Ganesh P. Bhattarai

Global networks for invasion science: benefits, challenges and guidelines

Much has been done to address the challenges of biological invasions, but fundamental questions (e.g., which species invade? Which habitats are invaded? How can invasions be effectively managed?) still need to be answered before the spread and impact of alien taxa can be effectively managed. Questions on the role of biogeography (e.g., how does biogeography influence ecosystem susceptibility, resistance and resilience against invasion?) have the greatest potential to address this goal by increasing our capacity to understand and accurately predict invasions at local, continental and global scales. This paper proposes a framework for the development of ‘Global Networks for Invasion Science’ to help generate approaches to address these critical and fundamentally biogeographic questions. We define global networks on the basis of their focus on research questions at the global scale, collection of primary data, use of standardized protocols and metrics, and commitment to long-term global data. Global networks are critical for the future of invasion science because of their potential to extend beyond the capacity of individual partners to identify global priorities for research agendas and coordinate data collection over space and time, assess risks and emerging trends, understand the complex influences of biogeography on mechanisms of invasion, predict the future of invasion dynamics, and use these new insights to improve the efficiency and effectiveness of evidence-based management techniques. While the pace and scale of global change continues to escalate, strategic and collaborative global networks offer a powerful approach to inform responses to the threats posed by biological invasions.

Cosmopolitan Species As Models for Ecophysiological Responses to Global Change: The Common Reed Phragmites australis

Phragmites australis is a cosmopolitan grass and often the dominant species in the ecosystems it inhabits. Due to high intraspecific diversity and phenotypic plasticity, P. australis has an extensive ecological amplitude and a great capacity to acclimate to adverse environmental conditions; it can therefore offer valuable insights into plant responses to global change. Here we review the ecology and ecophysiology of prominent P. australis lineages and their responses to multiple forms of global change. Key findings of our review are that: (1) P. australis lineages are well-adapted to regions of their phylogeographic origin and therefore respond differently to changes in climatic conditions such as temperature or atmospheric CO2; (2) each lineage consists of populations that may occur in geographically different habitats and contain multiple genotypes; (3) the phenotypic plasticity of functional and fitness-related traits of a genotype determine the responses to global change factors; (4) genotypes with high plasticity to environmental drivers may acclimate or even vastly expand their ranges, genotypes of medium plasticity must acclimate or experience range-shifts, and those with low plasticity may face local extinction; (5) responses to ancillary types of global change, like shifting levels of soil salinity, flooding, and drought, are not consistent within lineages and depend on adaptation of individual genotypes. These patterns suggest that the diverse lineages of P. australis will undergo intense selective pressure in the face of global change such that the distributions and interactions of co-occurring lineages, as well as those of genotypes within-lineages, are very likely to be altered. We propose that the strong latitudinal clines within and between P. australis lineages can be a useful tool for predicting plant responses to climate change in general and present a conceptual framework for using P. australis lineages to predict plant responses to global change and its consequences.

Do ploidy level and nuclear genome size and latitude of origin modify the expression of Phragmites australis traits and interactions with herbivores

We studied the relationship between genome size and ploidy level variation and plant traits for the reed grass Phragmites australis. Using a common garden approach on a global collection of populations in Aarhus, Denmark, we investigated the influence of monoploid genome size and ploidy level on the expression of P. australis growth, nutrition and herbivore-defense traits and whether monoploid genome size and ploidy level play different roles in plant trait expression. We found that both monoploid genome size and latitude of origin contributed to variation in traits that we studied for P. australis, with latitude of origin being generally a better predictor of trait values and that ploidy level and its interaction with monoploid genome size and latitude of origin also contributed to trait variation. We also found that for four traits, tetraploids and octoploids had different relationships with the monoploid genome size. While for tetraploids stem height and leaf water content showed a positive relationship with monoploid genome size, octoploids had a negative relationship with monoploid genome size for stem height and no relationship for leaf water content. As genome size within octoploids increased, the number of aphids colonizing leaves decreased, whereas for tetraploids there was a quadratic, though non-significant, relationship. Generally we found that tetraploids were taller, chemically better defended, had a greater number of stems, higher leaf water content, and supported more aphids than octoploids. Our results suggest trade-offs among plant traits mediated by genome size and ploidy with respect to fitness and defense. We also found that the latitude of plant origin is a significant determinant of trait expression suggesting local adaptation. Global climate change may favor some genome size and ploidy variants that can tolerate stressful environments due to greater phenotypic plasticity and to fitness traits that vary with cytotype which may lead to changes in population genome sizes and/or ploidy structure, particularly at species’ range limits.

Hurricane Activity and the Large-Scale Pattern of Spread of an Invasive Plant Species

Disturbances are a primary facilitator of the growth and spread of invasive species. However, the effects of large-scale disturbances, such as hurricanes and tropical storms, on the broad geographic patterns of invasive species growth and spread have not been investigated. We used historical aerial imagery to determine the growth rate of invasive Phragmites australis patches in wetlands along the Atlantic and Gulf Coasts of the United States. These were relatively undisturbed wetlands where P. australis had room for unrestricted growth. Over the past several decades, invasive P. australis stands expanded in size by 6–35% per year. Based on tropical storm and hurricane activity over that same time period, we found that the frequency of hurricane-force winds explained 81% of the variation in P. australis growth over this broad geographic range. The expansion of P. australis stands was strongly and positively correlated with hurricane frequency. In light of the many climatic models that predict an increase in the frequency and intensity of hurricanes over the next century, these results suggest a strong link between climate change and species invasion and a challenging future ahead for the management of invasive species.

