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.     

Underwater light attenuation inhibits native submerged plants and facilitates the invasive co-occurring plant Cabomba caroliniana

Aim

Decreasing in the diversity and distribution of native submerged plants have been widely observed in recent decades. Global underwater darkening, which is mainly caused by radiation dimming and a decrease in transparency due to, e.g. eutrophication, has emerged as a general trend that strongly hampers the growth of submerged plants in lakes by decreasing the light available for photosynthesis. However, few studies have attempted to compare the responses of native and invasive submerged plants to underwater darkening. In this study, we aimed to compare the effects of light attenuation on the growth and photosynthesis traits of native and invasive submerged plants.

Location

East China.

Method

Through field investigations and a mesocosm experiment, the responses of functional traits of two representative native [water thyme (Hydrilla verticillata), Eurasian watermilfoil (Myriophyllum spicatum)] and one invasive [Carolina fanwort (Cabomba caroliniana)] plant species to various environmental factors, notably to underwater light attenuation, were studied.

Results

Underwater photosynthetically active radiation (PAR) exerted a substantial effect on the relative coverage and abundance of the three studied submerged plants in their natural freshwater habitats. Invasive C. caroliniana showed relatively superior growth (total biomass and relative growth rate) and photosynthesis traits (maximum quantum yield of photosystem II F_v/F_m, chlorophyll a content, chlorophyll b content and the ratio of Chl a and b contents) compared to the two native plants under low underwater PAR conditions. In contrast, under high underwater PAR conditions, C. caroliniana showed the opposite response.

Main Conclusions

Light attenuation inhibits the growth of native submerged plants but facilitates the growth of invasive plant species. Restoration of freshwater lakes by reducing deterioration from underwater darkening (for instance, by reducing of external nutrients loading) may therefore constrain the growth and spread of the invasive C. caroliniana.     

Effects of soil nitrogen levels on growth and defense of the native and introduced genotypes of alligator weed

土壤氮水平对喜旱莲子草原产地和引入地基因型生长和防御的影响 同种植物生长在资源丰富生境中的个体,其防御水平被认为低于生长在资源匮乏生境中的个体。然而,生境的养分水平如何影响植物的诱导抗性和耐受性,以及这种影响在入侵植物的原产地和引入 地种群间是否存在差异,目前均知之甚少。本研究以入侵植物喜旱莲子草(Alternanthera philoxeroides)的原产地阿根廷和引入地美国的基因型为研究对象设计同质园实验,以探究土壤氮水平对植物的生长、组成和诱导性[莲草直胸跳甲(Agasicles hygrophila)取食诱导]化学防御以及耐受性的影响。实验中,我们测定了植物总生物量、伸长速率(生长速率的表征)以及叶片和根系中总碳、总氮和三萜皂苷(化学防御物质)的含量。研究结果显示,植物在低土壤氮水平下表现出较高的组成抗性(植物在低土壤氮水平下的叶片三萜皂苷含量高于其在高土壤氮水平的33%)和耐受性[植物被取食后总生物量下降的程度更低(植物在高土壤氮水平和低土壤氮水平下被取食后总生物量分别下降了24%和15%)],而在高土壤氮水平下表现出较高的诱导抗性(在高土壤氮水平下的植物被取食后叶片三萜皂苷含量与空白对照的植物相比升高了24%)。植物的组成抗性和耐受性与生长速率存在权衡,但诱导抗性与生长速率存在显著的正相关性。此外,引入地基因型在低土壤氮水平下叶片碳含量显著低于原产地基因型(-6%),但这种差异在高土壤氮水平下消失。这些结果表明,土壤氮水平 影响植物对不同防御策略的选择偏好,并且在决定引入地基因型的表现时与植食作用存在交互作用。     

