Jean H. Burns

More closely related species are more ecologically similar in an experimental test

The relationship between phylogenetic distance and ecological similarity is key to understanding mechanisms of community assembly, a central goal of ecology. The field of community phylogenetics uses phylogenetic information to infer mechanisms of community assembly; we explore, the underlying relationship between phylogenetic similarity and the niche. We combined a field experiment using 32 native plant species with a molecular phylogeny and found that closely related plant species shared similar germination and early survival niches. Species also competed more with close relatives than with distant relatives in field soils; however, in potting soil this pattern reversed, and close relatives might even have more mutalistic relationships than distant relatives in these soils. Our results suggest that niche conservatism (habitat filtering) and species interactions (competition or facilitation) structure community composition, that phylogenetic relationships influence the strength of species’ interactions, and that conserved aspects of plant niches include soil attributes.

Soil heterogeneity generated by plant–soil feedbacks has implications for species recruitment and coexistence

Summary n nMost studies of soil heterogeneity have focused on underlying abiotic factors such as soil nutrients. However, increasing recognition of plant–soil feedback (PSF) effects on plant growth, combined with the observation that PSFs operate at small spatial scales, suggests that heterogeneity due to PSF could affect plant population and community dynamics. The consequences of PSF-generated heterogeneity for coexistence depend on heterogeneitys effects on vital rates and how those vital rates influence population-level recruitment dynamics. nWe measured vital rates and recruitment dynamics of three congeneric pairs of introduced perennial plants grown as monocultures in experimental PSF-generated soil environments. Field soils collected from conspecifics and congeners were alternated in patches or mixed together to produce heterogeneous and homogeneous soils, respectively. nWe quantified the effects of PSF-generated heterogeneity on germination and establishment and determined how these vital rates affected recruitment. We calculated net pairwise interaction coefficients to predict whether PSFs could mediate coexistence between congeners. nSoil heterogeneity altered the relationship of vital rates to recruitment dynamics for some species. For example, Solanum dulcamara recruited later into heterogeneous than homogeneous soils, and germination was a stronger predictor of the timing of recruitment in heterogeneous soil, while mortality was a stronger predictor in homogeneous soil. Contrasts between soils of different origin suggest that mixing soils had non-additive effects on vital rates (e.g. Rumex crispus mortality was higher in homogeneous than in conspecific or congener soil). Interaction coefficients predicted that PSFs in heterogeneous soils might mediate stable coexistence only of Rumex congeners. nSynthesis. Heterogeneity generated by PSFs had species-specific effects on vital rates, with consequences for recruitment dynamics. Mixing soils of different origin often resulted in non-additive effects, which may indicate an interaction between soil abiotic and/or biotic properties and could predict non-additive responses to soil disturbance. Finally, quantifying the reciprocal effects of PSFs on congeners suggests that PSF-generated heterogeneity may promote coexistence of certain species, which was not evident from individual PSF responses. Future studies should determine whether such mechanisms might operate for more distantly related species.

Soil microbial community variation correlates most strongly with plant species identity, followed by soil chemistry, spatial location and plant genus

Soil ecologists have debated the relative importance of dispersal limitation (everything is not everywhere) and ecological factors in determining the structure of soil microbial communities. The relative explanatory power of spatial and ecological factors, including plant species identity and even plant relatedness, for different fractions of the soil microbial community (i.e. bacterial and fungal communities) are poorly understood. We sampled field soils in a northern California field site, and find that soil microbial community variation correlates most strongly with plant species identity, followed by soil chemistry, spatial location and plant genus.

Heterogeneity in plant–soil feedbacks and resident population dynamics affect mutual invasibility

