No evidence that modification of soil microbiota by woody invader facilitates subsequent invasion by herbaceous species

Many terrestrial ecosystems are co-invaded by multiple exotic species. The "invasional meltdown" hypothesis predicts that an initial invasive species will facilitate secondary invasions. In the plant kingdom, the potential underlying mechanisms of this hypothesis may be that modification of the soil properties by the initial invaders benefits for the subsequent exotic species invasion. In this study, we analyzed the composition of soil microbial communities and soil chemical properties from sites invaded by woody Rhus typhina, as well as uninvaded sites, to assess the impact of R. typhina invasion. Furthermore, we conducted a greenhouse experiment with multiple native-invasive pairs of herbaceous species to test whether R. typhina invasion facilitates subsequent exotic herb invasion. Our results showed that R. typhina invasion significantly altered the composition of soil fungal communities, especially pathogenic, endophytic, and arbuscular mycorrhizal fungi. However, this change in microbial composition led to neither direction nor magnitude changes in negative plant-soil feedback effects on both native and invasive species. This indicates that initial R. typhina invasion does not facilitate subsequent herb invasion, which does not support the "invasional meltdown" hypothesis. Additionally, R. typhina invasion significantly decreased soil total nitrogen and organic carbon contents, which may explain the significantly lower biomass of herbaceous roots grown in invaded soils compared with uninvaded soils. Alternately, although invasive herb growth was significantly more inhibited by soil microbiota compared with native herb growth, such inhibition cannot completely eliminate the risk of exotic herb invasion because of their innate growth advantages. Therefore, microbial biocontrol agents for plant invasion management should be combined with another approach to suppress the innate growth advantages of exotic species.

Inverse priority effects: A role for historical contingency during species losses

Communities worldwide are losing multiple species at an unprecedented rate, but how communities reassemble after these losses is often an open question. It is well established that the order and timing of species arrival during community assembly shapes forthcoming community composition and function. Yet, whether the order and timing of species losses can lead to divergent community trajectories remains largely unexplored. Here, we propose a novel framework that sets testable hypotheses on the effects of the order and timing of species losses-inverse priority effects-and suggests its integration into the study of community assembly. We propose that the order and timing of species losses within a community can generate alternative reassembly trajectories, and suggest mechanisms that may underlie these inverse priority effects. To formalize these concepts quantitatively, we used a three-species Lotka-Volterra competition model, enabling to investigate conditions in which the order of species losses can lead to divergent reassembly trajectories. The inverse priority effects framework proposed here promotes the systematic study of the dynamics of species losses from ecological communities, ultimately aimed to better understand community reassembly and guide management decisions in light of rapid global change.

Crossing the threshold: Invasive grasses inhibit forest restoration on Hawaiian islands

Forest removal for livestock grazing is a striking example of human-caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience to the alternative grass state can greatly benefit managers when planning forest restoration. We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawai'i Island, USA. Since the 1980s, over 400,000 Acacia koa (koa) trees have been planted across degraded pasture, and invasive grasses still dominate the understory with no native woody-plant recruitment. Between this koa/grass matrix are remnant native Metrosideros polymorpha ('ōhi'a) trees beneath which native woody plants naturally recruit. We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and 'ōhi'a) with a range of native woody understory basal area (BA). We found a positive relationship between total seed rain (but not bird-dispersed seed rain) and native woody BA and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (six levels) of two common understory species in plots under koa (n = 9) and remnant 'ōhi'a (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities two to 75 times higher than naturally occurring. However, seedling recruitment increased two to three times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses is not necessary to achieve system responses: Even moderate reductions (~50%) can increase rates of native woody recruitment. The nonlinear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources.

Dynamics and mechanisms of secondary invasion following biological control of an invasive plant

Secondary invasions in which nontarget invaders expand following eradication of a target invader commonly occur in habitats with multiple invasive plant species and can prevent recovery of native communities. However, the dynamics and mechanisms of secondary invasion remain unclear. Here, we conducted a common garden experiment to test underlying mechanisms of secondary invasion for 14 nontarget invaders after biological control of Ambrosia artemisiifolia in two consecutive years. We found secondary invasion for all tested nontarget invaders, but secondary invasiveness (change relative to natives) varied with species and time. Specifically, secondary invasiveness depended most strongly on phylogenetic relatedness between the target and nontarget invaders in the first year with closely related nontarget invaders being most invasive. By contrast, secondary invasiveness in the second year was mostly driven by functional traits with taller nontarget invaders or those with higher specific leaf area, or smaller seeds especially invasive. Our study indicates that secondary invasion is likely to occur wherever other invasive plants co-occur with an invasive species targeted for control. Furthermore, the most problematic invaders will initially be species closely related to the target invader but then species with rapid growth and high reproduction are most likely to be more aggressive secondary invaders.

