Ecology

Predation risk varies through space and time due to changing refuge quality, predator communities, and prey traits. Despite this, ecological research is often focused on measuring average predation risk at the community level. While this can give important information about overall trophic transfer and ecological efficiency, it ignores differences in predation risk among prey species within a community, which may be important determinants of species coexistence and local diversity. We used crustaceans associated with temperate seagrass in Northern California to explore the relationship between seasonal variation in among-species and community-level predation risk for a community of morphologically distinct prey. We measured predation risk of the four most abundant and widespread prey species at six field sites every two to six weeks for one year. At the community level, sites differed significantly in their annual variation in predation risk, and these differences were correlated with the amount of variation in the among-species predation risk. When there was more within-year variation in predation risk among the four prey species, predation risk at the community level was more stable across the year. On the other hand, when each prey species in the community had similar levels of predation risk throughout the year, predation as a community-level process was much more seasonal and variable. Variation in predation risk also changed across a gradient of seagrass cover, a proxy for refuge quality. Sites with greater seagrass cover had less annual variation in community-level predation risk and more variation in predation risk among the four species at any given time point. In contrast, at sites with less eelgrass, all species were consumed at the same rate throughout the year, suggesting previously demonstrated differences in antipredator strategies among species are less relevant in the absence of habitat-forming species. We suggest that larger species-specific differences in predation risk throughout a year result in a more stable level of predation risk for the whole community, and that this may be driven by increased refuge provided by seagrass habitat mediating different prey species relative levels of susceptibility to predation.

Biological rates, like growth, tend to have unimodal (hump-shaped) responses to temperature, and these relationships can vary among species and biological processes. In most systems, full thermal performance relationships are rarely characterized for interacting species (e.g., consumer-resource or host-pathogen pairs), making it challenging to predict how their interactions, and subsequently, how communities, will shift with climate change. We investigated how the thermal responses of eelgrass (Zostera marina, an important marine foundation species in the N. hemisphere) and an isopod grazer (Pentidotea resecata), which putatively acts as an indirect vector of eelgrass wasting disease, interact to affect eelgrass productivity and wasting disease dynamics. In a laboratory experiment crossing five temperatures, two grazing, and two disease exposure treatments, across various metrics, eelgrass growth responded unimodally to temperature in the absence of grazers. Grazers depressed plant growth and flattened its thermal performance curves. Thermal performance curves for isopods indicated that increases in grazing and survival at intermediate temperatures negated concurrent gains in plant growth at these temperatures, while decreased isopod survival at high temperatures reduced their top-down effect on eelgrass. Isopods had negligible effects on plant disease responses, but warming reduced the time to disease onset and increased final disease severity. Overall, whole-plant disease severity remained low and did not substantially affect eelgrass leaf elongation, net growth, or rhizome dry mass. However, disease-treatment plants grew more new leaves at intermediate temperatures, possibly to combat losses in photosynthetic capacity in diseased leaf tissue. These results indicate that climate change-associated warming will likely increase eelgrass vulnerability to wasting disease. However, in sublethal outbreaks, disease could have less of an impact on eelgrass productivity than warming-induced increases in grazing. Thus, ignoring grazer responses to temperature could result in unreliable predictions of eelgrass productivity under climate change.

Intraspecific behavioral variation in prey could alter predator-prey interactions, yet its effects on temporal dynamics and food-web persistence remain underexplored. Pea aphids (Acyrthosiphon pisum) exhibit dropping behavior in response to predators like the seven-spot ladybird (Coccinella septempunctata). This response could be an effective anti-predator defense but could be costly in terms of energy expenditure and time not available for feeding. To investigate the impact of behavioral variation on food-chain persistence and dynamics, we used a tri-trophic experimental system with Vicia faba (plant), pea aphids (prey), and seven-spot ladybirds (predator), implementing three aphid behavioral treatments: droppers, non-droppers, and a mix of both droppers and non-droppers. To minimize genetic differences, we used clonal aphid populations across all treatments. We then tracked predator-prey population dynamics and species persistence over 25 days. Our results showed that aphid dropping behavior reduced food-chain persistence, with extinction risk significantly higher in dropper treatments than in the mix or non-dropper treatments. Ladybirds persisted across treatments, although they showed a steeper decline in abundance in the dropper treatment. In the mixed behavioral treatment, they had an intermediate persistence, suggesting a buffering effect of behavioral variation. Trophic food-chain state transitions also differed by treatment, with tri-trophic states most stable in the non-dropper, and least frequent in the dropper treatment. Furthermore, our results showed a trend of dropper treatments becoming more stable and robust towards the end of the experiment. These results demonstrate that prey behavior influences the persistence and dynamics of food-chains, with important implications for behavior-driven community dynamics.

