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1. Interannual variation in seasonal weather patterns causes shifts in the relative timing of phenological events of species within communities, but we currently lack a mechanistic understanding of how these phenological shifts affect species interactions. Identifying these mechanisms is critical to predicting how interannual variation affects populations and communities. 2. Species' phenologies, particularly the timing of offspring arrival, play an important role in the annual cycles of community assembly. We hypothesize that shifts in relative arrival of offspring can alter interspecific interactions through a mechanism called size-mediated priority effects (SMPE), in which individuals that arrive earlier can grow to achieve a body size advantage over those that arrive later. 3. In this study, we used an experimental approach to isolate and quantify the importance of SMPE for species interactions. Specifically, we simulated shifts in relative arrival of the nymphs of two dragonfly species to determine the consequences for their interactions as intraguild predators. 4. We found that shifts in relative arrival altered not only predation strength but also the nature of predator–prey interactions. When arrival differences were great. SMPE allowed the early arriver to prey intensely upon the late arriver, causing exclusion of the late arriver from nearly all habitats. As arrival differences decreased, the early arriver's size advantage also decreased. When arrival differences were smallest, there was mutual predation, and the two species coexisted in similar abundances across habitats. Importantly, we also found a nonlinear scaling relationship between shifts in relative arrival and predation strength. Specifically, small shifts in relative arrival caused large changes in predation strength while subsequent changes had relatively minor effects. 5. These results demonstrate that SMPE can alter not only the outcome of interactions but also the demographic rates of species and the structure of communities. Elucidating the mechanisms that link phenological shifts to species interactions is crucial for understanding the dynamics of seasonal communities as well as for predicting the effects of climate change on these communities.

Plants often associate with specialized decomposer communities that increase plant litter breakdown, a phenomenon that is known as the 'home-field advantage' (HFA). Although the concept of HFA has long considered only the role of the soil microbial community, explicit consideration of the role of the microbial community on the foliage before litter fall (i.e. the phyllosphere community) may help us to better understand HFA. We investigated the occurrence of HFA in the presence vs absence of phyllosphere communities and found that HFA effects were smaller when phyllosphere communities were removed. We propose that priority effects and interactions between phyllosphere and soil organisms can help explain the positive effects of the phyllosphere at home, and suggest a path forward for further investigation.

BackgroundPlants condition the soil in which they grow, thereby altering the performance of subsequent plants growing in this soil. This phenomenon, known as plant-soil feedback (PSF), has garnered increasing interest. Experiments are moving from single species soil pairings in the glasshouse to community-level field trials. Consequently, our knowledge of the role PSF plays in shaping ecosystem functions has advanced. However, knowledge gaps remain.ScopeHere, we explore intrinsic and extrinsic abiotic and biotic drivers of PSF such as maternal effects, plant functional traits, self-DNA, plant-plant competition, herbivory, interactions between soil organisms, temperature, drought, flooding, greenhouse gases, (micro)nutrients, plant-litter-soil feedback and priority effects. These drivers have begun to feature in experiments, thereby increasing our mechanistic understanding of PSF. Nonetheless, many of these topics have received insufficient coverage to determine general principles across larger temporal and spatial scales. Further, conflicting terminology has excluded PSF studies from reviews and meta-analyses. We review terms such as soil sickness, Janzen-Connell hypothesis, soil-related invasive species work, soil legacies, allelopathy and soil-related succession that overlap with PSF but are generally not named as such.ConclusionHolistic experimental designs that consider the continual reciprocal feedback between the extrinsic environment, plants and soil, as well as the unification of terminologies are necessary if we are to realise the full potential of PSF for understanding and steering ecosystem processes. Here, we compile outstanding questions related to PSF research that emphasis the aforementioned topics and suggest ways to incorporate them into future research in order to advance plant-soil ecology.

