Where vectors collide: the importance of mechanisms shaping the realized niche for modeling ranges of invasive <Emphasis Type="Italic">Aedes</Emphasis> mosquitoes

The vector mosquitoes Aedes aegypti (L.), native to Africa, and Aedes albopictus (Skuse), native to Asia, are widespread invasives whose spatial distributions frequently overlap. Predictive models of their distributions are typically correlative rather than mechanistic, and based on only abiotic variables describing putative environmental requirements despite extensive evidence of competitive interactions leading to displacements. Here we review putative roles of competition contributing to distribution changes where the two species meet. The strongest evidence for competitive displacements comes from multiple examples of habitat segregation where the two species co-occur and massive reductions in the range and abundance of A. aegypti attributable to A. albopictus invasions in the southeastern U.S.A. and Bermuda (U.K). We summarize evidence to support the primacy of asymmetric reproductive interference, or satyrization, and larval resource competition, both favoring A. albopictus, as displacement mechanisms. Where evidence of satyrization or interspecific resource competition is weak, differences in local environments or alternative ecologies or behaviors of these Aedes spp. may explain local variation in the outcomes of invasions. Predictive distribution modeling for both these major disease vectors needs to incorporate species interactions between them as an important process that is likely to limit their realized niches and future distributions. Experimental tests of satyrization and resource competition are needed across the broad ranges of these species, as are models that incorporate both reproductive interference and resource competition to evaluate interaction strengths and mechanisms. These vectors exemplify how fundamental principles of community ecology may influence distributions of invasive species.     

Quantifying how acquired interactions with native and invasive insects influence population growth rates of a non-indigenous plant

Non-native species often acquire novel interspecific interactions, which are central to several hypotheses of invasion success, including biotic resistance and invasional meltdown. However, the outcome of these interactions is not often linked with the demographic evidence based on the full life cycle of the species. The Philippine Ground Orchid (Spathoglottis plicata) has invaded Puerto Rico and has acquired both negative and positive interspecific interactions involving the native weevil Stethobaris polita and the invasive red fire ant Solenopsis invicta, respectively. We studied a population in the Rio Abajo Forest, and asked how these interactions affect population demography by using a combination of field, experimental and modelling approaches. Stage-structured matrix population models based on four years of field observations showed that the population of S. plicata is growing at a rate (λ) of 1.05 under natural conditions. When we modified fecundity values based on experimental exclusion of weevils and ants, the control treatment showed a similar λ. Excluding weevils increased λ to 1.20, whereas the exclusion of ants decreased λ to 1.03. When we incorporate demographic and environmental stochasticity in our models, exclusion of invasive red fire ants significantly reduces the orchid abundance over time. Although weevils offer some biotic resistance to S. plicata, these effects do not prevent orchid population growth and expansion. On the other hand, invasive red fire ants have a positive effect on the invasive orchid’s λ, partially supporting the invasional meltdown hypothesis. This study presents a method that allows one to combine opposing mechanisms of species interactions within the same quantitative framework, and the results highlight the importance of considering acquired plant–animal interactions and stochastic processes when evaluating the population growth rates and dynamics of invasive plants.     

Fast seedling root growth leads to competitive superiority of invasive plants

The competitive superiority of invasive plants plays a key role in the process of plant invasions, enabling invasive plants to overcome the resistance of local plant communities. Fast aboveground growth and high densities lead to the competitive superiority of invasive species in the competition for light. However, little is understood of the role belowground root competition may play in invasion. We conducted an experiment to test the effect of root growth on the performance of an invasive shrub Cassia alata, a naturalized, non-invasive shrub Corchorus capsularis, and a native shrub Desmodium reticulatum. We compared seedling growth of the three species and their competitive ability in situ. The roots of the C. alata seedlings grew much faster than those of C. capsularis and D. reticulatum during the entire growth period although C. alata had shorter shoots than D. reticulatum. Furthermore, C. alata showed an apparent competition advantage compared to the other two species as evidenced by less biomass reduction in intraspecific competition and higher competitive effects in interspecific competition. Our study reveals that fast seedling root growth may be important in explaining the competitive advantages of invasive plants. Future studies should pay more attention to the belowground traits of invasive plants, the trade-off between shoot and root growth, and the role of root competition in affecting the population dynamics of invasive plants and the structures of invaded communities.     

