Exotic Range Research Articles

Niche conservatism and the invasive potential of the wild boar.

Niche conservatism, i.e. the retention of a species' fundamental niche through evolutionary time, is cornerstone for biological invasion assessments. The fact that species tend to maintain their original climate niche allows predictive maps of invasion risk to anticipate potential invadable areas. Unravelling the mechanisms driving niche shifts can shed light on the management of invasive species. Here, we assessed niche shifts in one of the world's worst invasive species: the wild boar Sus scrofa. We also predicted potential invadable areas based on an ensemble of three ecological niche modelling methods, and evaluated the performance of models calibrated with native vs. pooled (native plus invaded) species records. By disentangling the drivers of change on the exotic wild boar population's niches, we found strong evidence for niche conservatism during biological invasion. Ecological niche models calibrated with both native and pooled range records predicted convergent areas. Also, observed niche shifts are mostly explained by niche unfilling, i.e. there are unoccupied areas in the exotic range where climate is analogous to the native range. Niche unfilling is expected as result of recent colonization and ongoing dispersal, and was potentially stronger for the Neotropics, where a recent wave of introductions for pig-farming and game-hunting has led to high wild boar population growth rates. The invasive potential of wild boar in the Neotropics is probably higher than in other regions, which has profound management implications if we are to prevent their invasion into species-rich areas, such as Amazonia, coupled with expansion of African swine fever and possibly great economic losses. Although the originally Eurasian-wide distribution suggests a pre-adaptation to a wide array of climates, the wild boar world-wide invasion does not exhibit evidence of niche evolution. The invasive potential of the wild boar therefore probably lies on the reproductive, dietary and morphological characteristics of this species, coupled with behavioural thermoregulation.

Evidence for rapid evolutionary change in an invasive plant in response to biological control.

We present evidence that populations of an invasive plant species that have become re-associated with a specialist herbivore in the exotic range through biological control have rapidly evolved increased antiherbivore defences compared to populations not exposed to biocontrol. We grew half-sib families of the invasive plant Lythrum salicaria sourced from 17 populations near Ottawa, Canada, that differed in their history of exposure to a biocontrol agent, the specialist beetle Neogalerucella calmariensis. In a glasshouse experiment, we manipulated larval and adult herbivory to examine whether a population's history of biocontrol influenced plant defence and growth. Plants sourced from populations with a history of biocontrol suffered lower defoliation than naïve, previously unexposed populations, strongly suggesting they had evolved higher resistance. Plants from biocontrol-exposed populations were also larger and produced more branches in response to herbivory, regrew faster even in the absence of herbivory and were better at compensating for the impacts of herbivory on growth (i.e. they exhibited increased tolerance). Furthermore, resistance and tolerance were positively correlated among genotypes with a history of biocontrol but not among naïve genotypes. Our findings suggest that biocontrol can rapidly select for increased defences in an invasive plant and may favour a mixed defence strategy of resistance and tolerance without an obvious cost to plant vigour. Although rarely studied, such evolutionary responses in the target species have important implications for the long-term efficacy of biocontrol programmes.

Genetic patterns across an invasion's history: a test of change versus stasis for the Eurasian round goby in North America.

Biological invasions comprise accidental evolutionary experiments, whose genetic compositions underlie relative success, spread and persistence in new habitats. However, little is known about whether, or how, their population genetic patterns change temporally and/or spatially across the invasion's history. Theory predicts that most would undergo founder effect, exhibit low genetic divergence across the new range and gain variation over time via new arriving propagules. To test these predictions, we analyse population genetic diversity and divergence patterns of the Eurasian round goby Neogobius melanostomus across the two decades of its North American invasion in the Laurentian Great Lakes, comparing results from 13 nuclear DNA microsatellite lociand mitochondrial DNA cytochrome b sequences. We test whether 'genetic stasis', 'genetic replacement' and/or 'genetic supplement' scenarios have occurred at the invasion's core and expansion sites, in comparison with its primary native source population in the Dnieper River, Black Sea. Results reveal pronounced genetic divergence across the exotic range, with population areas remaining genetically distinct and statistically consistent across two decades, supporting 'genetic stasis' and 'founder takes most'. The original genotypes continue to predominate, whose high population growth likely outpaced the relative success of later arrivals. The original invasion core has stayed the most similar to the native source. Secondary expansion sites indicate slight allelic composition convergence towards the core population over time, attributable to some early 'genetic supplementation'. The geographic and temporal coverage of this investigation offers a rare opportunity to discern population dynamics over time and space in context of invasion genetic theory vs. reality.

