Invasive shrimp Cinetorhynchus erythrostictus (Decapoda: Caridea) misidentified in the marine aquarium trade: Niche overlap with a native congeneric species

In a recent article in Ecology, Leffler et al. (2014) presented a potentially new perspective on the importance of trait differences between native and invasive exotic plants in explaining invasions in local native communities. The new perspective brought forward is that, if trait differences between invasive and native species are likely to be important in explaining exotic plant invasion, the differences must be larger than those observed between native species in the new community. A meta-analysis of previous studies searching for trait differences was presented, with the general finding that the magnitudes of trait differences between invasive and native species tend not to differ from those observed between native species only. Leffler et al. (2014) interpret this result as evidence that trait differences are highly context dependent, and that mechanisms other than trait differences are likely to be more important in most cases of invasion. We acknowledge that there is no universal explanation of successful exotic invasion into native communities. Moreover, we do not believe that invasive plant species always have trait values that differ substantially from the traits present in the native community, or that trait differences are important for invasion in all cases. However, we cannot agree with the criterion stipulated by Leffler et al. (2014), namely that a trait difference between invasive and native species can only be important to invasion success if it is greater than the differences among natives. Leffler et al. (2014) do not explain the logic behind the criterion, but a flaw of the criterion is that it will discount cases when a successfully invading species has intermediate trait values that are not represented by native species. Leffler et al. (2014) seem to focus on trait differences as representing niche differences among species. Consider the scenarios of niche differences among native and exotic invasive species in Fig. 1. If a trait is related to the niche space occupied by native species in the community and the invader, for a trait difference to be important in invasion success under the criterion of Leffler et al. (2014), only the scenario in Fig. 1a would qualify. Here, the invader occupies a niche at the extreme of the niche space, compared to native species. The average niche-related trait difference between the invasive species and the natives will be greater than the average difference among natives. However, consider Fig. 1b. Here, the invader occupies a vacant niche that is intermediate between the native species (Stachowicz and Tilman 2005), and the invader would have an intermediate, niche-related trait value not represented by the native community. However, the average trait difference between the invader and native species in Fig. 1b will be smaller than the difference among native species, and under the criterion proposed, the native-invasive trait difference would be considered unimportant. Thus, the criterion proposed by Leffler et al. (2014) cannot distinguish between cases where trait values may lie between those of native species but are still distinct and cases where they are very similar to native species. Exotic species may not only invade a community by having different niche-related traits compared to native species. Some of the traits considered in the metaanalysis of Leffler et al. (2014), e.g., biomass, are arguably traits related to fitness. Such fitness-related traits also do not have to be more different between invasive and native species than among natives, for them to be important for invasions. All that is required is for the trait difference to be large enough for invasive species to have greater fitness than the native species (Fig. 1c). If this occurs and there is niche overlap between the invasive species and a native species, then the invasive species should displace the native species (MacDougall et al. 2009). The trait difference between invasive and native species should always be greater than the average native-native difference only when the trait is related to niche space and the invader is occupying a vacant niche at the extremes of the niche space available to the whole community. Thus, cases that meet the Leffler et al. (2014) criterion could be viewed as representing only one of three possible scenarios where differences in traits between native and invasive species are potentially important, and the only scenario where native-native differences are relevant. The challenge is to understand which of the many traits we can measure are actually related to fitness and niches of invasive and native species, and then to identify whether fitness or niche differences (or perhaps even both) have led to invasion. Manuscript received 15 July 2014; revised 7 August 2014; accepted 10 September 2014. Corresponding Editor: D. C. Laughlin. 1 Ecology Lab, Department of Biology, University of Konstanz, Universitaetsstrassse 10, Konstanz D78457 Germany. 2 E-mail: wayne.dawson@uni-konstanz.de

