Are invasive species drivers of native species decline or passengers of habitat modification? A case study of the impact of the common myna (Acridotheres tristis) on Australian bird species

Habitat modification and invasive species are significant drivers of biodiversity decline. However, distinguishing between the impacts of these two drivers on native species can be difficult. For example, habitat modification may reduce native species abundance, while an invasive species may take advantage of the new environment. This scenario has been described as the driver‐passenger model, with ‘passengers’ taking advantage of habitat modification and ‘drivers’ causing native species decline. Therefore, research must incorporate both habitat modification and invasive species impact to successfully investigate native species decline. In this paper, we used the common myna (Acridotheres tristis) as a case study to investigate the driver‐passenger model. We investigated changes in bird abundance, over 2 years, in relation to different habitat types and common myna abundance. We hypothesized that the common myna is both a passenger of habitat change and a driver of some bird species decline. Our results indicated that the abundance of many native species is greater in high tree density nature reserves, while the common myna was uncommon in these areas. Common myna abundance was almost three times higher in urban areas than nature reserves and declined rapidly as tree density in nature reserves increased. Our findings indicated that the common myna is primarily a passenger of habitat change. However, we also observed negative associations between common myna abundance and some bird species. We stress the importance of simultaneously investigating both invasive species impact and habitat modification. We suggest habitat restoration could be a useful tool for both native species recovery and invasive species control. Understanding the drivers of native species decline will help inform impact mitigation and direct further research.     

Invasive ring-necked parakeets Psittacula krameri in Belgium: habitat selection and impact on native birds

There is a large and growing number of alien species in ecosystems all over the world. In view of this, management efforts should concentrate on invasives that have detrimental effects on native biota. However, action against invasives is often hindered by a lack of relevant ecological information such as the expected distribution and impact of the invader. Our aims were to identify the habitat characteristics that influence ring-necked parakeet abundance in Belgium and to assess the effects of competition for nesting cavities with native hole-nesters. We determined the abundance of parakeets and native hole-nesters in 44 study sites using point counts. We examined the relationship between parakeet numbers and a set of habitat and landscape variables and to assess the effect of competition, we studied the relationships between the number of parakeets and the number of native hole-nesters. We found that parakeet abundance differs between vegetation types based on dominant tree species, and that abundance is higher in forests or parks surrounded by built-up areas. Parakeet numbers were also strongly associated with cavity density, suggesting that this may be a limiting factor. Of the two native hole-nesters considered to be the most vulnerable to competition (nuthatch and starling), we found a negative association with parakeet numbers for the nuthatch only. No relations were found with starlings or any of the other hole-nesting species examined. These findings should be considered in the debate concerning the possible need of action against this species.     

Investigations of density interactions among breeding birds in ponderosa pine forests: correlative and experimental evidence

Summary We assessed the importance of interspecific competition among insectivorous birds breeding in northcentral Arizona's ponderosa pine forests. We examined density interactions among species using two analytic approaches; correlative and experimental. The correlative approach examined patterns of change in breeding densities over four years at the community level and within two foraging guilds; picker-gleaners and aerial feeders. The relationships between morphological and behavioral similarity with pairwise density interactions were also assessed. Our experimental approach involved placement of nest boxes on two treatment plots to increase breeding densities of secondary cavity nesting birds that were in foraging guilds with open nesting insectivores.We found little evidence of interspecific competition. Patterns of density fluctuations indicated large positive covariances among species at both the community level and within guilds. Pairwise density interactions were independent of morphological or behavioral similarity. Nest boxes significantly increased breeding densities of the secondary cavity nesters. However, these increases did not induce reprocal density changes in the open nesting species. Interspecific competition for food during the breeding season appears to be unimportant in ponderosa pine bird communities.     

Spreading of alien species and diversity of communities

A new species invading a new area may cause a decrease in diversity of the community already present there. Comparison of temporal changes in species diversity of the “new” community (including alien species) with those of the “original” community (including only native species) may clarify our understanding of the effect of alien species. Using a simulation-based modelling approach we considered several scenarios describing the invasion of native communities by alien species and calculated the trends in Shannon-Wiener indices and in the numbers of species of the “original” and “new” communities during the course of the invasion. We found that despite a large increase in the population size of the invasive alien species the diversity of the original community may be little affected. Native species numbers may stay relatively constant for a long time and then suddenly collapse. The results indicate some possibly still concealed consequences of the spread of the invasive ladybird Harmonia axyridis (Pallas).     