Contrasting patterns of genetic diversity and spatial structure in an invasive symbiont-host association

Do host invaders and their associated symbiont co-invaders have different genetic responses to the same invasion process? To answer this question, we compared genetic patterns of native and exotic populations of an invasive symbiont-host association. This is an approach applied by very few studies, of which most are based on parasites with complex life cycles. We used the mitochondrial genetic marker cytochrome oxidase subunit I (COI) to investigate a non-parasitic freshwater ectosymbiont with direct life-cycle, low host specificity and well-documented invasion history. The study system was the crayfish Procambarus clarkii and its commensal ostracod Ankylocythere sinuosa, sampled in native (N American) and exotic (European) ranges. Results of analyses indicated: (1) higher genetic diversity in the symbiont than its host; (2) genetic diversity loss in the exotic range for both species, but less pronounced in the symbiont; (3) native populations genetically structured in space, with stronger patterns in the symbiont and (4) loss of spatial genetic structure in the exotic range in both species. The combination of historical, demographic and genetic data supports a higher genetic diversity of source populations and a higher propagule size that allowed the symbiont to overcome founder effects better than its host co-invader. Thus, the symbiont might be endowed with a higher adaptive potential to new hosts or off-host environmental pressures expected in the invasive range. We highlight the usefulness of this relatively unexplored kind of symbiont-host systems in the invasion context to test important ecological and evolutionary questions.

Annual monitoring reveals rapid upward movement of exotic plants in a montane ecosystem

There is increasing evidence that invasive species are threating montane ecosystems globally. However, trends in species distribution are difficult to observe directly due to a lack of data with suitable spatio-temporal resolution. Here, we aimed to detect spatio-temporal trends in exotic plants in a montane ecosystem, and to determine which drivers had a role affecting these trends. Each year, for a period of 7 years, we recorded the upper elevational range limit of exotic plant species in road verges along an elevational gradient of 1500–2874 m a.s.l. in southern Africa. We fitted repeated-measures ANOVA models to test if upper elevational range limits changed over time. Generalized least squares models showed that exotic richness of annuals increased by 3.9 species per year. Also, the upper elevational range limits of established exotics ascended by 24.5 m/year for annuals (N = 17 species), and by 9.7 m/year for perennials (N = 26). These upward trends were too rapid to be explained by slow-acting drivers such as climatic change or time since species introduction. The rates of increase indicate that many exotics were not yet in equilibrium with the environment and, therefore, had not been in the region long enough to have filled their potential niches. Exotic species could reach much higher elevations than expected, indicating that current prediction models are likely an underestimation of potential distributional ranges. The spatial clustering of upper elevational range limits around potential points of introduction indicates ongoing human-mediated propagule pressure as the major cause of rapid exotic range expansion, especially along roads and near dwellings. Montane road verges are regularly disturbed by erosion and maintenance, creating unoccupied habitats, while traffic in the form of vehicles and tourists facilitates the introduction of new species. This suggests that easily accessible montane ecosystems are much more susceptible to invasions than previously assumed, due to a combination of anthropogenic disturbance and ongoing propagule pressure.

Evidence for shifts to faster growth strategies in the new ranges of invasive alien plants.

