Increased compensatory ability of an invasive plant to above- and below-ground enemies in monocultures

Above- and below-ground enemies have prominent influence on plant invasions, and increasing evidence has shown that plant invasions are also affected by inter- or intraspecific interactions between individual plants. However, how these two factors interactively affect plant invasions has rarely been tested. Here, we examined the response of the invasive plant Alternanthera philoxeroides (Mart.) Griseb. to above- and below-ground enemies at varying plant densities in a greenhouse experiment in Wuhan, China. We found that both above- and below-ground enemies decreased the plant total and root mass at individual and population levels, but that of the two, below-ground enemies had a greater effect than above-ground enemies, and that the two guilds of enemies acted additively on the plant. However, their impacts decreased as the plant density increased, due to enhanced plant tolerance to both guilds of enemies. The increased plant tolerance may result from changes in plant resource allocation patterns, corresponding to a positive linear relationship between the ratio of fine root mass to total root mass and plant density. Given that forming dense monocultures in their new ranges is one of the most important characteristics of invasive plants, we propose that the high compensatory ability at dense monocultures may be an important mechanism underlying exotic species invasion.     

Rapid adaptation of insect herbivores to an invasive plant

Introduced plant success often is attributed to release from natural enemies in their new ranges. However, herbivores may accumulate over time and reduce invasiveness but evidence for this process to date is weak. We report here that enemy release is indeed limited to the early stages of introduction of the Chinese tallow tree (Sapium sebiferum). In bioassays and gardens along a geographical gradient of time since tallow tree introduction, herbivory was highest and tree performance was poorest where tallow tree has been present longer (i.e. introduced earlier). Additionally, Asian ecotypes (grown from seeds collected in Asia) had lower survival than North American ecotypes (seeds collected in North America), which is consistent with genetic responses to low herbivory in the introduced range (EICA Hypothesis). Release from insect herbivores appears to contribute to early success of the tallow tree, but accumulation of insect herbivores has apparently reduced this benefit over time.     

Parallel evolution in an invasive plant: effect of herbivores on competitive ability and regrowth of Jacobaea vulgaris

A shift in the composition of the herbivore guild in the invasive range is expected to select for plants with a higher competitive ability, a lower regrowth capacity and a lower investment in defence. We show here that parallel evolution took place in three geographically distinct invasive regions that differed significantly in climatic conditions. This makes it most likely that indeed the shifts in herbivore guilds were causal to the evolutionary changes. We studied competitive ability and regrowth of invasive and native Jacobaea vulgaris using an intraspecific competition set‐up with and without herbivory. Without herbivores invasive genotypes have a higher competitive ability than native genotypes. The invasive genotypes were less preferred by the generalist Mamestra brassicae but more preferred by the specialist Tyria jacobaeae, consequently their competitive ability was significantly increased by the first and reduced by the latter. Invasive genotypes showed a lower regrowth ability in both herbivore treatments.     

Reduced competitive ability in an invasive plant

One explanation for successful plant invaders is that they evolved to be more competitive. An intuitive prediction of this Evolution of Increased Competitive Ability (EICA) hypothesis never previously tested is that invasive populations should outcompete their native ‘ancestors’ in a common environment. We tested this idea in a diallel competition experiment with Alliaria petiolata where offspring from native and invasive populations were grown alone or in all pairwise combinations. While without competition, there were no differences between native and invasive populations, native populations outperformed invasive ones when competing against each other. Our results contradict the EICA hypothesis and we conclude that it does not not hold for Alliaria petiolata. Instead, we formulate a new ERCA (Evolutionary Reduced Competitive Ability) hypothesis: if there is less competition in the invasive range and competitive ability involves traits that have a fitness cost, then selection might act against it, thereby reducing intraspecific interactions too.     

