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Despite theoretical advances, the ecological factors and functional traits that enable species varying in seed size and fecundity to coexist remain unclear. Given inherent fecundity advantages, why don't small‐seeded species dominate communities?In perennial grasslands, we evaluated whether small‐seeded species are less tolerant of competition from the community dominant bunchgrass than large‐seeded species, but also less vulnerable to seed predation by mice. We also explored whether trade‐offs involving competitive tolerance include two other functional traits, height and leaf mass per area (LMA). We added seeds of 17 forb species to plots where bunchgrass competition and rodent seed predation were manipulated across sites varying in bunchgrass productivity and thus competitive intensity. Seeds were added at densities mimicking interspecific variation in fecundity among target species.Standardizing for differences in fecundity (i.e. seed input, which enabled us to evaluate inherent interspecific differences in susceptibility to biotic interactions), bunchgrass competition more greatly reduced recruitment and establishment of small‐ versus large‐seeded species, whereas rodent seed predation more greatly reduced the recruitment of large‐ versus small‐seeded species. Plant height and LMA were unrelated to the competition effect size.Small‐seeded species abundance decreased across sites increasing in bunchgrass productivity, whereas this was not the case for large‐seeded species. For adult plants but not seedlings, community‐weighted functional trait means (CWM) for seed size, height and LMA increased in plots with versus without bunchgrass competition and the CWM for seed size and height also increased at sites with greater bunchgrass productivity (for adults only). In contrast, rodent seed predation had no significant effects on CWM seed size.At the end of the experiment, adult abundance positively correlated with plant fecundity in plots lacking bunchgrass, indicating the inherent advantages accrued to high fecundity small‐seeded species. However, with bunchgrass competition, abundances were equalized across species due to reduced competitive tolerance of high fecundity small‐seeded species.Synthesis. Our results suggest that coexistence among subordinate forb species varying in seed size and fecundity is in‐part due to a trade‐off involving competitive tolerance and fecundity, mediated by seed size and associated functional traits. [ABSTRACT FROM AUTHOR]

Disentangling the effect of multiple ecological processes on plant trait composition is complicated by the fact that both top‐down and bottom‐up processes may affect similar traits.We examined the interacting role of resource variation and vole herbivory on functional trait patterns in an annual California grassland. We manipulated vole herbivory via exclosures at eight grassland sites along a steep resource gradient and measured plant composition and functional traits over 3 years.Plants with resource acquisitive functional traits were favoured at sites with increasing resources. Vole herbivory influenced community‐weighted mean (CWM) leaf nitrogen (N) and seed mass, suggesting these traits may mediate plant susceptibility to vole herbivory. After 3 years, CWM leaf N increased in the absence of the voles, as did CWM seed mass, although this increase in CWM seed mass only occurred at higher resource sites. Vole exclusion at high‐resources sites also increased the functional diversity of leaf N and seed mass by the end of the experiment.Overall, environmental filtering primarily structured the dominant plant trait strategies, but vole herbivory also influenced the functional diversity of traits that influence herbivore susceptibility, particularly at resource‐rich sites.Thus, habitat filtering and herbivory can operate on different dimensions of plant functional composition to influence the species and functional composition of communities. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Priority effects refer to the order or timing of species arrival, including how species that arrive early at a site either positively or negatively affect establishment, growth, or reproduction of species that arrive later. Despite the clear implications of priority effects for ecological restoration, there have been no reviews of how and where priority effects have been studied and the extent to which findings can be applied to restoration. Here, we systematically review the literature on priority effects by (1) synthesizing information from papers that compared simultaneous and nonsimultaneous planting or sowing; (2) discussing the mechanisms through which priority effects operate, (3) considering how these mechanisms might be manipulated to achieve restoration goals; and (4) highlighting future research needed to improve the use of priority effects in restoration. In a term‐targeted search, we found 43 studies that experimentally manipulated the order of arrival of different species. Overall, these concluded that even small delays in arrival time, as opposed to simultaneous arrival of species, can promote differences in subsequent community composition as well as ecosystem functions. There were very few studies on the long‐term stability of these priority effects, and the majority were conducted in temperate grasslands. Our findings suggest that creating alternative vegetation states via priority treatments is a promising avenue for restoration. However, for the concept to be best operationalized for restoration, we need research in more ecosystems that are priorities for restoration, and treatments that are followed over extended time periods. [ABSTRACT FROM AUTHOR]

