Local- Vs. Landscape-Scale Indirect Effects of an Invasive Weed on Native Plants 1 (original) (raw)
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Ecology, 2004
Landscape change has great, yet infrequently measured, potential to influence the susceptibility of natural systems to invasive species impacts. We quantified attack by an invasive biological control weevil (Rhinocyllus conicus) on native thistles in relation to two types of landscape change: agricultural intensification and invasion by an exotic thistle, Carduus nutans, the original target of biological control. Weevil egg load was measured on native thistles in three landscape types: (1) agriculture dominated, (2) grassland dominated with exotic thistles, and, (3) grassland dominated without exotic thistles. We found no difference in egg load on native thistles within grassland landscapes without exotic thistles vs. within agricultural landscapes, suggesting that agricultural intensification per se does not influence levels of weevil attack. However, attack on the native Cirsium undulatum increased significantly (three-to fivefold) with increasing exotic thistle density. Within-patch exotic thistle density explained Ͼ50% of the variation in both the intensity and frequency of weevil attack. Since R. conicus feeding dramatically reduces seed production, exotic thistles likely exert a negative indirect effect on native thistles. This study provides some of the first empirical evidence that invasion by an exotic plant can increase attack of native plants by shared insect herbivores.
Oecologia, 2012
The biotic resistance hypothesis is a dominant paradigm for why some introduced species fail to become invasive in novel environments. However, predictions of this hypothesis require further empirical field tests. Here, we focus on evaluating two biotic factors known to severely limit plants, interspecific competition and insect herbivory, as mechanisms of biotic resistance. We experimentally evaluated the independent and combined effects of three levels of competition by tallgrass prairie vegetation and two levels of herbivory by native insects on seedling regeneration, size, and subsequent flowering of the Eurasian Cirsium vulgare, a known invasive species elsewhere, and compared its responses to those of the ecologically similar and co-occurring native congener C. altissimum. Seedling emergence of C. vulgare was greater than that of C. altissimum, and that emergence was reduced by the highest level of interspecific competition. Insect leaf herbivory was also greater on C. vulgare than on C. altissimum at all levels of competition. Herbivory on seedlings dramatically decreased the proportion of C. vulgare producing flower heads at all competition levels, but especially at the high competition level. Competition and herbivory interacted to significantly decrease plant survival and biomass, especially for C. vulgare. Thus, both competition and herbivory limited regeneration of both thistles, but their effects on seedling emergence, survival, size and subsequent reproduction were greater for C. vulgare than for C. altissimum. These results help explain the unexpectedly low abundance recorded for C. vulgare in western tallgrass prairie, and also provide strong support for the biotic resistance hypothesis.
Invasive Insect Abundance Varies Across The Biogeographic Distribution Of A Native Host Plant
Ecological Applications, 2006
Studies of biogeographic variation in species abundances are fundamental to understanding and predicting the impacts of invasive exotic species. We quantified the abundance of the introduced and now invasive biocontrol weevil, Rhinocyllus conicus, on a newly adopted native host plant, Cirsium canescens (Platte thistle), across the plant's distributional range. We used regression and structural equation analyses to examine variation in weevil abundance at 92-108 sites over three years in relation to variation in abiotic and biotic parameters hypothesized to be important in insect or plant dynamics and distribution. We found that R. conicus now occurs throughout the majority of the range of C. canescens, even in the center of the native plant's distribution where its coevolved, targeted weed host (Carduus nutans, musk thistle) is absent. In fact, weevil densities were greater in the center of the native plant's distribution in the Sand Hills formation than in peripheral sand patches closer to areas where the targeted exotic thistle is common. None of the macroclimatic variables examined were consistent predictors of the large-scale variation in weevil abundance on C. canescens. In addition to biogeographic position, the only other consistent predictor of weevil densities across sites was the number of flower heads per C. canescens plant. These results exclude the ''spillover'' hypothesis to explain nontarget feeding on this newly adopted native host species. Instead, the results are consistent with the alternative hypothesis that exotic weevil abundance on C. canescens is related to the local availability of native floral resources. Because C. canescens densities have declined with increases in R. conicus at sites central in the plant's distribution, these results suggest that isolated, peripheral populations of C. canescens are likely to be critical for persistence of Platte thistle. More generally, this study suggests that the persistence of a native species that is impacted by an exotic natural enemy may require preservation of populations in habitats outside the optimal portion of the native species' distribution.
VARIATION IN HERBIVORE-MEDIATED INDIRECT EFFECTS OF AN INVASIVE PLANT ON A NATIVE PLANT
Ecology, 2007
Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species. To evaluate the importance of ecological factors, such as host plant proximity and density, in determining the direction and strength of such herbivore-mediated indirect effects, we quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie on loam soils in the upper Great Plains, USA. Over three years (2001)(2002)(2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0-220 m) from a musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density ( 5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle's co-occurrence with the coevolved preferred exotic host plant and not from the weevil's response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivoremediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants.
Background/Question/Methods Mechanisms underlying invasive species impacts remain incompletely understood in general, and the potential competitive effects of invasive insect herbivores in particular have been relatively under-studied. We combined a manipulative field experiment with a large scale survey to examine the potential competitive effects of an invasive biocontrol weevil, Rhinocyllus conicus, on the predominant native floral herbivore, Paracanta culta, associated with a native, non-target thistle host plant, Cirsium canescens. Results/Conclusions The field experiment demonstrated strong, asymmetric, competition with R. conicus out-competing P. culta within thistle flower heads. In addition, weevil priority access to floral resources increased the magnitude of its competitive suppression of P. culta. Evidence for competitive suppression with increasing weevil priority included decreases in both the numbers and the total biomass of native flies, plus decreases in individual ...