Time-series data for ecological communities are increasingly available from long-term studies designed to track species responses to environmental change. However, classical multivariate methods for analyzing community composition have limited applicability for time series, as they do not account for temporal autocorrelation in community-member abundances. Furthermore, traditional approaches often obscure the connections between responses at the community level and those for individual taxa, limiting their capacity to infer mechanisms of community change. We show how linear mixed models that account for group-specific temporal autocorrelation and observation error can be used to infer both taxon- and community-level responses to environmental predictors from replicated time-series data. Variation in taxon-specific responses to predictors is modeled using random effects, which can be used to characterize variation in community composition. Moreover, the degree of autocorrelation is estimated separately for each taxon, since this is likely to vary due to differences in their underlying population dynamics. We illustrate the utility of the approach by analyzing the response of a predatory arthropod community to spatiotemporal variation in allochthonous resources in a subarctic landscape. Our results show how mixed models with temporal autocorrelation provide a unified approach to characterizing taxon- and community-level responses to environmental variation through time.

O_LIThe enemy release hypothesis is a popular hypothesis to explain the success of invasive plants. Enemy release studies typically focus on single species or types of communities, feature indirect experimental manipulations that apply pesticides to whole communities not individual species, and only examine responses of established plants or plant populations, limiting their generality. Using a novel species-specific approach, we examine whether enemy release can enhance seedling survival and recruitment of 16 grassland species by experimentally linking enemy release with enhanced plant performance. C_LIO_LIWe planted seedlings of 16 native grassland species from two functional groups (C4 grasses and non-legume forbs) into two grassland sites (early and mid succession). We hand-painted 1,548 individual seedlings with pesticides (insecticide and fungicide) over the course of a growing season to enforce aboveground species-specific enemy release, and tested whether it enhanced survival relative to untreated controls. Using native species enabled us to directly test effects of enemy release, while avoiding confounding factors like unknown invasion histories. Of the 16 native study species, 13 are naturalised/invasive outside of their native ranges. C_LIO_LIRelease from insects increased seedling survival by 80% on average, with no additional benefit of release from fungal pathogens. This effect was consistent across functional groups and community successional stages, and was strongest in resource-acquisitive species. The size of species performance benefits from enemy release were positively correlated with the number of regions globally where each species has been introduced and naturalised. C_LIO_LISynthesis. Previous studies of enemy release have centred on adults and findings have varied among species. We found a positive effect of release from insect herbivores early in colonisation - a trend that held across functional groups and types of resident community. We posit that the consistent vulnerability of seedlings vis-a-vis later life stages leads to this more ubiquitous benefit of enemy release. Enemy release may therefore aid initial recruitment of most, if not all, plants during the invasion process, even if enemies rapidly accumulate. The positive correlations between the benefits of enemy release for seedlings, species life history strategies and global naturalisation patterns provide compelling hypotheses for future research. C_LI

There is considerable interest in understanding patterns of {beta}-diversity that measure the amount of change in species composition through space or time. Most hypotheses for {beta}-diversity evoke nonrandom processes that generate spatial and temporal within species aggregation; however, {beta}-diversity can also be driven by random sampling processes. Here, we describe a framework based on rarefaction curves that quantifies the non-random contribution of species compositional differences across samples to {beta}-diversity. We isolate the effect of within-species spatial or temporal aggregation on beta-diversity using a coverage standardized metric of {beta}-diversity ({beta}C). We demonstrate the utility of our framework using simulations and an empirical case study examining variation in avian species composition through space and time in engineered versus natural riparian areas. The primary strengths of our approach are that it provides an intuitive visual null model for expected patterns of biodiversity under random sampling that allows integrating analyses across -, {gamma}-, and {beta}-scales. Importantly, the method can accommodate comparisons between communities with different species pool sizes, and can be used to examine species turnover both within and between meta-communities. Open Research statement: all code and data used in this manuscript are available at the following link: https://github.com/MoBiodiv/beta_concept