Trait divergence and the ecosystem impacts of invading species

Flowering phenology is a potentially important component of success of alien species, since elevated fecundity may enhance invasiveness. The flowering patterns of invasive alien plant species and related natives were studied in three regions with Mediterranean-type climate: California, Spain and South Africa's Cape region. A total of 227 invasive-native pairs were compared for seven character types across the regions, with each pair selected on the basis that they shared the same habitat type within a region, had a common growth form and pollination type, and belonged to the same family or genus. Invasive alien plant species have different patterns of flowering phenology from native species in the three regions. Whether the alien species flower earlier, later or at the same time as natives depends on the climatic regime in the native range of the aliens and the proportion of species in the invasive floras originating from different regions. Species invading at least two of the regions displayed the same flowering pattern, showing that flowering phenology is a conservative trait. Invasive species with native ranges in temperate climates flower earlier than natives, those from Mediterranean-type climates at the same time, and species from tropical climates flower later. In California, where the proportion of invaders from the Mediterranean Basin is high, the flowering pattern did not differ between invasive and native species, whereas in Spain the high proportion of tropical species results in a later flowering than natives, and in the Cape region early flowering than natives was the result of a high proportion of temperate invaders. Observed patterns are due to the human-induced sympatry of species with different evolutionary histories whose flowering phenology evolved under different climatic regimes. The severity of the main abiotic filters imposed by the invaded regions (e.g. summer drought) has not been strong enough (yet) to shift the flowering pattern of invasive species to correspond with that of native relatives. It does, however, determine the length of the flowering season and the type of habitat invaded by summer-flowering aliens. Results suggest different implications for impacts at evolutionary time scales among the three regions.

Inter-specific interactions around resources, such as nesting sites, are an important factor by which invasive species impact native communities. As resource availability varies across different environments, competition for resources and invasive species impacts around those resources change. In urban environments, changes in habitat structure and the addition of introduced species has led to significant changes in species composition and abundance, but the extent to which such changes have altered competition over resources is not well understood. Australia’s cities are relatively recent, many of them located in coastal and biodiversity-rich areas, where conservation efforts have the opportunity to benefit many species. Australia hosts a very large diversity of cavity-nesting species, across multiple families of birds and mammals. Of particular interest are cavity-breeding species that have been significantly impacted by the loss of available nesting resources in large, old, hollow-bearing trees. Cavity-breeding species have also been impacted by the addition of cavity-breeding invasive species, increasing the competition for the remaining nesting sites. The results of this additional competition have not been quantified in most cavity breeding communities in Australia. Our understanding of the importance of inter-specific interactions in shaping the outcomes of urbanization and invasion remains very limited across Australian communities. This has led to significant gaps in the understanding of the drivers of interspecific interactions and how such interactions shape resource use in highly modified environments. This knowledge deficit limits the effectiveness of conservation and management efforts to mitigate the loss of nest sites for native species and the effectiveness of ongoing management actions, such as the addition of artificial nesting boxes.To address these gaps, I examined the changes in native and invasive alien species use of resources across urban gradients and examined the factors shaping inter-specific interactions to provide a mechanistic framework for understanding competition at the community scale and invasive species impact on native cavity-nesting species. The research presented here provides the first community-level description of competitive interactions, describes a mechanism driving interaction intensity, and provides a method to predict where and when invasive species impacts on nesting are likely to occur. Additionally, I show that both habitat structure and predation are important processes for urban cavity-nesting birds. My work highlights that many birds make some use of urban habitats. While there remains much to learn about how urban environments can be improved to support long term persistence of individual species, conservation and management efforts can enhance the opportunity to help a large number of birds in Australian cities.This study established a large-scale field project that included eight field sites across southeast QLD and NSW. The study was designed along both regional-scale invasion gradients, with focus on the Common (Indian) myna (Acridotheres tristis), and local-scale urbanization gradients. I discovered that interaction webs around tree hollows in urban environments are dominated by a small number of highly aggressive species, including the invasive common myna and that interaction frequency between species was positively correlated with the overlap in preferred breeding sites (breeding niche; Chapter 2). To investigate how breeding niche overlap can be applied to a community with many unstudied invasive species, I examined the extent of niche overlap between species in the cavity-breeding birds of Tasmania detailing the potential interaction network for the island (Chapter 3). I discovered that niche overlap predicted important inters-specific interactions and that most native cavity-nesting species potentially face increased competition for nesting sites as a result of the addition of multiple invasive species.In Chapter 4, I examined how birds use supplementary nesting sites (i.e, nest boxes) varied across urban environments in different landscape contexts across New South Wales and Queensland), and across cities which have been invaded for different lengths of time. This was aimed at understanding the importance of artificial nest boxes as supplementary resources in areas with limited breeding cavities due to human impacts. There was high variation in the rates of nest box use both by invasive and native species such that the importance of nest boxes as supplemental nesting sites will differ across cities. I found that many nesting attempts in the nest boxes failed, highlighting the need to account for both competition and predation when designing and deploying nest boxes.Finally, in Chapter 5, I explored patterns in the community composition of urban bird communities. This allowed me to test the impacts of regional invasion dynamics and local variation in habitat on avian community richness and abundance. I discovered that increasing habitat structure and decreasing abundance of despotic species (i.e., species which influence community composition through aggressive interactions such as the noisy miner) predicted greater native species richness. More broadly the work presented in this thesis highlights that urban conservation efforts will be improved by a better including into management actions and future work the factors influencing breeding dynamics and resource use.

