Existing and emerging high impact invasive species are characterized by higher functional responses than natives

Predicting ecological impacts of invasive species and identifying potentially damaging future invaders are research priorities. Since damage by invaders is characterized by their depletion of resources, comparisons of the ‘functional response’ (FR; resource uptake rate as a function of resource density) of invaders and natives might predict invader impact. We tested this by comparing FRs of the ecologically damaging ‘world''s worst’ invasive fish, the largemouth bass (Micropterus salmoides), with a native equivalent, the Cape kurper (Sandelia capensis), and an emerging invader, the sharptooth catfish (Clarias gariepinus), with the native river goby (Glossogobius callidus), in South Africa, a global invasion hotspot. Using tadpoles (Hyperolius marmoratus) as prey, we found that the invaders consumed significantly more than natives. Attack rates at low prey densities within invader/native comparisons reflected similarities in predatory strategies; however, both invasive species displayed significantly higher Type II FRs than the native comparators. This was driven by significantly lower prey handling times by invaders, resulting in significantly higher maximum feeding rates. The higher FRs of these invaders are thus congruent with, and can predict, their impacts on native communities. Comparative FRs may be a rapid and reliable method for predicting ecological impacts of emerging and future invasive species.     

Ecological impacts of an invasive predator explained and predicted by comparative functional responses

Forecasting the ecological impacts of invasive species is a major challenge that has seen little progress, yet the development of robust predictive approaches is essential as new invasion threats continue to emerge. A common feature of ecologically damaging invaders is their ability to rapidly exploit and deplete resources. We thus hypothesized that the ‘functional response’ (the relationship between resource density and consumption rate) of such invasive species might be of consistently greater magnitude than those of taxonomically and/or trophically similar native species. Here, we derived functional responses of the predatory Ponto-Caspian freshwater ‘bloody red’ shrimp, Hemimysis anomala, a recent and ecologically damaging invader in Europe and N. America, in comparison to the local native analogues Mysis salemaai and Mysis diluviana in Ireland and Canada, respectively. This was conducted in a novel set of experiments involving multiple prey species in each geographic location and a prey species that occurs in both regions. The predatory functional responses of the invader were generally higher than those of the comparator native species and this difference was consistent across invaded regions. Moreover, those prey species characterized by the strongest and potentially de-stabilizing Type II functional responses in our laboratory experiments were the same prey species found to be most impacted by H. anomala in the field. The impact potential of H. anomala was further indicated when it exhibited similar or higher attack rates, consistently lower prey handling times and higher maximum feeding rates compared to those of the two Mysis species, formerly known as ‘Mysis relicta’, which itself has an extensive history of foodweb disruption in lakes to which it has been introduced. Comparative functional responses thus merit further exploration as a methodology for predicting severe community-level impacts of current and future invasive species and could be entered into risk assessment protocols.     

Low resource availability limits weed invasion of tropical savannas

The savanna biome is one of the least invaded among global biomes, although the mechanisms underpinning its resistance to alien species relative to other biomes is not well understood. Invaders generally are at the resource acquisitive end of functional global plant trait variation and in low-resource savanna environments we might expect that successful invaders will only outperform native species under resource rich or highly disturbed conditions. However, invaders may also directly exploit resource stressed environments using resource conservative traits in some situations. It’s also possible that successful invaders and native species largely overlap in their trait profiles indicating site specific environmental factors are responsible for invader success in particular contexts rather than a general trait and functional divergence between invaders and native species. To address these various hypotheses, we compared a suite of morphological and physiological traits in graminoid and herbaceous native and co-occurring invasive plant species across a range of habitats in savannas of the Kimberley region of northern Australia. Invader grass species had traits associated with resource acquisition and fast growth rates, such as high SLA and leaf nutrient contents. In contrast, dominant native perennial grasses had traits characteristic of resource conservation and slow growth in resource stressed conditions. Trait profiles among invasive forbs and legumes exhibited stress tolerant traits relative to their native counterparts. Invaders also displayed strong divergence in reproductive traits, suggesting diverse responses to disturbance not indicated by leaf economic traits alone. These results suggest that savannas may be resistant to invaders with resource acquisitive traits due to their strong resource limitation.     