Tipping the balance: The role of seed density, abiotic filters, and priority effects in seed-based wetland restoration

Sowing native seeds is a common approach to reintroduce native plants to degraded systems. However, this method is often overlooked in wetland restoration despite the immense global loss of diverse native wetland vegetation. Developing guiding principles for seed-based wetland restoration is critical to maximize native plant recovery, particularly in previously invaded wetlands. Doing so requires a comprehensive understanding of how restoration manipulations, and their interactions, influence wetland plant community assembly. With a focus on the invader Phragmites australis, we established a series of mesocosm experiments to assess how native sowing density, invader propagule pressure, abiotic filters (water and nutrients), and native sowing timing (i.e., priority effects) interact to influence plant community cover and biomass in wetland habitats. Increasing the density of native seeds yielded higher native cover and biomass, but P. australis suppression with increasing sowing densities was minimal. Rather, community outcomes were largely driven by invader propagule pressure: P. australis densities of ≤500 seeds/m² maintained high native cover and biomass. Low-water conditions increased the susceptibility of P. australis to dominance by native competitors. Early sowing of native seeds showed a large and significant benefit to native cover and biomass, regardless of native sowing density, suggesting that priority effects can be an effective restoration manipulation to enhance native plant establishment. Given the urgent wetland restoration need combined with the limited studies on seed-based wetland restoration, these findings provide guidance on restoration manipulations that are grounded in ecological theory to improve seed-based wetland restoration outcomes.

The role of microtopography and resident species in post-disturbance recovery of arid habitats in Hawai'i

Habitat-suitability indices (HSI) have been employed in restoration to identify optimal sites for planting native species. Often, HSI are based on abiotic variables and do not include biotic interactions, even though similar abiotic conditions can favor both native and nonnative species. Biotic interactions such as competition may be especially important in invader-dominated habitats because invasive species often have fast growth rates and can exploit resources quickly. In this study, we test the utility of an HSI of microtopography derived from airborne LiDAR to predict post-disturbance recovery and native planting success in native shrub-dominated and nonnative, invasive grass-dominated dryland habitats in Hawai'i. The HSI uses high-resolution digital terrain models to classify sites' microtopography as high, medium, or low suitability, based on wind exposure and topographic position. We used a split-plot before-after-control-impact design to implement a disturbance experiment within native shrub (Dodonaea viscosa) and nonnative, invasive grass (Cenchrus clandestinus)-dominated ecosystems across three microtopography categories. In contrast to previous studies using the same HSI, we found that microtopography was a poor predictor of pre-disturbance conditions for soil nutrients, organic matter content, or foliar C:N, within both Dodonaea and Cenchrus vegetation types. In invader-dominated Cenchrus plots, microtopography helped predict cover, but not as expected (i.e., highest cover would be in high-suitability plots): D. viscosa had the greatest cover in low-suitability and C. clandestinus had the greatest cover in medium-suitability plots. Similarly, in native-dominated Dodonaea plots, microtopography was a poor predictor of D. viscosa, C. clandestinus, and total plant cover. Although we found some evidence that microtopography helped inform post-disturbance plant recovery of D. viscosa and total plant cover, vegetation type was a more important predictor. Important for considering the success of plantings, percent cover of D. viscosa decreased while percent cover of C. clandestinus increased within both vegetation types 20 months after disturbance. Our results are evidence that HSIs based on topographic features may prove most useful for choosing planting sites in harsh habitats or those already dominated by native species. In more productive habitats, competition from resident species may offset any benefits gained from "better" suitability sites.

The Impact of Multiple Species Invasion on Soil and Plant Communities Increases With Invasive Species Co-occurrence

Despite increasing evidence indicating that invasive species are harming biodiversity, ecological systems and processes, impacts of multiple species invasion and their links with changes in plant and soil communities are inadequately documented and remain poorly understood. Addressing multiple invaders would help to ward against community-wide, synergistic effects, aiding in designing more effective control strategies. In this work, correlative relationships are examined for potential impacts of three co-occurring invasive plant species, Amorpha fruticosa, Fraxinus pennsylvanica, and Acer negundo, on soil conditions and native plant diversity. The research was conducted in riparian ecosystems and included the following treatments: (1) co-occurrence of the three invasive plant species, (2) occurrence of a single invasive species, and (3) control, i.e., absence of invasive species. Co-occurrence of three invasive plant species caused higher direct impact on soil properties, soil functioning, and native plant diversity. Soil in mixed plots (those populated with all three invaders) contained higher levels of nitrifying bacteria, organic matter, nitrogen, and carbon as well as lower carbon to nitrogen ratio as compared to single species invaded plots and control plots. Furthermore, native plant diversity decreased with invasive plants co-occurrence. Differences in soil conditions and lower native plant diversity revealed the interactive potential of multiple invasive species in depleting biodiversity and eroding soil functionality, ultimately affecting ecological and biogeochemical processes both below and above ground. Our results highlight the need to prevent the impact of multispecies invasion, suggesting that riparian ecosystems affected by co-occurring invaders should be prioritized for invasion monitoring and ecological restoration.