Summary 1. Understanding the mechanisms governing coexistence is a central goal in ecology and has implications for conserving and restoring communities, yet the high diversity in many plant communities is difficult to explain. Theory suggests that plant–soil feedbacks (PSF) can lead to frequency-dependent coexistence by suppressing conspecifics more than heterospecifics, potentially helping to explain high-diversity plant communities. In addition, species-specific population dynamics, including the rate at which individuals are replaced in a population or population turnover rate, may influence coexistence outcomes. 2. We have created a rigorous test of the coexistence predictions of theory by generating a soil heterogeneity experiment in the field and testing for mutual invasibility by establishing resident populations, then experimentally invading them. Experimental tests of mutual invasibility can demonstrate coexistence because, if species are able to invade one another’s populations when at low density, they should exhibit long-term coexistence. We use pairs of congeners in this experiment that coexist at small spatial scales, sometimes within cm, at our field site. 3. We demonstrate that invader individuals established better in congener’s soils than in conspecific soils, consistent with plant–soil feedback mediated coexistence. This effect was often mediated by competition with established resident plants. 4. Further, we show that soil heterogeneity interacted with the population turnover rate of the resident population to influence invasibility (P < 0.10), consistent with the theoretical prediction that a plant’s population dynamics will interact with heterogeneity to influence coexistence. 5. Synthesis. Plant–soil feedbacks (PSF) can in theory lead to frequency-dependent coexistence, and reciprocally, negative feedback effects in glasshouse experiments are often consistent with this prediction. We provide the first field test of mutual invasibility structured by PSF, demonstrating that PSF can lead to coexistence when they create a patchy, or heterogeneous, soil environment. This work suggests that understanding the influence of PSF on diversity necessitates understanding the spatial scale at which soil heterogeneity emerges in the field. Thus, high diversity might be maintained in plant communities by heterogeneity created by plants’ influence on the soil, and this outcome depends strongly on population dynamics.

Spatial heterogeneity of plant–soil feedbacks increases per capita reproductive biomass of species at an establishment disadvantage

Plant–soil feedbacks have been widely implicated as a driver of plant community diversity, and the coexistence prediction generated by a negative plant–soil feedback can be tested using the mutual invasibility criterion: if two populations are able to invade one another, this result is consistent with stable coexistence. We previously showed that two co-occurring Rumex species exhibit negative pairwise plant–soil feedbacks, predicting that plant–soil feedbacks could lead to their coexistence. However, whether plants are able to reproduce when at an establishment disadvantage (“invasibility”), or what drivers in the soil might correlate with this pattern, are unknown. To address these questions, we created experimental plots with heterogeneous and homogeneous soils using field-collected conditioned soils from each of these Rumex species. We then allowed resident plants of each species to establish and added invader seeds of the congener to evaluate invasibility. Rumex congeners were mutually invasible, in that both species were able to establish and reproduce in the other’s resident population. Invaders of both species had twice as much reproduction in heterogeneous compared to homogeneous soils; thus the spatial arrangement of plant–soil feedbacks may influence coexistence. Soil mixing had a non-additive effect on the soil bacterial and fungal communities, soil moisture, and phosphorous availability, suggesting that disturbance could dramatically alter soil legacy effects. Because the spatial arrangement of soil patches has coexistence implications, plant–soil feedback studies should move beyond studies of mean effects of single patch types, to consider how the spatial arrangement of patches in the field influences plant communities.

Light heterogeneity interacts with plant-induced soil heterogeneity to affect plant trait expression

Phenotypic plasticity, the expression of traits dependent on an individual’s environment, should increase the range of communities in which a plant can establish, potentially influencing community assembly. Plasticity may be induced by either the mean or variance in conditions; however, plant responses to variance in the environment remain largely unquantified. We conducted a greenhouse experiment in northeastern Ohio using two congeneric pairs of perennial species in a design that crossed two soil heterogeneity and two light heterogeneity treatments. Nested within the soil heterogeneity treatment were patches of congener and conspecific origin, which tests the effects of possible plant-soil feedbacks on trait expression. We used plant biomass to measure performance and root:shoot ratio and specific leaf area (SLA) to measure resource acquisition and allocation. Plant biomass responded to the origin of the soil, consistent with the literature demonstrating that plant-soil feedbacks influence plant performance. Soil and light heterogeneity interacted to significantly affect resource acquisition traits (i.e., SLA) demonstrating that plants integrated both above- and belowground resource variance. All four species showed light-dependent, non-additive effects of soil mixture, though these effects were only marginally significant (Pxa0<xa00.10). This is important in the context of community assembly because studies find the variance in environmental drivers to be as, or more, important than the means in determining community structure. This study provides some of the first evidence that plant trait expression responds to the variance of environmental drivers, including those driven by plant-soil feedbacks, as well as the means.

