Theory predicts that native plant species should exhibit latitudinal gradients in the strength of their interactions with herbivores. We hypothesize that if an invasive plant species exhibits a different latitudinal gradient in response to herbivores (e.g., a nonparallel gradient), it can create large-scale heterogeneities in community resistance/susceptibility to the invasive species. We conducted a study of latitudinal variation in the strength of herbivory and defenses of native genotypes of Phragmites australis in North America (NA) and Europe (EU) and European invasive genotypes in NA. Within NA, we tested whether (1) invasive genotypes are better defended and suffer less herbivory than co-occurring native genotypes, (2) herbivory and defenses of native P. australis decreases with increasing latitude; and (3) invasive genotypes exhibit either no latitudinal gradient, or a nonparallel latitudinal gradient in herbivory and defenses compared to native genotypes. For the European genotypes, we tested two additional hypotheses: (4) defenses, nutritional condition, and herbivory would differ between the native (EU) and invasive ranges (NA) and (5) latitudinal gradients in defenses and herbivory would be similar between ranges. Within NA, chewing damage, internal stem-feeding incidence, and aphid abundance were 650%, 300%, and 70% lower, respectively, on invasive than native P. australis genotypes. Genotypes in NA also differed in nutritional condition (percent N, C:N ratio), but there was little support for invasive genotypes being better defended than native genotypes. For the European genotypes, herbivory was significantly lower in the invaded than native range, supporting the enemy-release hypothesis. Defense levels (leaf toughness and total phenolics) and tissue percent C and percent N were higher in the invaded than native range for European genotypes. Overall, latitudinal gradients in P. australis nutritional condition, defenses, and herbivory were common. Interestingly, chewing damage and stem-feeder incidence decreased with latitude for native P. australis genotypes in NA and EU, but no latitudinal gradients in response to herbivores were evident for invasive genotypes in NA. Nonparallel latitudinal gradients in herbivory between invasive and native P. australis suggest that the community may be more susceptible to invasion at lower than at higher latitudes. Our study points to the need for invasion biology to include a biogeographic perspective.

The juxtaposition of plant‐species invasions with latitudinal gradients in herbivore pressure is an important yet mostly unexplored issue in invasion biology. Latitudinal clines in defense and palatability to herbivores are expected to exist in native plant species but the evolution of these clines may lag behind for invasive plant species resulting in non‐parallel latitudinal clines that may impact invasion success. Our study focused on a native and European invasive lineages of the common reed Phragmites australis in North America. Using native and invasive genotypes of P. australis collected across a 17° latitudinal range, we performed experiments in replicate northern and southern common gardens to investigate whether these two lineages exhibited different genetically based latitudinal clines in defenses, nutritional condition, and palatability to their herbivores, the aphid Hyalopterus pruni and the fall armyworm Spodoptera frugiperda. We also tested whether invasive genotypes are more phenotypically plastic than native genotypes and whether plasticity varies with latitude. Although invasive genotypes did not exhibit higher defense levels (leaf toughness, phenolics, percent carbon), they were considerably less palatable to their herbivores than native genotypes. Genetically based latitudinal clines were evident for both native and invasive P. australis and for all defenses, nutrients, and at least one palatability trait for each herbivore. In 36% of the cases where clines were evident, they were non‐parallel between the two lineages. These data suggest that clines in the invasive genotypes of P. australis evolved within the past ~100 years. Moreover, our study showed that the occurrence and direction of latitudinal clines in plant traits were commonly dependent on where the study was conducted (north or south), indicating strong phenotypic plasticity in these genetic‐based clines. Finally, traits for invasive genotypes of P. australis were 2.5 times more plastic than traits for native genotypes. Interestingly, plasticity for native but not invasive genotypes was strongly dependent on latitude of origin. Such spatial heterogeneity within and between the native and invasive lineages of P. australis with respect to their interactions with herbivores can generate substantial spatial variability in biotic resistance that can have important implications for the establishment and spread of invasive genotypes and species.

Plant–microbe interactions play crucial roles in species invasions but are rarely investigated at the intraspecific level. Here, we study these interactions in three lineages of a globally distributed plant, Phragmites australis. We use field surveys and a common garden experiment to analyze bacterial communities in the rhizosphere of P. australis stands from native, introduced, and Gulf lineages to determine lineage-specific controls on rhizosphere bacteria. We show that within-lineage bacterial communities are similar, but are distinct among lineages, which is consistent with our results in a complementary common garden experiment. Introduced P. australis rhizosphere bacterial communities have lower abundances of pathways involved in antimicrobial biosynthesis and degradation, suggesting a lower exposure to enemy attack than native and Gulf lineages. However, lineage and not rhizosphere bacterial communities dictate individual plant growth in the common garden experiment. We conclude that lineage is crucial for determination of both rhizosphere bacterial communities and plant fitness.

Understanding and predicting biological invasions is challenging because of the complexity of many interacting players. A holistic approach is needed with the potential to simultaneously consider all relevant effects and effectors. Using networks to describe the relevant anthropogenic and ecological factors, from community-level to global scales, promises advances in understanding aspects of invasion from propagule pressure, through establishment, spread, and ecological impact of invaders. These insights could lead to development of new tools for prevention and management of invasions that are based on species' network characteristics and use of networks to predict the ecological effects of invaders. Here, we review the findings from network ecology that show the most promise for invasion biology and identify pressing needs for future research. Scaling up to a Network Approach in Invasion BiologyUnderstanding and predicting biological invasions and their impacts is a huge challenge in ecology that will become more important as the homogenization of Earth's biota increases [1]. Invasion biology's ability to predict invasions and their impacts has been limited by the lack of theoretical frameworks that can incorporate and quantify the formidable ecological complexity of direct and indirect species interactions over multiple trophic levels [2]. Ecological networks are a framework for holistic consideration of whole sets of organisms (nodes) (see Glossary) (usually species, individuals, higher taxa, or guilds) and their ecological interactions (links) that make up natural communities. We are now gaining a burgeoning understanding of how ecological networks relate to the abiotic environment [3], anthropogenic influences [4], ecosystem stability [5], and ecosystem functioning [6,7]. Further, ecological network data collection and analytical approaches are developing rapidly, but although many network studies have considered invasive species the findings from network ecology with the greatest potential utility in invasion prevention or management have so far not been incorporated into invasion biology. Here, we aim to help focus future attention on areas likely to advance invasion biology.Anthropogenic introductions of exotic species span a continuum from unsuccessful, through those that establish and spread, to a subset that inflict significant detrimental impacts on ecosystems, economic activity, and human wellbeing. Several definitions exist for invasive species, but here we consider an invasive species to be one that is introduced by humans outside of its natural distribution and that has since established and spread substantially [8,9]. Interspecific interactions are key to invasion processes, but their complexity renders simple food-chain models inadequate for studying introduced species [10]. Recent research has integrated networks into invasion biology, yet so far, their utility has been more explanatory than predictive. Challenges in understanding species invasions also stem from the complexity of anthropogenic...