Invasive plant alters community and ecosystem dynamics by promoting native predators

Restoration of native saltmarshes can reverse arthropod assemblages and trophic interactions changed by a plant invasion

Plant invasions profoundly impact both natural and managed ecosystems, and removal of the invasive plants addresses only part of the problem of restoring impacted areas. The rehabilitation of diverse communities and their ecosystem functions following removal of invasive plants is an important goal of ecological restoration. Arthropod assemblages and trophic interactions are important indicators of the success of restoration, but they have largely been overlooked in saltmarshes. We determined how arthropod assemblages and trophic interactions changed with the invasion of the exotic plant Spartina alterniflora and with the restoration of the native plant Phragmites australis following Spartina removal in a Chinese saltmarsh. We investigated multiple biotic and abiotic variables to gain insight into the factors underlying the changes in arthropod assemblages and trophic structure. We found that although Spartina invasion had changed arthropod diversity, community structure, feeding-guild composition, and the diets of arthropod natural enemies in the saltmarsh, these changes could be reversed by the restoration of native Phragmites vegetation following removal of the invader. The variation in arthropod assemblages and trophic structure were critically associated with four biotic and abiotic variables (aboveground biomass, plant density, leaf N, and soil salinity). Our findings demonstrate the positive effects of controlling invasive plants on biodiversity and nutrient cycling and provide a foundation for assessing the efficacy of ecological restoration projects in saltmarshes.

OUP accepted manuscript

The invasive plant Alliaria petiolata (garlic mustard) has spread throughout forest understory and edge communities in much of North America, but its persistence, density, and impacts have varied across sites and time. Surveying the literature since 2008, we evaluated both previously proposed and new mechanisms for garlic mustard's invasion success and note how they interact and vary across ecological contexts. We analyzed how and where garlic mustard has been studied and found a lack of multisite and longitudinal studies, as well as regions that may be under- or overstudied, leading to poor representation for understanding and predicting future invasion dynamics. Inconsistencies in how sampling units are scaled and defined can also hamper our understanding of invasive species. We present new conceptual models for garlic mustard invasion from a macrosystems perspective, emphasizing the importance of synergies and feedbacks among mechanisms across spatial and temporal scales to produce variable ecological contexts.

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.

Impacts of invasive plants on animal behaviour

The spread of invasive species is a threat to ecosystems worldwide. However, we know relatively little about how invasive species affect the behaviour of native animals, even though behaviour plays a vital role in the biotic interactions which are key to understanding the causes and impacts of biological invasions. Here, we explore how invasive plants - one of the most pervasive invasive taxa - impact the behaviour of native animals. To promote a mechanistic understanding of these behavioural impacts, we begin by introducing a mechanistic framework which explicitly considers the drivers and ecological consequences of behavioural change, as well as the moderating role of environmental context. We then synthesise the existing literature within this framework. We find that while some behavioural impacts of invasive plants are relatively well-covered in the literature, others are supported by only a handful of studies and should be explored further in the future. We conclude by identifying priority topics for future research, which will benefit from an interdisciplinary approach uniting invasion ecology with the study of animal behaviour and cognition.

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.

Biogeographical comparison of terrestrial invertebrates and trophic feeding guilds in the native and invasive ranges of Carpobrotus edulis

Plant invasions impact on biodiversity by altering the composition of native communities by disrupting taxonomic and functional diversity. Non-native plants are often released from their natural enemies, which might result in a reduction of the attack of primary consumers. However, they can also be exposed to the attack of new herbivores that they might not be able to tolerate. Hence, invertebrate communities can be influenced by invasive non-native plants, which in turn modify interactions and change environmental conditions. In this study, we examined the compositional and trophic diversity of invertebrate species, comparing ecosystems with and without the plant species Carpobrotus edulis in coastal areas in its native (South Africa) and introduced (Iberian Peninsula) ranges. Results show that C. edulis has a clear impact on invertebrate communities in its non-native range, reducing their abundance in invaded areas, and particularly affecting certain trophic groups. Invasive C. edulis also alters the invertebrate diversity by not only reducing abundance but also by altering species composition. Overall, the physical dominance of C. edulis modifies the co-occurrence of invertebrate assemblages, reducing the number of trophic groups and leading to substantial effects on primary consumers. Results suggest that the lack of natural enemies might be an important driver of the expansion of C. edulis in its introduced range. Further work is needed to examine long-term changes caused by non-native plants on invertebrate assemblages and the subsequent modification of biological interactions.