Limiting similarity and Darwin’s naturalization hypothesis: understanding the drivers of biotic resistance against invasive plant species

Several hypotheses have been proposed to explain biotic resistance of a recipient plant community based on reduced niche opportunities for invasive alien plant species. The limiting similarity hypothesis predicts that invasive species are less likely to establish in communities of species holding similar functional traits. Likewise, Darwin’s naturalization hypothesis states that invasive species closely related to the native community would be less successful. We tested both using the invasive alien Ambrosia artemisiifolia L. and Solidago gigantea Aiton, and grassland species used for ecological restoration in central Europe. We classified all plant species into groups based on functional traits obtained from trait databases and calculated the phylogenetic distance among them. In a greenhouse experiment, we submitted the two invasive species at two propagule pressures to competition with communities of ten native species from the same functional group. In another experiment, they were submitted to pairwise competition with native species selected from each functional group. At the community level, highest suppression for both invasive species was observed at low propagule pressure and not explained by similarity in functional traits. Moreover, suppression decreased asymptotically with increasing phylogenetic distance to species of the native community. When submitted to pairwise competition, suppression for both invasive species was also better explained by phylogenetic distance. Overall, our results support Darwin’s naturalization hypothesis but not the limiting similarity hypothesis based on the selected traits. Biotic resistance of native communities against invasive species at an early stage of establishment is enhanced by competitive traits and phylogenetic relatedness.     

Biological invasion into the nested assemblage of tree–beetle associations on the oceanic Ogasawara Islands

Invasion by alien organisms is a common worldwide phenomenon, and many alien species invade native communities. Invasion by alien species is especially likely to occur on oceanic islands. To determine how alien species become integrated into island plant–insect associations, we analyzed the structure of tree–beetle associations using host plant records for larval feeding by wood-feeding beetles (Coleoptera: Cerambycidae) on the oceanic Ogasawara Islands in the northwestern Pacific Ocean. The host plant records comprised 109 associations among 28 tree (including 8 alien) and 26 cerambycid (including 5 alien) species. Of these associations, 41.3% involved at least one alien species. Most native cerambycid species feed on host trees that have recently died. Alien trees were used by as many native cerambycid species (but by significantly more alien cerambycid species) as were native trees. Native cerambycid species used as many alien tree species (but significantly more native tree species) as did alien cerambycids. Thus, we observed many types of interactions among native and alien species. A network analysis revealed a significant nested structure in tree–cerambycid associations regardless of whether alien species were excluded from the analysis. The original nested associations on the Ogasawara Islands may thus have accepted alien species.     

Do alien predators pose a particular risk to duck nests in Northern Europe? Results from an artificial nest experiment

Several alien predator species have spread widely in Europe during the last five decades and pose a potential enhanced risk to native nesting ducks and their eggs. Because predation is an important factor limiting Northern Hemisphere duck nest survival, we ask the question, do alien species increase the nest loss risk to ground nesting ducks? We created 418 artificial duck nests in low densities around inland waters in Finland and Denmark during 2017–2019 and monitored them for seven days after construction using wildlife cameras to record whether alien species visit and prey on the nests more often than native species. We sampled various duck breeding habitats from eutrophic agricultural lakes and wetlands to oligotrophic lakes and urban environments. The results differed between habitats and the two countries, which likely reflect the local population densities of the predator species. The raccoon dog (Nyctereutes procyonoides), an alien species, was the most common mammalian nest visitor in all habitats and its occurrence reduced nest survival. Only in wetland habitats was the native red fox (Vulpes vulpes) an equally common nest visitor, where another alien species, the American mink (Neovison vison), also occurred among nest visitors. Although cautious about concluding too much from visitations to artificial nests, these results imply that duck breeding habitats in Northern Europe already support abundant and effective alien nest predators, whose relative frequency of visitation to artificial nests suggest that they potentially add to the nest predation risk to ducks over native predators.

