Arbuscular mycorrhizal fungi (AMF) can increase plant fitness under certain environmental conditions. Among the mechanisms that may drive this mutualism, the most studied is provisioning of nutrients ...by AMF in exchange for carbon from plant hosts. However, AMF may also provide a suite of non-nutritional benefits to plants including improved water uptake, disease resistance, plant chemical defense, soil aggregation, and allelochemical transport and protection. Here, we use a meta-analysis of 93 studies to assess the relative effect of AMF on nutritional and non-nutritional factors that may influence plant fitness. We find that the positive effects of AMF on soil aggregation, water flow and disease resistance are equal to the effect of AMF on plant nitrogen and phosphorus uptake. However, AMF had no effect on the uptake of other nutrients, plant water content, allelopathic transport or production of chemical defense compounds. We suggest future research directions, including experimentally assessing the relative contribution on plant fitness of AMF interactions by untangling the independence of alternative benefits of AMF from an increase in nutrient uptake. This will lead to a more holistic view of the mycorrhizal-plant association and a more accurate picture of the net impact on the plant or plant community in question.
Question: Plant–soil feedback (PSF) has emerged as a ubiquitous phenomenon and a potentially important predictor of plant community structure and dynamics. However, the predictive power of PSF in ...field contexts is mixed, and ecologists do not yet understand its relative importance compared to other factors that structure communities. Further progress requires a more nuanced understanding of how PSF interacts with other biotic and abiotic factors. Environmental factors (e.g. natural enemies, moisture, light, nutrients) are known to affect plant interactions with soil and soil organisms, leading to an expectation of environmental context dependence in plant–soil feedback. Such context dependence could explain when PSF is expected to be an important driver of community dynamics, and under what conditions PSF is likely to be positive (destabilizing) vs negative (stabilizing). Methods: We conducted a literature review of studies that examined PSF across biotic and abiotic gradients. Results: Although few relevant studies have been conducted in this emerging research area, results to date suggest that plant–soil feedback is sensitive to biotic factors, such as above- and below-ground herbivory, and abiotic factors, such as nutrients and light. Conclusions: We develop a conceptual framework to predict variation in the direction and strength of PSF depending on the biotic or abiotic drivers of feedback (e.g. pathogens, nutrients) and the environmental context (e.g. intensity of herbivory, soil fertility). We explore the utility of our predictive framework through discussion of case studies from the literature on context dependence in PSF. We also consider how different experimental approaches might yield different insights about PSF–environment interactions, and suggest key future research directions.
Placing invasion in a more complete food web context expands our understanding of species invasions to reflect the inherent complexity of ecological networks. Garlic mustard (Alliaria petiolata) has ...traditionally been predicted to dominate native communities through mechanisms embodied in popular hypotheses such as direct plant-plant interactions (allelopathy) and plant-herbivore interactions (enemy escape). However, garlic mustard also interacts directly with native predators by providing habitat for web-building spiders, which colonize the dry fruit structures (siliques) that garlic mustard leaves behind after it senesces. This interaction may lead to altered food web structure, resulting previously unexamined invasion consequences. This idea was tested in a field experiment including three treatments in which garlic mustard siliques were left intact (S+), removed (S−), or native species dominated and garlic mustard was absent (N). When siliques were intact, estimated insect abundance was locally reduced in invaded plots compared to native plots, but this relationship disappeared when siliques were removed. Phosphorus availability and the growth of one native plant species were both elevated in invaded plots where siliques were intact compared to plots where siliques were removed. Results indicate that garlic mustard's close association with web-building spiders initiates cascading invader impacts on the native community and ecosystem properties. This work supports recent theory suggesting that taking a broader food web perspective may help predict invasion impacts in different environmental contexts.
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.
Biological invasions are a key component of global change, and understanding the drivers of global invasion patterns will aid in assessing and mitigating the impact of invasive species. While ...invasive species are most often studied in the context of one or two trophic levels, in reality species invade communities comprised of complex food webs. The complexity and integrity of the native food web may be a more important determinant of invasion success than the strength of interactions between a small subset of species within a larger food web. Previous efforts to understand the relationship between food web properties and species invasions have been primarily theoretical and have yielded mixed results. Here, we present a synthesis of empirical information on food web connectance and species invasion success gathered from different sources (estimates of food web connectance from the primary literature and estimates of invasion success from the Global Invasive Species Database as well as the primary literature). Our results suggest that higher‐connectance food webs tend to host fewer invaders and exert stronger biotic resistance compared to low‐connectance webs. We argue that while these correlations cannot be used to infer a causal link between food web connectance and habitat invasibility, the promising findings beg for further empirical research that deliberately tests for relationships between food web connectance and invasion.
Climate change is expected to favor smaller-bodied organisms through effects of temperature on physiological performance and food-web interactions, so much so that smaller body size has been touted ...as a universal response to global warming alongside range shifts and changing phenology. However, climate change involves more than warming. It is multivariate, and the interplay between climate variables may result in less straightforward predictions. We present a model that considers the simultaneous effect of multiple variables (temperature, CO₂, and moisture) on herbivore body sizes within a tritrophic food web comprised of vegetation, herbivores, and a shared predator. The model accounts for climate effects on animal behavior, plant and animal metabolism, and plant quality to explore emergent effects on herbivore body size. Our analysis reveals that some common multivariate climate change scenarios may favor larger-bodied herbivores, challenging previous findings of shifts toward small-bodied herbivores in the face of rising temperatures.
Plant-soil feedback (PSF), the process by which plants influence con- or heterospecifics via alteration of abiotic or biotic soil properties, is a known driver of plant coexistence and invasion. Yet ...there is limited understanding of how PSF interacts with other important drivers of plant community structure and dynamics, such as aboveground herbivory. Aboveground herbivory and PSFs are ubiquitous processes in plant communities, but traditional PSF experiments in the greenhouse eliminate herbivory as an experimental factor. Aboveground herbivory can affect plant-soil systems in multiple ways and therefore is likely to strongly interact with PSF. Herbivores can be selective, preferring certain species over others, which could influence PSF dynamics. Aboveground herbivory could also affect PSF dynamics by influencing photosynthate allocation, defense compound production, and soil nutrient levels. An existing conceptual framework predicts that aboveground herbivory should generally weaken pathogen-, mutualist- and soil nutrient-driven feedbacks, and a logical extension of these predictions is that aboveground herbivory will weaken PSF as a driver of plant species invasion. Using a Midwest urban woodland study system, we first measured aboveground mammalian herbivore pressure on native woodland perennials used in local restoration efforts. We then simulated these levels of herbivory in a greenhouse experiment to assess whether and how aboveground herbivory alters net pairwise PSF interactions between these native species and Euonymus fortunei, a common invasive plant of Midwest urban woodlands. Results support predictions that aboveground herbivory weakens PSF interactions. In our experiment, simulated herbivory eliminated PSF among E. fortunei and a co-occurring community of native species, although this effect depended on competitive context. When the native community and the invasive species were grown separately, net feedback was neutral regardless of herbivory, but when grown in a competitive mixture, feedback between the native community and the invader switched from negative to neutral when herbivory was imposed. To assess the generality of these findings, future studies are needed that examine herbivory-PSF interactions across multiple native-native and native-invasive species combinations, and for a range of plant community types.
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