Predicting whether alien species will invade a native community is a key challenge in invasion ecology. One factor that may help predict invasion success is phylogenetic relatedness. Darwin proposed that closely related species tend to share similar niches, although this relationship may be influenced by various ecological and evolutionary factors. To test this, we classified alien Asteraceae species in China into three categories based on their invasion status and the extent of ecological damage: introduced, naturalized, and invasive. We then compared the genetic relationships and niche overlap between alien and native Asteraceae species. We found that invasive Asteraceae species are more closely related to native Asteraceae species than are introduced and naturalized species. However, alien Asteraceae species (including introduced, naturalized, and invasive species) exhibited relatively low niche overlap with native Asteraceae species. These findings suggest that the main premise underlying Darwin’s naturalization conundrum, namely, the universality of phylogenetic niche conservatism, may not hold true. Instead, our findings indicate that alien species are more likely to invade successfully when they are more closely related to native plants, exhibit less niche overlap, and maintain conservative niches during the invasion process. These findings provide new insights into the mechanisms of alien plant invasions, highlight the relationship between alien species invasions and native community vulnerability, and offer important insights into the development of effective biological invasion management strategies.

Latitudinal variation of life-history traits of an exotic and a native impatiens species in Europe

QuestionsInvasive species establish either by possessing traits, or trait trade‐offs similar to native species, suggesting pre‐adaptation to local conditions; or by having a different suite of traits and trait trade‐offs, which allow them to occupy unfilled niches. The trait differences between invasives and non‐invasives can inform on which traits confer invasibility. Here, we ask: (a) are invasive species functionally different or similar to native species? (b) which traits of invasives differ from traits of non‐invasive aliens and thus confer invasibility? and (c) do results from the sub‐Antarctic region, where this study was conducted, differ from findings from other regions?LocationSub‐Antarctic Marion Island.MethodsWe measured 13 traits of all terrestrial native, invasive and non‐invasive alien plant species. Using principal components analysis and phylogenetic generalized least‐squares models, we tested for differences in traits between invasive (widespread alien species) and native species. Bivariate trait relationships between invasive and native species were compared using standardized major axis regressions to test for differences in trait trade‐offs between the two groups. Second, using the same methods, we compared the traits of invasive species to non‐invasive aliens (alien species that have not spread).ResultsBetween invasive and native species, most traits differed, suggesting that the success of invasive species is mediated by being functionally different to native species. Additionally, most bivariate trait relationships differed either in terms of theiry‐intercept or their position on the axes, highlighting that plants are positioned differently along a spectrum of shared trait trade‐offs. Compared to non‐invasive aliens, invasive species had lower plant height, smaller leaf area, lower frost tolerance, and higher specific leaf area, suggesting that these traits are associated with invasiveness. The findings for the sub‐Antarctic corresponded to those of other regions, except lower plant height which provides a competitive advantage to invaders in the windy sub‐Antarctic context.ConclusionOur findings support the expectation that trait complexes of invasive species are predominantly different to those of coexisting native species, and that high resource acquisition and low defence investment are characteristic of invasive plant species.

