Marine invasion processes: interactions between native and introduced marsh snails

We test two hypotheses to explain the success of a biological invader in marshes on the Pacific coast of North America. The first hypothesis focuses on the competitive superiority of an invading species over native species; the second hypothesis focuses on the absence of ecological equivalents in the aboriginal biota, such that no competitive interactions are necessary to facilitate successful invasion. As a model system we experimentally manipulated the introduced pulmonate snail Ovatella myosotis and the native prosobranch snails Assiminea californica and Littorina subrotundata. Manipulations included pair-wise enclosures to determine of Ovatella depressed the growth of the native;species; pair-wise associations to determine whether or not snails demonstrated negative or positive clustering; removal experiments to see if native snails invaded regions occupied by Ovatella, and reciprocal transplants on a vertical gradient to determine the physiological breath of the target species. In addition to monitoring population abundances of all three species seasonally, dietary overlap of these gastropods was also examined. No evidence of competitive superiority by Ovatella was found. Ovatella possesses the physiological capacities to exist in a semiterrestrial environment and is therefore able to take advantage of resources unavailable to native gastropods. We conclude that the successful establishment of this Atlantic snail in the Pacific Northwest did not arise at the expense of native species.     

Predator–prey interactions between brown trout Salmo trutta and native and introduced ampbipods; tbeir implications for fisb diets

An attempt to improve the food base for brown trout Salmo trutta in Northern Ireland was made in 195859 by deliberately introducing English Gammarus pule.x into several Irish rivers. In addition, another amphipod Crangonyx pseudogracilis, was later accidently introduced into Irish waters. Our study represents the first attempt to examine the trophic interactions between a native fish predator ( S. trutta ) and an array of these native ( Gammarus dueheni celticus ) and introduced ( G. pulex and C pseudogracilis ) amphipods. Feeding experiments, involving young brown trout predators and amphipod prey, revealed that the fish actively selected C. pseudogracilis relative to two alternative Gammarus prey species. Although the trout encountered the Gammarus species more than C pseudogracilis, they were eaten less than Crangonyx. Difficulties in handling and ingestion of Gammarus by trout may be a key component of the preference for the smaller, more easily handled Crangonyx. The microdistribution of the species was altered by the fish, due to predation being greater in particular microhabitats. Our study showed that the introduction of the herbivorous C. pseudogracilis into Irish freshwaters may represent a useful addition to fish diets, particularly for small and or juvenile fish. The repercussions of the deliberate introduction of G. pule.x are less clear. It may improve feeding for fish. but only if it can coexist with indigenous macromvertebrates and thus ultimately improve the range and quantity of possible food items in predator diets. Alternatively, being highly predatory towards other macroinvertebrates including G. d. celticus and C. pseudogracilis, G. pule.x may be deleterious to the diversity of the resident benthic community and hence reduce the diversity of prey available to fish predators.     

Enhanced fitness and greater herbivore resistance: implications for dandelion invasion in an alpine habitat

Several hypotheses have been proposed to explain the defense strategies of invasive plants in new ranges. In the absence of specialist herbivores, it is believed that invasive plants may allocate fewer resources to resistance and more to growth and reproduction, thus increasing tolerance to damage in the invasive genotypes. In order to test these predictions, we compared both performance (growth and reproduction) and defense strategies (tolerance and resistance) of two populations of Taraxacum officinale, one from the native range in the French Alps, and one from the introduced range in the Chilean Andes. Individuals from the introduced population demonstrated improved reproductive traits relative to those from the native population, although there was no discernible difference in biomass accumulation. Additionally, reduced tolerance was evident in the case of the former; whereas fitness traits of native plants were unaffected by damage, invasive plants reduced growth and seed output by 25 and 30% respectively following damage treatments. Increases in levels of phenols and anthocyanins, produced as a defense response to herbivory, were observed in introduced plants. Our results suggest that reallocation of resources to reproduction may be an important factor favouring invasive success of T. officinale in Chile, and that a higher investment in chemical resistance traits in this population may also be a factor in this regard.     

