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Charles Darwin posited two alternative hypotheses to explain the success of nonnative species based on their relatedness to natives: nonnative species that are closely related to native species could experience (1) higher invasion success because of an increased probability of habitat suitability (conferred by trait similarity) or (2) lower invasion success due to biotic interference, such as competition and limiting similarity. The paradox raised by the opposing predictions of these two hypotheses has been termed \"Darwin's naturalization conundrum\" (DNC). Using plant communities measured repeatedly across an experimental fire gradient in an oak savanna (Minnesota, USA) over 31 yr, we evaluated the DNC by incorporating taxonomic, functional, and phylogenetic information. We used a \"focal-species\" approach, in which the taxonomic, functional, and phylogenetic structure of species co-occurring with a given nonnative (focal) species in local communities was quantified. We found three main results: first, nonnative species tended to co-occur most with closely related natives, except at the extreme ends of the fire gradient (i.e., in communities with no fire and those subjected to high fire frequencies); second, with increasing fire frequency, nonnative species were functionally more similar to native species in recipient communities; third, functional similarity between co-occurring nonnatives and natives was stable over time, but their phylogenetic similarity was not, suggesting that dynamic external forces (e.g., climate variability) influenced the phylogenetic relatedness of nonnatives to natives. Our results provide insights for understanding invasion dynamics across environmental gradients and highlight the importance of evaluating different dimensions of biodiversity in order to draw stronger inferences regarding species co-occurrence at different spatial and temporal scales.

Starry stonewort (Nitellopsis obtusa) is an alga that has emerged as an aquatic invasive species of concern in the United States. Where established, starry stonewort can interfere with recreational uses of water bodies and potentially have ecological impacts. Incipient invasion of starry stonewort in Minnesota provides an opportunity to predict future expansion in order to target early detection and strategic management. We used ecological niche models to identify suitable areas for starry stonewort in Minnesota based on global occurrence records and present-day and future climate conditions. We assessed sensitivity of forecasts to different parameters, using four emission scenarios (i.e., RCP 2.6, RCP 4.5, RCP 6, and RCP 8.5) from five future climate models (i.e., CCSM, GISS, IPSL, MIROC, and MRI). From our niche model analyses, we found that (i) occurrences from the entire range, instead of occurrences restricted to the invaded range, provide more informed models; (ii) default settings in Maxent did not provide the best model; (iii) the model calibration area and its background samples impact model performance; (iv) model projections to future climate conditions should be restricted to analogous environments; and (v) forecasts in future climate conditions should include different future climate models and model calibration areas to better capture uncertainty in forecasts. Under present climate, the most suitable areas for starry stonewort are predicted to be found in central and southeastern Minnesota. In the future, suitable areas for starry stonewort are predicted to shift in geographic range under some future climate models and to shrink under others, with most permutations indicating a net decrease of the species' suitable range. Our suitability maps can serve to design short-term plans for surveillance and education, while future climate models suggest a plausible reduction of starry stonewort spread in the long-term if the trends in climate warming remain.

To efficiently detect aquatic invasive species early in an invasion when control may still be possible, predictions about which locations are likeliest to be occupied are needed at fine scales but are rarely available. Occupancy modeling could provide such predictions given data of sufficient quality and quantity. We assembled a data set for the macroalga starry stonewort ( Nitellopsis obtusa ) across Minnesota and Wisconsin, USA, where it is a new and high-priority invader. We used these data to construct a multi-season, single-species spatial occupancy model that included biotic, abiotic, and movement-related predictors. Distance to the nearest access was an important occurrence predictor, highlighting the likely role boats play in spreading starry stonewort. Fetch and water depth also predicted occupancy. We estimated an average detection probability of 63% at sites with mean non- N. obtusa plant cover, declining to ~ 38% at sites with abundant plant cover, especially that of other Characeae. We recommend that surveyors preferentially search for starry stonewort in areas of shallow depth and high fetch close to boat accesses. We also recommend searching during late summer/early fall when detection is likelier. This study illustrates the utility of fine-scale occupancy modeling for predicting the locations of nascent populations of difficult-to-detect species.