Invasive species have been identified as a major threat to native biodiversity and ecosystem functioning worldwide due to their superiority in spread and growth. Such superiority is explained by the invasional meltdown phenomena, which suggests that invasive species facilitate the establishment of more invasive species rather than native species by modifying the plant-soil feedback (PSF). We conducted a two-phase plant-soil feedback experiment using the native Prosopis cineraria and the invasive Prosopis juliflora in Oman. Firstly, we conditioned the soil by planting seedlings of native species, invasive species, native and invasive species "mixed", and unconditioned soil served as a control. Secondly, we tested the feedback of these four conditioned soil on the two species separately by measuring the productivity (total biomass) and the performance in the form of plant functional traits (plant height, specific leaf area (SLA), leaf nitrogen content (Nmass), leaf carbon content (Cmass) and specific root length (SRL) of native and invasive species as well as the nutrient availability in soil (soil organic carbon (SOC) and soil total nitrogen (STN)). We found that the native species produced more biomass, best performance, and higher SOC and STN when grown in soil conditioned by native species, additionally, it gave lower biomass, reduced performance, and lower SOC and STN when grown in the soil conditioned by invasive and mixed species. These results suggest negative PSF for native species and positive PSF for invasive species in the soil conditioned by invasive species, which can be considered as red flag concerning the restoration of P. cineraria as an important native species in Oman, as such positive PSF of the invasive species P. juliflora will inhibit the regeneration of P. cineraria.

Invasive species may outperform native species by acquiring more resources or by efficiently using limited resources. Studies comparing leaf traits as a metric of carbon capture strategies in native and invasive species have come to different conclusions. Some studies suggest that invasive species are better at acquiring resources, but that native and invasive species use resources similarly. Other studies have found that native and invasive species differ in resource use efficiency, which implies different biochemical or physiological mechanisms of carbon capture. To resolve this debate, we examined relationships among four leaf traits (photosynthetic rate, specific leaf area, foliar nitrogen, foliar phosphorus) in co-occurring native and invasive species from eight plant communities across five Mediterranean-climate ecosystems. We performed standardized major axis regression for all trait combinations within and across sites, testing for slope homogeneity and shifts in elevation (y-intercept) or along a common slope between species groups. Across the global dataset, native and invasive species had similar carbon capture strategies (i.e., similar slopes), with invasive species occupying a position of greater resource acquisition. However, these patterns did not hold when regions were analyzed individually. Regional differences may be driven by differences in life form between native and invasive species, and variation in soil resource availability among regions. Our context-dependent results reveal not only that management of invasive species will differ across regions but also that global comparisons of invasive and native species can be misleading.

Invasive Prosopis juliflora harbours arbuscular mycorrhizal fungal communities distinct from its native congener P. cineraria

Native plants do not benefit from arriving early, but invasives pay to arrive late

Native forest species exhibit a well-known range of ecological roles with respect to natural disturbance regimes, from pioneer phase to mature phase, and they regenerate from a range of sources, including dormant seeds, seed rain, pre-established juveniles, and resprouts from damaged adults. In contrast, the ecological roles of invasive, non-indigenous species in forest communities after natural disturbances are not well understood. Some previous studies of invasive species have emphasized their weedy nature and their ability to colonize anthropogenic disturbances. Tropical hardwood hammock forests in southern Florida experience frequent disturbance by hurricanes. Our studies of forest regeneration during two years following a recent severe hurricane suggest that invasive non-indigenous forest species exhibit the same range of ecological roles as native forest species and compete with native species for particular kinds of regeneration opportunities. To study ecological roles of non-indigenous species in regenerating forests after Hurricane Andrew, we set up four large study areas at each of three study sites that had differing amounts of hurricane-caused canopy disturbance. There were two pairs of 30 × 60 m research plots per site, and in each pair there was one control plot and one restoration plot; restoration areas were subject to an aggressive management program, focused on reducing non-indigenous vine cover. Within these study areas we subsampled vegetation in small study plots that were regularly spaced, and conducted vegetation censuses in April (the end of the dry season) and October (the end of the rainy season) for 2 yr, beginning in April 1993. We found that the source of regeneration for forest species was dependent upon the amount of canopy disturbance, the time since disturbance, and the autecology of the constituent species. Overall, 28% of the 90 species were non-indigenous: 34% of the vines (N = 32) and 24% of other life-forms (N = 58). Non-indigenous vines seemed to have a special role; not only could they compete with native vines, but they could also negatively affect the regeneration of other natives from a diverse array of sources including pre-established juveniles and resprouts from damaged adults. Both native and non-indigenous vine cover in unmanipulated study areas increased following the hurricane. Non-indigenous vine species had higher cover than native vine species, and many species formed dense “blankets.” Non-indigenous species in general (not just vines) did not differ significantly from native species in seed mass, nor were they restricted to the pioneer type of life history. Many non-indigenous species had invaded forests prior to hurricane disturbance and had their own banks of pre-established juveniles; others recruited from dormant seeds, seed rain, and/or resprouts from pre-established adults. Based on information on source of regeneration and impact on native species, we propose a classification scheme for functional roles of non-indigenous invasive species in forests. To investigate whether non-indigenous taxa had roles in other geographic regions similar to those they had in Florida, we reviewed literature for 50 taxa belonging to genera that have species known to be invasive in southern Florida. We found that these taxa were invasive or had congeners that were invasive in other geographic regions (Western Australia, the Mariana Islands, Hawaii, the Mascarene Islands, and South Africa). We propose that taxa predominantly retain their invasive, functional-role type across regions. Thus, studies of ecological roles of invasive species with respect to natural disturbance regimes in one region may help us predict invasive roles in other regions.