入侵植物的繁殖策略以及对本土植物繁殖的影响

理解入侵生物的繁殖策略是阐明生物入侵机制的一个重要方面。入侵植物常表现出一些共同的繁殖特征, 如以两性花为主的性系统、自动自交为主的繁育系统或不依赖传粉媒介的无融合生殖和无性繁殖以及高生殖投资的资源配置策略等。成功入侵的外来植物通过影响本土的传粉者, 在种群和群落水平上影响本土植物的有性繁殖, 甚至促使某些本土植物在繁殖对策和表型性状上发生快速转变。目前, 入侵植物繁殖策略及其生态效应的研究多侧重于入侵种的快速演化, 而有关外来植物与本土植物间的相互影响及其可能存在的协同适应研究还较为缺乏。探讨本土植物在外来种入侵压力下的繁殖对策和响应机制, 将丰富人们对物种间竞争、共存及群落构建等机制的深入了解。从繁殖和适应的角度探求入侵植物与本土植物之间的复杂关系, 将有助于解析生物入侵的机制及人类干扰下的物种演化规律, 也为预测和防控入侵植物提供科学依据。     

Prey naïveté rather than enemy release dominates the relation of an invasive spider toward a native predator

Ecosystems may suffer from the impact of invasive species. Thus, understanding the mechanisms contributing to successful invasions is fundamental for limiting the effects of invasive species. Most intuitive, the enemy release hypothesis predicts that invasive species might be more successful in the exotic range than resident sympatric species owing to the absence of coevolution with native enemies. Here, we test the enemy release hypothesis for the invasion of Europe by the North American spider Mermessus trilobatus. We compare the susceptibility of invasive Mermessus trilobatus and a native species with similar life history to a shared predator with which both species commonly co‐occur in Europe. Contrary to our expectations, invasive Mermessus trilobatus were consumed three times more frequently by native predators than their native counterparts. Our study shows that invasive Mermessus trilobatus is more sensitive to a dominant native predator than local sympatric species. This suggests that the relation between the invasive spider and its native predator is dominated by prey naïveté rather than enemy release. Further studies investigating evolutionary and ecological processes behind the invasion success of Mermessus trilobatus, including testing natural parasites and rapid reproduction, are needed to explain its invasion success in Europe.     

The influence of host plant genotype on variation in population densities of the gorse thrips,Sericothrips staphylinus (Thysanoptera: Thripidae), and its consideration in relation to release strategies

Large variations in population densities of Sericothrips staphylinus were observed between gorse bushes at a field site in an earlier Tasmanian study. A replicated potted plant experiment was conducted to assess whether genetic variation between gorse bushes could affect densities of S. staphylinus. There were significant differences in the number of S. staphylinus recovered from the individual bushes sampled within sites and individual bushes across all sites. This provides evidence that genetic variation between gorse bushes is at least partly responsible for the observed variability in field populations. The result validates the current release strategy used for the field establishment of S. staphylinus in south-eastern Australia. This strategy involves making multiple small releases (ca. 250 adults per bush per site) instead of a small number of large releases (ca. 1000 per bush per site), thereby reducing variation in numbers due to genetic affects. Genetic variation between individual plants of the same species and associated effects on herbivore densities should be considered when devising release strategies for biological control agents.     

Divergent effects of leaf-cutting ant herbivory and soil engineering on the reproductive success of plants in a Caatinga dry forest

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Reciprocal interactions between native and introduced populations of common milkweed,Asclepias syriaca,and the specialist aphid,Aphis nerii

Following its introduction into Europe (EU), the common milkweed (Asclepias syriaca) has been free of most specialist herbivores that are present in its native North American (NA) range, except for the oleander aphid Aphis nerii. We compared EU and NA populations of A. nerii on EU and NA milkweed populations to test the hypothesis that plant–insect interactions differ on the two continents. First, we tested if herbivore performance is higher on EU plants than on NA plants, because the former have escaped most of their herbivores and have perhaps been selected for lower defence levels following introduction. Second, we compared two A. nerii lines (one from each continent) to test whether genotypic differences in the herbivore may influence species interactions in plant–herbivore communities in the context of species introductions. The NA population of A. nerii developed faster, had higher fecundity and attained higher population growth rates than the EU population. There was no overall significant continental difference in aphid resistance between the plants. However, milkweed plants from EU supported higher population growth rates and faster development of the NA line of A. nerii than plants from NA. In contrast, EU aphids showed similar (low) performance across plant populations from both continents. In a second experiment, we examined how chewing herbivores indirectly mediate interactions between milkweeds and aphids, and induced A. syriaca plants from each continent by monarch caterpillars (Danaus plexippus) to compare the resulting changes in plant quality on EU aphid performance. As specialist chewing herbivores of A. syriaca are only present in NA, we expected that plants from the two continents may affect aphid growth in different ways when they are challenged by a specialist chewing herbivore. Caterpillar induction decreased aphid developmental times on NA plants, but not on EU plants, whereas fecundity and population growth rates were unaffected by induction on both plant populations. The results show that genetic variation in the plants as well as in the herbivores can determine the outcome of plant–herbivore interactions.     