Potamogeton crispus (curlyleaf pondweed) and Myriophyllum spicatum (Eurasian watermilfoil) are widely thought to competitively displace native macrophytes in North America. However, their perceived competitive superiority has not been comprehensively evaluated. Coexistence theory suggests that invader displacement of native species through competitive exclusion is most likely where high niche overlap results in competition for limiting resources. Thus, evaluation of niche similarity can serve as a starting point for predicting the likelihood of invaders having direct competitive impacts on resident species. Across two environmental gradients structuring macrophyte communities—water depth and light availability—both P. crispus and M. spicatum are thought to occupy broad niches. For a third dimension, phenology, the annual growth cycle of M. spicatum is typical of other species, whereas the winter-ephemeral phenology of P. crispus may impart greater niche differentiation and thus lower risk of native species being competitively excluded. Using an unprecedented dataset comprising 3404 plant surveys from Minnesota collected using a common protocol, we modeled niches of 34 species using a probabilistic niche framework. Across each niche dimension, P. crispus had lower overlap with native species than did M. spicatum; this was driven in particular by its distinct phenology. These results suggest that patterns of dominance seen in P. crispus and M. spicatum have likely arisen through different mechanisms, and that direct competition with native species is less likely for P. crispus than M. spicatum. This research highlights the utility of fine-scale, abundance-based niche models for predicting invader impacts.

Aim Biotic homogenization (BH), a reduction in the distinctness of species composition between geographically separated ecological communities in a region, is an important but underappreciated potential consequence of biological invasions. While BH theory has always considered invasions, it has generally been in a relatively narrow context that the cosmopolitan nature of invasive species increases BH because of their shared presence across many locations. We sought to evaluate this component of BH as well as broader effects of invasive species on BH through changes in native communities, including overall reductions in species richness or shifts in species composition. Location Minnesota, USA. Time period 2002–2014. Major taxa studied Aquatic macrophytes, including both vascular plants and attached macroalgae. Methods We used surveys of aquatic macrophyte communities from 1,102 shallow lakes in Minnesota, USA (including 248 lakes with repeated surveys) to evaluate relationships between invasion, native species and BH. Results We found that the presence of invasive species was associated with BH and that this pattern was reflected in both the total community (i.e., with invasive species included) and in the composition of the native species community alone. We found that invaded lakes were more compositionally similar to each other than uninvaded lakes, but that both groups were becoming more similar over time—despite neither group exhibiting declines in species richness. This pattern was largely driven by shifts in the native community itself, with common species becoming more widespread and rare species becoming rarer. Main conclusions Invasive species increase measures of community similarity through their own presence in multiple locations, and also by influencing the composition of native species. These patterns have important implications for conservation and management and suggest that BH should be considered more widely in evaluating the impacts of biological invasions and developing response strategies.

Understanding how the characteristics of native plant communities influence invasion is a pressing question, with implications for theory and management. For decades, the primary native community characteristic used in tests of biotic resistance was species richness. However, previous studies have demonstrated that evolutionary history and functional traits shape the invasion process, as ecological theory predicts. Theoretically, restoration projects would benefit from designing seed mixtures around maximizing resistance to invasion. However, there is little empirical evidence on the importance of evolutionary diversity for management and still less guidance for practitioners on effective application of ecological theories. We empirically tested how several native community characteristics (phylogenetic diversity, functional diversity, phylogenetic relatedness, and mean trait values) affected the survival of three introduced invasive species. We explored this question in experimentally restored 15‐species prairie plots with three levels of phylogenetic diversity and two levels of functional diversity. Our experiment also included monocultures of all native species, which were also experimentally invaded. We found evidence that phylogenetic diversity conferred biotic resistance against one invasive species, contributing to reduced biomass in models explaining up to 10% of variance. Tall species better suppressed invaders, with height explaining up to 27% of variation in invader biomass. Surprisingly, we found patterns in leaf and seed traits linked to invasion resistance which were associated with both conservative and resource‐acquisitive strategies. We also found evidence in both the diversity and monoculture plots that invaders were more successful with more closely related native species. Taken together, our results indicate that invasion resistance emerges from nuanced interactions between phylogenetic diversity, functional traits, and community composition, rather than from any single community characteristic. Our results underscore the complexity of biotic resistance and suggest that practitioners should prioritize phylogenetic diversity and strategic species selection when designing restoration plantings to enhance invasion resistance. This experiment tests the effects of phylogenetic and functional trait attributes on invasion resistance in experimentally restored tallgrass prairie plots. We found that more diverse plots with varied ecological strategies are more likely to suppress invaders.

