversity and ecosystem function. However, the importance of intraspecific genetic variation and diversity to floral abundance and pollinator visitation has received little attention.Methodology/Principal FindingsUsing an experimental common garden that manipulated genotypic diversity (the number of distinct genotypes per plot) of Solidago altissima, we document that genotypic diversity of a dominant plant can indirectly influence flower visitor abundance. Across two years, we found that 1) plant genotype explained 45% and 92% of the variation in flower visitor abundance in 2007 and 2008, respectively; and 2) plant genotypic diversity had a positive and non-additive effect on floral abundance and the abundance of flower visitors, as plots established with multiple genotypes produced 25% more flowers and received 45% more flower visits than would be expected under an additive model.Conclusions/SignificanceThese results provide evidence that declines in genotypic diversity may be an important but little considered factor for understanding plant-pollinator dynamics, with implications for the global decline in pollinators due to reduced plant diversity in both agricultural and natural ecosystems.

Summary 1. Biological invasions threaten biodiversity, and understanding the factors that influence a community’s susceptibility to invasion informs both management of invasive species and conservation efforts towards promoting biodiversity. 2. In this study, we examined the native–exotic richness relationships (NERR) at two spatial scales and asked what variables mediate the relationship between native and exotic species richness in 1‐m2 plots among old fields. We also tested whether the favourable environment and spatial heterogeneity mechanisms alter the relationship between native and exotic richness. Additionally, we identified biotic, abiotic and landscape factors that accounted for patterns of exotic species richness across spatial scales, from 50‐m transects (i.e. local scales) to entire old fields (i.e. landscape scales). 3. We found that native and exotic richness were positively related at both local and landscape spatial scales, with the strength of their relationship greater at the landscape scale. In old‐field communities with lower foliar cover, native and exotic plant richness were negatively related across 1‐m2 plots, whereas in old fields with higher foliar cover, native and exotic plant richness were positively related across 1‐m2 plots. Overall, predictors of exotic species richness depended on spatial scale: at local scales, native plant richness, light and landscape factors accounted for most of the variation, but biotic factors alone accounted for most of the variation at the landscape scale. 4. Our findings suggest that fields favourable for native species are also suitable for exotic species, and management efforts towards exotic richness should be tailored to spatial scale.

No abstract

Understanding how ecological communities change over time is critical for biodiversity conservation, but few long‐term studies directly address decadal‐scale changes in both the within‐ and among‐community components of diversity. In this study, we use a network of permanent forest vegetation plots, established in Great Smoky Mountains National Park (USA) in 1978, to examine the factors that influence change in community composition within and among communities. In 2007, we resampled 15 plots that were logged in the late 1920s and 15 plots that had no documented history of intensive human disturbance. We found that understory species richness decreased by an average of 4.3 species over the 30‐yr study period in the logged plots, but remained relatively unchanged in the unlogged plots. In addition, tree density decreased by an average of 145 stems ha−1 in the logged plots, but was relatively stable in the unlogged plots. However, we found that historic logging had no effect on within‐community understory or tree compositional turnover during this time period. Instead, sites at lower elevations and sites with lower understory biomass in 1978 had higher understory compositional turnover than did sites at higher elevations and sites with higher understory biomass. In addition, sites with lower soil cation exchange capacity (CEC) and with lower tree basal area in 1978 had higher tree compositional turnover than did sites with higher soil CEC and higher tree basal area. Among‐community similarity was unchanged from 1978 to 2007 for both the logged and unlogged plots. Overall, our results indicate that human disturbance can affect plant communities for decades, but the extent of temporal change in community composition may nevertheless depend more on environmental gradients and community attributes.