Explaining variation in insect herbivore control over plant communities

Research has repeatedly demonstrated that herbivores can, at some times and in some places, control the distribution and abundance of plants. Consequently, explaining variation in herbivore control over plant communities is a central goal in ecology and evolutionary biology. Two major challenges have prevented theoretical progress in this area of research. First, although there are numerous hypotheses that attempt to explain variation in herbivore control over plant communities, theoretical reviews have focused on a single hypothesis. Thus, it has been unclear where these herbivore control hypotheses diverge in their predictions and rationale. Second, herbivore control hypotheses base their explanations on highly correlated vegetation characteristics, namely net primary productivity (NPP), plant vigor, plant apparency, plant tissue nitrogen, plant defenses, plant tolerance, and host plant concentration. Consequently, interpretations of field experiments and meta-analyses have been equivocal. To address the first problem, I simultaneously reviewed herbivore control hypotheses and their predictions and rationale. I demonstrate that these hypotheses can be synthesized into four central hypotheses based on NPP, plant size, resource availability, and host stem density. This provides researchers with few vs. many herbivore control hypotheses. To address the second problem, I simultaneously tested these hypotheses by experimentally manipulating resource availability, total stem density, plant species composition, and herbivore abundance under field conditions. I then monitored the response of herbivore abundance, damage to plants, and the reduction in plant mass due to herbivory. The experiments demonstrated that herbivory caused the strongest reductions in mean stem mass where per stem resource availability was lowest, regardless of where herbivore abundance and damage was greatest. This result supports the plant tolerance based resource availability hypothesis, which assumes that the ability of plants to tolerate herbivory increases as resource availability increases. In addition, herbivore control over both simple plant communities (i.e., monocultures) and complex plant communities (i.e., polycultures) was due to herbivory on the dominant plant species, Solidago canadensis. Together, these results suggest that future herbivore control hypotheses should focus on the effect of per-capita resource availability on the ability of dominant plants to tolerate herbivory

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. © 2014 Bhattarai, Cronin

The Reliability of a Rotational Power Assessment of the Core

Context: Most athletic upper-body power generation involveshigh levels of neuromuscular activation/coordination of a rotationalnature. Therefore, it is important to assess athletic ability thatreplicates the rotational activity of athletes. However, a paucity ofresearch currently measures rotational power of the core.Objective: Establish inter-day reliability of chop and lift mean poweroutput via a linear position transducer on rotational reliant powerathletes.Design: Controlled laboratory study.Setting: Professional cricket training facilities.Population: Eight male professional cricket players (age= 23±3.38years, height= 186±10.06 cm, mass= 89.71± 8.12 kg) with aresistance (>2 years) training background volunteered to participatein the study.Intervention: A linear position transducer was attached to theweight stack of a cable pulley system to determine the peak poweroutputs associated with a chop and lift movement. Assessmentoccurred on three occasions separated by at least seven days.Asymmetry, intraclass correlation coefficients (ICCs) and coefficientof variations (CV) were calculated and used to quantify the absoluteand relative consistency of the testing procedures.Results: The mean peak power outputs for chop and lift rangedfrom 404 - 494W and 277-314W respectively, the power outputsdiffering minimally (2.7-6.3%) between the left and right sides.Coefficients of variation of 7.4% - 19% were reported, with intraclass correlation coefficients of 0.54 - 0.94 observed betweentesting occasions.Conclusion: Mean muscular power output associated with the liftassessment reported greatest reliability in well trained athletes.The asymmetry between sides was relatively small suggestingbalanced multi-planar trunk development in the current throwingathletes. Equipment limitations (load related), training status andvariable selection (mean or peak power) need to be consideredprior to rotational assessment of the core.It is recommended that the lift movement is utilized in rotationalpower assessments, or that greater familiarization is undertakenwhen administering the chop assessment.KeywordsCore; Trunk; Assessment; Anaerobic; Transverse plan

Geographic variation in apparent competition between native and invasive \u3ci\u3ePhragmites australis\u3c/i\u3e