Invasive plants negatively impact native, but not exotic, animals

Despite our growing understanding of the impacts of invasive plants on ecosystem structure and function, important gaps remain, including whether native and exotic species respond differently to plant invasion. This would elucidate basic ecological interactions and inform management. We performed a meta-analytic review of the effects of invasive plants on native and exotic resident animals. We found that invasive plants reduced the abundance of native, but not exotic, animals. This varied by animal phyla, with invasive plants reducing the abundance of native annelids and chordates, but not mollusks or arthropods. We found dissimilar impacts among "wet" and "dry" ecosystems, but not among animal trophic levels. Additionally, the impact of invasive plants increased over time, but this did not vary with animal nativity. Our review found that no studies considered resident nativity differences, and most did not identify animals to species. We call for more rigorous studies of invaded community impacts across taxa, and most importantly, explicit consideration of resident biogeographic origin. We provide an important first insight into how native and exotic species respond differently to invasion, the consequences of which may facilitate cascading trophic disruptions further exacerbating global change consequences to ecosystem structure and function.

Invasive shrubs modify rodent activity timing, revealing a consistent behavioral rule governing diel activity

Animals adjust the timing of their activity to maximize benefits, such as access to resources, and minimize costs, such as exposure to predators. Despite many examples of invasive plants changing animal behavior, the potential for invasive plants to alter the timing of animal activity remains unexplored. In eastern North America, invasive shrubs might have particularly strong effects on animal activity timing during spring and fall, when many invasive shrubs retain their leaves long after native species’ leaves senesce. We experimentally removed an invasive shrub (buckthorn, Rhamnus cathartica) and monitored the activity timing of a ubiquitous small-mammal species (white-footed mouse, Peromyscus leucopus) in spring, summer, and fall. We captured nearly 3 times as many P. leucopus in plots invaded by R. cathartica compared with plots with R. cathartica removed, and P. leucopus were captured 2 h earlier in invaded plots. Regardless of invasion treatment, P. leucopus appear to follow a common rule to set activity timing: P. leucopus were only active below a threshold of ground-level moonlight illuminance (0.038 lux). Diel and monthly lunar cycles play an important role in regulating small-mammal activity, but our data suggest that decreased light penetration dampens the influence of moonlight illuminance in habitats invaded by R. cathartica, allowing P. leucopus to remain active throughout the night. By changing the temporal niche of ubiquitous native animals, invasive shrubs may have unappreciated effects on many ecological interactions, including processes that alter community diversity and affect human health.

How to Invade an Ecological Network

Invasion science is in a state of paradox, having low predictability despite strong, identifiable covariates of invasion performance. We propose shifting the foundation metaphor of biological invasions from a linear filtering scheme to one that invokes complex adaptive networks. We link invasion performance and invasibility directly to the loss of network stability and indirectly to network topology through constraints from the emergence of the stability criterion in complex systems. We propose the wind vane of an invaded network - the major axis of its adjacency matrix - which reveals how species respond dynamically to invasions. We suggest that invasion ecology should steer away from comparative macroecological studies, to rather explore the ecological network centred on the focal species.

Structural insights into the inactivation of CRISPR-Cas systems by diverse anti-CRISPR proteins

A molecular arms race is progressively being unveiled between prokaryotes and viruses. Prokaryotes utilize CRISPR-mediated adaptive immune systems to kill the invading phages and mobile genetic elements, and in turn, the viruses evolve diverse anti-CRISPR proteins to fight back. The structures of several anti-CRISPR proteins have now been reported, and here we discuss their structural features, with a particular emphasis on topology, to discover their similarities and differences. We summarize the CRISPR-Cas inhibition mechanisms of these anti-CRISPR proteins in their structural context. Considering anti-CRISPRs in this way will provide important clues for studying their origin and evolution.

Predators in the plant-soil feedback loop: aboveground plant-associated predators may alter the outcome of plant-soil interactions

Plant-soil feedback (PSF) can structure plant communities, promoting coexistence (negative PSF) or monodominance (positive PSF). At higher trophic levels, predators can alter plant community structure by re-allocating resources within habitats. When predator and plant species are spatially associated, predators may alter the outcome of PSF. Here, I explore the influence of plant-associated predators on PSF using a generalised cellular automaton model that tracks nutrients, plants, herbivores and predators. I explore key contingencies in plant-predator associations such as whether predators associate with live vs. senesced vegetation. Results indicate that plant-associated predators shift PSF to favour the host plant when predators colonise live vegetation, but the outcome of PSF will depend upon plant dispersal distance when predators colonise dead vegetation. I apply the model to two spider-associated invasive plants, finding that spider predators should shift PSF dynamics in a way that inhibits invasion by one forest invader, but exacerbates invasion by another.