2006
An important, yet poorly quantified mechanism to explain the failure of some exotic species to increase and spread is that indigenous natural enemies provide ecosystem resistance to invasiveness. To evaluate this idea, we hypothesized that spillover of native thistle-feeding floral insect herbivores onto Eurasian bull thistle (Cirsium vulgare) from the prairie native tall thistle (C. altissimum) helps limit bull thistle population growth and spread throughout the western tallgrass prairie region in Nebraska. To test this hypothesis, we quantified both the occurrence of bull thistle, a known invasive species worldwide, and the floral herbivory on it by native insect herbivores in relation to occurrence and floral herbivory on tall thistle across the region. We quantified plant occurrence by recording presence/absence of each thistle in 1600 m x 30 m (0.1 mile x ~100 ft) quadrats along four radiating 16 km (10 mile) transects at nine sites stratified across three longitudes and three ...
EVIDENCE FOR NATURAL BIOLOGICAL CONTROL: INSECTS DECREASE SURVIVAL AND GROWTH OF A NATIVE THISTLE
Ecological Applications, 1997
Native thistles, in contrast to exotic species, are seldom noxious weeds. In this study, we evaluated one hypothesis for this difference: that the growth and fitness of native thistles are limited by natural enemies. Specifically, we tested the effect of insect foliage feeding on the survival and growth of large and small rosettes of tall thistle, Cirsiurn altissirnurn, using an insecticide exclusion experiment. Large juveniles were proportionally more damaged than were small ones. Insecticide reduced insect feeding, including overall intensity of plant damage and area removed or damaged on each of the two longest leaves, compared to controls treated only with water. As a result, both leaf growth and plant size increased significantly by the end of the growing season. Growth parameters showing major increases with the reduction in insect herbivory included upper rootcrown diameter, the total number of leaves, and the length of the two longest leaves on both large and small rosettes. Insecticide also reduced the mortality of large juveniles to half that of the controls. Thus, the study shows that the suite of naturally occurring, coevolved, foliage-feeding insects significantly reduces the growth and survival of rosettes of this native thistle under field conditions. The results provide strong support for the hypothesis underlying biological control of weeds programs.
Plant Ecology, 2019
Exotic plants are involved in different interactions with the fauna of the invaded sites, which can facilitate or limit their successful establishment and spread. Here, we evaluated the impact of native aphid-tending ants on the reproductive consequences of the invasive thistle Carduus thoermeri in NW Patagonia, which is frequently infested by aphids. We estimated the number and proportion of viable seeds, seed weight, germination proportion, and mean germination time of thistles in the presence and absence of aphids and ants, and with or without pollinator access. Aphid-infested thistles had 57% less viable seeds and 29% lower seed weight than non-infested thistles. Although ants and aphids had no effect on germination proportion, the mean germination time was ca. 15% faster in seeds from aphid-infested thistles. Our results suggest that the potential indirect effects of aphid-tending ants on thistles (negative effects via pollinator deterrence and positive effects via driving away non-aphid herbivores) are less important than the direct negative effects of aphids. Interestingly, although harboring aphids and ants has negative reproductive consequences for C. thoermeri plants, it could also generate a competitive advantage by giving rise to small and fast germinated seeds. This study illustrates the complexity of novel interactions among exotic plants and native ants, reinforcing the need for more studies to fully understand the potential impact of ant-plant interactions mediated by Hemiptera on the invasion success of plants.
What controls the population dynamics of the invasive thistle Carduus nutans in its native range
Journal of Applied Ecology, 2006
The invasive thistle Carduus nutans causes major economic losses in the Americas, Australia and New Zealand. For the first time, we have modelled its population dynamics in its native range, Eurasia, where it rarely reaches problematic densities, in order to identify ways to improve management strategies for this weed in the invaded range. 2. In particular, we investigated whether specialist enemies in the thistle's native range suppress thistle populations, as predicted by the enemy-release hypothesis, and, if so, how this effect relates to other factors that may limit population growth. 3. We constructed population transition matrix models with data from three French populations. A vital rate elasticity analysis revealed that reproduction determines between 33% and 61% of the projected growth rate of the populations, and thus is a key driver of the population dynamics of this monocarpic short-lived perennial. 4. Decreases in population size were predicted by the models for all three populations ( λ < 1). Using limiting factor omission analyses, we showed that the suite of native insect herbivores causing seed losses had a larger impact than the joint effects of rosette damage by sheep and summer drought acting on seedling establishment. Removal of insect herbivores increased the native population growth rate by 166% on average; removal of sheep damage and summer drought from the model increased population growth by 51%. Specialist herbivores and drought interacted synergistically to affect reproduction. 5. Synthesis and applications. We show that vital rate elasticity analysis provides more management information than elasticity analysis on the level of matrix elements, particularly when management options affect only certain vital rates. For example, it can be used to make predictions about the effectiveness of predispersal floral herbivores in control management. The model can also help identify ways in which biocontrol can be augmented using integrated weed management that reduces seedling establishment probabilities, for example by preventing overgrazing. This method illustrates how native range studies of invasive species can be used to generate insights into managing populations in the invaded range.