Competing for crevices: interspecific conflict influences retreat-site selection in montane lizards

Direct interference competition between sympatric taxa affects habitat use and shelter-site selection in species within most major vertebrate lineages. However, studies on interspecific social interference in reptiles largely have been confined to research on interactions between non-native (invasive) species and native fauna. Does interspecific interference also influence habitat use within natural assemblages of reptiles? We studied five broadly sympatric species of viviparous montane skinks within the genera Egernia and Eulamprus in southeastern Australia. Previous work has shown strong interspecific overlap in abiotic attributes of shelter-sites for these taxa, but no joint occupancy of retreats. Laboratory trials in which we manipulated the identity of co-occurring lizards revealed frequent displacement from “preferred” (hotter) shelters, with interspecific interactions more intense than intraspecific conflicts. The five species displayed a linear interspecific dominance hierarchy, with larger species displacing smaller taxa. Field manipulations confirmed that interspecific interactions between these species affect shelter-site use. We conclude that direct agonistic encounters between individuals of different species strongly influence habitat use by lizards within this assemblage.     

Different ecological conditions support individual specialization in closely related,ecologically similar species

Individual generalist predators often have more specialized diets than their populations do. Individual specialization (IS) is influenced by ecological opportunity, intraspecific competition, and interspecific competition, although the effects of these parameters are inconsistent across studies. We investigated IS in five species of frogs and toads, Anaxyrus americanus, A. fowleri, Lithobates catesbeianus, L. clamitans, and L. sphenocephalus. We used the natural history and ecology of each species to predict which parameters would influence IS. Our predictions were supported for some species but not others. We predicted IS would be positively influenced by resource diversity in all species, but this prediction held for only three species, with the relationship significant in A. fowleri and L. catesbeianus and marginally significant in A. americanus. We also predicted that interspecific competition would have a negative relationship with IS in L. clamitans because L. catesbeianus is competitively superior to L. clamitans and likely to suppress its foraging options. This prediction was upheld. Finally, we predicted that IS in A. americanus, A. fowleri, and L. clamitans would be influenced by intraspecific competition. However, IS was not influenced by intraspecific competition in any species, a surprising result given that intraspecific competition has traditionally been assumed to be the ecological parameter with the strongest effects on IS. Many previous studies did not simultaneously consider all three ecological parameters, which may have increased the apparent importance of intraspecific competition for IS. Our results revealed that the ecological parameters affected IS differently even across closely related and ecologically similar species, and demonstrated that these differences are sometimes predictable based on natural history. This study also suggests that sympatric ecological speciation based on IS may be rare because the ecological parameters driving IS are inconsistent across species, and the strength of their effects on intraspecific diet variation varies in space.     

Pitcher plant facilitates prey capture in a sympatric congener

Carnivorous plants avoid below-ground competition for nitrogen by utilizing an alternative nitrogen resource—invertebrate prey, but it remains unclear if sympatric carnivorous plants compete for prey resources. The aim of this study was to investigate if exploitative prey-resource competition occurs between the two sympatric pitcher plant species, Nepenthes rafflesiana and N. gracilis in Singapore. We first investigated if prey-resource partitioning occurs between these two species, and then investigated niche shift in N. gracilis by examining its pitcher contents along an in situ gradient of N. rafflesiana interspecific competition. Our results showed clear evidence of resource partitioning between the two species, but contrary to the expectation of competition, proximity to N. rafflesiana pitchers correlated with higher total prey numbers in N. gracilis pitchers. Our multivariate model of prey assemblages further suggested that N. rafflesiana facilitates N. gracilis prey capture, especially in several ant taxa that are trapped by both species. Concurrently, we found strong evidence for intraspecific competition between N. gracilis pitchers, suggesting that prey resources are exhaustible by pitcher-predation. Our results show that resource partitioning can be associated with facilitative interactions, instead of competition as is usually assumed. Facilitation is more typically expected between phylogenetically distant species, but divergences in resource acquisition strategies can permit facilitation between congeners.     