Impacts of a native parasitic plant on an introduced and a native host species: implications for the control of an invasive weed

Background and Aims: While invasive species may escape from natural enemies in thenew range, the establishment of novel biotic interactions withspecies native to the invaded range can determine their success.Biological control of plant populations can be achieved by manipulationof a species' enemies in the invaded range. Interactions weretherefore investigated between a native parasitic plant andan invasive legume in Mediterranean-type woodlands of SouthAustralia. Methods: The effects of the native stem parasite, Cassytha pubescens,on the introduced host, Cytisus scoparius, and a co-occurringnative host, Leptospermum myrsinoides, were compared. The hypothesisthat the parasitic plant would have a greater impact on theintroduced host than the native host was tested. In a fieldstudy, photosynthesis, growth and survival of hosts and parasitewere examined. Key Results: As predicted, Cassytha had greater impacts on the introducedhost than the native host. Dead Cytisus were associated withdense Cassytha infections but mortality of Leptospermum wasnot correlated with parasite infection. Cassytha infection reducedthe photosynthetic rates of both hosts. Infected Cytisus showedslower recovery of photosystem II efficiency, lower transpirationrates and reduced photosynthetic biomass in comparison withuninfected plants. Parasite photosynthetic rates and growthrates were higher when growing on the introduced host Cytisus,than on Leptospermum. Conclusions: Infection by a native parasitic plant had strong negative effectson the physiology and above-ground biomass allocation of anintroduced species and was correlated with increased plant mortality.The greater impact of the parasite on the introduced host maybe due to either the greater resources that this host providesor increased resistance to infection by the native host. Thisdisparity of effects between introduced host and native hostindicates the potential for Cassytha to be exploited as a controltool.     

Habitat selection and foraging success by an endangered Mediterranean cyprinid: implications for habitat restoration

Hydrobiologia - Habitat loss and degradation are causing collapses in freshwater fish in the Mediterranean region, where habitat restoration actions are still hampered by poor understanding of fish...     

Differing interactions between an introduced beetle and a resident root nematode mediated by an invasive plant and its native congener

Interactions between above- and below-ground herbivores play an important role in shaping plant competition and invasion, while the effects of non-native species invasions on above- and below-ground interactions remain unexplored. In this study, we report the interactions between an above-ground introduced beetle and a resident root nematode hosted by an invasive plant or its native congener with a laboratory bioassay and a greenhouse experiment in Wuhan, China. Nematode infections decreased beetle food conversion rates and larval biomass on the native plant, and increased beetle food conversion rates with no detectable impact on the larval biomass on the invasive plant. Beetle defoliation decreased nematode egg production on both the native and invasive plants. The interactions of the introduced beetle and the nematode were different by the invasive and native plants, which suggests that invasive plants and their introduced herbivores have the potential to alter above- and below-ground interactions and affect associated community members, which may in turn affect invasion processes and the safety of classical biocontrol practices.     

Daytime habitat selection by introduced eastern cottontail Sylvilagus floridanus and native european hare Lepus europaeus in Northern Italy

We used radiotelemetry to investigate resting sites habitat selection by introduced eastern cottontail (Sylvilagus floridanus) and native European hare (Lepus europaeus) under sympatric conditions. We tracked 24 hares and 34 cottontails in a protected area of northwestern Italy. Hares were found in different sites every week, while cottontails used the same site for two weeks, and occasionally for longer. It is supposed that this periodic nest switching reduces the risk of predation and parasitism. Hares and cottontails forms were located in different habitats and characterized by dense vegetation cover near the ground. This cover increased from winter to summer in both species, while in autumn it continued to increase in cottontails only, and decreased in hares. Cottontails selected shrubby habitats near the river, and avoided crop fields in all seasons. Hares were more adaptive in their search, using high herbs and shrubs all year round, wheat fields in spring, maize in spring and summer, and stubbles in winter. Arguably, partial niche differentiation is necessary to allow the coexistence of similar species. In our study area, hares and cottontails differentiated in the use of resting sites habitats, presumably so as not to compete in this part of their ecological niche.     