O_LIPlant community succession is structured by priority effects, plant consumer pressure, and soil resource supply. Importantly, these drivers may interact, their effects may vary temporally, and they may influence different facets of plant community diversity by promoting different plant tradeoff strategies.\nC_LIO_LIIn an herbaceous successional system, we manipulated priority effects by altering initial plant richness, consumer pressure via pesticide spraying, and soil resource supply via fertilization. We examined how these processes jointly influenced succession, including taxonomic diversity and functional traits, over four years.\nC_LIO_LIDiversity decreased in different years in response to more diverse priority effects, lower consumer pressure, and increased soil resource supply. Functionally, higher soil resource supply increased community height, SLA, and seed mass; higher consumer pressure decreased intraspecific community height, and increased interspecific SLA; priority effects led to decreased seed mass only when plots were unplanted.\nC_LIO_LIOur results suggest species resource strategies underlie plant diversity responses. Resource addition promoted resource-acquisitive species, consumer pressure disadvantaged resource-conservative species, and diversity of priority effects altered subsequent community composition through persistence of early residents, not via traits. We show that community responses to drivers of succession depend on underlying trait tradeoffs of resident species, and these tradeoffs influence community diversity across succession.\nC_LI

Host and parasite richness are generally positively correlated, but the stability of this relationship during community assembly remains untested. The composition of host communities can alter parasite transmission, and the relationship between host and parasite richness is sensitive to parasite transmission. Thus, changes in composition during host community assembly could strengthen or weaken the relationship between host and parasite richness. Host community assembly, in turn, can be driven by many processes, including resource enrichment. To test the hypothesis that host community assembly can alter the relationship between host and parasite richness, we experimentally crossed host diversity and resource supply to hosts, then allowed communities to assemble. As previously shown, initial host diversity and resource supply determined the trajectory of host community assembly, altering post-assembly host species richness, richness-independent host phylogenetic diversity, and colonization by exotic host species. Throughout community assembly, host richness predicted parasite richness. As predicted, this effect was moderated by exotic abundance: communities dominated by exotic species exhibited a stronger positive relationship between post-assembly host and parasite richness. Ultimately, these results suggest that, by modulating parasite transmission, community assembly can modify the relationship between host and parasite richness, providing a novel mechanism to explain contingencies in this relationship.

O_LIBiodiversity loss can alter interactions not only through changes in tree species richness, but also through the loss of particular functional strategies from ecological communities. Working in a subtropical forest diversity experiment we asked whether tree species richness effects on arthropod herbivory and leaf pathogen infestation depend on community functional diversity, and whether trait dissimilarity-based, non-random species loss alters these relationships compared to random loss. To address this, we combined already established planted scenarios with newly constructed extinction pathways. C_LIO_LIWe tested the responses of herbivory and leaf pathogen infestation (i) to tree species richness, functional diversity (Raos Q), community structure and resource strategies (i.e. ever-greenness) and community-weighted trait means as well as predation, and (ii) trait dissimilarity-based extinction pathway analyses that contrasted directed loss of functionally similar versus functionally distinct tree species. C_LIO_LIHerbivory increased with tree species richness and this increase was significantly stronger in communities with higher tree functional diversity. Under directed species loss scenarios, herbivory differed most strongly from random-loss expectations when similar tree species were lost first. By contrast, losing functionally distinct species first produced richness effects that were much closer to the random-loss scenarios. Trait-based species loss will therefore modify trophic interactions more strongly than random loss. For pathogen infestation tree richness effects depended on evergreenness and among planted extinction scenarios (three-way interaction), with only minor deviations of trait-based extinction path-ways from random-loss expectations. Pathogen infestation also tended to increase with community-weighted mean leaf nitrogen. Predation showed no clear relationship with tree species richness or functional diversity but was positively associated with herbivory. The strength of this association differed among extinction scenarios, providing no evidence for consistent top-down regulation. C_LI SynthesisThe ecological consequences of biodiversity loss for leaf damage depend on which functional strategies are lost, not only on how many tree species remain. By integrating ob-served tree diversity gradients with trait-based extinction pathways, this study shows that functional diversity and host redundancy help explain why herbivores and pathogens are shaped by the same changes in tree diversity through different functional constraints and im-prove predictions of interaction strength under non-random species loss.

Herbivory shapes plant invasion outcomes, yet its role in aquatic plant invasions under changing environmental conditions, such as artificial light at night (ALAN), remains poorly understood. We conducted three experiments using invasive macrophyte Myriophyllum aquaticum, a native macrophyte community (Vallisneria natans, Hydrilla verticillata, M. spicatum), invasive snail Pomacea canaliculata, and native snail Cipangopaludina chinensis to test the combined effects of ALAN and herbivory on non-native macrophyte invasions. ALAN increased M. aquaticum height and total biomass, but had no effect on native species growth. In feeding assays, P. canaliculata consumed all three native species but consistently avoided M. aquaticum under both light treatments. C. chinensis showed no feeding in no-choice assays, but in choice assays, consumed H. verticillata and M. spicatum under No-ALAN and only M. spicatum under ALAN. In community mesocosms, P. canaliculata reduced native macrophyte biomass by 48.0% under No-ALAN and 87.2% under ALAN without affecting M. aquaticum. This selective feeding increased M. aquaticums proportional biomass, with a stronger effect under ALAN than No-ALAN. These results suggest that ALAN can indirectly facilitate non-native macrophyte invasion by amplifying their relative biomass within native communities, particularly in the presence of invasive herbivores, and may promote invasional meltdown through altered feeding preferences.