Climate change and invasion by intracontinental range-expanding exotic plants: the role of biotic interactions

Background and Aims

In this Botanical Briefing we describe how the interactions between plants and their biotic environment can change during range-expansion within a continent and how this may influence plant invasiveness.

Scope

We address how mechanisms explaining intercontinental plant invasions by exotics (such as release from enemies) may also apply to climate-warming-induced range-expanding exotics within the same continent. We focus on above-ground and below-ground interactions of plants, enemies and symbionts, on plant defences, and on nutrient cycling.

Conclusions

Range-expansion by plants may result in above-ground and below-ground enemy release. This enemy release can be due to the higher dispersal capacity of plants than of natural enemies. Moreover, lower-latitudinal plants can have higher defence levels than plants from temperate regions, making them better defended against herbivory. In a world that contains fewer enemies, exotic plants will experience less selection pressure to maintain high levels of defensive secondary metabolites. Range-expanders potentially affect ecosystem processes, such as nutrient cycling. These features are quite comparable with what is known of intercontinental invasive exotic plants. However, intracontinental range-expanding plants will have ongoing gene-flow between the newly established populations and the populations in the native range. This is a major difference from intercontinental invasive exotic plants, which become more severely disconnected from their source populations.     

Plant–soil biota interactions and spatial distribution of black cherry in its native and invasive ranges

One explanation for the higher abundance of invasive species in their non‐native than native ranges is the escape from natural enemies. But there are few experimental studies comparing the parallel impact of enemies (or competitors and mutualists) on a plant species in its native and invaded ranges, and release from soil pathogens has been rarely investigated. Here we present evidence showing that the invasion of black cherry (Prunus serotina) into north‐western Europe is facilitated by the soil community. In the native range in the USA, the soil community that develops near black cherry inhibits the establishment of neighbouring conspecifics and reduces seedling performance in the greenhouse. In contrast, in the non‐native range, black cherry readily establishes in close proximity to conspecifics, and the soil community enhances the growth of its seedlings. Understanding the effects of soil organisms on plant abundance will improve our ability to predict and counteract plant invasions.     

Oviposition strategy as a means of local adaptation to plant defence in native and invasive populations of the viburnum leaf beetle

Herbivores have been hypothesized to adapt locally to variation in plant defences and such adaptation could facilitate novel associations in the context of biological invasions. Here, we show that in the native range of the viburnum leaf beetle (VLB, Pyrrhalta viburni), two populations of geographically isolated hosts-Viburnum opulus and Viburnum tinus-have divergent defences against VLB oviposition: negative versus positive density-dependent egg-crushing wound responses, respectively. Populations of beetles coexisting with each host show an adaptive behavioural response: aggregative versus non-aggregative oviposition on V. opulus and V. tinus, respectively. In parallel, we show that in North America, where VLB is invasive, defences of three novel hosts are negatively density-dependent, and beetles' oviposition behaviour is aggregative. Thus, local adaptation to plant defences has the potential to facilitate the invasion of herbivores onto novel hosts.     