Light energy partitioning,photosynthetic efficiency and biomass allocation in invasive <Emphasis Type="Italic">Prunus serotina</Emphasis> and native <Emphasis Type="Italic">Quercus petraea</Emphasis> in relation to light environment,competition and allelopathy

This study addressed whether competition under different light environments was reflected by changes in leaf absorbed light energy partitioning, photosynthetic efficiency, relative growth rate and biomass allocation in invasive and native competitors. Additionally, a potential allelopathic effect of mulching with invasive Prunus serotina leaves on native Quercus petraea growth and photosynthesis was tested. The effect of light environment on leaf absorbed light energy partitioning and photosynthetic characteristics was more pronounced than the effects of interspecific competition and allelopathy. The quantum yield of PSII of invasive P. serotina increased in the presence of a competitor, indicating a higher plasticity in energy partitioning for the invasive over the native Q. petraea, giving it a competitive advantage. The most striking difference between the two study species was the higher crown-level net CO₂ assimilation rates (A_crown) of P. serotina compared with Q. petraea. At the juvenile life stage, higher relative growth rate and higher biomass allocation to foliage allowed P. serotina to absorb and use light energy for photosynthesis more efficiently than Q. petraea. Species-specific strategies of growth, biomass allocation, light energy partitioning and photosynthetic efficiency varied with the light environment and gave an advantage to the invader over its native competitor in competition for light. However, higher biomass allocation to roots in Q. petraea allows for greater belowground competition for water and nutrients as compared to P. serotina. This niche differentiation may compensate for the lower aboveground competitiveness of the native species and explain its ability to co-occur with the invasive competitor in natural forest settings.     

Mutual effects of several fouling organisms of the White Sea (<Emphasis Type="Italic">Mytilus edulis,Styela rustica</Emphasis>, and <Emphasis Type="Italic">Hiatella arctica</Emphasis>) on their growth rate and survival

The mutual effects of several fouling species (the bivalves Mytilus edulis and Hiatella arctica and a solitary ascidian Styela rustica) on their growth rate and mortality were studied through field experiments. The interactions between S. rustica and H. arctica appeared to be the least antagonistic. In contrast, the mussel was the most “aggressive” species with regard to both competitors. It was observed that the ascidians died, because they were intensively braided and gummed up with the byssus threads of the mussels. However, in some cases the intraspecific competition was stronger than the interspecific one.     

Timing of invasive pollen deposition influences pollen tube growth and seed set in a native plant

Invasive plants may threaten the reproductive success of native sympatric plants by modifying the pollination process. One potential mechanism takes place through the deposition of invasive pollen onto native stigmas when pollinators are shared among species. We explore how pollen from the invasive plant Brassica nigra influences pre- and post-fertilization stages in the native plant Phacelia parryi, through a series of hand pollination experiments. These two species share pollinators to a high degree. P. parryi flowers were hand-pollinated with either pure conspecific pollen (the control) or with B. nigra pollen applied prior to, simultaneously with, or following conspecific pollen. Application of B. nigra pollen lowered seed set, with the simultaneous application resulting in the highest reduction. Pollen tube growth was also influenced by the presence of invasive pollen, with fewer conspecific pollen tubes reaching the base of P. parryi styles in treatments where B. nigra pollen was applied prior to or simultaneously with conspecific pollen. The deleterious effects of invasive pollen on native seed set in this study are likely not due to loss of stigmatic receptivity since seed set was less affected when heterospecific pollen was applied prior to conspecific pollen, but may instead involve interactions between interspecific pollen grains on the stigma or within the style. Our study highlights the importance of timing of foreign pollen deposition on native stigmas and suggests that interspecific pollen transfer between native and exotic plants may be an important mechanism of competition for pollination in invaded plant communities.     

