Stacy B. Endriss

Spread of non‐native species can be important drivers of biodiversity declines, leading to precautionary management based on assumptions that (1) non‐native biota have negative impacts and are “guilty” of causing harm and (2) reducing a non‐native species' abundance will reduce these negative impacts, in turn, benefiting native species. However, we frequently lack data to gauge both negative impacts of non‐native species and success or failure of chosen management interventions to benefit native species. Addressing these knowledge gaps is critical to improving management outcomes for native species while maintaining public trust to sustain funding of management activities. Here, we investigated the response of Lythrum salicaria (purple loosestrife) and associated plant communities to implementation of biological control in more than 10 wetland sites in New York State for up to 28 years. Introduced to North America from Europe in the 1800s, L. salicaria is a prime example of a non‐native species with a continent‐wide distribution that could not be suppressed by mechanical and chemical treatments. In the 1980s, waterfowl biologists, wetland managers, and conservationists alike worried about the loss of diverse wetland plant communities associated with the rapid expansion of L. salicaria. In response, after careful assessments of safety, and potential costs and benefits, four highly host‐specific insect herbivores were released in North America in the early 1990s to reduce L. salicaria abundance and its negative ecological impacts. In a companion paper, Blossey et al. documented reduced L. salicaria occupancy and stem densities following insect releases over time (i.e., biological success), irrespective of site‐specific differences in starting plant communities or L. salicaria abundance. Here, we show that reduced abundance of L. salicaria leads to the ultimate goal of non‐native plant management: increased cover, abundance, and diversity of species, often of native species (i.e., ecological success). We also conduct analyses to provide inference about which plant species are most sensitive to L. salicaria, including changes in L. salicaria stem density. Overall, we provide an important conservation success story: our findings emphasize that biocontrol of non‐native plants can be effective and safe, allowing native species to recover as a dominant non‐native species gradually declines.

While successful plant invasions often occur in novel environments, invasive species usually occupy broad niches within their native and introduced ranges. A better understanding of the process of invasion therefore requires a wide sampling of ranges, and a good knowledge of introduction history. We tested for differentiation in herbivore resistance among 128 introduced (European, North American) and native (Chinese, Japanese) populations of the invasive Japanese knotweed Reynoutria japonica in two common gardens in the native range: one in Shanghai and the other in Yunnan. In both common gardens, we found that herbivore resistance of plants from introduced populations differed from that from native populations in China but not from native populations in Japan, the putative source of introduction. Compared to native Chinese populations, plants from native Japanese populations and introduced European and North American populations had thicker leaves in both common gardens, and a lower C:N ratio but higher flavonoids content in the Shanghai garden. Variation in herbivore resistance was more strongly associated with climate of collecting sites for populations from the native range than for those from introduced ranges. Our results support the hypothesis that introduction of particularly resistant plants from Japan may have played a key role in driving biogeographic variation in herbivore resistance. Our study highlights the importance of understanding introduction history to interpret the biogeographic divergence of global plant invaders.

Summary: Many widespread invasive plant species express high phenotypic variation across novel environments, providing a unique opportunity to examine ecological and evolutionary dynamics under global change. However, studies often lack information about the origin of introduced populations, limiting our understanding of post‐introduction evolution. We assessed the responses of Reynoutria japonica from 128 populations spanning latitudinal transects in the native (China and Japan), and introduced (North America and Europe) ranges when grown in two common gardens. Plants from introduced populations differed in almost all traits from those from Chinese populations, but were similar to plants from the putative origin in Japan. Compared to Chinese populations, North American, European and Japanese populations expressed lower trait values and plasticity in most traits. However, plants from both introduced and Japanese populations expressed higher clonality and plasticity in clonality than plants from Chinese populations. Further, introduced populations expressed higher plasticity in clonality but lower plasticity in basal diameter compared to Japanese populations. Our findings emphasize the potential role of clonality and plasticity in clonality for invasion success. In addition, our study highlights the importance of comparisons to source populations within the native range to identify evolutionary responses of introduced plants to novel environments.