Predicting the ecological impacts of a new freshwater invader: functional responses and prey selectivity of the ‘killer shrimp’, Dikerogammarus villosus,compared to the native Gammarus pulex

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Extended leaf phenology may drive plant invasion through direct and apparent competition

Invasive plants can inflict great harm, yet drivers of successful invasion remain unclear. Many invaders of North American deciduous forests exhibit extended leaf phenology (ELP), or longer growing season relative to natives. ELP may grant invaders competitive advantages, but we argue that ELP more potently drives invasion in the presence of herbivores. ELP invaders can support herbivores by lessening starvation during winter; consequently, native plants may suffer when attacked later through apparent competition. As modeled here, even short ELP can promote competitive success of invaders, and apparent competition sharply enhances ELP invader dominance. In ‘partial enemy escape’ scenarios, a less palatable ELP invader nearly excludes a preferred native where an invader without ELP could not. Together, ELP and apparent competition enhance invasion even when biotic resistance should suppress it, i.e. when the invader competes weakly or provides preferred forage. Thus, ELP‐apparent competition interactions grant invaders considerable success while challenging core tenets of invasion ecology.     

A functional trait perspective on plant invasion

Background and Aims

Global environmental change will affect non-native plant invasions, with profound potential impacts on native plant populations, communities and ecosystems. In this context, we review plant functional traits, particularly those that drive invader abundance (invasiveness) and impacts, as well as the integration of these traits across multiple ecological scales, and as a basis for restoration and management.

Scope

We review the concepts and terminology surrounding functional traits and how functional traits influence processes at the individual level. We explore how phenotypic plasticity may lead to rapid evolution of novel traits facilitating invasiveness in changing environments and then ‘scale up’ to evaluate the relative importance of demographic traits and their links to invasion rates. We then suggest a functional trait framework for assessing per capita effects and, ultimately, impacts of invasive plants on plant communities and ecosystems. Lastly, we focus on the role of functional trait-based approaches in invasive species management and restoration in the context of rapid, global environmental change.

Conclusions

To understand how the abundance and impacts of invasive plants will respond to rapid environmental changes it is essential to link trait-based responses of invaders to changes in community and ecosystem properties. To do so requires a comprehensive effort that considers dynamic environmental controls and a targeted approach to understand key functional traits driving both invader abundance and impacts. If we are to predict future invasions, manage those at hand and use restoration technology to mitigate invasive species impacts, future research must focus on functional traits that promote invasiveness and invader impacts under changing conditions, and integrate major factors driving invasions from individual to ecosystem levels.     

Biodiversity influences invasion success of a facultative epiphytic seaweed in a marine forest

The biotic resistance hypothesis predicts that more diverse communities should have greater resistance to invasions than species-poor communities. However for facultative and obligate epiphytic invaders a high native species richness, abundance and community complexity might provide more resources for the invader to thrive to. We conducted surveys across space and time to test for the influence of native algal species abundance and richness on the abundance of the invasive facultative epiphytic filamentous alga Lophocladia lallemandii in a Mediterranean Cystoseira balearica seaweed forest. By removing different functional groups of algae, we also tested whether these relationships were dependent on the complexity and abundance of the native algal community. When invasion was first detected, Lophocladia abundance was positively related to species richness, but the correlation became negative after two years of invasion. Similarly, a negative relationship was also observed across sites. The removal experiment revealed that more complex native communities were more heavily invaded, where also a positive relationship was found between native algal richness and Lophocladia, independently of the native algal abundance. Our observational and experimental data show that, at early stages of invasion, species-rich seaweed forests are not more resistant to invasion than species-poor communities. Higher richness of native algal species may increase resource availability (i.e. substrate) for invader establishment, thus facilitating invasion. After the initial invasion stage, native species richness decreases with time since invasion, suggesting negative impacts of invasive species on native biodiversity.     

After an invasion: understanding variation in grassland community recovery following removal of a high-impact invader

A pervasive problem in invasion ecology is the limited recovery of native communities following removal of invaders. Little evidence exists on the causes of variation in post-invasion recovery. In a 4-year experiment using 65 sets of matched plots, we imposed an invader removal treatment (with control) on heterogeneous grassland plots invaded or uninvaded by an aggressive recent arrival, Aegilops triuncialis (barb goatgrass). We tested the validity of plot matching using transplants and soil analyses. We analyzed the community-level correlates of invader impacts, removal treatment side effects, and community recovery, each defined in two ways: by compositional similarity to uninvaded plots, and by relative native species richness. Recovery of native species richness in invaded and treated plots was high (approaching 100 %) although recovery of composition was not high (median 71 % Bray–Curtis dissimilarity to uninvaded untreated plots). We measured resilience as the residuals of community recovery in models that controlled for invader impacts and removal treatment side effects. Compositional resilience was highest where the uninvaded communities had the least cover by other invaders in the same functional group as the focal invader. Richness resilience was highest where the uninvaded communities had the lowest native species richness. Our study suggests that the recovery of native species per se may be a more relevant goal than the recovery of the exact pre-invasion species composition of particular sites, particularly in cases where pre-invasion species composition included exotic species other than the focal invader.     

