A snail prefers native over exotic freshwater plants: implications for the enemy release hypotheses

1. One of the most popular single‐factor hypotheses that have been proposed to explain the naturalization and spread of introduced species is the enemy release hypothesis (ERH). Most studies have considered how specialist consumers might affect the success of exotics, but the importance of generalist herbivores has received little attention 2. In this study, we compared the palatability of native and exotic freshwater plants in both phylogenetic pairings and in region‐wide assays, using a generalist consumer (the pond snail Radix swinhoei). 3. Our study indicated that, when plants were paired by taxonomic relatedness, the snail preferred native over exotic freshwater plants by a 2 : 1 ratio. The snail also preferred native over exotic plants when tested across 20 native and seven exotic species found growing in Liangzi Lake. 4. Snails consumed more biomass of native than exotic plants, providing support for the ERH and suggesting that exotic species have a potential advantage in most lakes of the middle and lower reaches of the Yangtze River in China, thus helping their invasion.     

Comparison of invertebrate herbivores on native and non‐native Senecio species: Implications for the enemy release hypothesis

The enemy release hypothesis posits that non‐native plant species may gain a competitive advantage over their native counterparts because they are liberated from co‐evolved natural enemies from their native area. The phylogenetic relationship between a non‐native plant and the native community may be important for understanding the success of some non‐native plants, because host switching by insect herbivores is more likely to occur between closely related species. We tested the enemy release hypothesis by comparing leaf damage and herbivorous insect assemblages on the invasive species Senecio madagascariensis Poir. to that on nine congeneric species, of which five are native to the study area, and four are non‐native but considered non‐invasive. Non‐native species had less leaf damage than natives overall, but we found no significant differences in the abundance, richness and Shannon diversity of herbivores between native and non‐native Senecio L. species. The herbivore assemblage and percentage abundance of herbivore guilds differed among all Senecio species, but patterns were not related to whether the species was native or not. Species‐level differences indicate that S. madagascariensis may have a greater proportion of generalist insect damage (represented by phytophagous leaf chewers) than the other Senecio species. Within a plant genus, escape from natural enemies may not be a sufficient explanation for why some non‐native species become more invasive than others.     

Advancing the missed mutualist hypothesis,the under-appreciated twin of the enemy release hypothesis

Introduced species often benefit from escaping their enemies when they are transported to a new range, an idea commonly expressed as the enemy release hypothesis. However, species might shed mutualists as well as enemies when they colonize a new range. Loss of mutualists might reduce the success of introduced populations, or even cause failure to establish. We provide the first quantitative synthesis testing this natural but often overlooked parallel of the enemy release hypothesis, which is known as the missed mutualist hypothesis. Meta-analysis showed that plants interact with 1.9 times more mutualist species, and have 2.3 times more interactions with mutualists per unit time in their native range than in their introduced range. Species may mitigate the negative effects of missed mutualists. For instance, selection arising from missed mutualists could cause introduced species to evolve either to facilitate interactions with a new suite of species or to exist without mutualisms. Just as enemy release can allow introduced populations to redirect energy from defence to growth, potentially evolving increased competitive ability, species that shift to strategies without mutualists may be able to reallocate energy from mutualism toward increased competitive ability or seed production. The missed mutualist hypothesis advances understanding of the selective forces and filters that act on plant species in the early stages of introduction and establishment and thus could inform the management of introduced species.     

Damselfly eggs alter their development rate in the presence of an invasive alien cue but not a native predator cue

Biological invasions are a serious problem in natural ecosystems. Local species that are potential prey of invasive alien predators can be threatened by their inability to recognize invasive predator cues. Such an inability of prey to recognize the presence of the predator supports the naïve prey hypothesis. We exposed eggs of a damselfly, Ischnura elegans, to four treatments: water with no predator cue (control), water with a native predator cue (perch), water with an invasive alien predator cue (spinycheek crayfish) that is present in the damselfly sampling site, and water with an invasive alien predator cue (signal crayfish) that is absent in the damselfly sampling site but is expected to invade it. We measured egg development time, mortality between ovipositing and hatching, and hatching synchrony. Eggs took longer to develop in the signal crayfish group (however, in this group, we also observed high green algae growth), and there was a trend of shorter egg development time in the spinycheek crayfish group than in the control group. There was no difference in egg development time between the perch and the control group. Neither egg mortality nor hatching synchrony differed between groups. We suggest that egg response to signal crayfish could be a general stress reaction to an unfamiliar cue or an artifact due to algae development in this group. The egg response to the spinycheek crayfish cue could be caused by the predation of crayfish on damselfly eggs in nature. The lack of egg response to the perch cue could be caused by perch predation on damselfly larvae rather than on eggs. Such differences in egg responses to alternative predator cues can have important implications for understanding how this group of insects responds to biological invasions, starting from the egg stage.     