Comparison of damage to native and exotic tallgrass prairie plants by natural enemies

We surveyed the prevalence and amount of leaf damage related to herbivory and pathogens on 12 pairs of exotic (invasive and noninvasive) and ecologically similar native plant species in tallgrass prairie to examine whether patterns of damage match predictions from the enemy release hypothesis. We also assessed whether natural enemy impacts differed in response to key environmental factors in tallgrass prairie by surveying the prevalence of rust on the dominant C₄ grass, Andropogon gerardii, and its congeneric invasive exotic C₄ grass, A. bladhii, in response to fire and nitrogen fertilization treatments. Overall, we found that the native species sustain 56.4% more overall leaf damage and 83.6% more herbivore-related leaf damage when compared to the exotic species. Moreover, we found that the invasive exotic species sustained less damage from enemies relative to their corresponding native species than the noninvasive exotic species. Finally, we found that burning and nitrogen fertilization both significantly increased the prevalence of rust fungi in the native grass, while rust fungi rarely occurred on the exotic grass. These results indicate that reduced damage from enemies may in part explain the successful naturalization of exotic species and the spread of invasive exotic species in tallgrass prairie.     

Inhibitory effects of Eucalyptus globulus on understorey plant growth and species richness are greater in non‐native regions

Aim

We studied the novel weapons hypothesis in the context of the broadly distributed tree species Eucalyptus globulus. We evaluated the hypothesis that this Australian species would produce stronger inhibitory effects on species from its non‐native range than on species from its native range.

Location

We worked in four countries where this species is exotic (U.S.A., Chile, India, Portugal) and one country where it is native (Australia).

Time period

2009–2012.

Major taxa studied

Plants.

Methods

We compared species composition, richness and height of plant communities in 20 paired plots underneath E. globulus individuals and open areas in two sites within its native range and each non‐native region. We also compared effects of litter leachates of E. globulus on root growth of seedlings in species from Australia, Chile, the U.S.A. and India.

Results

In all sites and countries, the plant community under E. globulus canopies had lower species richness than did the plant community in open areas. However, the reduction was much greater in the non‐native ranges: species richness declined by an average of 51% in the eight non‐native sites versus 8% in the two native Australian sites. The root growth of 15 out of 21 species from the non‐native range were highly suppressed by E. globulus litter leachates, whereas the effect of litter leachate varied from facilitation to suppression for six species native to Australia. The mean reduction in root growth for Australian plants was significantly lower than for plants from the U.S.A., Chile and India.

Main conclusions

Our results show biogeographical differences in the impact of an exotic species on understorey plant communities. Consistent with the novel weapons hypothesis, our findings suggest that different adaptations of species from the native and non‐native ranges to biochemical compounds produced by an exotic species may play a role in these biogeographical differences.     

Controls on pathogen species richness in plants' introduced and native ranges: roles of residence time, range size and host traits

Introduced species escape many pathogens and other enemies, raising three questions. How quickly do introduced hosts accumulate pathogen species? What factors control pathogen species richness? Are these factors the same in the hosts' native and introduced ranges? We analysed fungal and viral pathogen species richness on 124 plant species in both their native European range and introduced North American range. Hosts introduced 400 years ago supported six times more pathogens than those introduced 40 years ago. In hosts' native range, pathogen richness was greater on hosts occurring in more habitat types, with a history of agricultural use and adapted to greater resource supplies. In hosts' introduced range, pathogen richness was correlated with host geographic range size, agricultural use and time since introduction, but not any measured biological traits. Introduced species have accumulated pathogens at rates that are slow relative to most ecological processes, and contingent on geographic and historic circumstance.     

Breeding system and pollination ecology of introduced plants compared to their native relatives

Identifying how plant-enemy interactions contribute to the success of introduced species has been a subject of much research, while the role of plant-pollinator interactions has received less attention. The ability to reproduce in new environments is essential for the successful establishment and spread of introduced species. Introduced plant species that are not capable of autonomous self-fertilization and are unable to attract resident pollinators may suffer from pollen limitation. Our study quantifies the degree of autogamy and pollination ecology of 10 closely related pairs of native and introduced plant species at a single site near St. Louis, Missouri, USA. Most of these species pairs had similar capacities for autogamy; however, of those that differed, the introduced species were more autogamous than their native congeners. Most introduced plants have pollinator visitation rates similar to those of their native congeners. Of the 20 species studied, only three had significant pollen limitation. We suggest that the success of most introduced plant species is because they are highly autogamous or because their pollinator visitation rates are similar to those of their native relatives. Understanding and identifying traits related to pollination success that are key in successful introductions may allow better understanding and prediction of biological invasions.