Biogeographic origins of plant lineages are often reflected in species functional traits, with consequences for community assembly, diversity, and ecosystem function. The climatic and environmental conditions in which species evolved have lasting influence (legacy effects) through phylogenetic conservatism of traits that underlie community assembly and drive ecosystem processes. Legacy effects that influence community assembly may have direct consequences for ecosystem function or may be linked, owing to lineage history, to traits that impact ecosystems. Evolutionary priority effects, driven by the order of colonization and lineage diversification, as well as migration barriers and historical environmental changes, have shaped the diversity and composition of regional floras and their ecosystem functions. We examine the likely consequences of biogeographic history for plant responses to global change and consider how understanding linkages between biogeographic origins, functional traits, and ecosystem consequences can aid the management and restoration of ecosystems globally in the face of rapid environmental change.

Traditionally, resistance and resilience are associated with good ecological health, often underpinning restoration goals. However, degraded ecosystems can also be highly resistant and resilient, making restoration difficult: degraded communities often become dominated by hyper-tolerant species, preventing recolonization and resulting in low biodiversity and poor ecosystem function. Using streams as a model, we undertook a mesocosm experiment to test if degraded community presence hindered biological recovery. We established 12 mesocosms, simulating physically healthy streams. Degraded invertebrate communities were established in half, mimicking the post-restoration scenario of physical recovery without biological recovery. We then introduced a healthy colonist community to all mesocosms, testing if degraded community presence influenced healthy community establishment. Colonists established less readily in degraded community mesocosms, with larger decreases in abundance of sensitive taxa, likely driven by biotic interactions rather than abiotic constraints. Resource depletion by the degraded community likely increased competition, driving priority effects. Colonists left by drifting, but also by accelerating development, reducing time to emergence but sacrificing larger body size. Since degraded community presence prevented colonist establishment, our experiment suggests successful restoration must address both abiotic and biotic factors, especially those that reinforce the ‘negative’ resistance and resilience which perpetuate degraded communities and are typically overlooked.

Priority effects occur when the order of species arrival affects the final community structure. Mutualists often interact with multiple partners in different orders, but if or how priority effects alter interaction outcomes is an open question. In the field, we paired the legume Medicago lupulina with two nodulating strains of Ensifer bacteria that vary in nitrogen-fixing ability. We inoculated plants with strains in different orders and measured interaction outcomes. The first strain to arrive primarily determined plant performance and final relative abundances of rhizobia on roots. Plants that received effective microbes first and ineffective microbes second grew larger than plants inoculated with the same microbes in the opposite order. Our results show that mutualism outcomes can be influenced not just by partner identity, but by the interaction order. Furthermore, hosts receiving high-quality mutualists early can better tolerate low-quality symbionts later, indicating that priority effects may help explain the persistence of ineffective symbionts.

Priority effects – the impact of a species' arrival on subsequent community development – have been shown to influence species composition in many organisms. Whether priority effects among arbuscular mycorrhizal fungi (AMF) structure fungal root communities is not well understood. Here, we investigated whether priority effects influence the success of two closely related AMF species (Rhizophagus irregularis and Glomus aggregatum), hypothesizing that a resident AMF suppresses invader success, this effect is time‐dependent and a resident will experience reduced growth when invaded. We performed two glasshouse experiments using modified pots, which permitted direct inoculation of resident and invading AMF on the roots. We quantified intraradical AMF abundances using quantitative PCR and visual colonization percentages. We found that both fungi suppressed the invading species and that this effect was strongly dependent on the time lag between inoculations. In contrast to our expectations, neither resident AMF was negatively affected by invasion. We show that order of arrival can influence the abundance of AMF species colonizing a host. These priority effects can have important implications for AMF ecology and the use of fungal inocula in sustainable agriculture.