A central goal of ecology is understanding the determinants of species' distributions. "Metapopulation" models for the existence of distributional boundaries predict that species' geographic ranges arise from the landscape‐scale deterioration of habitat suitability towards the range edge (i.e., niche mechanisms), which simultaneously hinders demographic performance and limits dispersal to suitable habitat beyond the edge (i.e., dispersal limitation). However, few studies have examined both of these mechanisms for the same species by examining abundance and comprehensive measures of demographic performance across the distribution and beyond its boundary.We tested the predictions of metapopulation models for range limits by contrasting abundance, demographic performance, and population growth (λ) of the perennial forb Astragalus utahensis in central and northern range edge populations. We tested for dispersal limitation by transplanting individuals at and beyond the northern range boundary and monitoring their demographic performance.Astragalus abundance and stochastic λ decreased from the range centre to the northern range edge, with stochastic λ falling to or below replacement in range edge populations. However, transplants at some sites beyond the northern range edge survived and reproduced at levels similar to transplants within the range. Thus, in addition to deteriorating conditions at the range edge, dispersal limitation appears to contribute to limiting A. utahensis' northern distribution.Synthesis. Our results support metapopulation models for range limits by suggesting that decreased demographic performance constrains the viability of range edge populations while contributing to dispersal limitation of Astragalus utahensis' distribution. Thus, the tandem action of niche and dispersal processes appears to play an important role in constraining this species' northern latitudinal range. [ABSTRACT FROM AUTHOR]

Abstract: Propagule size and number often vary by several orders of magnitude among co‐occurring plant species. Explaining the maintenance of this variation and understanding how propagule size contributes to coexistence remain a central challenge for community ecologists. The dominant paradigm is that a competition–colonization trade‐off maintains interspecific variation in seed size, but empirical support is limited and other coexistence mechanisms, such as size‐dependent seed predation, have not been examined. We examined how seed size, fecundity, and other functional traits of 18 co‐occurring perennial forbs trade‐off with both vulnerability to rodent seed predation and competitive response to a community‐dominant perennial bunchgrass. We added seeds of these species to 1 m2 plots at 10 sites where we factorially manipulated rodent seed predation and competition from the community dominant, Festuca campestris. For a given plot, seeds of each species were added at densities that reflected the fecundity (i.e., per capita seed production) for each species. Moreover, we varied total seed density among plots by adding seeds at one of five relative densities (0, 0.25. 0.5, 1.0, and 1.5 times each species’ fecundity) to examine how fecundity affected overall recruitment for each species. There was a trade‐off between seed size and fecundity, as expected, but larger seed size was also associated with greater plant height, lower C/N ratios, and lower water use efficiency, suggesting that seed size represented a “resource acquisitive” trait syndrome (as quantified by trait principal component scores). In the field experiment, the suppressive effects of seed predation on seed recruitment rate increased with increasing seed size. In contrast, smaller seeded species with less resource‐acquisitive traits were more negatively affected by competition from F.  campestris than were species with more resource acquisitive traits. Synthesis. While the competition–colonization trade‐off has been the predominant mechanism thought to maintain coexistence among species that vary in fecundity and seed size, our work suggests that susceptibility to rodent seed predation and competitive response to community dominants represent alternative mechanisms that can differentially influence plant recruitment of species based on their seed size and associated traits. [ABSTRACT FROM AUTHOR]