Removal of invasive Scotch broom increases its negative effects on soil chemistry and plant communities

Recovery of ecosystem properties following removal of invasive plants likely varies with characteristics of the plant and the relative soil quality at a given site. These factors may influence the occurrence of soil legacies and secondary invasions, hindering the effectiveness of restoration strategies. We assessed the potential for ecosystem recovery following removal of N-fixing Scotch broom for 4 years at two sites that contrasted strongly in soil quality in western Washington and Oregon, USA. Comparisons were made among plots, where Scotch broom was never present (uninvaded), retained, or removed. Scotch broom removal increased PAR and soil temperature but had limited effects on soil moisture. Concentrations of soil Ca, Mg, K, and P were significantly lower with Scotch broom removal, with the effect being most pronounced at the low-quality site. NMS ordinations indicated that the treatments differed in vegetation composition, with limited recovery following broom removal. Non-native and native species varied inversely in their abundance responses, where non-native species abundance was greatest in the removal treatment, intermediate in the retained treatment, and lowest in the uninvaded treatment, indicating occurrence of a secondary invasion following removal. As with the soil response, effects were more pronounced at the low-quality site. Our findings indicate that Scotch broom removal exacerbates negative effects on soil chemistry and plant communities, with little evidence of recovery over our study period. These findings highlight the importance of controlling Scotch broom invasions immediately after the species establishes, especially on low-quality sites that are more susceptible to Scotch broom invasion.

Species Home-Making in Ecosystems: Toward Place-Based Ecological Metrics of Belonging

Globalization has undeniably impacted the Earth’s ecosystems, but it has also influenced how we think about natural systems. Three fourths of the world’s forests are now altered by human activity, which challenges our concepts of native ecosystems. The dichotomies of pristine vs. disturbed as well as our view of native and non-native species, have blurred; allowing us to acknowledge new paradigms about how humans and nature interact. We now understand that the use of militaristic language to define the perceived role of a plant species is holding us back from the fact that novel systems (new combinations of all species) can often provide valuable ecosystem services (i.e., water, carbon, nutrients, cultural, and recreation) for creatures (including humans). In reality, ecosystems exist in a gradient from native to intensely managed – and “non-nativeness” is not always a sign of a species having negative effects. In fact, there are many contemporary examples of non-native species providing critical habitat for endangered species or preventing erosion in human-disturbed watersheds. For example, of the 8,000–10,000 non-native species introduced to Hawai‘i, less than 10% of these are self-sustaining and 90 of those pose a danger to native biota and are considered invasive. In this paper, we explore the native/non-native binary, the impacts of globalization and the political language of invasion through the lens of conservation biology and sociology with a tropical island perspective. This lens gives us the opportunity to offer a place-based approach toward the use of empirical observation of novel species interactions that may help in evaluating management strategies that support biodiversity and ecosystem services. Finally, we offer a first attempt at conceptualizing a site-specific approach to develop “metrics of belonging” within an ecosystem.

Environmental gradients influence differences in leaf functional traits between native and non-native plants

Determining the characteristics of non-native plants that can successfully establish and spread is central to pressing questions in invasion ecology. Evidence suggests that some non-native species establish and spread in new environments because they possess characteristics (functional traits) that allow them to either successfully compete with native residents or fill previously unfilled niches. However, the relative importance of out-competing native species vs. filling empty niche space as potential mechanisms of invasion may depend on environmental characteristics. Here, we measured plant functional traits, proxies indicative of competitive and establishment strategies, to determine if these traits vary among native and invasive species and if their prevalence is dependent on environmental conditions. Using a natural environmental gradient in Hawai'i Volcanoes National Park, we evaluated how functional traits differ between native and non-native plant communities and if these differences change along an environmental gradient from hot, dry to cool, wet conditions. Functional trait differences suggested that both competition and open niche space may be important for invasion. Non-native communities tended to have traits associated with faster growth strategies such as higher specific leaf area and lower leaf thickness. However, native and non-native community traits became more dissimilar along the gradient, suggesting that non-native species may be occupying previously unfilled niche space at the hot, dry end of the gradient. We also found that most of the variation in functional trait values amongst plots was due to species turnover rather than intraspecific variation. These results highlight the role of environmental context when considering invasion mechanisms.