The enemy release and EICA hypothesis revisited: incorporating the fundamental difference between specialist and generalist herbivores

The success of invasive plants has been attributed to their escape from natural enemies and subsequent evolutionary change in allocation from defence to growth and reproduction. In common garden experiments with Senecio jacobaea, a noxious invasive weed almost worldwide, the invasive populations from North America, Australia, and New Zealand did indeed allocate more resources to vegetative and reproductive biomass. However, invasive plants did not show a complete change in allocation from defence to growth and reproduction. Protection against generalist herbivores increased in invasive populations and pyrrolizidine alkaloids, their main anti‐herbivore compounds, did not decline in invasive populations but were higher overall compared with native populations. In contrast, invasive plants lost additional protection against specialist herbivores adapted to pyrrolizidine alkaloids. Hence, the absence of specialist herbivores in invasive populations resulted in the evolution of lower protection against specialists and increased growth and reproduction, but also allowed a shift towards higher protection against generalist herbivores.     

Responses of morphological and physiological traits to herbivory by snails of three invasive and native submerged plants

3种入侵和本地沉水植物形态和生理性状对螺类牧食的响应 沉水植物水盾草(Cabomba caroliniana)已成为中国太湖流域的优势入侵水生植物。与外来物种的原产地环境相比,引入地新环境中存在的专食性天敌较少。外来物种可能会逃避其原产地环境中的天敌牧食,又因为它们的适口性相对较差,从而导致在引入地外来物种通常比本地物种遭受的牧食者影响更低(天敌逃逸假说)。本研究的目的是比较水盾草与共生的本地沉水植物对本地牧食者的响应。我们进行了一个中宇宙尺度实验,研究了水盾草和两种共生的本地沉水植物黑藻(Hydrilla verticillata)和穗花狐尾藻(Myriophyllum spicatum)对两种本地广食性腹足纲螺类萝卜螺(Radix swinhoei)和环棱螺(Sinotaia quadrata)的牧食响应。记录了它们的形态性状指标(总生物量、冠根比和相对生长率)和生理性状指标(叶片总非结构性碳、木质素和纤维素)。研究结果表明,环棱螺对3种沉水植物性状指标的影响较少。随着本地广食性螺类萝卜螺数量的增加,黑藻和穗花狐尾藻大部分植物性状发生了改变,而水盾草的植物性状表现出相对稳定的趋势。水盾草对萝卜螺的牧食更具抵抗力,这与天敌逃逸假说的假设一致。这一发现说明牧食性螺类促进了水盾草的入侵,这可能会改变沉水植物群落中的物种组成。     

土著昆虫素毒蛾在本地植物芦苇与入侵植物互花米草上的生活史

互花米草(Spartina alterniflora)入侵盐沼生态系统导致了土著广食性昆虫素毒蛾(Laelia coenosa)发生宿主转移。但是, 目前对素毒蛾在互花米草和芦苇(Phragmites australis)上的生活史规律的比较尚未见报道, 而此信息对评价素毒蛾能否持续利用互花米草具有重要的参考价值。为了明确素毒蛾在其原始宿主芦苇和外来新宿主互花米草上的生活史动态, 我们在上海崇明长江口盐沼湿地中, 对素毒蛾在芦苇和互花米草生境中的发生情况进行了系统调查。结果显示, 虽然素毒蛾在芦苇和互花米草上均一年发生3代, 并都以幼虫越冬, 但在互花米草生境中, 越冬代幼虫开始越冬的时间要比芦苇中晚20 d左右, 其他世代中的多个虫态在互花米草上持续出现的时间也比芦苇上延迟了约10 d。这些结果表明, 互花米草入侵后素毒蛾在外来植物上出现的持续时间虽然发生了延长, 但其年世代数未变化。我们推测素毒蛾在互花米草和芦苇上生长发育规律的差异可能受生境的资源可利用性及两种植物的营养与防御水平调控。     