Intra- and interspecific hybridization in invasive Siberian elm

Hybridization creates unique allele combinations which can facilitate the evolution of invasiveness. Frequent interspecific hybridization between the Siberian elm, Ulmus pumila, and native elm species has been detected in the Midwestern United States, Italy and Spain. However, Ulmus pumila also occurs in the western United States and Argentina, regions where no native elm species capable of hybridizing with it occurs. We examined whether inter- or intraspecific hybridization could be detected in these regions. Nuclear markers and the program STRUCTURE helped detect interspecific hybridization and determine the population genetic structure in both the native and the two non-native ranges. Chloroplast markers identified sources of introduction into these two non-native ranges. No significant interspecific hybridization was detected between U. pumila and U. rubra in the western United States or between U. pumila and U. minor in Argentina and vice versa. However, the genetic findings supported the presence of intraspecific hybridization and high levels of genetic diversity in both non-native ranges. The evidence presented for intraspecific hybridization in the current study, combined with reports of interspecific hybridization from previous studies, identifies elm as a genus where both inter- and intraspecific hybridization may occur and help maintain high levels of genetic diversity potentially associated with invasiveness.     

Spatial pattern and association of tree species in a mixed <Emphasis Type="Italic">Abies holophylla</Emphasis>-broadleaved deciduous forest in Odaesan National Park

The mixed Abies holophylla-broadleaved deciduous forest is mature relative to other forest types in the midland of South Korea. The spatial distribution patterns of eight dominant canopy tree species were analyzed using Ripley’s K function. This study was conducted to clarify interspecific and intraspecific associations among growth stages and to interpret the coexistence mechanism among such species, by extension, to forecast their future. Disturbance-driven site heterogeneity has spatially separated disturbance-resistant Magnolia sieboldii from the other seven species. Spatial distribution of other species is affected by dispersal mechanisms and interspecific and intraspecific competition. These species were classified into three groups. The first group, composed of A. holophylla, Tilia amurensis, Acer pseudo-sieboldianum, and Quercus mongolica, was the most dominant and intraspecifically affinitive. Additionally, it seemed that they were established before the others. Q. mongolica and T. amurensis are poorly resistant to shade and are likely to be crowded out. In contrast, the other two species may continue, as they are highly resistant to shade and have high reproductivity. The second group was composed of Carpinus cordata, Acer tegmentosum, and Acer mono, i.e., late-successional species that wait for chances with shade tolerance and high reproductivity. These species are expected to occupy much of the Q. mongolica and T. amurensis space. M. sieboldii, i.e., the third group, were negatively related with other species and have dominated the valleys where intense disturbances are repeated. Understories have poor reproductivity, but a stationary population is expected to be maintained if canopy gaps are created by occasional disturbances.     

Behavioral interactions under multiple stressors: temperature and salinity mediate aggression between an invasive and a native fish

Interspecific aggression is a critical determinant of the success and competitive superiority of many invasive over native species. While single abiotic stressors can alter aggression levels, the manner in which multiple stressors may alter the strength and outcome of interspecific interactions and hence the invasion potential of a species is still poorly understood, even though multiple stressors are prevalent in many ecosystems. Furthermore, multiple stressors may interact to produce synergistic or antagonistic effects on individual level behaviors, thereby modulating invasive-native species interactions in unexpected ways. Here we examined the effect of two key abiotic stressors in freshwater ecosystems-temperature and salinity-on interspecific aggression between the invasive eastern mosquito fish (Gambusia holbrooki) and juveniles of the native Australian bass (Macquaria novemaculeata). Under controlled laboratory conditions, individuals were exposed to low or high salinity levels (15 and 35‰), and low or high temperatures (21 and 28 °C), and the frequency of interspecific aggressive behaviors was scored. The effect of temperature and salinity on interspecific aggression was antagonistic for both M. novemaculeata and G. holbrooki. While elevated temperature promoted aggression, elevated salinity partially or entirely negated this effect. Moreover, regardless of temperature, M. novemaculeata was more aggressive than G. holbrooki under elevated salinity. In addition to this, the native displayed more aggression to smaller than larger heterospecifics when exposed to elevated salinity alone, while G. holbrooki showed no such preference. These results highlight the importance of considering the interplay between multiple abiotic stressors and behavioral interactions between invasive and native species, combined with the modulating effect of species-specific and size based responses to those stressors.     

