Comparative impacts of aboveground and belowground enemies on an invasive thistle

This is the published version of an article published by Wiley Open Access.Most research examining how herbivores and pathogens affect performance of invasive plants focuses on aboveground interactions. Although important, the role of belowground communities remains poorly understood, and the relative impact of aboveground and belowground interactions is still debated. As well, most studies of belowground interactions have been carried out in controlled environments, so little is known about the role of these interactions under natural conditions or how these relationships may change across a plant’s range. Using the invasive plant Cirsium arvense, we performed a reciprocal transplant experiment to test the relative impacts of above- and belowground interactions at three sites across a 509-km latitudinal gradient in its invaded range in Ontario, Canada. At each site, C. arvense seedlings were protected with above-and/or belowground exclosures in a factorial design. Plant performance (biomass, height, stem thickness, number of leaves, length of longest leaf, maximum rhizome length) was greatest when both above-and belowground exclosures were applied and lowest when no exclosures were applied. When only one type of exclosure was applied, biomass generally improved more with belowground exclosures than with aboveground exclosures. Despite site-to-site differences in foliar damage, root damage, and mesofaunal populations, belowground interactions generally had a greater negative impact on performance than aboveground herbivory alone. These results stress the importance of including both aboveground enemy interactions and plant–soil interactions in studies of plant community dynamics and invader performance.This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (PMK), a University of Toronto Centre for Global Change Science Summer Internship (PMK), and an Ontario Graduate Scholarship (KAN)

Data from: Comparative impacts of aboveground and belowground enemies on an invasive thistle

1. Most research examining how herbivores and pathogens affect performance of invasive plants focuses on aboveground interactions. Although important, the role of belowground communities remains poorly understood, and the relative impact of aboveground and belowground interactions is still debated. As well, most studies of belowground interactions have been carried out in controlled environments, so little is known about the role of these interactions under natural conditions or how these relationships may change across a plant’s range. 2. Using the invasive plant Cirsium arvense, we performed a reciprocal transplant experiment to test the relative impacts of above- and belowground interactions at three sites across a 509 km latitudinal gradient in its invaded range in Ontario, Canada. At each site, C. arvense seedlings were protected with above- and/or belowground exclosures in a factorial design. 3. Plant performance (biomass, height, stem thickness, number of leaves, length of longest leaf, maximum rhizome length) was greatest when both above- and belowground exclosures were applied and lowest when no exclosures were applied. When only one type of exclosure was applied, biomass generally improved more with belowground exclosures than with aboveground exclosures. 4. Synthesis: Despite site-to-site differences in foliar damage, root damage, and mesofaunal populations, belowground interactions generally had a greater negative impact on performance than aboveground herbivory alone. These results stress the importance of including both aboveground enemy interactions and plant-soil interactions in studies of plant community dynamics and invader performance

Does Local Isolation Allow an Invasive Thistle to Escape Enemy Pressure?

This is an accepted manuscript originally published by Springer Nature.Enemy release often is invoked to explain the success of invasive plants: an invader benefts from reduced attack as it escapes specialized enemies through the invasion process. Enemy release typically is thought of as occurring at large geographic scales, but local-scale interactions may also be important for invader establishment and success. Furthermore, most tests of local enemy release have been conducted over a single year even though release may be a transient phenomenon, especially at small scales. In this study, we used a multi-year field experiment to investigate whether locally isolated populations of the noxious non-native weed Cirsium arvense beneft from reduced levels of aboveground damage, and whether any initial advantage is lost over subsequent growing seasons. Populations of C. arvense were grown in plots at set distances from established source populations for 4 years. In the frst year of the experiment, folivory signifcantly declined with host isolation, but damage from specialist stem gallers and seed predators did not. However, in subsequent years of observation, folivoresbegan colonizing isolated C. arvense populations while stem gallers exhibited very slow colonization of more isolated plots;seed predation showed no pattern with distance in any year. Local enemy escape did not result in increased plant performance, which instead negatively correlated with degree of isolation. Nonetheless, our results stress the importance of multi-year observations in tests of enemy release, since the herbivory patterns initially observed often changed within subsequent years depending on the dispersal ability and biology of the causal organism involved.This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (PMK), an NSERC CGS-M Award (KAN), a Zimmerman and Weis PhD Scholarships in Field Biology (KAN), and an Ontario Graduate Scholarship (KAN)

