Summary1. Ecological restoration is a global priority that holds great potential for benefiting natural ecosystems, but restoration outcomes are notoriously unpredictable. Resolving this unpredictability represents a major, but critical challenge to the science of restoration ecology. 2. In an effort to move restoration ecology toward a more predictive science, we consider the key issue of variability. Typically, restoration outcomes vary relative to goals (i.e. reference or desired future conditions) and with respect to the outcomes of other restoration efforts. The field of restoration ecology has largely considered only this first type of variation, often focusing on an oversimplified success vs. failure dichotomy. The causes of variation, particularly among restoration efforts, remain poorly understood for most systems. 3. Variation associated with restoration outcomes is a consequence of how, where and when restoration is conducted; variation is also influenced by how the outcome of restoration is measured. We propose that variation should decrease with the number of factors constraining restoration and increase with the specificity of the goal. When factors (e.g. harsh environmental conditions, limited species reintroductions) preclude most species, little variation will exist among restorations, particularly when goals are associated with metrics such as physical structure, where species may be broadly interchangeable. Conversely, when few constraints to species membership exist, substantial variation may result and this will be most pronounced when restoration is assessed by metrics such as taxonomic composition. 4. Synthesis and applications. The variability we observe during restoration results from both restoration context (how, where and when restoration is conducted) and how we evaluate restoration outcomes. To advance the predictive capacity of restoration, we outline a research agenda that considers metrics of restoration outcomes, the drivers of variation among existing restoration efforts, experiments to quantify and understand variation in restoration outcomes, and the development of models to organise, interpret and forecast restoration outcomes.

Summary1. The outcomes of restoration efforts are contingent on the specifics of the restoration practices utilized, but also on uncontrolled contingencies such as site effects and year effects. Although restoration practitioners have long been aware that the successes of their projects vary from site to site and from year to year, there have been few direct experimental tests of these contingencies. 2. We established grassland restoration plots identically across three sites in northern California, in each of four establishment years (for 12 site-year combinations). 3. The resulting plant communities differed significantly across sites and across establishment years. As a consequence of these community differences, there were 'forb years' and 'grass years', although these sometimes differed among sites. Multivariate analysis identified mean annual temperature and total precipitation as likely drivers of some of these differences. 4. Synthesis and applications. Our results not only confirm the idiosyncratic nature of the results of restoration efforts (and ecological experiments in general) but also demonstrate that some of this variation can potentially be related to measurable environmental conditions. Understanding the drivers of this variability can ultimately aid restoration practitioners by allowing them to focus restoration efforts on years and sites most likely to yield desired outcomes.

Restoration success is often hampered by the failure of less dominant competitors to establish. An emerging literature on priority effects suggests the manipulation of community assembly as a useful technique to help overcome these difficulties by altering competitive relationships. We present data from a set of four priority experiments, carried out at each of three sites in restoration settings in California grasslands. These data, combined with patterns summarized from the literature, indicate that both short‐term priority (1–3 weeks) and long‐term priority (1 year) can profoundly shift interspecific relationships and benefit otherwise subordinate plant species, but that these effects are sometimes transitory, asymmetric, and contingent on environmental conditions and species composition. Restoration interventions that can produce priority effects include staggered planting times, weed control, seed pre‐germination, plug planting, and spatial aggregation. Such interventions are likely to be at least initially effective, but their strength and persistence can differ considerably across systems in space and time. Further research may help identify the conditions that maximize the strength and persistence of priority effects in restoration settings.