Between‐lake variation in the trophic ecology of an invasive crayfish

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Can limiting similarity increase invasion resistance? A meta‐analysis of experimental studies

Synthesis We used meta‐analyses to examine experimental evidence that functional similarity between invaders and resident communities reduces invasion. We synthesized evidence from studies that experimentally added seed to resident communities in which the functional group composition had been manipulated. We found communities containing functionally similar resident species reduced invasion of forb but not grass invaders. However, experimental design dramatically influenced the results – with evidence for limiting similarity only found in artificially assembled communities, and not when studies used functional group removal from more ‘natural communities’. We suggest that functional group similarity plays a limited role in biotic resistance in established communities. The principle of limiting similarity suggests that species must be functionally different to coexist; based on the assumption that inter‐specific competition should be greatest between functionally similar species. There has been controversy over the generality of this assembly rule for plant communities with some studies finding evidence for limiting similarity and others not. One approach to testing this is to examine the ‘invasion’ success of species into communities in which the functional group composition has been manipulated. Using a meta‐analysis approach, we examined the generality of limiting similarity for plant communities based on published experimental studies. We asked – is establishment of an invading species less successful if it belongs to a functional group that is already present in the community compared to a community in which that functional group is absent? We explored separately colonisation (i.e. germination, establishment or seedling survival) and performance (i.e. biomass, cover or growth) of different functional groups (forbs and grasses) and experimental designs (removal experiments of more or less natural communities and synthetic‐assemblage experiments). We found that communities containing functionally similar resident species did reduce invader colonisation and performance of forb invaders, but did not reduce colonisation or performance of grass invaders. Evidence in support of limiting similarity was only detected in synthetic‐assemblage experiments and not when studies used functional group removal from ‘natural’ communities. Functional similarity is an important aspect of biotic resistance for forb invaders, but was only found in artificial communities. This has implications for restoration ecology especially when communities are built de novo. However, we suggest that limiting similarity plays a limited role in biotic resistance, because no evidence was detected in established communities.     

Replacement of a native freshwater macroinvertebrate species by an invader: implications for biological water quality monitoring

The rate of freshwater invasions may be increasing, and macroinvertebrate invaders can have significant impacts on native macroinvertebrate assemblage structure through biotic interactions. More pollution-tolerant invaders can often replace native species. We examined implications of a species replacement for accurate biological monitoring of river systems using biotic indices. Our study uses Northern Ireland and the Isle of Man as examples of countries that possess river networks with many riverine macroinvertebrate assemblages subject to invasion. The introduced amphipod crustacean Gammarus pulex has replaced the native species G. duebeni celticus in many rivers in N. Ireland and the Isle of Man. Extensive seasonal data sets (119 sites) from three river networks, Lough Neagh and the Lagan in N. Ireland, and island-wide in the Isle of Man, were used to investigate the assumed equivalence of the native and invader in biotic indices concerned with the water quality monitoring system. Based on the derivation of the Biological Monitoring Working Party (BMWP) score, the Average Score Per Taxon (ASPT), as an example of a commonly used biotic index of water quality, we found index scores were lower in G. pulex sites compared to G. d. celticus-only sites. This indicated that assemblages were dominated by taxa more tolerant of organic pollution in the invader sites and more sensitive in the native sites. Inclusion of the invader in generation of the ASPT index, overinflated the ASPT values obtained compared to those with the native’s inclusion. This questions the accuracy of the ASPT and similar indices in rivers where the invader had replaced the native. We argue that with invasion pressures increasing, the validity of water quality indices such as the BMWP/ASPT needs to be re-examined in catchments where invaders have replaced natives. Indices such as the BMWP/ASPT are based on family level taxa and are inevitably coarse in their resolution given the wide range of water qualities tolerated by different genera within families. We argue that this resolution is even more compromised by the presence of very pollution-tolerant invaders, who may have replaced natives in disturbed or degraded river systems. The whole structure of water quality indices such as the BMWP/ASPT may need revising to take into account the presence of invasive species within monitored assemblages.     

Is biotic resistance enhanced by natural variation in diversity?