     

Native richness and species level trophic traits predict establishment of alien freshwater fishes

Community level ecological traits are thought to affect invasibility as more diverse communities with complex trophic interactions may be associated with greater biotic resistance. Elucidation of the nature of this relationship is often hampered by difficulties in characterising food webs, particularly where field data are lacking. We attempted to overcome this by coupling food web modelling with information-theoretic analysis of the modelled webs. In addition, we also investigated the possibility that species level trends in trophic traits of established aliens might reflect exploitation of empty niches. We constructed hypothetical food webs of 26 natural and artificial lentic habitats from a data set consisting of 370 fish species representing 71 families. Using these food webs, we investigated associations at the community level between food web traits and network topology and number of alien fish species using an information-theoretic approach based on a set of competing a priori hypotheses. At the species level, we similarly tested for trends in trophic traits of established alien fishes using the information-theoretic approach in addition to nMDS of diet data. We found that native species richness in a community was the most important determinant of the number of alien fish taxa, displaying an inverse relationship. Our data also show that alien fish generally feed lower down the food web. Our findings suggest that the biotic resistance hypothesis, though scale dependent, can result in observable effects in animal communities. Moreover, we also found that the ability to exploit low energy yield food sources could favour the establishment of alien species via avoidance of resistive forces from native taxa.     

Insect-Flower Interaction Network Structure Is Resilient to a Temporary Pulse of Floral Resources from Invasive Rhododendron ponticum

Invasive alien plants can compete with native plants for resources, and may ultimately decrease native plant diversity and/or abundance in invaded sites. This could have consequences for native mutualistic interactions, such as pollination. Although invasive plants often become highly connected in plant-pollinator interaction networks, in temperate climates they usually only flower for part of the season. Unless sufficient alternative plants flower outside this period, whole-season floral resources may be reduced by invasion. We hypothesized that the cessation of flowering of a dominant invasive plant would lead to dramatic, seasonal compositional changes in plant-pollinator communities, and subsequent changes in network structure. We investigated variation in floral resources, flower-visiting insect communities, and interaction networks during and after the flowering of invasive Rhododendron ponticum in four invaded Irish woodland sites. Floral resources decreased significantly after R. ponticum flowering, but the magnitude of the decrease varied among sites. Neither insect abundance nor richness varied between the two periods (during and after R. ponticum flowering), yet insect community composition was distinct, mostly due to a significant reduction in Bombus abundance after flowering. During flowering R. ponticum was frequently visited by Bombus; after flowering, these highly mobile pollinators presumably left to find alternative floral resources. Despite compositional changes, however, network structural properties remained stable after R. ponticum flowering ceased: generality increased, but quantitative connectance, interaction evenness, vulnerability, H’₂ and network size did not change. This is likely because after R. ponticum flowering, two to three alternative plant species became prominent in networks and insects increased their diet breadth, as indicated by the increase in network-level generality. We conclude that network structure is robust to seasonal changes in floral abundance at sites invaded by alien, mass-flowering plant species, as long as alternative floral resources remain throughout the season to support the flower-visiting community.     

Invasive mutualists erode native pollination webs

Plant–animal mutualisms are characterized by weak or asymmetric mutual dependences between interacting species, a feature that could increase community stability. If invasive species integrate into mutualistic webs, they may alter web structure, with consequences for species persistence. However, the effect of alien mutualists on the architecture of plant–pollinator webs remains largely unexplored. We analyzed the extent of mutual dependency between interacting species, as a measure of mutualism strength, and the connectivity of 10 paired plant–pollinator webs, eight from forests of the southern Andes and two from oceanic islands, with different incidences of alien species. Highly invaded webs exhibited weaker mutualism than less-invaded webs. This potential increase in network stability was the result of a disproportionate increase in the importance and participation of alien species in the most asymmetric interactions. The integration of alien mutualists did not alter overall network connectivity, but links were transferred from generalist native species to super-generalist alien species during invasion. Therefore, connectivity among native species declined in highly invaded webs. These modifications in the structure of pollination webs, due to dominance of alien mutualists, can leave many native species subject to novel ecological and evolutionary dynamics.     