Mortality-inducing disturbances are important, ubiquitous drivers of community composition and function. Importantly, human activities and climate change are increasingly altering disturbance regimes. Most disturbance studies focus on the effects of current disturbance regimes, rarely considering those of historical regimes. However, recent theoretical work predicts that historical regimes can leave persistent legacies, modulating the communitys response to novel disturbances and invasive species. Here, we complement this theoretical approach using a model bacterial system that experienced disturbance regimes for [~]120 generations, followed by novel regimes and invasions for another [~]120 generations. Our results show persistent effects of historical legacies on disturbance-diversity relationships. Furthermore, some combinations of past and novel regimes promote invasion with increasing resident diversity, while others prevent it; legacies may explain conflicting diversity-invasibility relationships. These findings demonstrate the importance of historical legacies in disturbance-prone ecosystems, and underscore the challenges in predicting future community responses to disturbance regime changes.

AimThree fundamental and inter-related concepts have accrued debates: ecological communities, beta diversity ({beta}), and spatial scale. Spatial scaling of {beta} informs the community concept because the scale of maximal {beta} corresponds to the most apparent size of an ecological community (without invoking external features of habitat, etc.). Here we test five alternative hypotheses about spatial scaling of {beta} for ants, birds, diatoms, and trees across the contiguous USA, using spatial grains from 1 to 106 km2. We compare {beta} scaling among clades and test hypotheses about repeatability where data permit for: (a) summer and winter bird {beta} in six consecutive years; (b) trees through time (4 years, spaced 5 years apart). Finally, we compare different forms of {beta} (i.e., observed and deviations from null models based on spatial heterogeneity and spatial homogeneity). LocationThe contiguous United States of America Time PeriodRecent but varying with clade Taxa Studiedants, birds, diatoms, and trees MethodsWe obtained data from publicly-available sources and assigned point locations to hexagonal grids ranging from 1 to 106 km2. At each spatial grain, we calculated mean pairwise {beta} between each hexagon and its neighboring grids. We also compared alternative {beta} measures and evaluated potential confounding effects of neighborhood size and species richness on results. ResultsSpatial scaling of {beta} repeatedly supported the regional community concept among clades, though with different spatial scales per clade. Based on peak mean {beta}, community size for trees ([~]300 km2) < winter birds ([~]500 km2) < summer birds ([~]2000 km2) {approx} ants ([~]2000 km2) < diatoms ([~]11,000 km2). We note that community scales represent peaks on gradients rather than definitive one-size-fits-all scales. Spatial scaling of {beta} was sensitive to seasonality (birds) and consistent among years for both birds and trees. Also, {beta} deviation from a null model based on spatial heterogeneity adjusted observed {beta} but was less sensitive to neighborhood size and species richness than {beta} deviation based on spatial homogeneity. Main conclusionsResults here indicate that: (a) similar patterns should occur across the tree of life; (b) local ecological and evolutionary forces scale up to form repeatable regional community patterns in ways not yet fully understood; (c) local biodiversity conservation efforts need to be coordinated at biogeographical scales to best achieve goals; and (d) a recent method to calculate {beta} deviation from a null model based on spatial heterogeneity improves {beta} research.

1. The physical structure of an environment potentially influences feeding interactions among organisms, for instance, by providing refuge for prey. We examined how habitat complexity affects the functional feeding response of an ambush predator (damselfly larvae Ischnura elegans) and a pursuit predator (backswimmer Notonecta glauca) feeding on the isopod Asellus aquaticus. 2. We ran experiments in aquatic microcosms with an increasing number of structural elements (0, 2, or 3 rings of plastic plants in different spatial configurations), resulting in five habitat complexity levels. Across these levels, predators were presented with different prey densities to determine the functional response pattern. The experimental design and analysis allowed us to test for effects of structure presence, amount, and complexity level on functional response in one pass, without confounding predictors. 3. The feeding for both predators across all complexity levels was best described by a type II functional response model, and habitat drove feeding strength. Regarding the latter, the predators showed different responses to the complexity treatments. The overall feeding rate of I. elegans was mainly explained by the absence vs. presence of structure. Yet, in the case of N. glauca, feeding rate was strongly dependent on habitat complexity with the predator showing a unique maximum feeding rate (i.e. the inverse of the handling time) for each complexity level and a decreasing attack rate with increasing amount of habitat. 4. On average, prey consumption by both predators was reduced when complex structures were present, compared to the no habitat structure environment (e.g. consumption more than halved for some treatments). Our findings demonstrate that habitat complexity dampens feeding rates and therefore plays a key role in the stability of freshwater ecosystems.

