Invasional meltdown mediated by plant-soil feedbacks may depend on community diversity

Increasing and fluctuating resource availability enhances invasional meltdown

Exotic plant invaders can promote others via direct or indirect facilitation, known as "invasional meltdown." Increased soil nutrients can also promote invaders by increasing their competitive impacts, but how this might affect meltdown is unknown. In a mesocosm experiment, we evaluated how eight exotic plant species and eight Eurasian native species responded individually to increasing densities of the invasive plant Conyza canadensis, while varying the supply and fluctuations of nutrients. We found that increasing density of C. canadensis intensified competitive suppression of natives but intensified facilitation of other exotics. Higher and fluctuating nutrients exacerbated the competitive effects on natives and facilitative effects on exotics. Overall, these results show a pronounced advantage of exotics over native target species with increased relative density of C. canadensis under high nutrient availability and fluctuation. We integrate these results with the observation that exotic species commonly drive increases in soil resources to suggest the Resource-driven Invasional Meltdown and Inhibition of Natives hypothesis in which biotic acceleration of resource availability promotes other exotic species over native species, leading to invasional meltdown.

Co-Invasion of Congeneric Invasive Plants Adopts Different Strategies Depending on Their Origins

Plant communities may be co-invaded by invasive plants, sometimes even by congeneric invasive plants (CIPs). Despite the growing understanding of co-invasion in the environment, little is known about how CIP interactions and mechanisms regulate co-invasion. Darwin's naturalisation conundrum predicts that the coexistence of closely related species is difficult due to their structural and behavioural similarities. Nevertheless, communities containing closely related species are more susceptible to being invaded because close relatives may favour similar environments; therefore, this hypothesis should be followed in the co-invasion of CIPs. To explore whether the phylogenetic relatedness and origins of invasive species to CIPs can promote or hinder co-invasion, we conducted a controlled interaction and soil-legacy greenhouse experiment to quantify the growth response of invasive plants and their congeners. We consistently found that CIPs of identical origin were more likely to co-invade compared to CIPs of distinct origins. CIPs of distinct origins exhibited an antagonistic effect on co-invasion by allelopathy. Invasive plant-conditioned soil was more conducive to the growth of CIPs of identical origin than CIPs of distinct origins. Our results revealed the different effects of invader-invader phylogenetic relatedness on co-invader success and impact, suggesting the operation of different mechanisms across co-invasion.

A nature-positive future with biological invasions: theory, decision support and research needs

In 2050, most areas of biodiversity significance will be heavily influenced by multiple drivers of environmental change. This includes overlap with the introduced ranges of many alien species that negatively impact biodiversity. With the decline in biodiversity and increase in all forms of global change, the need to envision the desired qualities of natural systems in the Anthropocene is growing, as is the need to actively maintain their natural values. Here, we draw on community ecology and invasion biology to (i) better understand trajectories of change in communities with a mix of native and alien populations, and (ii) to frame approaches to the stewardship of these mixed-species communities. We provide a set of premises and actions upon which a nature-positive future with biological invasions (NPF-BI) could be based, and a decision framework for dealing with uncertain species movements under climate change. A series of alternative management approaches become apparent when framed by scale-sensitive, spatially explicit, context relevant and risk-consequence considerations. Evidence of the properties of mixed-species communities together with predictive frameworks for the relative importance of the ecological processes at play provide actionable pathways to a NPF in which the reality of mixed-species communities are accommodated and managed. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Impacts of Soil Properties on Species Diversity and Structure in Alternanthera philoxeroides-Invaded and Native Plant Communities