SummaryUnderstanding the processes underlying the transition from introduction to naturalization and spread is an important goal of invasion ecology. Release from pests and pathogens in association with capacity for rapid growth is thought to confer an advantage for species in novel regions.We assessed leaf herbivory and leaf-level traits associated with growth strategy in the native and exotic ranges of 13 invasive plant species from 256 populations. Species were native to either the Western Cape region of South Africa, south-western Australia or south-eastern Australia and had been introduced to at least one of the other regions or to New Zealand. We tested for evidence of herbivore release and shifts in leaf traits between native and exotic ranges of the 13 species.Across all species, leaf herbivory, specific leaf area and leaf area were significantly different between native and exotic ranges while there were no significant differences across the 13 species found for leaf mass, assimilation rate, dark respiration or foliar nitrogen.Analysis at the species- and region-level showed that eight out of 13 species had reduced leaf herbivory in at least one exotic region compared to its native range.Six out of 13 species had significantly larger specific leaf area (SLA) in at least one exotic range region and five of those six species experienced reduced leaf herbivory. Increases in SLA were underpinned by increases in leaf area rather than reductions in leaf mass.No species showed differences in the direction of trait shifts from the native range between different exotic regions. This suggests that the driver of selection on these traits in the exotic range is consistent across regions and hence is most likely to be associated with factors linked with introduction to a novel environment, such as release from leaf herbivory, rather than with particular environmental conditions.Synthesis. These results provide evidence that introduction of a plant species into a novel environment commonly results in a reduction in the top-down constraint imposed by herbivores on growth, allowing plants to shift towards a faster growth strategy which may result in an increase in population size and spread and consequently to invasive success.

General hypotheses in invasion ecology

General hypotheses in invasion ecology

New microsatellite markers for the ectomycorrhizal fungus Pisolithus tinctorius sensu stricto reveal the genetic structure of US and Puerto Rican populations

Abstract The ectomycorrhizal fungus Pisolithus tinctorius has been introduced to many areas around the world as a source of inoculum for pine plantations. However, little is known about the genetic structure of fungal populations in their introduced habitats. To study the genetics of exotic P. tinctorius populations, we developed and employed seven new microsatellite markers and compared samples from Puerto Rico (exotic range) and the eastern United States (native range). Bayesian cluster analysis, neighbor joining analysis and FST values all strongly separated Puerto Rican populations from North American populations. Consistent with a founder effect, populations from Puerto Rico had reduced allelic richness when compared to samples from the United States. The genetic structure of P. tinctorius populations in Puerto Rico is weak to modest and is not correlated with geographic distance reflecting anthropogenic movement of inoculum with forestry practices.

The hidden world of Asian tiger mosquitoes: immature Aedes albopictus (Skuse) dominate in rainwater corrugated extension spouts.

The primary sources of Aedes albopictus (Skuse) in its exotic range in North America are artificial containers in backyards, which vary widely in accessibility. In this study we examined their prevalence in two container types that are difficult to inspect: catch basins and corrugated extension spouts (CES), the latter used to divert rainwater in downspouts away from house foundations. We conducted larval and pupal surveys in catch basins, CES and open containers such as buckets and plant saucers in three urban locations in Mercer County, New Jersey, USA. We found that Ae. albopictus were rare in catch basins but prevalent in CES, and were often the only species collected in CES. Specific characteristics of the CES were not significantly associated with the presence or number of Ae. albopictus in them, but those longer and closer to the ground were significantly more likely to contain water, and therefore mosquitoes. During peak season (July-August), the abundance of immature Ae. albopictus was significantly higher in CES than open containers. We found that CES are an important source of Ae. albopictus in our region and propose that effective control strategies should be implemented to minimize mosquito populations from these cryptic habitats.

Hierarchy of hypotheses or cascade of predictions? A comment on Heger et al. (2013).

Hierarchy of hypotheses or cascade of predictions? A comment on Heger et al. (2013).

Are exotic species red queens?

We combined two general hypotheses from the fields of invasion biology and evolutionary biology, the enemy release hypothesis and the Red Queen hypothesis, into the new invasive queens hypothesis. This hypothesis predicts that if species with the principal ability to reproduce both sexually and asexually are introduced to an exotic range, they show a shift towards asexual reproduction in the exotic range. The reasoning behind this hypothesis is that (1) species tend to leave behind their natural enemies when being introduced to an exotic range (enemy release hypothesis) and (2) sexual reproduction is less advantageous in the absence of natural enemies due to its high costs (inverted Red Queen hypothesis). We tested the invasive queens hypothesis against data from 70 animal species, based on information from the literature and unpublished information provided by 105 experts from various continents. As predicted, the investigated species showed a significant shift towards asexual reproduction in their exotic as compared to their native range.