Phylogenetic isolation increases plant success despite increasing susceptibility to generalist herbivores

Aim Theory suggests that introduced species that are phylogenetically distant from their recipient communities should be more successful than closely related introduced species because they can exploit open niches and escape enemies in their new range, i.e. Darwin’s Naturalization Hypothesis. Alternatively, it has also been hypothesized that closely related invaders might be more successful than novel invaders because they are pre‐adapted to conditions in their new range; a paradox coined Darwin’s Naturalization Conundrum. To date, these hypotheses have been tested primarily at the regional scale, not within local plant communities where introduced species colonize, compete and encounter herbivores. Location Global. Methods and Results We used community phylogenetics to analyse data from 49 published experiments to examine the importance of phylogenetic relatedness and generalist herbivory on native and exotic plant success at the community level. Plants that were categorized as ‘invasive’ were indeed less related to the recipient community than ‘non‐pest’ exotic plants. Distantly related exotic plants were also more abundant than closely related species. Phylogenetic relatedness predicted herbivore impact, but in a way that was opposite to predictions, as herbivores had stronger, not lesser, impacts on distantly related plants. Importantly, these same patterns generally held for native plants, as distantly related native plants were more abundant and more susceptible to herbivores than closely related species, ultimately resulting in herbivores suppressing community‐level phylogenetic diversity. Main conclusions Distantly related plants were more locally successful despite experiencing stronger control by generalist herbivores, a finding that was robust across native and exotic species. To our knowledge, this is the first evidence that phylogenetic matching influences the local success of both native and exotic species and that herbivores can influence community phylodiversity. Phylogenetic relatedness explained a relatively small portion of the variance in the data even after taking herbivory into account, however, suggesting that phylogenetic matching works in combination with other factors to influence community assembly.     

Aboveground herbivory increases soil nematode abundance of an invasive plant

入侵植物往往可以影响土壤线虫的群落结构。然而,入侵植物在入侵地并未完全逃逸地上植食性昆虫的取食危害,而我们对昆虫取食是否会调节入侵植物对土壤线虫群落的影响还知之甚少。在本研究中,我们探讨了地上植食性昆虫取食危害如何调节入侵植物空心莲子草(Alternanthera philoxeroides)对土壤线虫群落的影响,并研究了昆虫调节作用的时间效应。我们开展温室实验,对盆栽的空心莲子草施加三种不同的昆虫取食处理[无昆虫取食、专食性昆虫莲草直胸跳甲(Agasicles hygrophila)取食、广食性昆虫虾钳菜披龟甲(Cassida piperata)取食],并在移除所有植食性昆虫后的第1、10和20天测定土壤线虫的群落结构。此外,我们还测定了空心莲子草根系及根系分泌物的碳含量。研究发现,在植食性昆虫被移除的第1天,昆虫取食处理显著提高了空心莲子草根围土壤线虫的丰度,特别是植食性线虫。然而,随着时间推移,这种影响逐渐减退,并在后期(第10和20天)完全消失。此外,专食性昆虫莲草直胸跳甲和广食性昆虫虾钳菜披龟甲的取食危害对土壤线虫丰度的影响并无显著性差异。综上所述,本研究表明地上植食性昆虫的取食危害可以改变入侵植物对土壤线虫群落的影响,但昆虫的调节作用是短暂的。此外,我们的研究强调了在评估入侵植物的影响时,需要同时考虑地上和地下生物群落。     

Flooding compromises compensatory capacity of an invasive plant: implications for biological control

Plant compensatory growth is proposed to be insidious to biological control and known to vary under different environmental conditions. However, the effects of microsite conditions on compensation capacity and its indirect impacts on biological control of plant invaders have received little attention. Alligator weed, Alternanthera phioxeroides, is an invasive plant worldwide, growing in both aquatic and terrestrial habitats that are often affected by flooding. Biological control insects have been successful in suppressing the plant in many aquatic habitats but have failed in terrestrial habitats. To evaluate the impact of flooding on compensation capacity, we conducted common garden and greenhouse experiments in which plants were grown under different moisture conditions (aquatic versus terrestrial). Our results show that plants were able to fully recover from continued herbivory in the terrestrial habitat, but failed in the aquatic habitat, indicating a flooding-regulated plant compensatory capacity. Also, the grazed plants increased below-ground growth and reproductive root bud formation in the terrestrial habitat, but there was no such difference in the aquatic habitat. Our findings suggest that the differing plant compensatory capacity, affected by flooding, may explain the different biological control efficacy of alligator weed in aquatic and terrestrial habitats. Understanding mechanisms in plant invader compensation in different microsite conditions is important for improving management efficiency.     