Aggression and competition for shelter between a native and an introduced crayfish in Europe

1. The introduced North American crayfish, Pacifastacus leniusculus Dana, is expanding its range in Europe and locally often replaces two native crayfish species, Astacus astacus L. and Austropotamobius pallipes Lereboullet. Pacifastacus leniusculus is also expected to invade the habitat of a third native crayfish, the endangered Austropotamobius torrentium Schrank. Interspecific aggressive interactions and competition for shelter were experimentally studied in the laboratory to assess the potential impact of P. leniusculus on A. torrentium .
2. Neither species was inherently dominant in aggressive interactions, but dominance was strongly size-dependent, favouring the larger and faster growing species, P. leniusculus .
3. Access to limited shelter was generally determined by aggressive dominance, although species-specific preference also influenced the outcome of competition for shelter. Austropotamobius torrentium had a higher preference for experimental shelters and often defended these even against larger P. leniusculus .
4. In accordance with theoretical models of animal conflicts, agonistic interactions between equally sized contestants were more severe than between animals of different size.
5. The P. leniusculus used in the present experiment were infected with the crayfish plague, Aphanomyces astaci , to which the animals are resistant. The crayfish transmitted the disease to non-resistant A. torrentium which died ≈ 2 weeks after contact with P. leniusculus .     

Physicochemical habitat association of a native and a non-native crayfish in the lower Flint river, Georgia: implications for invasion success

Invasive species must cope with a suite of environmental conditions that are different from those in their native ranges. We examine how the physicochemical environment contributes to the invasion success of Orconectes palmeri, a non-native crayfish, in the lower Flint River basin, Georgia, USA. We examined the distribution of Procambarus spiculifer, a native crayfish, and O. palmeri within the lower Flint River basin, and examined associations between species relative abundance and physicochemical variables. Within the lower Flint River, O. palmeri was found almost exclusively at upstream sites, while P. spiculifer was more abundant downstream. We did not detect small O. palmeri downstream, suggesting little recruitment in this area. Within tributaries, only native crayfish were observed. O. palmeri was more abundant at sites with warmer water temperatures, and P. spiculifer was more abundant at sites with cooler water temperatures, which were maintained by ground water seepage. P. spiculifer abundance was also positively associated with abundance of coarse wood. Laboratory studies demonstrated that O. palmeri selected warmer temperatures than P. spiculifer, suggesting that warmer temperatures in the upstream area favor O. palmeri. Water temperatures may have increased in recent years due to human withdrawals from the aquifer and the installation of upstream dams, creating a thermal regime that P. spiculifer has not historically encountered. Our findings suggest that the maintenance of groundwater inputs and a forested riparian buffer is crucial to protect populations of P. spiculifer and to prevent further spread by O. palmeri, especially if climatic changes result in warmer waters.     

Scale-dependent habitat selection by wintering geese: implications for landscape management

The management of Arctic migrant geese is complex, because they frequently use landscapes under intensive human use, and are conflict species in multiple respects. Some populations are of high conservation concern, but they also cause agricultural damage, are quarry for hunters, and may be particularly sensitive to infrastructure developments. In Bulgarian Dobrudzha, large wintering populations of greater white-fronted geese Anser albifrons and red-breasted geese Branta ruficollis feed in agricultural land, and cause management dilemmas. We developed linear models to investigate fine- and meso-scale foraging habitat selection of geese foraging on winter wheat in the area, and used these models to make suggestions for zoning landscape use in order to reduce conflict and conserve geese. Habitat selection was scale-dependent. Geese selected fields that were near to major roosts and had low proximity to roads and tree-lines, which may be a proxy for hunting disturbance. We found some evidence for selection of wheat fields with high nutritional quality. Within fields, geese strongly avoided features which cause landscape ‘clutter’: power-lines, tree-lines and wind-turbines, but primarily over distances of less than a few hundred metres. Optimal management might involve encouraging goose populations to feed in areas close to roosts, by means of agri-environmental measures and creation of hunting-free refuges. This would allow efficient use of agri-environment funds, might reduce conflict with farmers, and would mean that infrastructure development—notably wind farms—could be sited at greater distance from roosts with relatively minor impact on foraging habitat availability.     

Behavioural and genetic interactions between an endangered and a recently-arrived hummingbird

The invasion or expansion of non-native species into new geographic areas can pose a major threat to the conservation of biodiversity. These threats are augmented when the newly-arrived species interacts with native species that are already threatened by other ecological or anthropogenic processes. Potential interactions can include both competition for scarce resources and reproductive interference, including hybridisation. Understanding the dynamics of these interactions forms a crucial component of conservation management strategies. A recent contact zone occurs in the north of Chile between the endangered Chilean woodstar (Eulidia yarrellii) and the closely-related and recently-arrived Peruvian sheartail (Thaumastura cora), which expanded its range from Peru into Chile during the 1970s. We characterised the interactions between the species by combining population size estimates with molecular, morphological and behavioural data. We show that a low degree of hybridisation, but not introgression, is occurring between the two species. Despite interspecific morphological similarities, behavioural observations indicate that food niche overlap between the species is relatively low, and that the dietary breadth of sheartails is larger, which may have aided the species’ range expansion. Finally, woodstars dominate the sheartails in male–male territorial interactions. However, potentially increased energetic costs for woodstars associated with frequent territorial chases and courtship displaying with sheartails may exacerbate the effects of other threats on woodstar viability, such as human-induced habitat modification. This study highlights the value of implementing multidisciplinary approaches in conservation biology to gain a more complete understanding of interactions between recently-arrived and endangered species.     