Apparent competition, the negative interaction between species mediated by shared natural enemies, is thought to play an important role in shaping the structure and dynamics of natural communities. However, its importance in driving species invasions, and whether the strength of this indirect interaction varies across the latitudinal range of the invasion, has not been fully explored. We performed replicated field experiments at four sites spanning 900 km along the Atlantic Coast of the United States to assess the presence and strength of apparent competition between sympatric native and invasive lineages of Phragmites australis. Four herbivore guilds were considered: stem‐feeders, leaf‐miners, leaf‐chewers and aphids. We also tested the hypothesis that the strength of this interaction declines with increasing latitude. Within each site, native and invasive plants of P. australis were cross‐transplanted between co‐occurring native and invasive patches in the same marsh habitat and herbivore damage was evaluated at the end of the growing season. Apparent competition was evident for both lineages and involved all but the leaf‐chewer guild. For native plants, total aphids per plant was 296% higher and the incidence of stem‐feeding and leaf‐mining herbivores was 34% and 221% higher, respectively, when transplanted into invasive than native patches. These data suggest that invasive P. australis has a negative effect on native P. australis via apparent competition. Averaged among herbivore types, the indirect effects of the invasive lineage on the native lineage was 57% higher than the reverse situation, suggesting that apparent competition was asymmetric. We also found that the strength of apparent competition acting against the native lineage was comparable to the benefits to the invasive lineage from enemy release (i.e., proportionately lower mean herbivory of the invasive relative to the native taxa). Finally, we found the first evidence that the strength of apparent competition acting against the native lineage (from stem‐feeders only) decreased with increasing latitude. These results suggest that not only could apparent competition be of tantamount importance to enemy release in enhancing the establishment and spread of invasive taxa, but also that these indirect and direct herbivore effects could vary over the invasion range

Working Papers: Astronomy and Astrophysics Panel Reports

The papers of the panels appointed by the Astronomy and Astrophysics survey Committee are compiled. These papers were advisory to the survey committee and represent the opinions of the members of each panel in the context of their individual charges. The following subject areas are covered: radio astronomy, infrared astronomy, optical/IR from ground, UV-optical from space, interferometry, high energy from space, particle astrophysics, theory and laboratory astrophysics, solar astronomy, planetary astronomy, computing and data processing, policy opportunities, benefits to the nation from astronomy and astrophysics, status of the profession, and science opportunities

The matrix enhances the effectiveness of corridors and stepping stones

Conservation strategies often call for the utilization of corridors and/or stepping stones to promote dispersal among fragmented populations. However, the extent to which these strategies increase connectivity for an organism may depend not only on the corridors and stepping stones themselves, but also on the composition of the surrounding matrix. Using an herbivore-host-plant system consisting of the planthopper Prokelisia crocea and its sole host plant, prairie cordgrass (Spartina pectinata), we show that the effectiveness of corridors and stepping stones for promoting planthopper dispersal among patches depended strongly on the intervening matrix habitat. In a low-resistance matrix (one that facilitates high rates of interpatch dispersal), both stepping stones and corridors promoted high connectivity, increasing the number of colonists by threefold relative to patches separated by matrix habitat only. The effectiveness of stepping stones and corridors was significantly lower in a high-resistance matrix (one that promotes low rates of interpatch dispersal), with stepping stones failing to improve connectivity for the planthoppers relative to controls. Thus, we conclude that the matrix is an integral component of landscapes and should be considered together with corridors and stepping stones in strategies designed to increase dispersal among fragmented populations

Biogeography of a Plant Invasion: Plant–Herbivore Interactions

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

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

Phragmites australis biological control is intended to address a major invasion in North America and is likely to change the ecology of vast areas of coastal and inland wetlands. However, the real risks to the native North American genotypes of P. australis (as indicated by recent research summarized above) may not have been fully considered, particularly the extirpation of native populations or the eventual extinction of the native North American lineage altogether. The concerns we raise need to be considered in the process of developing and approving the release of biological control agents and the entire approval process would benefit from greater transparency and wider input from Phragmites researchers globally

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)