The enemy release hypothesis as a hierarchy of hypotheses

In ecology, a hypothesis is usually not discarded if a few studies reject it, as long as there are other studies supporting it. How to assess the usefulness of ecological hypotheses is therefore not straightforward. Using the enemy release hypothesis as an example, we show how creating a hierarchy of hypotheses (HoH) can help reviewing and evaluating evidence for and against an ecological hypothesis. In a HoH, a broad, overarching hypothesis branches into more specific and better testable sub‐hypotheses. The enemy release hypothesis is a major hypothesis in invasion ecology and posits that the absence of enemies in the exotic range of an alien species is a cause of its invasion success. Based on a systematic review of empirical tests of this hypothesis, we divided it into sub‐hypotheses, differentiating among 1) indicators for enemy release, 2) types of comparisons, and 3) types of enemies. We identified 176 empirical tests and weighted each test according to the number of alien species studied and the research method (experimental vs observational, field vs enclosure vs laboratory). For the broadly formulated enemy release hypothesis, we found nearly as much supporting (36%) as questioning evidence (43%). At the sub‐hypotheses level, however, we found that some sub‐hypotheses are strongly supported by empirical evidence, whereas others receive hardly any support. These differences are further emphasized for some types of habitat and focal taxonomic groups. Our findings suggest that several specific formulations (i.e. sub‐hypotheses) of the broad enemy release hypothesis are useful, whereas other formulations should be viewed more critically. In general, the approach outlined here can help evaluate major ecological hypotheses and their specific sub‐hypotheses. Our study also highlights the need for a scientific debate on how much supporting evidence is sufficient to consider an ecological hypothesis to be useful.

EARLY SPRING FLOWERING IN NOVA SCOTIA: AN EXTREME SPRING IS REFLECTED IN ADVANCED FLOWERING

Twenty species of herbaceous plants and four non-amentiferous shrubs were found in flower in March-April in Nova Scotia during the spring of 2012. Plants were observed primarily in Kings and Antigonish Counties, with several observations from Inverness County. The precocious flowering is attributed to an abnormally warm late winter and spring (February-March) in which climate normals for monthly average temperature were exceeded by a minimum of 1.2°C in February (Tracadie) to a maximum of 8.5°C in March (Kentville). Flowering was an average of 17 days earlier than herbarium records in the largest regional herbaria (ACAD, NSAC). Proportional contribution to the early flowering guild was greater for exotic species which featured weedy families not represented in the native group. These observations of spring climate conditions and flowering phenology are consistent with regional climate change associated with milder and earlier springs.Key Words: climate change, Nova Scotia, phenology, spring flowering, exotic range expansion.

Invasive belowground mutualists of woody plants

Most plants require mutualistic associations to survive, which can be an important limitation on their ability to become invasive. There are four strategies that permit plants to become invasive without being limited by a lack of mutualists. One is to not be dependent on mutualists. The other three strategies are to form novel mutualisms, form associations with cosmopolitan species, or co-invade with mutualists from their native range. Historically there has been a bias to study mutualisms from a plant perspective, with little consideration of soil biota as invasive species in their own right. Here we address this by reviewing the literature on belowground invasive mutualists of woody plants. We focus on woody invaders as ecosystem-transforming plants that frequently have a high dependence on belowground mutualists. We found that co-invasions are common, with many ectomycorrhizal plant species and N-fixing species co-invading with their mutualists. Other groups, such as arbuscular mycorrhizal plants, tend to associate with cosmopolitan fungal species or to form novel associations in their exotic range. Only limited evidence exists of direct negative effects of co-invading mutualists on native mutualist communities, and effects on native plants appear to be largely driven by altered environmental conditions rather than direct interactions. Mutualists that introduce novel ecosystem functions have effects greater than would be predicted based solely on their biomass. Focusing on the belowground aspects of plant invasions provides novel insights into the impacts, processes and management of invasions of both soil organisms and woody plant species.

Climate change may boost the invasion of the Asian needle ant.