Abstract: A central focus of ecology is to understand the conditions under which biotic interactions affect species’ abundance and distribution. Classic and recent studies have shown that biotic interactions can strongly impact local or regional patterns of species abundance, but two fundamental questions remain largely unaddressed for non‐competitive biotic interactions. First, do the effects of these interactions on population performance change predictably with environmental context? Second, to what extent do population‐scale effects contribute to limiting species’ geographic distributions? To address these questions, we experimentally assessed the extent to which pollen limitation and insect seed predators affected the fecundity and projected population growth rate (λ) of the native forb Astragalus utahensis. We studied populations at the centre and northern edge of the latitudinal range of A. utahensis that occur across a gradient in abiotic harshness characterized primarily by declining mean annual precipitation. Supplementing pollen and suppressing pre‐dispersal seed predators increased seed production similarly within A. utahensis populations at the centre and northern edge of the range. Integral projection population models revealed that relaxing these checks on seed production tended to increase λ in most populations, regardless of their location within the range. Synthesis. Our results suggest that pollen limitation and insect herbivores limit population growth in A. utahensis similarly across the centre‐to‐north portion of its latitudinal distribution. However, because A. utahensis population growth barely reaches the level of replacement at the northern range edge, the reduction in λ resulting from these interactions may contribute to limiting expansion at the northern edge of A. utahensis’ latitudinal range. [ABSTRACT FROM AUTHOR]

Abstract: Many clonal organisms occasionally outcross, but the long‐term consequences of such infrequent events are often unknown. During five years, representing three to five plant generations, we followed 16 experimental field populations of the forb, Oenothera biennis, originally planted with the same 18 original genotypes. Oenothera biennis usually self fertilizes, which, due to its genetic system (permanent translocation heterozygosity), results in seeds that are clones of the maternal plant. However, rare outcrossing produces genetically novel offspring (but without recombination or increased heterozygosity). We sought to understand whether novel genotypes produced through natural outcrossing had greater fecundity or different multigenerational dynamics compared to our original genotypes. We further assessed whether any differences in fitness or abundances through time between original and novel genotypes were exaggerated in the presence vs. absence of insect herbivores. Over the course of the experiment, we genotyped >12,500 plants using microsatellite DNA markers to identify and track the frequency of specific genotypes and estimated fecundity on a subset (>3,000) of plants. The effective outcrossing rate was 7.3% in the first year and ultimately 50% of the plants were of outcrossed origin by the final year of the experiment. Lifetime fruit production per plant was on average 32% higher across all novel genotypes produced via outcrossing compared to the original genotypes, and this fecundity advantage was significantly enhanced in populations lacking herbivores. Among 43 novel genotypes that were abundant enough to phenotype with replication, plants produced nearly 30% more fruits than the average of their specific two parental genotypes, and marginally more fruits (8%) than their most fecund parent. Mean per capita fecundity of novel genotypes predicted their relative frequencies at the end of the experiment. Novel genotypes increased more dramatically in herbivore‐present compared to suppressed populations (45% vs. 27% of all plants), countering the increased competition from dandelions (Taraxacum officinale) that resulted from herbivore suppression. Increased interspecific competition likely also lead to the lower realized fitness of novel vs. original genotypes in herbivore‐suppressed populations. These results demonstrate that rare outcrossing and the generation of novel genotypes can create high‐fecundity progeny, with the biotic environment influencing the dynamical outcome of such advantages. [ABSTRACT FROM AUTHOR]

PREMISE OF THE STUDY: Arbuscular mycorrhizal (AM) fungi can promote plant growth and reproduction, but other plant physiological traits or traits that provide defense against herbivores can also be affected by AM fungi. However, whether responses of different traits to AM fungi are correlated and whether these relationships vary among plants from different populations are unresolved. METHODS: In a common garden experiment, we grew Asclepias speciosa plants from seed collected from populations found along an environmental gradient with and without AM fungi to assess whether the responses of six growth and defense traits to AM fungi are correlated. KEY RESULTS: Although there was strong genetic differentiation in mean trait values among populations, AM fungi consistently increased expression of most growth and defense traits across all populations. Responses of biomass and root to shoot ratio to AM fungi were positively correlated, suggesting that plants that are more responsive to AM fungi allocated more biomass belowground. Responses of biomass and trichome density to AM fungi were negatively correlated, indicating a trade-off in responsiveness between a growth and defensive trait. CONCLUSIONS: Our results suggest that while there is substantial population differentiation in many traits of A. speciosa, populations respond similarly to AM fungi, and both positive and negative correlations among trait responses occur. [ABSTRACT FROM AUTHOR]