Arbuscular mycorrhiza of introduced and native grasses colonizing zinc wastes: implications for restoration practices

The analysis of mycorrhizal status of grasses introduced under conventional phytoremediation programmes (including covering of the waste area with a layer of uncontaminated soil and watering) and of grasses that spontaneously established in later successional stages, was carried out on the slopes of the 1- to 30-year-old tailings of ZG Trzebionka Mining Company located near Chrzanów (southern Poland). Most grasses were mycorrhizal and the parameters of mycorrhization were higher on the older parts of the waste. Grasses selected for restoration practices were well developed only if the waste was covered with a layer of soil and continuous irrigation took place, both being expensive practices. The field experiment showed that the introduction of standard inoculum containing mycorrhizal fungi indeed improved the development of grasses during early stages, but still was not effective enough due to high erosion of the substratum. A slight improvement was observed when AgroHydroGel was used to lock moisture in the soil, but the plants did not survive for longer time periods. On the contrary, much better results were obtained when vegetatively multiplied grasses selected from specimens originating from natural succession were used to stabilize the vegetation on the bare industrial waste.     

Comparison of competitive ability between native and introduced salmonids: evidence from pairwise contests

Brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, have been introduced to freshwaters in Hokkaido, Japan. Today, it is recognized that these introduced salmonids have negative impacts on native salmonids such as white-spotted charr, Salvelinus leucomaenis, and masu salmon, O. masou. In particular, interspecific competition may be an important mechanism that could contribute to the exclusion for native salmonids. In this study, experimental pairwise contests were conducted to compare interference competitive ability between native and introduced salmonids. We demonstrated that brown trout were competitively superior to white-spotted charr and masu salmon whereas rainbow trout were superior to white-spotted charr. We suggest that introduced brown trout negatively impact both white-spotted charr and masu salmon, and introduced rainbow trout negatively impact white-spotted charr.     

Small founding number and low genetic diversity in an introduced species exhibiting limited invasion success (speckled dace, Rhinichthys osculus)

Molecular evaluations of successful invaders are common, however studies of introduced species that have had limited invasion success, or have died out completely, are rare. We studied an introduced population of speckled dace (Rhinichthys osculus) from northern California, USA that has rapidly increased in abundance but remained restricted to a 25-km stretch of river since its introduction in the mid-1980s. Field and laboratory analyses indicate that invasion success of speckled dace is constrained by the combined effects of multiple predators. The role of bottleneck effects associated with the introduction has not been studied. We assayed variation in seven microsatellite loci and one mitochondrial DNA gene in the introduced population and nine putative source populations to identify the source population and evaluate bottleneck effects. The Trinity River system was supported as the source owing to its genetic similarity and geographic proximity to the introduced population. Consistent with a bottleneck, the introduced population exhibited reduced allelic and haplotype richness in comparison to source populations. Estimates of the genetically effective number of individuals founding the introduced population using nuclear coalescent analyses and a mitochondrial simulation procedure were highly concordant in suggesting that the initial colonizing group was comprised of about 10 individuals. A bottleneck effect in an exotic species exhibiting limited invasion success has rarely been documented and thus introduction of speckled dace represents an important model system for future investigation. Establishing a relationship between genetic diversity and factors limiting invasion success in this system (e.g., predator avoidance) will help determine the extent to which genetic diversity loss has constrained invasion success in speckled dace.     