Following its introduction from Asia to the USA, the Asian needle ant (Pachycondyla chinensis) is rapidly spreading into a wide range of habitats with great negative ecological affects. In addition, the species is a concern for human health because of its powerful, sometimes deadly, sting. Here, we assessed the potential of P. chinensis to spread further and to invade entirely new regions. We used species distribution models to assess suitable areas under current climatic conditions and in 2020, 2050 and 2080. With a consensus model, combining five different modelling techniques, three Global Circulation (climatic) Models and two CO2 emission scenarios, we generated world maps with suitable climatic conditions. Our models suggest that the species currently has a far greater potential distribution than its current exotic range, including large parts of the world landmass, including Northeast America, Southeast Asia and Southeast America. Climate change is predicted to greatly exacerbate the risk of P. chinensis invasion by increasing the suitable landmass by 64.9% worldwide, with large increases in Europe (+210.1%), Oceania (+75.1%), North America (+74.9%) and Asia (+62.7%). The results of our study suggest P. chinensis deserves increased attention, especially in the light of on-going climate change.

Origins and distribution of invasive Rubus fruticosus L. agg. (Rosaceae) clones in the Western United States

In facultatively asexual invasive species, an understanding of the origins and diversity of clones can reveal introduction and invasion pathways and inform management efforts. In this study, we use microsatellite and chloroplast DNA markers to infer clonal diversity of the Rubus fruticosus agg. invasion in the Western United States, determine the relationship of these clones to clones found in other exotic ranges, and determine the geographic and genetic origins of the invasive clones. We found two invasive clones in the Western United States, where the invasion had previously been thought to consist of a single asexual lineage. The most common clone was genetically identical to the microspecies R. armeniacus from the native range of Germany, while the second clone was identical to the microspecies R. anglocandicans in the invaded range of Australia and closely related to samples from the native ranges of England and Serbia. A third distinct clone was identified in a collection from the exotic range of Chile. Our results demonstrate that cryptic genetic diversity may be present in asexual invasions that are thought to be homogeneous. However, the asexual relationships between R. fruticosus agg. clones in the native and multiple exotic ranges indicate that preadaptation has played an important role in invasion success in this species aggregate.

Plant evolutionary ecology: molecular genetics, global warming and invasions, and the novel approaches we are using to study adaptations

Plant evolutionary ecology: molecular genetics, global warming and invasions, and the novel approaches we are using to study adaptations

Commentary: Geographic patterns of establishment success among exotic bird populations

ISSN 1948-6596 the capacity to deal with numerous statistical models, ranges of initial conditions, different cli- mate projections, and many more effects. Person- ally, I never liked the idea of automatically proc- essing hundreds of species without careful consid- eration of predictor sets with regard to a specific species. However, I will now use BIOMOD even for single species projections, simply because the general framework makes ensemble forecasting so easy for us. And if one chooses to use only a few carefully chosen predictors, then BIOMOD can successfully process many species simultane- ously. news and update Thuiller, W., Lafourcade, B., Engler, R., & Araujo, M.B. (2009) BIOMOD – a platform for ensemble fore- casting of species distributions. Ecography, 32, Niklaus E. Zimmermann Swiss Federal Research Institute WSL e-mail: niklaus.zimmermann@wsl.ch http://www.wsl.ch/staff/ niklaus.zimmermann/ Edited by Thiago F. Rangel commentary Geographic patterns of establishment success among exotic bird populations Human-mediated species invasions are a major component of global environmental change and there remains a pressing need to understand the mechanisms by which a species becomes invasive. With this in mind, the key stage in the invasion pathway (sensu Williamson and Brown 1986) is species establishment. It is at this stage, when a species has been transported outside its native geographic range and released in a novel recipient environment, that a species either fails or suc- ceeds to establish an exotic population. However, while much attention has been focussed on what species-level traits determine establishment suc- cess in exotic bird populations (Blackburn et al. 2009), relatively less research has concerned the role of location-level variables. The most obvious reason for this relative ‘lack of research’ is that in most cases, compara- tive analyses have consisted of large datasets of good quality records of different species to single specific locations (e.g. Hawaii, Moulton et al. 2001; Australia, Duncan et al. 2001; New Zealand, Duncan et al. 2006) rather than repeated intro- ductions of the same species to multiple locations. Bird species are conspicuous elements of the envi- ronment and their distribution and abundance have attracted considerable research attention in the discipline of biogeography (e.g. Gaston and Blackburn 2000). It is therefore somewhat surpris- ing that autecological studies of exotic birds have received disproportionately limited research at- tention within invasion biology (Pysek et al. 2008). A recent article in the Journal of Biogeography (Strubbe and Matthysen 2009), provides a timely example of a study that addresses this bias. Diederik Strubbe and Erik Matthysen com- piled data on the introductions and establishment success of two parakeet species (ring-necked Psit- tacula krameri and monk Myiopsitta monachus parakeets) across their exotic European distribu- tion. In doing so, the authors provide one of the first comparative biogeographic studies to test environment-level features of establishment suc- cess among exotic birds. In total, 181 introduction events were used for their analysis in the exotic range. In their native ranges, parakeet occurrence was estimated using the presence-only method MAXENT (Phillips et al. 2006). For both parakeet species, individually, their establishment success in Europe was 53%. Data from environmental and climatological variables were used to test the rela- tive influence of alternative 'climate-matching' and 'human-activity' hypotheses. The authors found that parakeet establishment success was greater in areas of more dense human population settlement and, both in the native and exotic ranges, their distribution was associated with a smaller number of annual frost days. Further ex- amination revealed that both species were equally sensitive to frost in their exotic range, the major- ity of failed introductions occurring in regions with more than 50 frost days per year. Human activity can influence establishment success both indi- rectly, through habitat modification and provision frontiers of biogeography 1.2, 2009 — © 2009 the authors; journal compilation © 2009 The International Biogeography Society

