Warwick J. Allen

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.

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.

Lineage overwhelms environmental conditions in determining rhizosphere bacterial community structure in a cosmopolitan invasive plant

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.Environmental factors often outweigh host heritable factors in structuring host-associated microbiomes. Here, Bowen et al. show that host lineage is crucial for determination of rhizosphere bacterial communities in Phragmites australis, a globally distributed invasive plant.

Intraspecific variation in indirect plant-soil feedbacks as a driver of a wetland plant invasion

Plant-soil feedbacks (PSFs) can influence plant competition via direct interactions with pathogens and mutualists or indirectly via apparent competition/mutualisms (i.e., spillover to cooccurring plants) and soil legacy effects. Presently, it is unknown how intraspecific variation in PSFs interacts with the environment (e.g., nutrient availability) to influence competition between native and invasive plants. We conducted a fully crossed multi-factor greenhouse experiment to determine the effects of soil biota, interspecific competition, and nutrient availability on biomass of replicate populations from one native and two invasive lineages of common reed (Phragmites australis) and a single lineage of native smooth cordgrass (Spartina alterniflora). Harmful soil biota consistently dominated PSFs involving all three P. australis lineages, reducing biomass by 10%, regardless of nutrient availability or S. alterniflora presence as a competitor. Spillover of soil biota derived from the rhizosphere of the two invasive P. australis lineages reduced S. alterniflora biomass by 7%, whereas soil biota from the native P. australis lineage increased S. alterniflora biomass by 6%. Interestingly, regardless of lineage, P. australis soil biota negatively affected S. alterniflora biomass when grown alone (i.e., a soil legacy), but had a positive impact when grown with P. australis, suggesting that P. australis is preferred by harmful generalist soil biota or facilitates S. alterniflora via spillover (i.e., apparent mutualism). Soil biota also reduced the negative impacts of interspecific competition on S. alterniflora by 13%, although it remained competitively inferior to P. australis across all treatments. Moreover, competitive interactions and the response to nutrients did not differ among P. australis lineages, indicating that interspecific competition and nutrient deposition may not be key drivers of P. australis invasion in North America. Taken together, although soil biota, interspecific competition, and nutrient availability appear to have no direct impact on the success of invasive P. australis lineages in North America, indirect spillover and soil legacies from P. australis occur and may have important implications for co-occurring native species and restoration of invaded habitats. Our study integrates multiple factors linked to plant invasions, highlighting that indirect interactions are likely commonplace in driving successful invasions and their impacts on the local community.

Development of an efficient trapping system for New Zealand flower thrips, Thrips obscuratus.

BACKGROUND New Zealand flower thrips (NZFT), Thrips obscuratus (Crawford), is an economic pest of various horticultural crops in New Zealand and is recognised as a quarantine pest globally. Two chemical attractants (ethyl nicotinate and 6-pentyl-2H-pyran-2-one), three dispensers, three trap designs and four trap heights were investigated to determine the most effective method for monitoring NZFT. Phenology of NZFT at two locations was compared. RESULTS 6-Pentyl-2H-pyran-2-one in a polyethylene bag dispenser was the most attractive lure formulation and exhibited high stability in release rate trials. There was no difference in NZFT catch between vertical-panel and cross-panel traps, but both caught significantly more than delta traps. However, both types of panel trap had unacceptably high by-catch of native insects. Catch of thrips increased with height from 0 to 3 m. Phenology of NZFT showed similar population trends at both locations, but with a timing difference of around 50 days. CONCLUSIONS Delta traps containing 6-pentyl-2H-pyran-2-one in a polyethylene bag at 2 m above the ground is the recommended method for monitoring NZFT, significantly improving the sensitivity, accuracy and labour input compared with prior methods. Long-term monitoring of NZFT could lead to more accurate economic damage thresholds and timing for when to apply insecticides.