Decreased resistance and increased tolerance to native herbivores of the invasive plant Sapium sebiferum

Release from natural enemies may favor invasive plants evolving traits associated with reduced herbivore‐resistance and faster‐growth in introduced ranges. Given a genetic trade‐off between resistance and tolerance, invasive plants could also become more tolerant to herbivory than conspecifics in the native range. We conducted a field common garden study in the native range of Sapium sebiferum using seeds from native Chinese populations and invasive North American populations to compare their growth and herbivory resistance. We also performed a cage‐pot experiment to compare their resistance and tolerance to Bikasha collaris beetles that are specialist feeders on S. sebiferum trees in China. Results of the common garden study showed that Sapium seedlings of invasive populations relative to native populations were more frequently attacked by native herbivores. Growth and leaf damage were significantly higher for invasive populations than for native populations. Growth of invasive populations was not significantly affected by insecticide spray, but insecticide spray benefited that of native populations. In the bioassay trial, beetles preferentially consumed leaf tissue of invasive populations compared to native populations when beetles had a choice between them. Regression of percent leaf damage on biomass showed that invasive populations tolerated herbivory more effectively than native populations. Our results suggest that S. sebiferum from the introduced range had lower resistance but higher tolerance to specialist herbivores. Both defense strategies could have evolved as a response to the escape from natural enemies in the introduced range.     

Timing of exposure to ozone affects reproductive sensitivity and compensatory ability in Brassica campestris

It is well established that exposure to ozone (O₃) may impair vegetative growth and reproductive development in plants, although the consequences for yield depend on the effectiveness of the compensatory processes induced. This study examined the effects of exposing the terminal inflorescence of Brassica campestris L. to 100 ppb O₃ for 6 h d⁻¹ on four consecutive days during early flowering while the vegetative organs received charcoal-filtered air. The ordered predictability of development in B. campestris is ideal for studies of the impact of abiotic stress factors such as O₃ on reproductive development and seed production. Effects on reproductive development and seed yield characteristics were determined for floral sites exposed at different developmental stages. Flower and pod numbers on the terminal raceme were unaffected by exposure, but effects on pod length varied depending on the developmental stage of floral sites during exposure. Increased ovule abortion and precocious seed germination in the pods of O₃-treated plants reduced mature seed number pod⁻¹. Although the individual weight of mature seeds was slightly greater in O₃-treated plants, seed yield pod⁻¹ and seed yield plant⁻¹ were reduced due to the lower seed number pod⁻¹. Seed from O₃-treated plants germinated more rapidly than control seed irrespective of the stage of floral development during exposure. The results show that exposure to realistic O₃ episodes during the early stages of flowering significantly reduced seed yield without impairing the vigour of the seeds produced. The physiological origins of these effects are discussed.     

A facultative mutualism between aphids and an invasive ant increases plant reproduction

1. The consequences to plants of ant–aphid mutualisms, particularly those involving invasive ants, are poorly studied. Ant–aphid mutualisms may increase or decrease plant fitness depending on the relative cost of herbivory by ant‐tended aphids versus the relative benefit of increased ant suppression of other (non‐aphid) herbivores. 2. We conducted field and greenhouse experiments in which we manipulated the presence and absence of cotton aphids (Aphis gossypii) on cotton plants to test the hypothesis that a mutualism between cotton aphids and an invasive ant, the red imported fire ant (Solenopsis invicta), benefits cotton plants by increasing fire ant suppression of caterpillars. We also manipulated caterpillar abundance to test whether the benefit of the mutualism varied with caterpillar density. 3. We found that more fire ants foraged on plants with cotton aphids than on plants without cotton aphids, which resulted in a significant reduction in caterpillar survival and caterpillar herbivory of leaves, flower buds, and bolls on plants with aphids. Consequently, cotton aphids indirectly increased cotton reproduction: plants with cotton aphids produced 16% more bolls, 25% more seeds, and 10% greater seedcotton mass than plants without aphids. The indirect benefit of cotton aphids, however, varied with caterpillar density: the number of bolls per plant at harvest was 32% greater on plants with aphids than on plants without aphids at high caterpillar density, versus just 3% greater at low caterpillar density. 4. Our results highlight the potential benefit to plants that host ant–hemipteran mutualisms and provide the first experimental evidence that the consequences to plants of an ant–aphid mutualism vary at different densities of non‐aphid herbivores.     