Experimental Protocol for Manipulating Plant-induced Soil Heterogeneity

Coexistence theory has often treated environmental heterogeneity as being independent of the community composition; however biotic feedbacks such as plant-soil feedbacks (PSF) have large effects on plant performance, and create environmental heterogeneity that depends on the community composition. Understanding the importance of PSF for plant community assembly necessitates understanding of the role of heterogeneity in PSF, in addition to mean PSF effects. Here, we describe a protocol for manipulating plant-induced soil heterogeneity. Two example experiments are presented: (1) a field experiment with a 6-patch grid of soils to measure plant population responses and (2) a greenhouse experiment with 2-patch soils to measure individual plant responses. Soils can be collected from the zone of root influence (soils from the rhizosphere and directly adjacent to the rhizosphere) of plants in the field from conspecific and heterospecific plant species. Replicate collections are used to avoid pseudoreplicating soil samples. These soils are then placed into separate patches for heterogeneous treatments or mixed for a homogenized treatment. Care should be taken to ensure that heterogeneous and homogenized treatments experience the same degree of soil disturbance. Plants can then be placed in these soil treatments to determine the effect of plant-induced soil heterogeneity on plant performance. We demonstrate that plant-induced heterogeneity results in different outcomes than predicted by traditional coexistence models, perhaps because of the dynamic nature of these feedbacks. Theory that incorporates environmental heterogeneity influenced by the assembling community and additional empirical work is needed to determine when heterogeneity intrinsic to the assembling community will result in different assembly outcomes compared with heterogeneity extrinsic to the community composition.

Plant performance was greater in the soils of more distantly related plants for an herbaceous understory species

We performed a glasshouse experiment to test whether degree of phylogenetic relatedness between Aquilegia canadensis and six co-occurring heterospecifics affects A. canadensis biomass through soil legacy effects. We found that A. canadensis performed significantly better in distant relatives soils than in close relatives soils, and this effect disappeared with soil sterilization. The greater performance of A. canadensis in soils of more versus less distant relatives is consistent with a hypothesis of phylogenetically-constrained pathogen escape, a phenomenon expected to promote coexistence of phylogenetically distant species.

Plant trait expression responds to establishment timing

Trait divergence between co-occurring individuals could decrease the strength of competition between these individuals, thus promoting their coexistence. To test this hypothesis, we manipulated establishment timing for four congeneric pairs of perennial plants and assessed trait plasticity. Because soil conditions can affect trait expression and competition, we grew the plants in field-collected soil from each congener. Competition was generally weak across species, but the order of establishment affected divergence in biomass between potmates for three congeneric pairs. The type of plastic response differed among genera, with trait means of early-establishing individuals of Rumex and Solanum spp. differing from late-establishing individuals, and trait divergence between potmates of Plantago and Trifolium spp. depending on which species established first. Consistent with adaptive trait plasticity, higher specific leaf area (SLA) and root–shoot ratio in Rumex spp. established later suggest that these individuals were maximizing their ability to capture light and soil resources. Greater divergence in SLA correlated with increased summed biomass of competitors, which is consistent with trait divergence moderating the strength of competition for some species. Species did not consistently perform better in conspecific or congener soil, but soil type influenced the effect of establishment order. For example, biomass divergence between Rumex potmates was greater in R. obtusifolius soil regardless of which species established first. These results suggest that plant responses to establishment timing act in a species-specific fashion, potentially enhancing coexistence in plant communities.

Reflections on, and visions for, the changing field of pollination ecology

Since the launch of Ecology Letters in 1998, the field of Pollination Ecology has changed considerably in its focus. In this review, we discuss the major discoveries across the past two decades. We quantitatively synthesise the frequency by which different concepts and topics appeared in the peer-reviewed literature, as well as the connections between these topics. We then look forward to identify pressing research frontiers and opportunities for additional integration in the future. We find that there has been a shift towards viewing plant-pollinator interactions as networks and towards understanding how global drivers influence the plants, pollinators and the ecosystem service of pollination. Future frontiers include moving towards a macroecological view of plant-pollinator interactions, understanding how ecological intensification and urbanisation will influence pollination, considering other interactions, such as plant-microbe-pollinator networks, and understanding the causes and consequences of extinctions. Pollination Ecology is poised to advance our basic understanding of the ecological and evolutionary factors that shape plant-animal interactions and to create applied knowledge that informs conservation decision making.