Stability in ecosystem function is an important but poorly understood phenomenon. Anthropogenic perturbations alter communities, but how they change stability and the strength of stabilizing mechanisms is not clear. We examined temporal stability (invariability) in aboveground productivity in replicated 18-year time series of experimentally perturbed grassland plant communities. We found that disturbed annual-dominated communities were more stable than undisturbed perennial communities, coincident with increases in the stabilizing effect of mean-variance scaling. We also found that nitrogen-fertilized communities maintained stability despite losses in species richness, probably because of increased compensatory dynamics and increased dominance by particularly stable dominant species. Among our communities, slight variation in diversity was not the strongest mechanism driving differences in stability. Instead, our study suggests that decreases in individual species variabilities and increases in the relative abundance of stable dominant species may help maintain stability in the functioning of ecosystems confronted with eutrophication, disturbance, and other global changes.

Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species-level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1-315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species' populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

Understanding priority effects, in which one species in a habitat decreases the success of later species, may be essential for restoring native communities. Priority effects can operate in two ways: size-asymmetric competition and creation of ''soil legacies,'' effects on soil that may last long after the competitive effect. We examined how these two types of priority effects, competition and soil legacies, drive interactions between seedlings of native and exotic California grassland plants. We established native and exotic communities in a mesocosm experiment. After 5 weeks, we removed the plants from half the treatments (soil legacy treatment) and retained the plants in the other half (priority effect treatment, which we interpret to include both competition and soil legacies). We then added native or exotic seed as the colonizing community. After 2 months, we measured the biomass of the colonizing community. When germinating first, both natives and exotics established priority effects, reducing colonist biomass by 86 and 92%, respectively. These priority effects were predominantly due to size-asymmetric competition. Only exotics created soil legacies, and these legacies only affected native colonizers, reducing biomass by 74%. These results imply that exotic species priority effects can affect native grassland restorations. Although most restorations focus on removing exotic seedlings, amending soil to address soil legacies may also be critical. Additionally, because native species can exclude exotics if given a head start, ensuring that natives germinate first may be a costeffective restoration technique.

The outcomes of ecological restoration are notoriously unpredictable, but we have no general predictive understanding of this contingency. Management decisions can have strong effects on restoration outcomes, but in other cases may be overwhelmed by site characteristics (e.g. soil conditions), landscape context (e.g. abundance of similar habitat) or historical factors (e.g. priority effects). However, we generally cannot predict which of these four classes of drivers will affect restoration outcomes. Disparate aspects of restoration outcomes (e.g. species richness, beta diversity and community composition) and their unique responses further complicate our understanding. Finally, these four classes of drivers might differentially affect subsets of the restored community, where, for example, management might shape the abundance and distribution of species of the target community, while other species are more contingent on site, landscape or historical factors. 2. Here, we used variation partitioning to compare the relative importance of management, site, landscape and historical factors for determining the plant community outcomes of 27 prairie restorations in south-west Michigan. 3. We found that management, especially the composition, diversity and density of seed mixes applied, and history, especially site age, were the most important drivers of prairie restoration species richness, beta diversity and composition. Site and landscape factors were only rarely important for restoration outcomes. 4. Finally, we found that comparing the unique responses of sown and non-sown species typically increased our understanding of the dynamics contributing to community-wide restoration outcomes. 5. Synthesis and applications. This is, to our knowledge, the first quantitative comparison of how four major classes of drivers determine the outcome of restoration. Historical legacies and management decisions, but generally not landscape context or local site conditions, shaped plant communities at restored sites. These findings represent an important step towards developing a more predictive framework for understanding contingency in restoration outcomes.

No abstract

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.

Abstract. Theory suggests that cheaters threaten the persistence of mutualisms, but that sanctions to prevent cheating can stabilize mutualisms. In the arbuscular mycorrhizal symbiosis, reports of parasitism suggest that reductions in plant carbon allocation are not universally effective. I asked whether plant species differences in mycorrhizal responsiveness would affect both their susceptibility to parasitism and their reduction in allocation to nonbeneficial arbuscular mycorrhizal fungi (AMF) in high-phosphorus soils. In a greenhouse experiment, I found that two C 3 grasses, Bromus inermis and Elymus repens, effectively suppressed root colonization and AMF hyphal abundance. Increases in soil phosphorus did not reduce the degree to which AMF increased plant biomass. In contrast, two C 4 grasses, Andropogon gerardii and Schizachyrium scoparium, more weakly reduced root colonization and failed to suppress AMF hyphal abundance. Consequently, they experienced strong declines in their response to AMF, and one species suffered parasitism. Thus, species differ in susceptibility to parasitism and their reduction in allocation to non-beneficial AMF. These differences may affect the distribution and abundance of plants and AMF, as well as the stability of the mutualism.

Summary Recovering biological diversity and ecosystem functioning are primary objectives of ecological restoration, yet these outcomes are often unpredictable. Assessments based on functional traits may help with interpreting variability in both community composition and ecosystem functioning because of their mechanistic and generalizable nature. This promise remains poorly realized, however, because tests linking environmental conditions, functional traits, and ecosystem functioning in restoration are rare. Here, we provide such a test through what is to our knowledge the first empirical application of the ‘response–effect trait framework’ to restoration. This framework provides a trait‐based bridge between community assembly and ecosystem functioning by describing how species respond to environmental conditions based on traits and how the traits of species affect ecosystem functioning. Our study took place across 29 prairies restored from former agricultural fields in southwestern Michigan. We considered how environmental conditions affect ecosystem functioning through and independently of measured functional traits. To do so, we paired field‐collected trait data with data on plant community composition and measures of ecosystem functioning and used structural equation modelling to determine relationships between environmental conditions, community‐weighted means of functional traits and ecosystem functioning. Environmental conditions were predictive of trait composition. Sites restored directly from tillage (as opposed to those allowed to fallow) supported taller species with larger seeds and higher specific leaf area (SLA). Site age and fire frequency were both negatively related to SLA. We also found a positive relationship between soil moisture and SLA. Both trait composition and environmental conditions predicted ecosystem functioning, but these relationships varied among the measured functions. Pollination mode (animal pollination) increased and fire frequency decreased floral resource availability, seed mass had a negative effect on below‐ground biomass production, and vegetative height increased decomposition rate. Soil moisture and fire frequency both increased while site age decreased above‐ground biomass production, and site age and soil moisture both increased decomposition rate. Synthesis and applications. Our results suggest that both trait composition and environmental conditions play a role in shaping ecosystem function during restoration, and the importance of each is dependent on the function of interest. Because of this, environmental heterogeneity will be necessary to promote multiple ecosystem functions across restored landscapes. A trait‐based approach to restoration can aid interpretation of variable outcomes through insights into community assembly and ecosystem functioning.