Effects of elevated temperature on chemistry of an invasive plant,its native congener and their herbivores

增温对莲子草属入侵植物与本地同属植物化学物质组成和天敌昆虫的影响 气候变暖影响植物生长和生理活动,然而气候变暖如何改变入侵植物化学物质组成并间接影响其与植食性昆虫互作还少有报道。本研究以入侵植物空心莲子草(Alternanthera philoxeroides)及其本地同属 植物莲子草(A. sessilis)为对象,探究增温对其叶片化学物质组成的影响并进一步检验这些变化如何影响两 种植食性昆虫虾钳菜披龟甲(Cassida piperata)和斜纹夜蛾(Spodoptera litura)的生长发育。通过模拟增温实验,探究增温对空心莲子草和莲子草13个叶片化学物质的影响,并用其饲养两种植食性昆虫,测量它们的生长和发育时间。研究结果显示,增温显著改变了空心莲子草和莲子草叶化学物质组成;增温降低了空心莲子草叶片氮浓度,增加了莲子草叶片总黄酮和总酚浓度;增温对其它营养物质(果糖、蔗糖、总可溶性糖和淀粉)随物种和具体物质发生改变;采用增温处理的莲子草饲养的虾钳菜披龟甲蛹重和斜纹夜蛾幼虫重量,以及增温处理的空心莲子草饲养的斜纹夜蛾幼虫重量,显著低于对照不增温处理;此外,采用增温处理的莲子草饲养的斜纹夜蛾幼虫发育时间显著延长。这些结果表明,增温对植物化学物质组成的影响随物种发生变化,增温对入侵植物和本地植物化学物质组成的影响间接改变了其与植食性昆虫的互作关系。     

Shift in competitive ability mediated by soil biota in an invasive plant

Understanding the shifts in competitive ability and its driving forces is key to predict the future of plant invasion. Changes in the competition environment and soil biota are two selective forces that impose remarkable influences on competitive ability. By far, evidence of the interactive effects of competition environment and soil biota on competitive ability of invasive species is rare. Here, we investigated their interactive effects using an invasive perennial vine, Mikania micrantha. The competitive performance of seven M. micrantha populations varying in their conspecific and heterospecific abundance were monitored in a greenhouse experiment, by manipulating soil biota (live and sterilized) and competition conditions (competition‐free, intraspecific, and interspecific competition). Our results showed that with increasing conspecific abundance and decreasing heterospecific abundance, (1) M. micrantha increased intraspecific competition tolerance and intra‐ vs. interspecific competitive ability but decreased interspecific competition tolerance; (2) M. micrantha increased tolerance of the negative soil biota effect; and (3) interspecific competition tolerance of M. micrantha was increasingly suppressed by the presence of soil biota, but intraspecific competition tolerance was less affected. These results highlight the importance of the soil biota effect on the evolution of competitive ability during the invasion process. To better control M. micrantha invasion, our results imply that introduction of competition‐tolerant native plants that align with conservation priorities may be effective where M. micrantha populations are long‐established and inferior in inter‐ vs. intraspecific competitive ability, whereas eradication may be effective where populations are newly invaded and fast‐growing.     

Experience matters: prior exposure to plant toxins enhances diversity of gut microbes in herbivores

For decades, ecologists have hypothesised that exposure to plant secondary compounds (PSCs) modifies herbivore‐associated microbial community composition. This notion has not been critically evaluated in wild mammalian herbivores on evolutionary timescales. We investigated responses of the microbial communities of two woodrat species (Neotoma bryanti and N. lepida). For each species, we compared experienced populations that independently converged to feed on the same toxic plant (creosote bush, Larrea tridentata) to naïve populations with no exposure to creosote toxins. The addition of dietary PSCs significantly altered gut microbial community structure, and the response was dependent on previous experience. Microbial diversity and relative abundances of several dominant phyla increased in experienced woodrats in response to PSCs; however, opposite effects were observed in naïve woodrats. These differential responses were convergent in experienced populations of both species. We hypothesise that adaptation of the foregut microbiota to creosote PSCs in experienced woodrats drives this differential response.     