Invasive plant integration into native plant–pollinator networks across Europe

The structure of plant–pollinator networks has been claimed to be resilient to changes in species composition due to the weak degree of dependence among mutualistic partners. However, detailed empirical investigations of the consequences of introducing an alien plant species into mutualistic networks are lacking. We present the first cross-European analysis by using a standardized protocol to assess the degree to which a particular alien plant species (i.e. Carpobrotus affine acinaciformis, Impatiens glandulifera, Opuntia stricta, Rhododendron ponticum and Solanum elaeagnifolium) becomes integrated into existing native plant–pollinator networks, and how this translates to changes in network structure.Alien species were visited by almost half of the pollinator species present, accounting on average for 42 per cent of the visits and 24 per cent of the network interactions. Furthermore, in general, pollinators depended upon alien plants more than on native plants. However, despite the fact that invaded communities received more visits than uninvaded communities, the dominant role of alien species over natives did not translate into overall changes in network connectance, plant linkage level and nestedness. Our results imply that although supergeneralist alien plants can play a central role in the networks, the structure of the networks appears to be very permeable and robust to the introduction of invasive alien species into the network.     

Whole community invasions and the integration of novel ecosystems

The impact of invasion by a single non-native species on the function and structure of ecological communities can be significant, and the effects can become more drastic–and harder to predict–when multiple species invade as a group. Here we modify a dynamic Boolean model of plant-pollinator community assembly to consider the invasion of native communities by multiple invasive species that are selected either randomly or such that the invaders constitute a stable community. We show that, compared to random invasion, whole community invasion leads to final stable communities (where the initial process of species turnover has given way to a static or near-static set of species in the community) including both native and non-native species that are larger, more likely to retain native species, and which experience smaller changes to the topological measures of nestedness and connectance. We consider the relationship between the prevalence of mutualistic interactions among native and invasive species in the final stable communities and demonstrate that mutualistic interactions may act as a buffer against significant disruptions to the native community.     

Allelopathy of a native grassland community as a potential mechanism of resistance against invasion by introduced plants

Successful plant invasions depend, at least partly, on interactions between introduced plants and native plant communities. While allelopathic effects of introduced invaders on native resident species have received much attention, the reverse, i.e. allelopathic effects of native residents on introduced plants, have been largely neglected. Therefore, we tested whether allelopathy of native plant communities decreases their invasibility to introduced plant species. In addition, we tested among the introduced species whether the invasive ones are more tolerant to allelopathy of native plant communities than the non-invasive ones. To test these hypotheses, we grew nine pairs of related (congeneric or confamilial) invasive and non-invasive introduced plant species (i.e. 18 species) in the presence or absence of a native grassland community, which consisted of three common forbs and three common grasses, with or without activated carbon in the soil. Activated carbon reduced the survival percentage and growth of introduced plants in the absence of the native plant community. However, its net effect on the introduced plants was neutral or even slightly positive in the presence of the native community. This might suggest that the native plant community imposed allelopathic effects on the introduced plants, and that these effects were neutralized or reduced by activated carbon. The invasive and non-invasive introduced plants, however, did not differ in their tolerance to such allelopathic effects of the native plant community. Thus, although allelopathy of native plant communities might increase their resistance against introduced plants, there was no evidence that tolerance to allelopathy of native plant communities contributes to the degree of invasiveness of introduced plants.     

Relative abundance of an invasive alien plant affects native pollination processes

One major characteristic of invasive alien species is their occurrence at high abundances in their new habitat. Flowering invasive plant species that are visited by native insects and overlap with native plant species in their pollinators may facilitate or disrupt native flower visitation and fertilisation by forming large, dense populations with high numbers of flowers and copious rewards. We investigated the direction of such a proposed effect for the alien invasive Rhododendron ponticum in Irish habitats. Flower visitation, conspecific and alien pollen deposition, fruit and seed set were measured in a self-compatible native focal plant, Digitalis purpurea, and compared between field sites that contained different relative abundances of R. ponticum. Flower visitation was significantly lower at higher alien relative plant abundances than at lower abundances or in the absence of the alien. Native flowers experienced a significant decrease in conspecific pollen deposition with increasing alien abundance. Heterospecific pollen transfer was very low in all field sites but increased significantly with increasing relative R. ponticum abundance. However, lower flower visitation and lower conspecific pollen transfer did not alter reproductive success of D. purpurea. Our study shows that indirect interactions between alien and native plants for pollination can be modified by population characteristics (such as relative abundance) in a similar way as interactions among native plant species. In D. purpurea, only certain aspects of pollination and reproduction were affected by high alien abundances which is probably a result of high resilience due to a self-compatible breeding system. Native species that are more susceptible to pollen limitation are more likely to experience fitness disadvantages in habitats with high relative alien plant abundances.     