Soil properties can affect plant population dynamics and the coexistence of native and invasive plants, thus potentially affecting community structure and invasion trends. However, the different impacts of soil physicochemical properties on species diversity and structure in native and invaded plant communities remain unclear. In this study, we established a total of 30 Alternanthera philoxeroides-invaded plots and 30 control plots in an area at the geographical boundary between North and South China. We compared the differences in species composition between the invaded and native plant communities, and we then used the methods of regression analysis, redundancy analysis (RDA), and canonical correspondence analysis (CCA) to examine the impacts of soil physicochemical properties on four α-diversity indices and the species distribution of these two types of communities. We found that A. philoxeroides invasion increased the difference between the importance values of dominant plant species, and the invasion coverage had a negative relationship with the soil-available potassium (R2 = 0.135; p = 0.046) and Patrick richness index (R2 = 0.322; p < 0.001). In the native communities, the species diversity was determined with soil chemical properties, the Patrick richness index, the Simpson dominance index, and the Shannon-Wiener diversity index, which all decreased with the increase in soil pH value, available potassium, organic matter, and ammonium nitrogen. However, in the invaded communities, the species diversity was determined by soil physical properties; the Pielou evenness index increased with increasing non-capillary porosity but decreased with increasing capillary porosity. The determinants of species distribution in the native communities were soil porosity and nitrate nitrogen, while the determinants in the invaded communities were soil bulk density and available potassium. In addition, compared with the native communities, the clustering degree of species distribution in the invaded communities intensified. Our study indicates that species diversity and distribution have significant heterogeneous responses to soil physicochemical properties between A. philoxeroides-invaded and native plant communities. Thus, we need to intensify the monitoring of soil properties in invaded habitats and conduct biotic replacement strategies based on the heterogeneous responses of native and invaded communities to effectively prevent the biotic homogenization that is caused by plant invasions under environmental changes.

Plant-soil feedback and plant invasion: effect of soil conditioning on native and invasive Prosopis species using the plant functional trait approach

Introduction

Invasive species have been identified as a major threat to native biodiversity and ecosystem functioning worldwide due to their superiority in spread and growth. Such superiority is explained by the invasional meltdown phenomena, which suggests that invasive species facilitate the establishment of more invasive species rather than native species by modifying the plant-soil feedback (PSF).

Methods

We conducted a two-phase plant-soil feedback experiment using the native Prosopis cineraria and the invasive Prosopis juliflora in Oman. Firstly, we conditioned the soil by planting seedlings of native species, invasive species, native and invasive species "mixed", and unconditioned soil served as a control. Secondly, we tested the feedback of these four conditioned soil on the two species separately by measuring the productivity (total biomass) and the performance in the form of plant functional traits (plant height, specific leaf area (SLA), leaf nitrogen content (Nmass), leaf carbon content (Cmass) and specific root length (SRL) of native and invasive species as well as the nutrient availability in soil (soil organic carbon (SOC) and soil total nitrogen (STN)).

Results and discussion

We found that the native species produced more biomass, best performance, and higher SOC and STN when grown in soil conditioned by native species, additionally, it gave lower biomass, reduced performance, and lower SOC and STN when grown in the soil conditioned by invasive and mixed species. These results suggest negative PSF for native species and positive PSF for invasive species in the soil conditioned by invasive species, which can be considered as red flag concerning the restoration of P. cineraria as an important native species in Oman, as such positive PSF of the invasive species P. juliflora will inhibit the regeneration of P. cineraria.

Soil legacy effects on biomass allocation depend on native plant diversity in the invaded community

The biodiversity of aboveground plants and belowground microbes is key for plant communities resisting exotic plant invasion. Whether the soil legacy effects after the invasion are related to the diversity of the invaded community is less studied. Soils from invaded communities were collected and potted to investigate the effects of the invasive community's legacy on the biomass allocation of plants that later grew in these soils. The plots where native plants were present had relatively high nutrient levels (except for available nitrogen) compared to the monodominance communities invaded by Chromolaena odorata. This also indirectly suggests that the severe invasion of C. odorata depleted the nutrients in the soil to a greater extent. When soils were from communities with only C. odorata or one native plant, their biotic legacies showed a significantly positive effect on biomass accumulation of subsequent invasive plants, but this positive effect became negative when more than two native plants were present in the invaded community. This result indicated that the effect of biological resistance increases with the number increase of native species in the invaded communities. The soil legacy effect of the invaded communities on subsequent plants depended on the diversity of native plants. This study can provide insights into the mechanisms of soil biological resistance to exotic plant invasion and provide a theoretical basis for the removal of soil legacy effects after the exotic plant invasion.