A model for habitat selection and species distribution derived from central place foraging theory

We have developed a habitat selection model based on central place foraging theory. An individual’s decision to include a patch in its habitat depends on the marginal fitness contribution of that patch, which is characterized by its quality and distance to the central place. The essence of the model we have developed is a fitness isocline which is a function of patch quality and travel time to the patch. It has two parameters: the maximum travel distance to a patch of infinite quality and a coefficient that appropriately scales quality by travel time. Patches falling below the isocline will have positive marginal fitness values and should be included in the habitat. The maximum travel distance depends on the availability and quality of patches, as well as on the forager’s life history, whereas the scaling parameter mostly depends on life history properties. Using the model, we derived a landscape quality metric (which can be thought of as a connectivity measure) that sums the values of available habitat in the landscape around a central place. We then fitted the two parameters to foraging data on breeding white storks (Ciconia ciconia) and estimated landscape quality, which correlated strongly with reproductive success. Landscape quality was then calculated for a larger region where re-introduction of the species is currently going on in order to demonstrate how this model can also be regarded as a species distribution model. In conclusion, we have built a general habitat selection model for central place foragers and a novel way of estimating landscape quality based on a behaviorally scaled connectivity metric.     

Breeding habitat selection behaviors in heterogeneous environments: implications for modeling reintroduction

Animal movement and habitat selection behavior are important considerations in ecology, and remain a major issue for successful animal reintroductions. However, simple rules are often used to model movement or focus only on intrinsic environmental cues, neglecting recent insights in behavioral ecology on habitat selection processes. In particular, social information has been proposed as a widespread source of information for habitat evaluation.
We investigated the role of explicit breeding habitat selection strategies on the establishment pattern of reintroduced populations and their persistence. We considered local movement at the scale of a single population. We constructed a spatially-implicit demographic model that considered five breeding habitat selection rules: 1) random, 2) intrinsic habitat quality, 3) avoidance of conspecifics, 4) presence of conspecifics and 5) reproductive success of conspecifics. The impact of breeding habitat selection was examined for different release methods under various levels of environmental heterogeneity levels, for both long and short-lived monogamous species.
When heterogeneity between intrinsic habitat patch qualities is high, the persistence of reintroduced populations strongly depends on habitat selection strategies. Strategies based on intrinsic quality and conspecific reproductive success lead to a lower reintroduction failure risk than random, conspecific presence or avoidance-based strategies. Conspecific presence or avoidance-based strategies may aggregate individuals in suboptimal habitats. The release of adults seems to be more efficient independent of habitat selection strategy.
We emphasize the crucial role of oriented habitat selection behavior and non ideal habitat selection in movement modeling, particularly for reintroduction.     

Competition between introduced and native spiders (Araneae: Linyphiidae)

The European sheet-web spider Linyphia triangularis (Araneae: Linyphiidae) has become established in Maine, where it often reaches very high densities. Two lines of evidence from previous work suggest that L. triangularis affects populations of the native linyphiid spider Frontinella communis. First, F. communis individuals are relatively scarce in both forest and coastal habitat where L. triangularis is common, but more common where L. triangularis is at low density. Second, in field experiments, F. communis species are less likely to settle in experimental plots when L. triangularis is present, and F. communis disappears from study plots when L. triangularis is introduced. Here we test two mechanisms that may underlie these patterns. First, we tested whether L. triangularis invades and usurps the webs of F. communis. When spiders were released onto webs of heterospecifics, L. triangularis was more likely to take over or share webs of F. communis than the reverse. We also observed natural takeovers of F. communis webs. Second, we explored the hypothesis that L. triangularis reduces prey availability for native species. We sampled flying prey in areas with L. triangularis and those where it had been removed, and found no effect of spider presence on measured prey density. We also found no effect of prey supplementation on web tenacity in F. communis, suggesting that F. communis movements are not highly dependent on prey availability. We conclude that web takeover is likely more important than prey reduction in driving negative effects of L. triangularis on F. communis.     

Cast adrift on an island: introduced populations experience an altered balance between selection and drift

A long-standing question in evolutionary biology is what becomes of adaptive traits when a species expands its range into novel environments. Here, we report the results of a study on an adaptive colour pattern polymorphism (stripes) of the coqui frog, Eleutherodactylus coqui, following its introduction to Hawaii from Puerto Rico. We compared population differentiation ( ) for the stripes locus-which underlies this colour pattern polymorphism-with neutral microsatellite loci to test for a signature of selection among native and introduced populations. Among native populations, for stripes were lower than expected under the neutral model, suggesting uniform balancing selection. Alternatively, among introduced populations, for stripes did not differ from the neutral model. These results suggest that the evolutionary dynamics of this previously adaptive trait have become dominated by random genetic drift following the range expansion.