Potential resource competition between an invasive mammal and native birds: overlap in tree cavity preferences of feral raccoons and Ural owls

Invasive mammals include good tree climbers that use tree cavities for resting and nesting. Tree cavities are important but limited resources in most forests; thus, some invasive mammals can be serious competitors for native cavity-using species, especially cavity-nesting birds. Despite the potential impact, such inter-class competition has rarely been considered. We examined the possibility of resource competition for tree cavities between the invasive raccoon Procyon lotor and the native Ural owl Strix uralensis. Both species are nocturnal and use tree cavities during daytime. We assessed an overlap in cavity use for both species as an indication of potential competition by monitoring 341 cavities during their breeding season in a natural park in Hokkaido, Japan. Of 341 potentially available cavities, raccoons and Ural owls used 37 and 32, respectively. The characteristics of 58 cavities used by raccoons or owls were compared to 49 random cavities to determine if they selected cavities with certain characteristics. As predicted from a large amount of tree cavities and a low raccoon density in this managed forest, we did not find direct evidence of competition, such as physical interaction, intrusion to cavities, or habitat segregation. Cavity types used by both species overlapped considerably in terms of height, entrance size, depth, and other characteristics: their habitats were widely overlapped. Further, in four cavities, one species was replaced by the other. Given the similar habitat requirements, the invasive raccoon could be a potential competitor for Ural owl when raccoon density increases and/or cavity availability decreases, which is the case for many forests in Japan. This study suggests that potential threats of resource competition among not only closely but also distantly related taxa should be taken into consideration when studying the impacts of invasive species.     

Impact of Alien Plant Invaders on Pollination Networks in Two Archipelagos

Mutualistic interactions between plants and animals promote integration of invasive species into native communities. In turn, the integrated invaders may alter existing patterns of mutualistic interactions. Here we simultaneously map in detail effects of invaders on parameters describing the topology of both plant-pollinator (bi-modal) and plant-plant (uni-modal) networks. We focus on the invader Opuntia spp., a cosmopolitan alien cactus. We compare two island systems: Tenerife (Canary Islands) and Menorca (Balearic Islands). Opuntia was found to modify the number of links between plants and pollinators, and was integrated into the new communities via the most generalist pollinators, but did not affect the general network pattern. The plant uni-modal networks showed disassortative linkage, i.e. species with many links tended to connect to species with few links. Thus, by linking to generalist natives, Opuntia remained peripheral to network topology, and this is probably why native network properties were not affected at least in one of the islands. We conclude that the network analytical approach is indeed a valuable tool to evaluate the effect of invaders on native communities.     

Investigating responses to control: a comparison of common myna behaviour across areas of high and low trapping pressure

Harvesting of wildlife by humans is commonly known to impact target species’ demography, life history and behaviour. Yet in the context of invasive and pest species control, behavioural shifts have received very little attention. The introduced common myna, Acridotheres tristis, is undergoing intensive control efforts in some areas of its Australian distribution. In order to investigate whether myna populations respond to harvesting, we compared the behaviour of free-ranging common mynas in areas of high and low trapping pressure. Behavioural analyses revealed that mynas in high  trapping risk areas stayed closer to refuges, tended to form smaller groups, and were overall less detectable. Behavioural differences between high and lowly trapped areas were not attributable to variation in population density. Overall, these results are consistent with the hypothesis that common mynas have the potential to adjust their behaviour in response to heightened anthropic risk in the environment. Behavioural shifts in invasive alien species could modify their ecological impact and may interfere not only with the effectiveness of control measures, but also with how their effectiveness is assessed. Future research should aim to isolate behavioural mechanisms underpinning compensatory responses to control so that any potential effects can be mitigated.     

