Hannah E. Marx’s research while affiliated with University of New Mexico and other places

Phylogenetic and functional diversity are theorised to increase invasion resistance. Experimentally testing whether plant communities higher in these components of diversity are less invasible is an important step for guiding restoration designs. To investigate how phylogenetic and functional diversity of vegetation affect invasion resistance in a restoration setting, we used experimental prairie restoration plots. The experiment crossed three levels of phylogenetic diversity with two levels of functional diversity while species richness was held constant. We allowed invaders to colonise plots; these included native species from neighbouring plots and non‐native invasive species from a surrounding old field. We tested if invader biomass was influenced by phylogenetic and functional diversity, and phylogenetic and hierarchical trait distances between invaders and planted species. We binned each invader into three categories: native species from neighbouring experimental plots (site‐specific invaders), native species not part of the experimental species pool (native invaders) or non‐native species (non‐native invaders). Counter to expectation, both non‐native and native invaders became more abundant in more phylogenetically diverse plots. However, plots with higher abundance of planted Asteraceae, a dominant family of the tallgrass prairie, had lower invader biomass for both native and non‐native invaders. We also found that hierarchical trait differences shaped invasion. The species that became most abundant were non‐native invaders that were taller, and native invaders with low specific leaf area relative to planted species. Site‐specific invaders were not influenced by any plot‐level diversity metrics tested. Synthesis and application: Our results suggest that greater phylogenetic diversity may lower resistance to invasion. This effect may be due to more even but sparser niche packing in high‐diversity plots, associated with greater availability of unsaturated niche space for colonisation. However, trait composition fostered invasion resistance in two ways in our study. First, establishment of native species with strongly dominant traits may confer invasion resistance. Second, species mixes that optimise trait differences between planted vegetation and likely invaders may enhance invasion‐resistance.

The prediction that higher biodiversity leads to denser niche packing and thus higher community resistance to invasion has long been studied, with species richness as the predominant measure of diversity. However, few studies have explored how phylogenetic and functional diversity, which should represent niche space more faithfully than taxonomic diversity, influence community invasibility, especially across longer time frames and over larger spatial extents. We used a 15‐year, 150‐site grassland dataset to assess relationships between invasive plant abundance and phylogenetic, functional and taxonomic diversity of recipient native plant communities. We analysed the dataset both pooled across all surveys and longitudinally, leveraging time‐series data to compare observed patterns in invasion with those predicted by two community assembly processes: biotic resistance and competitive exclusion. We expected more phylogenetically and functionally diverse communities to exhibit greater resistance to invasion. With the pooled dataset, we found support for the long‐standing observation that communities with more native species have lower abundance of invasive species, and a more novel finding that more phylogenetically diverse communities had higher abundance of invasive species. We found no influence of aggregate (multivariate) functional diversity on invasion, but assemblages with taller plants, lower variability in plant height and lower seed mass were less invaded. Viewed longitudinally, the phylogenetic diversity relationship was reversed: the most phylogenetically diverse communities were most resistant to invasion. This apparent discrepancy suggests invasion dynamics are influenced by both site attributes and biotic resistance and emphasizes the value in studying invasion across time. Synthesis. Our results provide insight into the nuances of the diversity–invasibility relationship: invasion dynamics differed for different dimensions of diversity and depending on whether the relationship was evaluated longitudinally. Our findings highlight the limitations of using single time‐point ‘snapshots’ of community composition to infer invasion mechanisms.

Background Plant–pollinator community diversity has been found to decrease under conditions of drought stress; however, research into the temporal dimensions of this phenomenon remains limited. In this study, we investigated the effect of seasonal drought on the temporal niche dynamics of entomophilous flowering plants in a water-limited ecosystem. We hypothesized that closely related native and exotic plants would tend to share similar life history and that peak flowering events would therefore coincide with phylogenetic clustering in plant communities based on expected phenological responses of plant functional types to limitations in soil moisture availability. Location Galiano Island, British Columbia, Canada. Methods Combining methods from pollinator research and phylogenetic community ecology, we tested the influence of environmental filtering over plant community phenology across gradients of landscape disturbance and soil moisture. Floral resource availability and community structure were quantified by counts of flowering shoots. We constructed a robust phylogeny to analyze spatial and temporal variation in phylogenetic patterns across the landscape, testing the significance of the observed patterns against a randomly generated community phylogeny. Phylogenetic metrics were then regressed against factors of disturbance and soil moisture availability. Results Critical seasonal fluctuations in floral resources coincided with significant phylogenetic clustering in plant communities, with decreasing plant diversity observed under conditions of increasing drought stress. Exotic plant species in the Asteraceae became increasingly pervasive across the landscape, occupying a late season temporal niche in drought-stressed environments. Main conclusion Results suggest that environmental filtering is the dominant assembly process structuring the temporal niche of plant communities in this water-limited ecosystem. Based on these results, and trends seen elsewhere, the overall diversity of plant–pollinator communities may be expected to decline with the increasing drought stress predicted under future climate scenarios.