Theories linking diversity to ecosystem function have been challenged by the widespread observation of more exotic species in more diverse native communities. Few studies have addressed the underlying processes by dissecting how biotic resistance to new invaders may be shaped by the same environmental influences that determine diversity and other community properties. In grasslands with heterogeneous soils, we added invaders and removed competitors to analyze the causes of invasion resistance. Abiotic resistance was measured using invader success in the absence of the resident community. Biotic resistance was measured as the reduction in invader success in the presence of the resident community. Invaders were most successful where biotic resistance was lowest and abiotic resistance was highest, confirming the dominant role of biotic resistance. Contrary to theory, though, biotic resistance was highest where both species richness and functional diversity were lowest. In the multivariate framework of a structural equation model, biotic resistance was independent of community diversity, and was highest where fertile soils led to high community biomass. Seed predation slightly augmented biotic resistance without qualitatively changing the results. Soil‐related genotypic variation in the invader also did not affect the results. We conclude that in natural systems, diversity may be correlated with invasibility and yet have little effect on biotic resistance to invasion. More generally, the environmental causes of variation in diversity should be considered when examining the potential functional consequences of diversity.     

Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

Co-invasion of similar invaders results in analogous ecological impact niches and no synergies

Exotic species can cause changes to their invaded ecosystems, which can be large and long lasting. Despite most landscapes being invaded by multiple exotic plant species, >90 % of impact studies only characterize the impacts of single species. Therefore, our knowledge of invasive plant impacts does not reflect the co-invaded nature of most landscapes, potentially ignoring complex interactions among exotic species. Our objective was to characterize potential invader synergies (positive interactions) on biotic and abiotic ecological parameters among the important forest invaders Japanese stiltgrass (Microstegium vimineum) and wavyleaf basketgrass (Oplismenus undulatifolius), which co-invade eastern US deciduous forests. To characterize synergies, we used a factorial selective removal study, as well as an observational study to further explore invader cover-impact relationships. Although both invaders can reduce native plant richness by 70 % individually or in combination, there were no impact synergies. Total cover of any combination of the two invaders had a negative quadratic effect on total, exotic, and native plant richness; i.e., all community metrics were greatest at intermediate levels of total invader cover and lowest at maximum invader cover. Native richness was more greatly affected than exotic richness by the co-invasion. Soil metrics had no clear trend in either study. Japanese stiltgrass and wavyleaf basketgrass appear to have overlapping impact niches—the number, magnitude, and direction of biotic and abiotic changes to the invaded ecosystem—that only vary in impact magnitude, not breadth. As a result of their overlapping impact niches and non-synergies in this co-invaded system, the addition of the recent invader wavyleaf basketgrass has not resulted in additional changes to the invaded forests. Future impact studies should focus on multiple species and identifying synergies, especially as they relate to invader cover, which informs ecological interactions and management prioritization.     

Contrasting the effects of environment,dispersal and biotic interactions to explain the distribution of invasive plants in alpine communities

Despite considerable efforts devoted to investigate the community assembly processes driving plant invasions, few general conclusions have been drawn so far. Three main processes, generally acting as successive filters, are thought to be of prime importance. The invader has to disperse (1st filter) into a suitable environment (2nd filter) and succeed in establishing in recipient communities through competitive interactions (3rd filter) using two strategies: competition avoidance by the use of different resources (resource opportunity), or competitive exclusion of native species. Surprisingly, despite the general consensus on the importance of investigating these three processes and their interplay, they are usually studied independently. Here we aim to analyse these three filters together, by including them all: abiotic environment, dispersal and biotic interactions, into models of invasive species distributions. We first propose a suite of indices (based on species functional dissimilarities) supposed to reflect the two competitive strategies (resource opportunity and competition exclusion). Then, we use a set of generalised linear models to explain the distribution of seven herbaceous invaders in natural communities (using a large vegetation database for the French Alps containing 5,000 community-plots). Finally, we measure the relative importance of competitive interaction indices, identify the type of coexistence mechanism involved and how this varies along environmental gradients. Adding competition indices significantly improved model’s performance, but neither resource opportunity nor competitive exclusion were common strategies among the seven species. Overall, we show that combining environmental, dispersal and biotic information to model invasions has excellent potential for improving our understanding of invader success.     

Biotic resistance buffers the effects of nutrient enrichment on the success of a highly invasive aquatic plant

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Evolutionary responses of natives to introduced species: what do introductions tell us about natural communities?