Biological control of invasive Phragmites australis will be detrimental to native P. australis

European Phragmites australis is widespread as a nonnative genotype in North America, abundant in many places, and often considered a pest. There is also a much less common North American native genotype of P. australis, and a ‘‘Gulf Coast’’ genotype (Saltonstall et al. 2004). The genetics of Phragmites are complex, and in North America there are hybrids between P. australis and other species of Phragmites as well as between the European and North American native genotypes of P. australis (Paul et al. 2010; Lambertini et al. 2012; Meyerson et al. 2012). P. australis is one of the best-studied plants globally (Hulme et al. 2013). European P. australis can become highly dominant in marshes, with effects on plant communities, birds, fishes, insects, and other organisms, as well as ecosystem processes (Meyerson et al. 2000a, b; Kiviat 2013). Some of these effects are considered negative and others positive, depending upon a stakeholder’s interests or management goals. Besides habitat functions, P. australis provides a number of non-habitat ecosystem services in both its native and introduced ranges related to its high above and belowground biomass and productivity. Among these services are formation and stabilization of tidal wetland soils for protection against sea level rise, carbon sequestration, wave attenuation, evapotranspirational cooling of the microclimate, and removal of macronutrients and trace metals from surface waters (Meyerson 2000; Meyerson et al. 1999, 2000a, b; Hershner and Havens 2008; Kiviat 2013). A group of researchers has been developing classical biological control for European P. australis in North America (Schwarzländer and Häfliger 2000; Tewksbury et al. 2002; Häfliger et al. 2005, 2006; Blossey 2014). Currently, at least two species of European noctuid moths are being tested as potential biological control agents. The proposed biological control is intended to affect only the European P. Guest editors: Laura A. Meyerson & Kristin Saltonstall/ Phragmites invasion.

Response to Blossey and Casagrande: ecological and evolutionary processes make host specificity at the subspecies level exceedingly unlikely

We agree with Blossey and Casagrande (2016) that absolute host-specificity is a necessity for successful biological control of invasive plants without unintended consequences for native species. However, inclusion of non-target native species in the diet of a biological control agent is a relatively common phenomenon with native congeners of the target plant species at greatest risk (Pemberton 2000). As our concerns relate to North American native and invasive Phragmites australis, and host-shifts at the subspecific level, the risk is substantially greater. We reiterate our previous arguments that the literature is replete with examples of how environmental context (e.g., spillover effects, apparent competition, biogeographic variation in herbivore resistance) and evolution (e.g., via the hybrid bridge hypothesis) can lead to incorporation of new hosts into the diet. None of these possibilities were addressed by Blossey and Casagrande (2016), nor can they be in a simple laboratory/greenhouse study of host specificity. Herbivore specificity for the invasive genotype is hardly as clear cut as Blossey and Casagrande (2016) claim. First, contrary to their claim, Lipara pullitarsis infests and successfully develops in native Phragmites (Allen et al. 2015). Second, another purported specialist, Lasioptera hungarica, attacks native-invasive hybrids (Saltonstall et al. 2014), a potential first step to a host shift (i.e., the hybrid bridge hypothesis). Finally, the noctuid moths (Archanara geminipuncta and Archanara neurica) they propose for biological control of invasive Phragmites in North America do not show the absolute host-specificity required to prevent damage to native genotypes. Fundamental host-ranges of these species include native genotypes of Phragmites and several other wetland grasses, including some economically important species (Blossey et al. 2013). Although leaf-sheaths, overwintering sites for Archanara eggs and larvae, ‘‘often’’ drop off from native stems, this is far from absolute and provides no assurance that native genotypes will not be attacked. Archanara also attack multiple stems during development, so can easily move to adjacent native Phragmites patches. Relying on managers to control Guest editors: Laura A. Meyerson and Kristin Saltonstall/ Phragmites invasion.

Release and distribution of Lilioceris cheni (Coleoptera: Chrysomelidae), a biological control agent of air potato (Dioscorea bulbifera: Dioscoreaceae), in Florida

ABSTRACT From 2012 to 2015, 429,668 Lilioceris cheni Gressit and Kimoto (Coleoptera: Chrysomelidae) were released in Florida for biological control of air potato Dioscorea bulbifera L. (Dioscoreaceae). In the fall of 2015, a state-wide survey was conducted at 113 randomly selected air potato infestations in order to determine the spatial distribution of L. cheni. Damage due to L. cheni was found at 86% of locations and was low in the Florida panhandle where air potato was relatively uncommon and fewer beetles had been released, and in far south Florida, despite high numbers of beetle releases. On average, beetles travelled 9.5 km from the nearest release site to survey sites from the date of release to the time of the survey, with a maximum distance of nearly 67 km. The rate of spread was estimated at 8.2 km/year under the assumption that beetles present at survey sites migrated or were moved from the nearest release site. Air potato vines produced fewer aerial tubers, the vegetative propagule of air potato, as foliar damage due to L. cheni increased. The results suggest that future research efforts should focus on determining the biotic and abiotic factors that may be limiting establishment in some areas of Florida.

Diversity and impact of herbivorous insects on Brazilian peppertree in Florida prior to release of exotic biological control agents

ABSTRACT Brazilian peppertree, Schinus terebinthifolia Raddi (Anacardiaceae), is a South American plant that is highly invasive in Florida. The impact of insect herbivores on the performance of Brazilian peppertree was evaluated at two locations in Florida using an insecticide exclusion method. Although 38 species of insect herbivores were collected on the invasive tree, there were no differences in growth or reproductive output of insecticide protected and unprotected trees, providing evidence that insect feeding had no measurable impact on tree performance. The majority of insects collected on Brazilian peppertree were generalists, and several were serious agricultural pests.