Disentangling the mechanisms underpinning disturbance-mediated invasion

Idiosyncratic invasion trajectories of human bacterial pathogens facing temperature disturbances in soil microbial communities

Current knowledge about effects of disturbance on the fate of invaders in complex microbial ecosystems is still in its infancy. In order to investigate this issue, we compared the fate of Klebsiella pneumoniae (Kp) and Listeria monocytogenes (Lm) in soil microcosms. We then used environmental disturbances (freeze-thaw or heat cycles) to compare the fate of both invaders and manipulate soil microbial diversity. Population dynamics of the two pathogens was assessed over 50 days of invasion while microbial diversity was measured at times 0, 20 and 40 days. The outcome of invasion was strain-dependent and the response of the two invaders to disturbance differed. Resistance to Kp invasion was higher under the conditions where resident microbial diversity was the highest while a significant drop of diversity was linked to a higher persistence. In contrast, Lm faced stronger resistance to invasion in heat-treated microcosms where diversity was the lowest. Our results show that diversity is not a universal proxy of resistance to microbial invasion, indicating the need to properly assess other intrinsic properties of the invader, such as its metabolic repertoire, or the array of interactions between the invader and resident communities.

Effects of nutrient pulses on exotic species shift from positive to neutral with decreasing water availability

Temporal fluctuation in nutrient availability generally promotes the growth of exotic plant species and has been recognized as an important driver of exotic plant invasions. However, little is known about how the impact of fluctuating nutrients on exotic species is dependent on the availability of other resources, although most ecosystems are experiencing dramatic variations in a wide variety of resources due to global change and human disturbance. Here, we explored how water availability mediates the effect of nutrient pulses on the growth of six exotic and six native plant species. We subjected individual plants of exotic and native species to well watered or water stressed conditions. For each level of water availability, we added equivalent amounts of nutrients at a constant rate, as a single large pulse, or in multiple small pulses. Under well watered conditions, nutrient pulses promoted exotic plant growth relative to nutrients supplied constantly, while they had no significant effect on natives. In contrast, under water stressed conditions, water deficiency inhibited the growth of all exotic and native species. More importantly, nutrient pulses did not increase plant growth relative to nutrients supplied constantly and these phenomena were observed for both exotic and native species. Taken together, our study shows that the impact of fluctuating nutrient availability on the growth of exotic plant species strongly depends on the variation of other resources, and that the positive effect of nutrient pulses under well watered conditions disappears under water stressed conditions. Our findings suggest that the variation in multiple resources may have complex feedback on exotic plant invasions and, therefore, it is critical to encompass multiple resources for the evaluation of fluctuating resource availability effects on exotic plant species. This will allow us to project the invasive trajectory of exotic plant species more accurately under future global change and human disturbance.

Diversity loss from multiple interacting disturbances is regime-dependent

Anthropogenic activities expose many ecosystems to multiple novel disturbances simultaneously. Despite this, how biodiversity responds to simultaneous disturbances remains unclear, with conflicting empirical results on their interactive effects. Here, we experimentally test how one disturbance (an invasive species) affects the diversity of a community over multiple levels of another disturbance regime (pulse mortality). Specifically, we invade stably coexisting bacterial communities under four different pulse frequencies, and compare their final resident diversity to uninvaded communities under the same pulse mortality regimes. Our experiment shows that the disturbances synergistically interact, such that the invader significantly reduces resident diversity at high pulse frequency, but not at low. This work therefore highlights the need to study simultaneous disturbance effects over multiple disturbance regimes as well as to carefully document unmanipulated disturbances, and may help explain the conflicting results seen in previous multiple-disturbance work.

Exotic asphyxiation: interactions between invasive species and hypoxia

Non-indigenous species (NIS) and hypoxia (<2 mg O₂ l^-1 ) can disturb and restructure aquatic communities. Both are heavily influenced by human activities and are intensifying with global change. As these disturbances increase, understanding how they interact to affect native species and systems is essential. To expose patterns, outcomes, and generalizations, we thoroughly reviewed the biological invasion literature and compiled 100 studies that examine the interaction of hypoxia and NIS. We found that 64% of studies showed that NIS are tolerant of hypoxia, and 62% showed that NIS perform better than native species under hypoxia. Only one-quarter of studies examined NIS as creators of hypoxia; thus, NIS are more often considered passengers associated with hypoxia, rather than drivers of it. Paradoxically, the NIS that most commonly create hypoxia are primary producers. Taxa like molluscs are typically more hypoxia tolerant than mobile taxa like fish and crustaceans. Most studies examine individual-level or localized responses to hypoxia; however, the most extensive impacts occur when hypoxia associated with NIS affects communities and ecosystems. We discuss how these influences of hypoxia at higher levels of organization better inform net outcomes of the biological invasion process, i.e. establishment, spread, and impact, and are thus most useful to management. Our review identifies wide variation in the way in which the interaction between hypoxia and NIS is studied in the literature, and suggests ways to address the number of variables that affect their interaction and refine insight gleaned from future studies. We also identify a clear need for resource management to consider the interactive effects of these two global stressors which are almost exclusively managed independently.