Novel habitats can support biodiversity by amending what has been lost through urban development. However, the effects of fragmentation, disturbance, and altered availability of resources in cities can prevent many local plant species from establishing and persisting. Novel habitats like green roofs could be colonized by native plants if species could overcome particularly harsh environmental conditions. To do so, green roofs could be designed using a habitat analog approach wherein natural habitats with similar abiotic characteristics inform the potential species pool. In this study, we tested the efficacy of using a habitat analog approach to establish native plant communities on green roofs. We surveyed vegetation in 18 dry prairies of three subtypes (gravel hill, dolomite, and sand) and planted replicates of two communities (sand prairie and rock prairie, which combined gravel hill and dolomite) on green roofs to determine which prairie was the most suitable analog. We investigated the effect of three environmental variables on plant establishment, survival, and growth: soil continuity (continuous soil vs. isolated trays), planting method (seeds vs. pre‐grown seedlings), and the addition of native arbuscular mycorrhizal (AM) fungi. We determined the most suitable habitat analog and measured the effect of the environmental variables by conducting vegetation surveys in the experimental roof plots for three years. The experimental rock prairie communities more closely resembled the target than did the sand prairies, supporting the hypothesis that shared soil properties are important for establishing analogous plant communities in novel habitats. At the community level, survival and growth were higher in continuous soil, highlighting the importance of belowground components of constructed habitats. We found no effect of planting method or addition of native AM fungi on plant survival or growth. Overall, our results support using a habitat analog approach to select native species that support biodiversity in constructed novel habitats like green roofs.

1. Pollinator conservation is of increasing interest in the light of managed honeybee (Apis mellifera) declines, and declines in some species of wild bees. Much work has gone into understanding the effects of habitat enhancements in agricultural systems on wild bee abundance, richness and pollination services. However, the effects of ecological restoration targeting \"natural\" ecological endpoints (e.g. restoring former agricultural fields to historic vegetation types or improving degraded natural lands) on wild bees have received relatively little attention, despite their potential importance for countering habitat loss. 2. We conducted a meta-analysis to evaluate the effects of ecological restoration on wild bee abundance and richness, focusing on unmanaged bee communities in lands restored and managed to increase habitat availability and quality. Specifically, we assessed bee abundance and/or richness across studies comparing restored vs. unrestored treatments and studies investigating effects of specific habitat restoration techniques, such as burning, grazing, invasive plant removal and seeding. 3. We analysed 28 studies that met our selection criteria: these represented 11 habitat types and 7 restoration techniques. Nearly all restorations associated with these studies were performed without explicit consideration of habitat needs for bees or other pollinators. The majority of restorations targeted plant community goals, which could potentially have ancillary benefits for bees. 4. Restoration had overall positive effects on wild bee abundance and richness across multiple habitat types. Specific restoration actions, tested independently, also tended to have positive effects on wild bee richness and abundance. 5. Synthesis and applications. We found strong evidence that ecological restoration advances wild bee conservation. This is important given that habitat loss is recognized as a leading factor in pollinator decline. Pollinator responses to land management are rarely evaluated in non-agricultural settings and so support for wild bees may be an underappreciated benefit of botanically focused management. Future restoration projects that explicitly consider the needs of wild bees could be more effective at providing nesting, foraging and other habitat resources. We encourage land managers to design and evaluate restoration projects with the habitat needs of wild bee species in mind.