Multispecies interactions among a plant parasite,a pollinator and a seed predator affect the reproductive output of an invasive plant,Cytisus scoparius

Interactions between species pairs are almost always mediated by interactions with other species. The outcomes of these multispecies interactions are often difficult to predict and are rarely studied. In addition to their theoretical importance, multispecies interactions are also important for management situations. Where multiple agents are used to control invasive species, interactions between agents may either enhance or reduce the impacts on the target species, or may simply have additive effects. In this study, conducted in a Mediterranean‐type woodland in Australia, we examined how the interaction between an invasive legume, Cytisus scoparius (Leguminosae), its pollinator, Apis mellifera and a seed predator, Bruchidius villosus (Bruchidae), are modified by a native hemiparasitic vine, Cassytha pubescens (Loranthaceae). The parasite had a direct negative effect on C. scoparius, reducing flowering by 50% and consequently fruit and seed production. Despite having fewer flowers, infected plants had the same proportion of ‘tripped’ flowers, an indirect measure of pollinator visitation, as uninfected plants; although fruit formed on infected plants it was more likely to abort prematurely. Seed predation by B. villosus was lower on parasite‐infected C. scoparius plants than in uninfected plants. Although Ca. pubescens had an antagonistic effect on B. villosus, in consort, the two agents reduced overall seed production by an average of 62%. The acquired parasite Ca. pubescens was more effective in reducing reproductive output than the introduced seed predator, B. villosus, and shows potential as a biocontrol agent for C. scoparius. We documented a subadditive effect of two biological enemies on the invasive species, where the acquired parasite had a stronger effect than the introduced seed predator.     

Prediction and validation of the potential global distribution of a problematic alien invasive species — the American bullfrog

Predicting the probability of successful establishment and invasion of alien species at global scale, by matching climatic and land use data, is a priority for the risk assessment. Both large- and local-scale factors contribute to the outcome of invasions, and should be integrated to improve the predictions. At global scale, we used climatic and land use layers to evaluate the habitat suitability for the American bullfrog Rana catesbeiana , a major invasive species that is among the causes of amphibian decline. Environmental models were built by using Maxent, a machine learning method. Then, we integrated global data with information on richness of native communities and hunting pressure collected at the local scale. Global-scale data allowed us to delineate the areas with the highest suitability for this species. Predicted suitability was significantly related to the invasiveness observed for bullfrog populations historically introduced in Europe, but did not explain a large portion of variability in invasion success. The integration of data at the global and local scales greatly improved the performance of models, and explained > 57% of the variance in introduction success: bullfrogs were more invasive in areas with high suitability and low hunting pressure over frogs. Our study identified the climatic factors entailing the risk of invasion by bullfrogs, and stresses the importance of the integration of biotic and abiotic data collected at different spatial scales, to evaluate the areas where monitoring and management efforts need to be focused.     

Geographic mosaics of plant–soil microbe interactions in a global plant invasion

Aim Our aim in this study was to document the global biogeographic variation in the effects of soil microbes on the growth of Centaurea solstitialis (yellow starthistle; Asteraceae), a species that has been introduced throughout the world, but has become highly invasive only in some introduced regions. Location  To assess biogeographic variation in plant–soil microbe interactions, we collected seeds and soils from native Eurasian C. solstitialis populations and introduced populations in California, Argentina and Chile. Methods To test whether escape from soil‐borne natural enemies may contribute to the success of C. solstitialis, we compared the performance of plants using seeds and soils collected from each of the biogeographic regions in greenhouse inoculation/sterilization experiments. Results  We found that soil microbes had pervasive negative effects on plants from all regions, but these negative effects were significantly weaker in soils from non‐native ranges in Chile and California than in those from the non‐native range in Argentina and the native range in Eurasia. Main conclusions The biogeographic differences in negative effects of microbes in this study conformed to the enemy‐release hypothesis (ERH) overall, but the strong negative effect of soil biota in Argentina, where C. solstitialis is invasive, and weaker effects in Chile where it is not, indicated that different factors influencing invasion are likely to occur in large scale biogeographic mosaics of interaction strengths.