Pollinator-mediated impacts of alien invasive plants on the pollination of native plants: the role of spatial scale and distinct behaviour among pollinator guilds

Alien invasive plant species can affect pollination, reproductive success and population dynamics of co-flowering native species via shared pollinators. Consequences may range from reproductive competition to facilitation, but the ecological drivers determining the type and magnitude of such indirect interactions remain poorly understood. Here, we examine the role of the spatial scale of invader presence and spatially contingent behavioural responses of different pollinator groups as potential key drivers, using the invasive Oxalis pes-caprae and the self-incompatible native annual Diplotaxis erucoides as a model system. Three treatments were assigned to native focal plants: (1) invader present at the landscape scale (hectares) but experimentally removed at the floral neighbourhood scale (pa); (2) invader present at both scales (pp); (3) invader absent at both scales (aa). Interestingly, we found pronounced spatially contingent differences in the responses of pollinators: honeybees and bumblebees were strongly attracted into invaded sites at the landscape scale, translating into native plant visitation facilitation through honeybees, while bumblebees almost exclusively visited Oxalis. Non-corbiculate wild bees, in contrast, showed less pronounced responses in foraging behavior, primarily at the floral neighborhood scale. Average heterospecific (Oxalis) pollen deposition onto stigmas of Diplotaxis was low (<1 %), but higher in the pp than in the pa treatment. Hand-pollination of Diplotaxis with Oxalis and conspecific pollen, however, reduced seed set by more than half when compared to hand-pollination with only conspecific pollen. Seed set of Diplotaxis, finally, was increased by 14 % (reproductive facilitation) in the pp treatment, while it was reduced by 27 % (reproductive competition) in the pa treatment compared to uninvaded populations. Our study highlights the crucial role of spatial scale and pollinator guild driving indirect effects of invasive on co-flowering native plant species.     

<Emphasis Type="Italic">Euonymus fortunei</Emphasis> dominance over native species may be facilitated by plant–soil feedback

Positive plant–soil feedback (PSF) may be a mechanism of invader dominance, whereas PSF is often negative for native species. Previous work in Eastern deciduous forests of North America has shown that the invasive liana Euonymus fortunei participates in a net positive PSF with native groundcover Asarum canadense, indicating that PSF may contribute to invader dominance. However, to identify PSF as a general invasion driver for Euonymus, we must consider the average net pairwise feedback for multiple native–invasive species pairs, and compare this to the average net pairwise feedback amongst native–native pairs. Here, we test E. fortunei in net pairwise feedback against five native species, comparing native–invader feedback to feedback amongst natives over a gradient of light availability. PSF was on average neutral for invader–native pairs and on average negative for native–native pairs, indicating that Euonymus does not face the same constraints that limit the growth of native species. Because even neutral feedback can facilitate invasion, results indicate that PSF may facilitate invader dominance over a broad range of native functional groups and light conditions in Eastern deciduous forest.     

