Invasive Plants Differentially Impact Forest Invertebrates, Providing Taxon-Specific Benefits by Enhancing Structural Complexity

Exotic plant species often negatively affect native herbivores due to the lack of palatability of the invading plant. Although often unsuitable as food, certain invasive species may provide non-nutritional ecological benefits through increased habitat structural complexity. To understand the potential for common invasive forest plant species of the eastern United States to benefit invertebrate communities, we examined the functional and taxonomic community composition of forest insects and spiders in long-term monitoring plots that contained invasive plant species. The extent of invasive plant species ground cover significantly altered spider community composition as categorized by hunting guild. Areas with higher invasive herbaceous and grass cover contained a higher abundance of space web-weaving and hunting spiders, respectively. Spider species richness and total invertebrate abundance also increased with greater invasive grass cover. Still, these trends were driven by just two invasive plant species, garlic mustard and Japanese stiltgrass, both of which have previously been shown to provide structural benefits to native invertebrate taxa. While these two species may improve the structural component of understory forest habitat, many invertebrate groups were not significantly correlated with other prevalent invasive plants and one species, mock strawberry, negatively affected the abundance of certain insect taxa. Particularly in forests with reduced native plant structure, invasive plant management must be conducted with consideration for holistic habitat quality, including both plant palatability and structure.

Invasive grass (Microstegium vimineum) indirectly benefits spider community by subsidizing available prey

Invasive plant species cause a suite of direct, negative ecological impacts, but subsequent, indirect effects are more complex and difficult to detect. Where identified, indirect effects to other taxa can be wide-ranging and include ecological benefits in certain habitats or locations.Here, we simultaneously examine the direct and indirect effects of a common, invasive grass species (Microstegium vimineum) on the invertebrate communities of understory deciduous forests in the eastern United States. To do this, we use two complementary analytic approaches to compare invaded and reference plots: (a) community composition analysis of understory arthropod taxa and (b) analysis of isotopic carbon and nitrogen ratios of a representative predatory spider species.Invaded plots contained a significantly greater abundance of nearly all taxa, including predators, herbivores, and detritivores. Spider communities contained over seven times more individuals and exhibited greater species diversity and richness in invaded plots.Surprisingly, however, the abundant invertebrate community is not nutritionally supported by the invasive plant, despite 100% ground cover of M. vimineum. Instead, spider isotopic carbon ratios showed that the invertebrate prey community found within invaded plots was deriving energy from the plant tissue of C3 plants and not the prevalent, aboveground M. vimineum. Synthesis and applications. We demonstrate that invasive M. vimineum can create non-nutritional ecological benefits for some invertebrate taxa, with potential impacts to the nutritional dynamics of invertebrate-vertebrate food webs. These positive impacts, however, may be restricted to habitats that experience high levels of ungulate herbivory or reduced vegetative structural complexity. Our results highlight the importance of fully understanding taxon- and habitat-specific effects of invading plant species when prioritizing invasive species removal or management efforts.

Colony size and brood investment of Myrmica rubra ant colonies in habitats invaded by goldenrods

Ant richness and abundance are negatively affected by the invasion of alien goldenrods (Solidago sp.). However, little is known about the mechanisms standing behind the impact of the invaders on ant life history, such as colony investments in growth and reproduction. We examined this problem of the investments of Myrmica rubra ant colonies living in different grasslands invaded and non-invaded by goldenrods. Altogether, 47 colonies were analysed; and for each colony, we calculated the number of queens, workers and the production of young workers, gynes, and males. We found that colonies from invaded meadows are smaller in size, but have a similar number of adult queens compared to colonies from non-invaded sites. We also found different brood investments among colonies from invaded and non-invaded meadows-colonies from non-invaded meadows produce more young workers and invest more in growth, whereas colonies from invaded meadows invest more in reproduction through higher gyne production. Male production was at a similar level in colonies from both habitat types. The observed patterns may be explained by the effect of various environmental factors occurring in both grassland types, such as stress in changed habitats, higher competition among gynes in non-invaded grasslands, or finally, by the adaptive colony-level response of ants to stress. The higher production of gynes observed in the invaded grasslands may support dispersal and enhance the probability of establishing a colony in a more favourable location.

Effects of invasive plants on arthropods

Non-native plants have invaded nearly all ecosystems and represent a major component of global ecological change. Plant invasions frequently change the composition and structure of vegetation communities, which can alter animal communities and ecosystem processes. We reviewed 87 articles published in the peer-reviewed literature to evaluate responses of arthropod communities and functional groups to non-native invasive plants. Total abundance of arthropods decreased in 62% of studies and increased in 15%. Taxonomic richness decreased in 48% of studies and increased in 13%. Herbivorous arthropods decreased in response to plant invasions in 48% of studies and increased in 17%, likely due to direct effects of decreased plant diversity. Predaceous arthropods decreased in response to invasive plants in 44% of studies, which may reflect indirect effects due to reductions in prey. Twenty-two percent of studies documented increases in predators, which may reflect changes in vegetation structure that improved mobility, survival, or web-building for these species. Detritivores increased in 67% of studies, likely in response to increased litter and decaying vegetation; no studies documented decreased abundance in this functional group. Although many researchers have examined effects of plant invasions on arthropods, sizeable information gaps remain, specifically regarding how invasive plants influence habitat and dietary requirements. Beyond this, the ability to predict changes in arthropod populations and communities associated with plant invasions could be improved by adopting a more functional and mechanistic approach. Understanding responses of arthropods to invasive plants will critically inform conservation of virtually all biodiversity and ecological processes because so many organisms depend on arthropods as prey or for their functional roles, including pollination, seed dispersal, and decomposition. Given their short generation times and ability to respond rapidly to ecological change, arthropods may be ideal targets for restoration and conservation activities.