Disturbance-mediated invasions are dependent on community resource abundance

Disturbances can facilitate biological invasions, with the associated increase in resource availability being a proposed cause. Here, we experimentally tested the interactive effects of disturbance regime (different frequencies of biomass removal at equal intensities) and resource abundance on invasion success using a factorial design containing five disturbance frequencies and three resource levels. We invaded populations of the bacterium Pseudomonas fluorescens with two ecologically different invader morphotypes: a fast‐growing “colonizer” type and a slower growing “competitor” type. As resident populations were altered by the treatments, we additionally tested their effect on invader success. Disturbance frequency and resource abundance interacted to affect the success of both invaders, but this interaction differed between the invader types. The success of the colonizer type was positively affected by disturbance under high resources but negatively under low. However, disturbance negatively affected the success of the competitor type under high resource abundance but not under low or medium. Resident population changes did not alter invader success beyond direct treatment effects. We therefore demonstrate that the same disturbance regime can either be beneficial or detrimental for an invader depending on both community resource abundance and its life history. These results may help to explain some of the inconsistencies found in the disturbance‐invasion literature.

The distribution and impact of an invasive plant species (Senecio inaequidens) on a dune building engineer (Calamagrostis arenaria)

Disturbance is thought to enhance the probability of invasive species establishment, a prerequisite for naturalisation. Coastal dunes are characterised by disturbance in the form of sand dynamics. We studied the effect of this disturbance on the establishment and spread of an invasive plant species (Senecio inaequidens) in European coastal dunes. Local sand dynamics dictate the spatial configuration of marram grass (Calamagrostis arenaria). Therefore, marram grass configuration was used as a reliable proxy for disturbance. Since marram grass plays a crucial role in natural dune formation, we evaluated the possible effects S. inaequidens could have on this process, if it is able to naturalise in European coastal dunes.

We expected the highest probability of S. inaequidens establishment at intermediate marram grass cover because too low cover would increase sand burial, whereas high cover would increase competition. However, our results indicate that S. inaequidens is quite capable of handling higher levels of sand burial. Thus, the probability of S. inaequidens establishment was high under low marram cover but slightly lowered when marram cover was high, hinting at the importance of competition.

We expected a negative impact of Senecio-altered soils on marram grass growth mediated by soil biota. However, marram grass grew better in sand gathered underneath Senecio plants due to abiotic soil modifications. This enhanced growth may be caused by Senecio leaf litter elevating nutrient concentrations in an otherwise nutrient-poor substrate. If such increased plant growth is a general phenomenon, further expansion of S. inaequidens could accelerate natural succession in European coastal dunes.

The impact of propagule pressure on whole community invasions in biomethane-producing communities

Microbes can invade as whole communities, but the ecology of whole community invasions is poorly understood. Here, we investigate how invader propagule pressure (the number of invading organisms) affects the composition and function of invaded laboratory methanogenic communities. An invading community was equally successful at establishing itself in a resident community regardless of propagule pressure, which varied between 0.01 and 10% of the size resident community. Invasion resulted in enhanced biogas production (to the level of the pure invading community) but only when propagule pressure was 1% or greater. This inconsistency between invasion success and changes in function can be explained by a lower richness of invading taxa at lower propagule pressures, and an important functional role of the taxa that were absent. Our results highlight that whole community invasion ecology cannot simply be extrapolated from our understanding of single species invasions. Moreover, we show that methane production can be enhanced by invading poorly performing reactors with a better performing community at levels that may be practical in industrial settings.

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Complex communities can successfully invade other communities

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Invasions change the community composition independently of propagule pressure

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Number of taxa invading increases with increased propagule pressure

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Larger propagule pressure impacts both the community structure and function

Artificial modification on lateral hydrological connectivity promotes range expansion of invasive Spartina alterniflora in salt marshes of the Yellow River delta, China

'Invasibility', or the extent to which a habitat is prone to being invaded by plants, is a measure of the resistance of that ecosystem to biological invasion: a limited extent represents abiotic conditions unsuitable for invasion by invasive species; however, human activity can change that and make a habitat prone to rapid invasion. Field surveys and greenhouse experiments were carried out to explore, using spatial analysis, how a strong invader, namely Spartina alterniflora, is assisted by such activities as constructing levees and digging trenches, ditches, and pits in a tidal salt marsh. These activities changed the lateral hydrological connectivity of a salt marsh. The invasibility was then estimated based on the probability of seed dispersal and retention using the classical probabilistic method, and the rate of seedling emergence using threshold analysis. Changes in lateral hydrological connectivity led to more seeds of the invading species being retained, especially in high marshes, and promoted the emergence of its seedlings by making the soil more moist and less saline. The results suggest that changes in the lateral hydrological connectivity in a salt marsh can make it more prone to being invaded. The results have important implications for the control of invasive plants by limiting human activity and thereby regulating lateral hydrological connectivity in coastal ecosystems.