Variation in herbivory along a latitudinal gradient for native and exotic Asteraceae

This is the accepted manuscript of an article published by Springer Verlag.It has long been hypothesized that biotic interactions, including herbivory, are most intense at lower latitudes. However, this generalization has recently been challenged with studies showing that latitudinal gradients in damage may be rarer than previously believed. Additionally, most studies have focused on herbivory of native species, so it remains unknown whether natives and exotics follow similar patterns. This study compares rates of aboveground herbivory of multiple native and non-native Asteraceae across a latitudinal gradient, with a more detailed investigation of a focal exotic, Cirsium arvense. Herbivory of multiple tissue types was quantified for all species across an 815 kilometre transect in Ontario, Canada. The native Asteraceae included in the survey typically experienced a decline in folivory with increasing latitude. Herbivory patterns for the exotic species were less clear; while most experienced high damage at the southernmost site, some also experienced high damage rates at mid-latitudes. For the focal species Cirsium arvense, leaf and stem herbivory declined with increasing latitude, although seed damage showed strong regional variation across the invaded range. These results show that latitudinal variation in herbivory is highly dependent on the plant species being investigated, the tissue type being measured, and the type of herbivore(s) causing the damage. In some cases, populations in marginal areas might benefit from reduced damage by some groups of herbivores. In other cases, factors such as the availability of suitable habitat, the biology of specific enemies, and the origin of the host plant may override the influence of latitude on host performance.This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC)

Soil biota composition and the performance of a noxious weed across its invaded range

This is an accepted manuscript of an article originally published by Wiley.The success of invasive plant species is driven, in part, by feedback with soil ecosystems. Yet, how variation in belowground communities across latitudinal gradients affects invader distributions remains poorly understood. To determine the effect of soil communities on the performance of the noxious weed Cirsium arvense across its invaded range, we grew seedlings for 40 days in soils collected across a 699 km linear distance from both inside and outside established populations. We also described the mesofaunal and bacterial communities across all soil samples. We found that C. arvense typically performed better when grown in soils sourced from northern populations than from southern locations where it has a longer invasion history. We also found evidence that C. arvense performed best in soils sourced from outside invaded patches, although this was not consistent across all sites. The bacterial community showed a significant increase in the magnitude of compositional change in invaded sites at higher latitudes, while the mesofaunal community showed the opposite pattern. Bacterial community composition was significantly correlated with C. arvense performance, although mesofaunal community composition was not. Our results demonstrate that the interactions between an invasive plant and associated soil communities change across the invaded range, and the bacterial community in particular may affect variation in plant performance. Observed patterns may be caused by C.arvense presence and time since invasion allowing for an accumulation of species‐specific pathogens in southern soils, while the naïveté of northern soils to invasion results in a more responsive bacterial community. Although these interactions are difficult to predict, such effects could possibly facilitate the establishment of this exotic species to novel locations.This research was supported by the Univ. of Toronto Research and Scholarly Activity Fund, a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (PMK) and a Queen Elizabeth II/Pfizer Canada Graduate Scholarship in Science and Technology (KAN)

Sources of controversy surrounding latitudinal patterns in herbivory and defence

This is the accepted manuscript of an article published by Elsevier.Both herbivory and plant defences against herbivores have been predicted to increase toward tropical regions. Early tests of this latitudinal herbivory-defence hypothesis (LHDH) were supportive, but accumulating evidence has been mixed. We argue that the lack of clarity might be due to heterogeneity in methodology and problems with study design and interpretation. Here we suggest possible solutions. Latitudinal studies need to carefully consider spatial and phylogenetic scale, to link plant defence measurements to herbivore performance, and to incorporate additional concepts from plant defence theory such as tolerance and induced defence. Additionally, we call for consistent measures of herbivory to standardize comparisons across biomes. Improving methodology in future studies of LHDH can resolve much of the current controversy.This work was supported by a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant to PMK, an NSERC Vanier grant to DNA, and a Michigan State University Distinguished Fellowship to CAB