Introduction and spread of non‐native plants provide ecologists and evolutionary biologists with abundant scientific opportunities. However, land managers charged with preventing ecological impacts face financial and logistical challenges to reduce threats by introduced species. The available toolbox (chemical, mechanical, or biological) is also rather limited. Failure to permanently suppress introduced species by mechanical and chemical treatments may result in biocontrol programs using host‐specific insect herbivores. Regardless of the chosen method, long‐term assessment of management outcomes on both the target species and associated biota should be an essential component of management programs. However, data to assess whether management results in desirable outcomes beyond short‐term reductions of the target plant are limited. Here, we use implementation of a biocontrol program targeting a widespread wetland invader, Lythrum salicaria (purple loosestrife), in North America to track outcomes on the target plant over more than two decades in New York State. After extensive testing, two leaf‐feeding beetles (Galerucella calmariensis and Galerucella pusilla; hereafter “Galerucella”), a root‐feeding weevil (Hylobius transversovittatus) and a flower‐feeding weevil (Nanophyes marmoratus), were approved for field releases. We used a standardized monitoring protocol to record insect abundance and L. salicaria stem densities and heights in 1‐m² permanent quadrats at 33 different wetlands and followed sites for up to 28 years. As part of this long‐term monitoring, in 20 of these wetlands, we established a factorial experiment releasing either no insects (control), only root feeders, only leaf beetles, or root and leaf feeders. We documented reduced L. salicaria occupancy and stem densities following insect releases over time, irrespective of site‐specific differences in starting plant communities or L. salicaria abundance. We could not complete our factorial experiment because dispersal of leaf beetles to root‐feeder‐only and control sites within 5 years invalidated our experimental controls. Our data show that it took time for significant changes to occur, and short‐term studies may provide misleading results, as L. salicaria stem densities initially increased before significantly decreasing. Several decades after insect releases, prerelease predictions of significant purple loosestrife declines have been confirmed.

Many widespread invasive plant species express high phenotypic variation across novel environments, providing a unique opportunity to examine ecological and evolutionary dynamics under global change. However, biogeographical studies often lack information about the origin of introduced populations, limiting our understanding of post-introduction evolution of introduced species in the new range. Here, we assessed the responses of Reynoutria japonica from 128 populations spanning three latitudinal transects in the native ranges of China and Japan, and the introduced ranges of North America and Europe. When grown in two common gardens in the native range, plants from introduced populations in North America and Europe differed in almost all traits from those from native Chinese populations, but were similar to plants from native Japanese populations. Compared to Chinese populations, North American, European and Japanese populations expressed lower trait values and plasticity in most traits. However, plants from both introduced ranges and from Japanese populations expressed higher clonality and plasticity in clonality than plants from Chinese populations. In addition, introduced populations expressed higher plasticity in clonality but lower plasticity in basal diameter compared to Japanese populations. Our study showed heritable differences in phenotypes between plants from the native ranges of Japan and China, and those from the introduced ranges were similar to the putative source of origin in Japan. However, we found that introduced populations may have evolved higher plasticity in clonal growth. Our findings emphasize the critical role of clonality and plasticity in invasion success, demonstrating the importance of discriminating between source and non-source native populations to identify ecological and evolutionary responses of invasive plants to novel environments.

Introduced plants create lasting abiotic and biotic changes to soil that are increasingly implicated as major drivers of invasion success, yet models for investigating patterns in the impacts of these soil legacies are lacking. Here, we used Bohemian knotweed (Reynoutria × bohemica) to present a novel approach for testing whether accounting for diverse evolutionary drivers can clarify how an invader's soil legacy mediates its success. For most tested species, R. × bohemica's soil legacy did not influence seedling performance irrespective of its evolutionary history or the evolutionary novelty of species that share its soil. However, we found the relative importance of legacy effects versus other mechanisms of invader dominance depends on the life stage of the plants growing in soil conditioned by the invader. While the effects of R. × bohemica's soil legacy on seedling performance were context dependent, emergence was, on average, higher in R. × bohemica‐conditioned soils than in unconditioned soils. Thus, contrary to theory, R. × bohemica's soil legacy not only fails to impede potential competitors but may also facilitate them. Our results are particularly powerful: we detected novel insights in the face of biological complexity, even for R. × bohemica, which likely does not depend on heterospecific plant–soil feedbacks for its competitive dominance in its introduced range. Designing rigorous investigations that explore how plant species' evolutionary histories mediate an invader's soil legacy across diverse abiotic environments, as shown here, is key for producing insights in other systems.