Biological invasions dramatically affect the distribution, abundance and reproduction of many native species. Because of these ecological effects, exotic species can also influence the evolution of natives exposed to novel interactions with invaders. Evolutionary changes in natives in response to selection from exotics are usually overlooked, yet common responses include altered anti-predator defenses, changes in the spectrum of resources and habitats used, and other adaptations that allow native populations to persist in invaded areas. Whether a native population is capable of responding evolutionarily to selection from invaders will depend on the demographic impact of the invader, the genetic architecture and genetic variability of the native population and potentially the history of previous invasions. In some cases, natives will fail to evolve or otherwise adapt, and local or global extinction will result. In other cases, adaptive change in natives may diminish impacts of invaders and potentially promote coexistence between invaders and natives. Here, we review the evidence for evolutionary responses of native species to novel community members. We also discuss how the effects of introduced species may differ from those caused by natural range expansions of native species. Notably, introduced species may come from remote biotas with no previous evolutionary history with the native community. In addition, the rate of addition of introduced species into communities is much greater than all but the most extreme cases of historical biotic exchange. Understanding the evolutionary component of exotic/native species interactions is critical to recognizing the long-term impacts of biological invasions, and to understanding the role of evolutionary processes in the assembly and dynamics of natural communities.     

Community modification by a grass invader has differing impacts for marine habitats

There is increasing recognition of invasive species impacts but less is known about how modifications may differ under variable environmental contexts. In particular, it is generally unknown whether impacts of single invasive species can vary among habitats and what the consequences of this variability may be. We used a multi-site comparative approach to examine the impacts of a single invader, the marine grass Spartina anglica , on estuarine habitats with different native species assemblages and physical conditions. We found that range (extent), abundance, and effects on sediment and native plant species vary depending on the habitat invaded. S. anglica has by far the greatest range and abundance in mudflats and low salinity marshes compared to high salinity marshes and cobble beaches. Changes in sediment characteristics also substantially differed among habitats, with invaded areas in some habitats experiencing greater sediment accretion, water content, and salinity than other habitats. In addition, in opposition to the theory that strong invaders decrease species diversity, we found that native plant diversity in our plots increased within invaded areas in some habitats while it declined in others. These variable modifications suggest that single invaders, even species that are considered strong interactors, do not produce the same effect in all habitats. We suggest that understanding impact variability can help predict how invasive species will respond to environmental changes, new habitats, and management strategies.     

Darwin’s naturalization hypothesis up-close: Intermountain grassland invaders differ morphologically and phenologically from native community dominants

Darwin’s naturalization hypothesis predicts that successful invaders will tend to differ taxonomically from native species in recipient communities because less related species exhibit lower niche overlap and experience reduced biotic resistance. This hypothesis has garnered substantial support at coarse scales. However, at finer scales, the influence of traits and niche use on invasibility and invader impacts is poorly understood. Within grasslands of western Montana, USA, we compared morphological and phenological traits for five top exotic invasive forbs and five dominant native forbs using multivariate techniques to examine niche separation between exotics and natives. Exotic forbs differed from native forbs in multivariate space. Phenologically, native forbs synchronized vegetative growth with bolting and flowering early in spring. In contrast, exotics initiated vegetative growth concurrent with natives but bolted and flowered later. Morphologically, vegetative growth of exotics was three times shorter and narrower, but flowering stem growth was 35% taller and 65% wider than the natives. Collectively, these patterns suggest different strategies of resource uptake and allocation. Additionally, following wildfire, survival was four times higher for exotics compared to natives, and three times more of the surviving exotics flowered. The exotics we examined appeared to be exploiting an empty community-level niche. The resulting pattern of trait differences between exotics and natives suggests a predictable pattern of invasion and a predictable trajectory of community change. Our results illustrate how quantifying trait differences between invading exotics and natives at the within-community scale can improve understandings of community invasibility and invader impacts.     

A call for an end to calls for the end of invasion biology

Calls for the end of invasion biology are misguided. There is no evidence that modern invasion biology has progressed slowly in its short life. Although some aspects of biological invasions fit comfortably in the framework of ecological succession, many others do not. Some native species, particularly in the wake of various anthropogenic impacts, behave like invasive non‐native species, but the probability and degree of harmful impact are greater for non‐native than for native species. Neither native nor non‐native species suffer lack of attention and research by virtue of the fact that invasion biology focuses on the latter. Basing management solely on current observed impact is highly risky because impacts may be subtle but nonetheless important, and impacts often change, as they are contingent on the physical or biotic environment. The known harmful impacts of many non‐native species suggest that recent introductions warrant attention even if impacts are not evident. Neither is the focus of modern invasion biology on non‐native species motivated by xenophobia. Rather, it reflects the recognition of their likelihood of harmful impact. A related call for the end of traditional restoration ecology shares many features with calls to terminate invasion biology, not least because management of invasive non‐native species is a key component of restoration ecology. Such species are a dominant element in generating the ‘novel ecosystems’ that are said to render traditional restoration ecology obsolete. The argument that both invasion management and traditional restoration are largely futile endeavors is contradicted by substantial and growing successes in both fields.