Priority effect theory, a foundational concept from community ecology, states that the order and timing of species arrival during species assembly can affect species composition. Although this theory has been applied to co-infecting parasite species, it has almost always been with a single time lag between co-infecting parasites. Thus, how the timing of parasite species arrival affects co-infections and disease remains poorly understood. To address this gap in the literature, we exposed postmetamorphic Cuban tree frogs (Osteopilus septentrionalis) to Ranavirus, the fungus Batrachochytrium dendrobatidis (Bd), a nematode Aplectana hamatospicula, or pairs of these parasites either simultaneously or sequentially at a range of time lags and quantified load of the secondary parasite and host growth, survival, and parasite tolerance. Prior exposure to Bd or A. hamatospicula significantly increased viral loads relative to hosts singly infected with Ranavirus, whereas A. hamatospicula loads in hosts were higher when coexposed to Bd than when coexposed to Ranavirus. There was a significant positive relationship between time since Ranavirus infection and Bd load, and prior exposure to A. hamatospicula decreased Bd loads compared to simultaneous co-infection with these parasites. Infections with Bd and Ranavirus either singly or in co-infections decreased host growth and survival. This research reveals that time lags between co-infections can affect parasite loads, in line with priority effects theory. As co-infections in the field are unlikely to be simultaneous, an understanding of when co-infections are impacted by time lags between parasite exposures may play a major role in controlling problematic co-infections.

Sowing native seeds is a common approach to reintroduce native plants to degraded systems. However, this method is often overlooked in wetland restoration despite the immense global loss of diverse native wetland vegetation. Developing guiding principles for seed-based wetland restoration is critical to maximize native plant recovery, particularly in previously invaded wetlands. Doing so requires a comprehensive understanding of how restoration manipulations, and their interactions, influence wetland plant community assembly. With a focus on the invader Phragmites australis, we established a series of mesocosm experiments to assess how native sowing density, invader propagule pressure, abiotic filters (water and nutrients), and native sowing timing (i.e., priority effects) interact to influence plant community cover and biomass in wetland habitats. Increasing the density of native seeds yielded higher native cover and biomass, but P. australis suppression with increasing sowing densities was minimal. Rather, community outcomes were largely driven by invader propagule pressure: P. australis densities of ≤500 seeds/m² maintained high native cover and biomass. Low-water conditions increased the susceptibility of P. australis to dominance by native competitors. Early sowing of native seeds showed a large and significant benefit to native cover and biomass, regardless of native sowing density, suggesting that priority effects can be an effective restoration manipulation to enhance native plant establishment. Given the urgent wetland restoration need combined with the limited studies on seed-based wetland restoration, these findings provide guidance on restoration manipulations that are grounded in ecological theory to improve seed-based wetland restoration outcomes.

Plant community assembly outcomes can be contingent upon establishment year (year effects) due to variations in the environment. Stochastic events such as interannual variability in climate, particularly in the first year of community assembly, contribute to unpredictable community outcomes over the short term, but less is known about whether year effects produce transient or persistent states on a decadal timescale. To test for short-term (5-year) and persistent (decadal) effects of establishment year climate on community assembly outcomes, we restored prairie in an agricultural field using the same methods in four different years (2010, 2012, 2014, and 2016) that captured a wide range of initial (planting) year climate conditions. Species composition was measured for 5 years in all four restored prairies and for 9 and 11 years in the two oldest restored prairies established under average precipitation and extreme drought conditions. The composition of the four assembled communities showed large and significant differences in the first year of restoration, followed by dynamic change over time along a similar trajectory due to a temporary flush of annual volunteer species. Sown perennial species eventually came to dominate all communities, but communities remained distinct from each other in year five. Precipitation in June and July of the establishment year explained short-term coarse community metrics (i.e., species richness and grass/forb cover), with wet establishment years resulting in a higher cover of grasses and dry establishment years resulting in a higher cover of forbs in restored communities. Short-term differences in community composition, species richness, and grass/forb cover in restorations established under average precipitation and drought conditions persisted for 9–11 years, with low interannual variability in the composition of each prairie over the long term, indicating persistently different states on a decadal timescale. Thus, year effects resulting from stochastic variation in climate can have decadal effects on community assembly outcomes.