Land managers spend substantial resources managing invasive species to mitigate their negative ecological and economic impacts. However, invasive species management is seldom coupled with empirical assessments of management outcomes or the ecological impacts of the target species. Additionally, the study of invasive species has, until recently, neglected to acknowledge co‐stressors on native communities, which often are the drivers of ecological degradation. We assessed the effects of the invasive plant Vincetoxicum rossicum and its removal on understory plant communities, as well as whether these effects were mediated by deer and invasive earthworms. We used paired fenced and open plots with V. rossicum and an open plot without V. rossicum at each of the three forested sites in New York State Parks from 2017 to 2020. Following a nested design, we located paired sets of open and fenced plots in areas where V. rossicum was experimentally removed and in areas where it was unmanaged. After three years of V. rossicum removal, V. rossicum significantly declined. However, the ultimate goal of management was native plant recovery. Contrary to this aim, native plant diversity and cover increased if V. rossicum was left unmanaged but did not change with V. rossicum removal. Thus, we provide strong evidence that reducing target species abundance does not always translate into native plant recovery. This disparity may be because deer and invasive earthworms are stronger drivers of understory plant communities than V. rossicum, as native plant cover and diversity were lower the more earthworms were in a plot and invasive plant species declined after fencing to exclude deer. Management that prioritizes reducing deer and earthworm impacts over invasive plant removal is therefore worth exploring, especially when coupled with continued monitoring to assess if these actions result in the desired management outcomes. Overall, our findings underscore the need to collect data to test management assumptions: reducing the abundance of an invader may not always result in native plant recovery, especially if other stressors are the ultimate drivers of change in invaded communities.

Development of a biological control program for invasivePhagmites australis australisin North America required 20 years of careful research, and consideration of management alternatives. A recent paper by Kiviat et al. (Biol Invasions 21:2529–2541, 2019.10.1007/s10530-019-02014-9) articulates opposition to this biocontrol program and questions the ethics and thoroughness of the researchers. Here we address inaccuracies and misleading statements presented in Kiviat et al. (2019), followed by a brief overview of why biological control targetingPhragmitesin North America can be implemented safely with little risk to native species. Similar to our colleagues, we are very concerned about the risks invasivePhragmitesrepresent to North American habitats. But to protect those habitats and the species, includingP. australis americanus, we come to a different decision regarding biological control. Current management techniques have not been able to reverse the invasiveness ofP. australis australis, threats to native rare and endangered species continue, and large-scale herbicide campaigns are not only costly, but also represent threats to non-target species. We see implementation of biocontrol as the best hope for managing one of the most problematic invasive plants in North America. After extensive review, our petition to release two host specific stem miners was approved by The Technical Advisory Group for the Release of Biological Control Agents in the US and Canadian federal authorities.

Dispersal is a key ecological process that is strongly influenced by both phenotype and environment. Here, we show that juvenile environment influences dispersal not only by shaping individual phenotypes, but also by changing the phenotypes of neighbouring conspecifics, which influence how individuals disperse. We used a model system (Tribolium castaneum, red flour beetles) to test how the past environment of dispersing individuals and their neighbours influences how they disperse in their current environment. We found that individuals dispersed especially far when exposed to a poor environment as adults if their phenotype, or even one‐third of their neighbours’ phenotypes, were shaped by a poor environment as juveniles. Juvenile environment therefore shapes dispersal both directly, by influencing phenotype, as well as indirectly, by influencing the external social environment. Thus, the juvenile environment of even a minority of individuals in a group can influence the dispersal of the entire group.