Plant invasions result in biodiversity losses and altered ecological functions, though quantifying loss of multiple ecosystem functions presents a research challenge. Plant phylogenetic diversity correlates with a range of ecosystem functions and can be used as a proxy for ecosystem multifunctionality. Laurentian Great Lakes coastal wetlands are ideal systems for testing invasive species management effects because they support diverse biological communities, provide numerous ecosystem services, and are increasingly dominated by invasive macrophytes. Invasive cattails are among the most widespread and abundant of these taxa. We conducted a three‐year study in two Great Lakes wetlands, testing the effects of a gradient of cattail removal intensities (mowing, harvest, complete biomass removal) within two vegetation zones (emergent marsh and wet meadow) on plant taxonomic and phylogenetic diversity. To evaluate native plant recovery potential, we paired this with a seed bank emergence study that quantified diversity metrics in each zone under experimentally manipulated hydroperiods. Pretreatment, we found that wetland zones had distinct plant community composition. Wet meadow seed banks had greater taxonomic and phylogenetic diversity than emergent marsh seed banks, and high‐water treatments tended to inhibit diversity by reducing germination. Aboveground harvesting of cattails and their litter increased phylogenetic diversity and species richness in both zones, more than doubling richness compared to unmanipulated controls. In the wet meadow, harvesting shifted the community toward an early successional state, favoring seed bank germination from early seral species, whereas emergent marsh complete removal treatments shifted the community toward an aquatic condition, favoring floating‐leaved plants. Removing cattails and their litter increased taxonomic and phylogenetic diversity across water levels, a key environmental gradient, thereby potentially increasing the multifunctionality of these ecosystems. Killing invasive wetland macrophytes but leaving their biomass in situ does not address their underlying mechanism of dominance and is less effective than more intensive treatments that also remove their litter. We conducted a 3‐year study in two Laurentian Great Lakes wetlands, testing the effects of dominant invasive plant removal techniques within two vegetation zones on plant taxonomic and phylogenetic diversity. Removal of invasive cattails and their litter increased phylogenetic diversity and species richness across water levels, more than doubling richness compared to unmanipulated controls, thereby potentially increasing the multifunctionality of these ecosystems.

Traits are important for understanding how plant communities assemble and function, providing a common currency for studying ecological processes across species, locations, and habitat types. However, the majority of studies relating species traits to community assembly rely upon vegetative traits of mature plants. Seed traits, which are understudied relative to whole‐plant traits, are key to understanding assembly of plant communities. This is particularly true for restored communities, which are typically started de novo from seed, making seed germination a critical first step in community assembly and an early filter for plant establishment. We experimentally tested the effects of seed traits (mass, shape, and embryo to seed size ratio) and phylogeny on germination response in 32 species commonly used in prairie grassland restoration in the Midwestern USA, analyzing data using time‐to‐event (survival) analysis. As germination is also influenced by seed dormancy, and dormancy break treatments are commonly employed in restoration, we also tested the effects of two pretreatments (cold stratification and gibberellic acid application) on time to germination. Seed traits, phylogeny, and seed pretreatments all affected time to germination. Of all traits tested, variables related to seed shape (height and shape variance) best predicted germination response, with high‐variance (i.e., pointier and narrower) seeds germinating faster. Phylogenetic position (the location of species on the phylogenetic tree relative to other tested species) was also an important predictor of germination response, that is, closely related species showed similar patterns in time to germination. This was true despite the fact that all measured seed traits showed phylogenetic signal, therefore phylogeny provided residual information that was not already captured by measured seed traits. Seed traits, phylogenetic position, and germination pretreatments were important predictors of germination response for a suite of species commonly used in grassland restoration. Shape traits were especially important, while mass, often the only seed trait used in studies of community assembly, was not a strong predictor of germination timing. These findings illustrate the ecological importance of seed traits that are rarely incorporated into functional studies of plant communities. This information can also be used to advance restoration practice by guiding restoration planning and seed mix design. We found that seed traits, phylogeny, and germination pretreatment were predictive of germination timing in 32 species commonly used in prairie restoration. While studies of community assembly typically use vegetative traits of mature plants, seed traits are key for understanding assembly, especially in restorations, which are often started from seed. Our findings highlight the need to incorporate evolutionary knowledge and traits of earlier life stages—beyond just seed mass—into community assembly research.