Genotypic differences in phosphorus use physiology in producers (<Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>) and consumers (<Emphasis Type="Italic">Daphnia pulex</Emphasis>) interact to alter primary and secondary production

By considering the relative abundance of elements in trophic interactions, ecological stoichiometry makes predictions about key ecological processes such as biomass production and consumer-driven nutrient recycling. Theoretical and empirical work has focused on interspecific variation in elemental composition, and stoichiometric imbalances between resources and consumers in determining productivity, particularly at the base of foodwebs. Recent work has found considerable intraspecific variation in elemental composition. We know little about the ecological relevance of such variation, and whether predictions of stoichiometric theory hold at the intraspecific level. Here, we used two genotypes of a primary producer Chlamydomonas reinhardtii, and two genotypes of a primary consumer Daphnia pulex, which are already known to vary considerably in their phosphorus (P) use physiology, under conditions of P abundance and limitation, to explore whether such intraspecific differences alter primary as well as secondary production. Specifically, we tested whether there are intraspecific differences in the carbon: phosphorus (C:P) stoichiometry of Chlamydomonas genotypes, whether such differences affect growth and abundance of the two Daphnia genotypes, and whether the two Daphnia genotypes had distinct effects on primary production and growth of the two Chlamydomonas genotypes. We found significant differences in C:P stoichiometry between the two Chlamydomonas genotypes in both P supply conditions. Such intraspecific differences altered the growth of Daphnia genotypes, and affected the outcome of genotypic competition. Finally, Daphnia genotype affected primary production, and interacted with P supply to distinctly affect the growth of the two Chlamydomonas genotypes. Together, our results highlight the potential ecological relevance of intraspecific differences in nutrient use physiology and elemental composition, and the utility of ecological stoichiometry in understanding such consequences.     

Voles (<Emphasis Type="Italic">Microtus</Emphasis> Schrank, 1798) of the Russian Far East: Allozymic and karyological divergence

The intraspecific and interspecific differentiations of voles of the Russian Far East with respect to allozymic and karyotypic characteristics have been studied. The morphologically similar but karyotypically different species Microtus oeconomus, M. fortis, M. maximowiczii, and M. sachaliensis are characterized by considerable allozymic differentiation. In these voles, the allozymic differentiation has been found to increase in the order adjacent populations-subspecies-species. The interspecific allozymic differentiation of the chromosomally polymorphic M. maximowiczii-M. evoronensis-M. mujanensis does not exceed the intraspecific differences in M. oeconomus, M. fortis, and M. maximowiczii. The results are analyzed in terms of the allopatric and stasipatric models of speciation. The taxonomic status of M. maximowiczii gromovi is considered.     

Red imported fire ants (<Emphasis Type="Italic">Solenopsis invicta</Emphasis>) and seasonality influence community refuge use

Refuges are fundamental to animal ecology as refuge availability affects many levels of biological organization—from the behavior and physiology of individuals to the interspecific dynamics of a community. Although frequently studied in the context of predator–prey interactions, refuges may also mediate interspecific competition between native and invasive taxa given the role of refuges as a valuable resource. Because interspecific interactions (e.g., competition and predation) can be modulated by temporal and biotic (e.g., trophic level) factors, we used a manipulative approach to investigate community-wide refuge-use patterns in the context of two important ecological factors: invasive species and seasonality. We surveyed refuge (artificial cover object) use of ants and vertebrates in a forest community for 2 years, and we systematically suppressed an established invasive species (red imported fire ant, Solenopsis invicta) to examine its impact on community refuge use. Native Camponotus ants appeared to co-exist and share refuges with S. invicta, but we found evidence for a negative effect of S. invicta on vertebrate refuge use that was also influenced by season. Vertebrates were more abundant under refuges undergoing suppression of S. invicta, and they were less abundant under refuges during the fall (the season characterized by the highest occupancy of refuges by S. invicta). Thus, researchers must continue to examine the entire community and to incorporate the effects of season when assessing the impact of invasive species (e.g., at our site, a survey conducted only in the summer or only on native ants would have indicated a negligible effect of S. invicta on community refuge use).     