Diversified grasslands that contain native plant species are being recognized as important elements of agricultural landscapes and for production of biofuel feedstocks as well as a variety of other ecosystem services. Unfortunately, establishment of such grasslands is often difficult, unpredictable, and highly vulnerable to interference and invasion by weeds. Evidence suggests that soil-microbial “legacies” of invasive perennial species can inhibit growth of native grassland species. However, previous assessments of legacy effects of soil occupancy by invasive species that invade grasslands have focused on single invasive species and on responses to invasive soil occupancy in only a few species. In this study, we tested the hypothesis that legacy effects of invasive species differ qualitatively from those of native grassland species. In a glasshouse, three invasive and three native grassland perennials and a native perennial mixture were grown separately through three cycles of growth and soil conditioning in soils with and without arbuscular mycorrhizal fungi (AMF), after which we assessed seedling growth in these soils. Native species differed categorically from invasives in their response to soil conditioning by native or invasive species, but these differences depended on the presence of AMF. When AMF were present, native species largely had facilitative effects on invasive species, relative to effects of invasives on other invasives. Invasive species did not facilitate native growth; neutral effects were predominant, but strong soil-mediated inhibitory effects on certain native species occurred. Our results support the hypothesis that successful plant invaders create biological legacies in soil that inhibit native growth, but suggest also this mechanism of invasion will have nuanced effects on community dynamics, as some natives may be unaffected by such legacies. Such native species may be valuable as nurse plants that provide cost-effective restoration of soil conditions needed for efficient establishment of diversified grasslands.

Effects of co-occurring non-native invasive plant species on old-field succession

Native plant communities are often invaded by multiple alien species. It is still unclear how increasing diversity of alien invasive species suppresses the growth of native species and thus contributes to invasion success. In the subtropical monsoon region of southeast China, we experimentally created a native plant community with 18 herbaceous species. One week later, we let it be invaded by either zero (controls without invasion), one, two, four or eight alien plant species, with either high or low species evenness. After a four‐month growth period we harvested the aboveground biomass of each species. We found that increasing invasive species richness significantly increased invasive plant biomass, the biomass of all invasive and native plant species within the community, and invasion success (the ratio of invasive plant biomass to the biomass of all native and invasive plants), but it did not significantly reduce native plant biomass. Experimentally manipulating invasive species evenness did not influence invasion success and did not show any differential suppression effects on native plants. One invasive species, Sesbania cannabina , became dominant in terms of plant biomass, irrespective of its proportion in the alien plant mixtures. Throughout this experiment, effects of invasive species richness on invasion success were mainly due to such selection effects among the invasive species. On the other hand, the unchanged biomass of native species under increasing invasive plant richness suggests the presence of at least partly complementary resource niches between invasive and native species.