Plant functional group identity and diversity determine biotic resistance to invasion by an exotic grass

Abstract

Summary1. Biotic resistance, the ability of species in a community to limit invasion, is central to our understanding of how communities at risk of invasion assemble after disturbances, but it has yet to translate into guiding principles for the restoration of invasion-resistant plant communities. We combined experimental, functional, and modelling approaches to investigate processes of community assembly contributing to biotic resistance to an introduced lineage of Phragmites australis, a model invasive species in North America. We hypothesized that (i) functional group identity would be a good predictor of biotic resistance to P. australis, while species identity effect would be redundant within functional group (ii) mixtures of species would be more invasion resistant than monocultures. 2. We classified 36 resident wetland plants into four functional groups based on eight functional traits. We conducted two competition experiments based on the additive competition design with P. australis and monocultures or mixtures of wetland plants. As an indicator of biotic resistance, we calculated a relative competition index (RCI avg ) based on the average performance of P. australis in competition treatment compared with control. To explain diversity effect further, we partitioned it into selection effect and complementarity effect and tested several diversity-interaction models. 3. In monoculture treatments, RCI avg of wetland plants was significantly different among functional groups, but not within each functional group. We found the highest RCI avg for fast-growing annuals, suggesting priority effect. 4. RCI avg of wetland plants was significantly greater in mixture than in monoculture mainly due to complementarity-diversity effect among functional groups. In diversity-interaction models, species interaction patterns in mixtures were described best by interactions between functional groups when fitted to RCI avg or biomass, implying niche partitioning. 5. Synthesis. Functional group identity and diversity of resident plant communities are good indicators of biotic resistance to invasion by introduced Phragmites australis, suggesting niche preemption (priority effect) and niche partitioning (diversity effect) as underlying mechanisms. Guiding principles to understand and/or manage biological invasion could emerge from advances in community theory and the use of a functional framework. Targeting widely distributed invasive plants in different contexts and scaling up to field situations will facilitate generalization.

Management of invasive Phragmites australis in the Adirondacks: a cautionary tale about prospects of eradication

“…Large populations may have also reached local limits of available wetland habitat within the forest matrix despite [240,000 ha of wetlands in the Adirondacks. We cannot exclude the possibility that biotic resistance by wetland plants (Sakai et al 2001;Levine et al 2004;Byun et al 2013;Zenni and Nuñez 2013), native herbivores (Parker and Hay 2005), or even negative soil feedback (van der Putten et al 2013) are contributing to reduced local and regional P. australis expansion.…”

Bird assemblage vulnerability depends on the diversity and biogeographic histories of islands

“…resistance to invasion (7)(8)(9)(10), and resistance to pathogen spread (11), this prediction is complicated by decreased populationlevel stability of individual species within diverse communities (30)(31)(32)(33)(34). In order for diverse islands to accumulate vulnerable species, the beneficial effects of ecosystem stability and increased ecosystem function that come with greater diversity must outweigh the costs of increased species-level instability as a result of high diversity.…”

“…Biodiversity is widely acknowledged to significantly influence the magnitude and stability of a large array of ecosystem properties, with stronger impacts evident when larger scales (1) and multiple functions are considered (2,3). These findings provide compelling support for biodiverse systems being functionally robust, with greater stability of function (4)(5)(6), resistance to invasion (7)(8)(9)(10), and resistance to pathogen spread (11). Some of the impacts of higher levels of biodiversity on ecosystem properties that are apparent at ecological timescales (e.g., greater resistance to invasion) may influence evolutionary processes, like extinction rates.…”

Extreme climate events and wet grasslands: plant traits for ecological resilience

“…Ecosystem functions related to biogeochemical processes, ecosystem services, resilience to disturbance and longterm stability in a community have all been closely allied to functional diversity (Loreau, 2000;Hooper et al, 2005;Díaz et al, 2007;Byun et al, 2013;Butterfield & Suding, 2013). This indicates that plant traits and functional groups are a powerful tool for assessing community responses to, and effects of, perturbation such as through extreme climate events.…”