No consistent legacy effects of invasion by giant goldenrod (Solidago gigantea) via soil biota on native plant growth

Aims Changes in soil microbial communities after occupation by invasive alien plants can represent legacy effects of invasion that may limit recolonization and establishment of native plant species in soils previously occupied by the invader. In this study, for three sites in southern Germany, we investigated whether invasion by giant goldenrod (Solidago gigantea) leads to changes in soil biota that result in reduced growth of native plants compared with neighbouring uninvaded soils.Methods We grew four native plant species as a community and treated those plants with soil solutions from invaded or uninvaded soils that were sterilized, or live, with live solutions containing different fractions of the soil biota using a decreasing sieve mesh-size approach. We measured aboveground biomass of the plants in the communities after a 10-week growth period.Main Findings Across all three sites and regardless of invasion, communities treated with <20 μm soil biota or sterilized soil solutions had significantly greater biomass than communities treated with the complete soil biota solution. This indicates that soil biota>20 μm are more pathogenic to the native plants than smaller organisms in these soils. Across all three sites, there was only a non-significant tendency for the native community biomass to differ among soil solution types, depending on whether or not the soil was invaded. Only one site showed significant differences in community biomass among soil solution types, depending on whether or not the soil was invaded; community biomass was significantly lower when treated with the complete soil biota solution than with soil biota <20 μm or sterilized soil solutions, but only for the invaded soil. Our findings suggest that efforts to restore native communities on soils previously invaded by Solidago gigantea are unlikely to be hindered by changes in soil microbial community composition as a result of previous invasion.     

High invasive pollen transfer, yet low deposition on native stigmas in a Carpobrotus-invaded community

Background and Aims

Invasive plants are potential agents of disruption in plant–pollinator interactions. They may affect pollinator visitation rates to native plants and modify the plant–pollinator interaction network. However, there is little information about the extent to which invasive pollen is incorporated into the pollination network and about the rates of invasive pollen deposition on the stigmas of native plants.

Methods

The degree of pollinator sharing between the invasive plant Carpobrotus affine acinaciformis and the main co-flowering native plants was tested in a Mediterranean coastal shrubland. Pollen loads were identified from the bodies of the ten most common pollinator species and stigmatic pollen deposition in the five most common native plant species.

Key Results

It was found that pollinators visited Carpobrotus extensively. Seventy-three per cent of pollinator specimens collected on native plants carried Carpobrotus pollen. On average 23 % of the pollen on the bodies of pollinators visiting native plants was Carpobrotus. However, most of the pollen found on the body of pollinators belonged to the species on which they were collected. Similarly, most pollen on native plant stigmas was conspecific. Invasive pollen was present on native plant stigmas, but in low quantity.

Conclusions

Carpobrotus is highly integrated in the pollen transport network. However, the plant-pollination network in the invaded community seems to be sufficiently robust to withstand the impacts of the presence of alien pollen on native plant pollination, as shown by the low levels of heterospecific pollen deposition on native stigmas. Several mechanisms are discussed for the low invasive pollen deposition on native stigmas.Key words: Alien plant, Carpobrotus aff. acinaciformis, competition for pollinators, invasion, Mediterranean shrubland, plant-pollinator network, pollen loads, pollinator visits, stigma     

An invasive gull displaces native waterbirds to breeding habitats more exposed to native predators

The effect of invasive opportunistic predators may include population changes in both native prey and native predators as well as alteration of predator–prey interactions. We analyzed the activity of native magpie Pica pica and changes in population, nest sites and nesting success probability of native waterbirds (namely: grebes, ducks, rails and native gulls) in response to the population growth of the invasive Caspian gull Larus cachinnans. The study was carried out at a reservoir in southern Poland and at a similar control reservoir where the Caspian gull was absent. Both the invasive gulls and the native magpie are opportunistic predators of nests of native waterbirds. The population increase of the invasive gull led to a decline in the population of native black-headed gulls Larus ridibundus only. However, the invasive gull displaced all the native species from the breeding islets located in the central part of the reservoir to islets located close to the shoreline. The latter were frequently visited by magpies, which depredated on nests along the shores, leading to an up to threefold decrease in nesting success as compared with nests located in the central area of the invaded reservoir. Predation by Caspian gulls was rarely observed. Thus, the invasion of Caspian gull caused complex direct and indirect effects on the waterbird community that included competition for breeding sites, changes in the spatial distribution of nests and alteration of predation rate by native predators. Moreover, the effects of invasion may not be reflected by changes in population size of native species.