Climatic warming is altering the behavior of individuals and the composition of communities. However, recent studies have shown that the impact of warming on ectotherms varies geographically: species at warmer sites where environmental temperatures are closer to their upper critical thermal limits are more likely to be negatively impacted by warming than are species inhabiting relatively cooler sites. We used a large-scale experimental temperature manipulation to warm intact forest ant assemblages in the field and examine the impacts of chronic warming on foraging at a southern (North Carolina) and northern (Massachusetts) site in eastern North America. We examined the influence of temperature on the abundance and recruitment of foragers as well as the number of different species observed foraging. Finally, we examined the relationship between the mean temperature at which a species was found foraging and the critical thermal maximum temperature of that species, relating functional traits to behavior. We found that forager abundance and richness were related to the experimental increase in temperature at the southern site, but not the northern site. Additionally, individual species responded differently to temperature: some species foraged more under warmer conditions, whereas others foraged less. Importantly, these species-specific responses were related to functional traits of species (at least at the Duke Forest site). Species with higher critical thermal maxima had greater forager densities at higher temperatures than did species with lower critical thermal maxima. Our results indicate that while climatic warming may alter patterns of foraging activity in predictable ways, these shifts vary among species and between sites. More southerly sites and species with lower critical thermal maxima are likely to be at greater risk to ongoing climatic warming.

Ecologists have long sought to explain the coexistence of multiple potentially competing species in local assemblages. This is especially challenging in species-rich assemblages in which interspecific competition is intense, as it often is in ant assemblages. As a result, a suite of mechanisms has been proposed to explain coexistence among potentially competing ant species: the dominance-discovery tradeoff, the dominance-thermal tolerance tradeoff, spatial segregation, temperature-based niche partitioning, and temporal niche partitioning. Through a series of observations and experiments, we examined a deciduous forest ant assemblage in eastern North America for the signature of each of these coexistence mechanisms. We failed to detect evidence for any of the commonly suggested mechanisms of coexistence, with one notable exception: ant species appear to temporally partition foraging times such that behaviourally dominant species foraged more intensely at night, while foraging by subdominant species peaked during the day. Our work, though focused on a single assemblage, indicates that many of the commonly cited mechanisms of coexistence may not be general to all ant assemblages. However, temporal segregation may play a role in promoting coexistence among ant species in at least some ecosystems, as it does in many other organisms.

The order of species arrival can dramatically alter the trajectory of community development. While there is experimental evidence that priority effects can be important drivers of community structure early on, the persistence and duration of these effects is unclear. Here we report on a community assembly experiment in which a mix of four native grasses and a mix of four native forbs were planted on their own, together, or with one-year priority over the other guild. We found positive effects of priority for both grasses and forbs in the initial years of the experiment. However, 6-8 yr after planting, the effectiveness of priority treatments were mixed. Some species became rare, persisting only in treatments in which they had been given priority; others continued to maintain high cover and exhibit a strong positive signal of priority effects; still others remained common but no longer showed a signature of the initial priority effects; and finally, some species became locally extinct across all experimental plots. Grass priority over forbs was strong and persistent, but not forb priority over grasses. Our results demonstrate that the long-term benefits of temporal priority can persist for at least 8 yr for some, but not all species, and these continued effects result in distinct community composition. Manipulating the trajectory of community assembly through priority in seeding has potential as a useful tool for restoration.

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Environmental conditions that vary from year to year can be strong drivers of ecological dynamics, including the composition of newly assembled communities. However, ecologists often chalk such dynamics up to “noise” in ecological experiments. Our lack of attention to such “year effects” hampers our understanding of contingencies in ecological assembly mechanisms and limits the generalizability of research findings. Here, we provide examples from published research demonstrating the importance of year effects during community assembly across study systems. We further quantify these year effects with two case studies—a grassland restoration experiment and a study of postfire conifer recruitment—finding that the effects of initiation year on community composition can dictate community as much, if not more, than the effects of experimental treatments or site. The evidence strongly suggests that year effects are pervasive and profound, and that year effects early in community assembly can drive strong and enduring divergence in community structure and function. Explicit attention to year effects in ecological research serves to illuminate basic ecological principles, allowing for better understanding of contingencies in ecology. These dynamics also have strong implications for applied ecological research, offering new insights into ecological restoration as well as future climate change.