Temperature,space availability,and species assemblages impact competition in global fouling communities

Competition drives community composition in many ecosystems and can influence the spread of invasive species. Marine fouling communities are excellent study systems for competition because of space limitation and the abundance of invasive species. While many studies have examined individual or site-specific responses to changes in temperature or presence of invasive species, it is difficult to predict ecological impacts without assessing interspecific interactions over a wide geographic range. This study compared interactions between several globally distributed invasive fouling species over a broad geographic range. Weekly examination of photographs of settlement panels in marinas at 18 sites around the world allowed for the quantification of competitive outcomes. In the north Atlantic, experimental panels became covered with fouling organisms exponentially faster at warmer temperatures, while northeast and south Pacific sites did not. An invasive ascidian (Diplosoma listerianum) and bryozoan (Bugula neritina) were strong competitors, but most species displayed a negative response in high competition settings where there was little available space. Two species (Botryllus schlosseri and Botrylloides violaceus) had better competitive outcomes at cooler temperatures, possibly due to fewer strong competitors at these sites. Thus, warmer sites with little open space and multiple strong competitors are likely most resistant to future invasions, while colder sites with more open space and weaker competitors would be more susceptible to invasive species. These results suggest that the establishment and spread of invasive fouling species is likely to be influence by seawater temperature, available space, and the competitive abilities of community members.     

Conquerors or exiles? Impact of interference competition among invasive Ponto-Caspian gammarideans on their dispersal rates

Ponto-Caspian gammarids have invaded European waters, affecting local communities by predation and competition. Their ranges and dispersal rates vary across Europe, which may result from their interspecific interactions, accelerating or reducing migrations. We checked this hypothesis by testing interference competition among co-occurring invaders: Dikerogammarus villosus, D. haemobaphes and Pontogammarus robustoides. We used 140-cm long tanks (gravel substratum), divided into seven compartments. We introduced 25 “residents” into the outermost compartment, separated with a barrier. After 1 h, we introduced 25 “intruders”. After the next 1 h, we removed the barrier and the gammarids dispersed in the tank. After 4 or 20 h, we counted the gammarids in the compartments. We tested all pairwise species combinations and single-species controls. Dikerogammarus villosus displaced other species (P. robustoides only after 4 h) and reduced its own motility after 20 h in their presence. Pontogammarus robustoides stimulated the short-time migrations of D. villosus intruders and of D. haemobaphes. As P. robustoides migrated spontaneously much more than Dikerogammarus spp., its impact decreased after longer time. Dikerogammarus haemobaphes stimulated the short-time movement of P. robustoides intruders but reduced the long-time relocation of this species. In general, gammarid dispersal increased in the presence of stronger competitors (D. villosus and P. robustoides, especially residents) and decreased in response to weaker competitors (D. haemobaphes). Thus, competitive interactions may affect dispersal of invasive gammarids and contribute to the fastest spread of the weakest competitor, D. haemobaphes observed in the field, whereas the strongest species, D. villosus was the latest newcomer in many novel areas.     

Ant interactions with native and exotic seeds in the Patagonian steppe: Influence of seed traits,disturbance levels and ant assemblage

Invasive plants may establish strong interactions with species in their new range which could limit or enhance their establishment and spread. These interactions depend upon traits of the invader and the recipient community, and may alter interactions among native species. In the Patagonian steppe we studied interactions of native ant assemblages with seeds of native and exotic plants, and asked whether ant–seed interactions differ with seed types and disturbance levels and whether the amount and type of ant–seed interactions can be predicted if both ant and seed traits are known. To characterize and quantify ant–seed interactions, we offered baits with large seeds of Pappostipa speciosa (native) and medium-sized elaiosome-bearing seeds of Carduus thoermeri (exotic), near and far from a road (high vs. low disturbed areas), and compared ant abundance and composition between areas. Interaction frequency was the highest for C. thoermeri seeds far from the road. Composition of ants interacting with C. thoermeri in these areas differed from that near the road and from that interacting with native seeds. Ant composition and abundance were similar between areas, but some species interacted more with exotic seeds in low disturbed areas. Ant foraging type predicted ant–seed interactions since the abundance of seed harvesters was positively correlated to interactions with P. speciosa, and that of generalists and predators, with interactions with C. thoermeri. The high interaction of ants with exotic seeds in low invaded areas suggests that ant activity could influence plant invasion, either by predating or dispersing seeds of invasive plants.