Post-dispersal seed predators contribute substantially to seed loss across many ecosystems. Most research has focused on understanding sources of variation in seed loss, without appreciating the implications of seed predation for plant coexistence, community assembly and broader community theory. Meanwhile, research aimed at understanding coexistence and community assembly processes in plant communities has focused on axes of dispersal and resource competition and the traits influencing these processes, without accounting for the role of generalist seed predators., We review the unique features of post-dispersal seed predation and assess the implications of seed loss on three critical components of plant community organization - coexistence, community structure and plant invasions - pointing to both important gaps in theory and empirical knowledge. We highlight how understanding fundamental controls on plant recruitment is central to determining how seed predation affects plant recruitment and coexistence. We discuss how accounting for seed predator foraging strategies may shift trait-based inferences of community assembly., Synthesis. We argue that seed predation by generalist consumers, which is pervasive in temperate communities, should be better incorporated into plant community theory. Experiments that specifically incorporate the presence and attributes of the seed predator community and that follow seed fate would fill important knowledge gaps. Particularly needed are studies focused on strengthening the connections between seed removal and plant establishment and linking selective and density-dependent foraging strategies to plant traits. Advancing our understanding of the processes regulating plant coexistence and community assembly requires that future research not only acknowledge but also incorporate generalist consumers' effects on plant communities., A is available for this article. [ABSTRACT FROM AUTHOR]

Determining how changes in abiotic conditions influence community interactions is a fundamental challenge in ecology. Meeting this challenge is increasingly imperative in the Anthropocene where climate change and exotic species introductions alter abiotic context and biotic composition to reshuffle natural systems., We created plant assemblages consisting of monocultures or equal abundance of the native community dominant bluebunch wheatgrass ( Pseudoroegneria spicata) and the exotic spotted knapweed ( Centaurea stoebe), a co-occurring invasive forb that has overtaken grasslands across the western United States. We subjected these composition treatments to drought (20% of average precipitation vs. average) and herbivory on C. stoebe by its biocontrol agent Cyphocleonus achates to explore how reduced precipitation might influence the effects of competition and biocontrol herbivory on C. stoebe's abundance., At the end of 7 years, C. stoebe dominated mixed-species plots under normal precipitation conditions, with biomass 50% greater than that of the native P. spicata. However, under drought stress, P. spicata's biomass was >200% greater than C. stoebe's. Interestingly, both species were impervious to drought in monoculture, indicating the importance of the drought by competition interaction. The biocontrol herbivore reduced C. stoebe abundance and indirectly increased P. spicata biomass in mixed-species drought plots, but these effects were only marginally significant and relatively weak. Overall, C. stoebe abundance was primarily driven by the drought by competition interaction, with negatively additive but weak effects of the drought by herbivory interaction., The response of the exotic to the treatments was driven by rapid changes in population density linked to its fast life-history strategy, while the native's response was driven by changes in per capita plant biomass linked to its slower life-history strategy. Individual plant performance metrics did not predict overall population responses for the invader, indicating the importance of longer term population measures., Synthesis. These results demonstrate that reduced precipitation inputs linked to climate change can dramatically shift the balance of plant competition, even toggling the advantage from exotic to native dominance. They also illustrate the importance of biotic interactions in predicting species responses to abiotic change. [ABSTRACT FROM AUTHOR]

Predicting changes in species' distributions is a crucial problem in ecology, with leading methods relying on information about species' putative climatic requirements. Empirical support for this approach relies on our ability to use observations of a species' distribution in one region to predict its range in other regions (model transferability). On the basis of this observation, ecologists have hypothesized that climate is the strongest determinant of species' distributions at large spatial scales. However, it is difficult to reconcile this claim with the pervasive effects of biotic interactions. Here, we resolve this apparent paradox by demonstrating how biotic interactions can affect species' range margins yet still be compatible with model transferability. We also identify situations where small changes in species' interactions dramatically shift range margins. [ABSTRACT FROM AUTHOR]