European rabbit survival and recruitment are linked to epidemiological and environmental conditions in their exotic range

AbstractThe European rabbit, Oryctolagus cuniculus, is threatened within its native range, yet it is a highly successful colonizing pest species across its worldwide introduced range, causing large economic losses and widespread environmental degradation. To date, there has been no long‐term empirical evidence documenting the relative roles of climatic, epidemiological and biological factors in limiting life‐history determinants of rabbit range and abundance. Using 12 years of capture–mark–recapture data from their exotic range in Australia, we constructed candidate Cormack–Jolly–Seber models to test the influence of environmental, competition and disease conditions on rabbit survival and recruitment. Our results show that: (i) population‐level disease infection rate has the largest overall impact on rabbit survival, explaining 80% of variance in survival rates; (ii) environmental as well as epidemiological conditions constrain rabbit survival, especially for younger animals; (iii) temporal variation in rabbit kitten recruitment patterns are best described by a combination of climate, competition and disease settings (accounting for 68% of variance), while temperature alone has a strong negative influence on kitten recruitment; and (iv) recruitment responds positively to rabbit haemorrhagic disease, but negatively to myxomatosis – the former, probably being mediated through a disease driven effect on intraspecific competition for food. A strengthened understanding of climate change impacts on rabbit range and abundance can be achieved by accounting explicitly for potential synergisms between disease dynamics and climate. In this analysis, we provide the first step towards such an attempt for this important mammal species. Integrated approaches of this kind are essential for future forecasts of rabbit range and abundance, offsetting the conservation threat faced by O. cuniculus in its native range, and achieving effective management in exotic habitats.

Latitudinal shifts of introduced species: possible causes and implications

This study aims to document shifts in the latitudinal distributions of non-native species relative to their own native distributions and to discuss possible causes and implications of these shifts. We used published and newly compiled data on inter-continentally introduced birds, mammals and plants. We found strong correlations between the latitudinal distributions occupied by species in their native and exotic ranges. However, relatively more non-native species occur at latitudes higher than those in their native ranges, and fewer occur at latitudes lower than those in their native ranges. Only a small fraction of species examined (i.e. <20% of animals and <10% of plants) have expanded their distributions in their exotic range beyond both high- and low-limits of their native latitudes. Birds, mammals and plants tended to shift their exotic ranges in similar ways. In addition, most non-native species (65–85% in different groups) have not reached the distributional extent observed in their native ranges. The possible drivers of latitudinal shifts in the exotic range may include climate change, greater biotic resistance at lower latitudes, historical limitations on ranges in native regions, and the impacts of humans on species distributions. The relatively restricted distribution of most species in their exotic range highlights the great potential of future spread of most introduced species and calls for closely monitoring their directional spread under climate change.