Feeding responses of specialist herbivores to plant extracts and pure allelochemicals: effects of prolonged exposure

We investigated the possibility of decreased feeding deterrent response following prolonged exposure in three specialist herbivores, Plutella xylostella (larvae), Pseudaletia unipuncta (larvae), and Epilachna varivestis (adults) through leaf disc choice and no‐choice tests. Neonate larvae (<24 h old) of the three species were reared on their host plants sprayed with the antifeedants until tested. Our results demonstrated that Ps. unipuncta did not show a decrease in feeding deterrent response to extracts of Melia volkensii (choice and no‐choice tests) or oregano (Origanum vulgare) (choice test) following prolonged exposure. Plutella xylostella exhibited a decrease in feeding deterrent response to M. volkensii in a choice test only, but not to oregano. Although not significant, we did find a trend toward decreasing feeding deterrent response to M. volkensii by both species in no‐choice tests. However, both species exhibited a significant decrease in feeding deterrent response to pure allelochemicals (Ps. unipuncta to thymol, and P. xylostella to thymol and toosendanin) compared with the naïve groups, following prolonged exposure in leaf disc choice tests. Epilachna varivestis showed a significant decrease in feeding deterrent responses to both oregano and thymol in leaf disc choice tests. We conclude from our present and previous studies that not only are there interspecific differences between generalist and specialist species but also among specialist species.     

Warming increases the spread of an invasive thistle

Background

Global warming and shifted precipitation regimes increasingly affect species abundances and distributions worldwide. Despite a large literature on species'' physiological, phenological, growth, and reproductive responses to such climate change, dispersal is rarely examined. Our study aims to test whether the dispersal ability of a non-native, wind-dispersed plant species is affected by climate change, and to quantify the ramifications for future invasion spread rates.

Methodology/Principal Findings

We experimentally increased temperature and precipitation in a two-cohort, factorial field study (n = 80). We found an overwhelming warming effect on plant life history: warming not only improved emergence, survival, and reproduction of the thistle Carduus nutans, but also elevated plant height, which increased seed dispersal distances. Using spatial population models, we demonstrate that these empirical warming effects on demographic vital rates, and dispersal parameters, greatly exacerbate spatial spread. Predicted levels of elevated winter precipitation decreased seed production per capitulum, but this only slightly offset the warming effect on spread. Using a spread rate decomposition technique (c*-LTRE), we also found that plant height-mediated changes in dispersal contribute most to increased spread rate under climate change.

Conclusions/Significance

We found that both dispersal and spread of this wind-dispersed plant species were strongly impacted by climate change. Dispersal responses to climate change can improve, or diminish, a species'' ability to track climate change spatially, and should not be overlooked. Methods that combine both demographic and dispersal responses thus will be an invaluable complement to projections of suitable habitat under climate change.     

Terrestrial deposition of aquatic insects increases plant quality for insect herbivores and herbivore density

1. Mobile organisms such as emergent aquatic insects can subsidise land with aquatic nutrients, creating a link between terrestrial and aquatic ecosystems. 2. Deposition of aquatic insects on land produces bottom‐up effects in arthropod detritivore communities and may also affect plants and plant–herbivore interactions. 3. To investigate the effects of insect deposition on plant–herbivore interactions, we conducted a field experiment and surveys of tealeaf willow (Salicaceae; Salix phylicifolia Coste) and July highflyer caterpillars (Geometridae; Hydriomena furcata Thunberg) at lakes in Northeast Iceland with either high‐ or low‐midge density and deposition to land. 4. It was found that willow at high‐midge lakes had 8–11% higher nitrogen content compared with willow at low‐midge lakes. In addition, natural caterpillar density was 4–6 times higher and caterpillars were 72% heavier at high‐midge lakes than low‐midge lakes. 5. A fully reciprocal caterpillar transplant experiment among willow at high‐ and low‐midge lakes was performed to separate the influence of habitat and midge effects on caterpillar performance. 6. After transplant, pupae of July Highflyer caterpillars were on average 11% heavier at high‐midge sites compared with low‐midge sites. However, this difference was not statistically significant. 7. The present findings indicate that cross‐ecosystem subsidies in the form of aquatic insects can increase plant foliar quality and the abundance of insect herbivores in recipient ecosystems.