Unexpected sensitivity of the highly invasive spider Mermessus trilobatus to soil disturbance in grasslands

The dwarf spider Mermessus trilobatus (Araneae: Linyphiidae), native to North America, has expanded its range over large parts of Europe within less than fifty years. It is notable for occurring in a wide range of mostly agricultural habitats, while most other invasive spiders in Europe are associated with human buildings. As in other invasive invertebrates and plants, the tremendous colonisation success of Mermessus trilobatus might be related to anthropogenic habitat disturbance. Here we aim to test if the invasion success of Mermessus trilobatus in Europe is associated with high tolerance towards soil disturbance. We sampled spiders from eight grasslands experimentally disturbed with superficial soil tillage and eight undisturbed grasslands without tillage. Opposite to our expectation, Mermessus trilobatus densities decrease sharply with soil disturbance. This is in contrast to several native species such as Oedothorax apicatus, which becomes more abundant in the fields after superficial soil tillage. Our study suggests that invasion success of Mermessus trilobatus is not connected to a ruderal strategy. The ecological and evolutionary processes behind colonisation success of Mermessus trilobatus need to be further investigated.

Relationships between community composition, productivity and invasion resistance in semi-natural bacterial microcosms

Common garden experiments that inoculate a standardised growth medium with synthetic microbial communities (i.e. constructed from individual isolates or using dilution cultures) suggest that the ability of the community to resist invasions by additional microbial taxa can be predicted by the overall community productivity (broadly defined as cumulative cell density and/or growth rate). However, to the best of our knowledge, no common garden study has yet investigated the relationship between microbial community composition and invasion resistance in microcosms whose compositional differences reflect natural, rather than laboratory-designed, variation. We conducted experimental invasions of two bacterial strains (Pseudomonas fluorescens and Pseudomonas putida) into laboratory microcosms inoculated with 680 different mixtures of bacteria derived from naturally occurring microbial communities collected in the field. Using 16S rRNA gene amplicon sequencing to characterise microcosm starting composition, and high-throughput assays of community phenotypes including productivity and invader survival, we determined that productivity is a key predictor of invasion resistance in natural microbial communities, substantially mediating the effect of composition on invasion resistance. The results suggest that similar general principles govern invasion in artificial and natural communities, and that factors affecting resident community productivity should be a focal point for future microbial invasion experiments.

Double trouble: the implications of climate change for biological invasions

The implications of climate change for biological invasions are multifaceted and vary along the invasion process. Changes in vectors and pathways are likely to manifest in changes in transport routes and destinations, together with altered transit times and traffic volume. Ultimately, changes in the nature of why, how, and where biota are transported and introduced will pose biosecurity challenges. These challenges will require increased human and institutional capacity, as well as proactive responses such as improved early detection, adaptation of present protocols and innovative legal instruments. Invasion success and spread are expected to be moderated by the physiological response of alien and native biota to environmental changes and the ensuing changes in biotic interactions. These in turn will likely affect management actions aimed at eradicating, containing, and mitigating invasions, necessitating an adaptive approach to management that is sensitive to potentially unanticipated outcomes.

Community coalescence: an eco-evolutionary perspective

Community coalescence, the mixing of different communities, is widespread throughout microbial ecology. Coalescence can result in approximately equal contributions from the founding communities or dominance of one community over another. These different outcomes have ramifications for community structure and function in natural communities, and the use of microbial communities in biotechnology and medicine. However, we have little understanding of when a particular outcome might be expected. Here, we integrate existing theory and data to speculate on how a crucial characteristic of microbial communities-the type of species interaction that dominates the community-might affect the outcome of microbial community coalescence. Given the often comparable timescales of microbial ecology and microevolution, we explicitly consider ecological and evolutionary dynamics, and their interplay, in determining coalescence outcomes. This article is part of the theme issue 'Conceptual challenges in microbial community ecology'.

Disentangling the mechanisms underpinning disturbance-mediated invasion

Disturbances can play a major role in biological invasions: by destroying biomass, they alter habitat and resource abundances. Previous field studies suggest that disturbance-mediated invader success is a consequence of resource influxes, but the importance of other potential covarying causes, notably the opening up of habitats, have yet to be directly tested. Using experimental populations of the bacterium Pseudomonas fluorescens, we determined the relative importance of disturbance-mediated habitat opening and resource influxes, plus any interaction between them, for invader success of two ecologically distinct morphotypes. Resource addition increased invasibility, while habitat opening had little impact and did not interact with resource addition. Both invaders behaved similarly, despite occupying different ecological niches in the microcosms. Treatment also affected the composition of the resident population, which further affected invader success. Our results provide experimental support for the observation that resource input is a key mechanism through which disturbance increases invasibility.