Biodiversity is expected to enhance the stability of ecological communities under environmental stress, but the relative roles of functional, interspecific, and intraspecific diversity remain poorly resolved, particularly under multiple concurrent stressors. We tested how these diversity dimensions shaped the growth and recovery of marine Synechococcus communities in a microcosm experiment manipulating strain composition across four strain-richness levels and two interspecific diversity levels under control, atrazine, warming, and combined atrazine-plus-warming treatments. Functional diversity was quantified from flow-cytometric trait data and analyzed as initial functional diversity during the stress phase and assembled functional diversity during recovery. Contrary to our expectations, higher initial functional diversity was associated with lower community growth during stress, while higher assembled functional diversity was generally associated with weaker recovery. However, these relationships depended on stressor identity and interspecific diversity: in two-species communities, the negative effects of functional diversity were reduced, and under combined stress, higher assembled functional diversity was associated with improved recovery. In contrast, intraspecific diversity consistently enhanced community growth and recovery, while interspecific diversity primarily promoted functional recovery. Together, our results show that functional, interspecific, and intraspecific diversity can influence stress responses through distinct pathways.

The maintenance of parasite diversity has historically taken a host-centric approach. Yet, many parasites are host generalists, and most parasites spend at least some time outside of their hosts. So, what mechanisms besides host-associated niche partitioning allow parasites to coexist? Using a year-long field survey and lab mesocosms, we examined whether environmental niche partitioning or aggregation could enable coexistence among soil-dwelling entomopathogenic nematodes. Field patterns along an elevational gradient reveal that species abundances differentiate with soil structure and moisture levels. Yet, most species strongly overlap within-sites throughout the year. Thus, niche partitioning alone is not sufficient to explain the coexistence of these species and other mechanisms are necessary to explain their coexistence. Aggregation at the within-site scale provides evidence for such a mechanism. Each species showed significant intraspecific clumping and largely random associations with other species. A mesocosm test of the consequences of intraspecific aggregation found that parasites at low or high densities limit their own population growth. Aggregation can promote negative feedback facilitating species coexistence. Our findings offer field-based evidence that spatiotemporal niche partitioning and aggregation both play a critical role in maintaining parasite species diversity, illustrating the importance of extending out view of parasites beyond their hosts.

O_LIForests are vertically structured ecosystems where light attenuation, microclimatic buffering and resource availability occur along continuous gradients from the canopy to the understorey layers. Despite this complexity, studies rarely integrate trophic interactions across vertical layers, overlooking how vertical forest structure shapes consumer communities through abiotic and biotic pathways. C_LIO_LIIn this study, we combined layer-specific measurements of plant diversity, structure and microclimate with arthropod sampling in the canopy and understorey, as well as bird survey data, in a temperate forest. Applying Bayesian structural equation models with explicitly defined directional pathways, we modelled both consumer biomass and abundance across trophic levels and vertical layers. C_LIO_LIAbundance measures were predominantly filtered by local layer conditions, while biomass responded to conditions across layers, reflecting stand-level energy flow. This suggests that these two metrics capture fundamentally different ecological processes. Canopy conditions consistently predicted understorey arthropod abundance across trophic levels, while the reverse was not observed, suggesting a strong asymmetric downward propagation of canopy-driven effects. Furthermore, trophic interactions between arthropod primary and secondary consumers remained largely stratified within vertical layers, suggesting a vertical food web compartmentalisation rarely shown in structurally complex aboveground systems. C_LIO_LIPlant diversity, structure and microclimate shaped consumer communities mainly through the modulation of resource availability and plant apparency, with effects varying across vertical layers, trophic levels and taxonomic groups. Through complementary mechanisms, plant diversity likely determined the variability of resources available to consumers at different trophic levels. Structural properties, in contrast, potentially drove the spatial redistribution of these resources through light attenuation and microclimatic buffering, which in turn influenced the physiological capacity of consumers to access and exploit available resources. C_LIO_LIThese findings demonstrate that vertical stratification mediates trophic pathways in a highly directional manner, with canopy characteristics playing a disproportionate role in structuring the forest community across layers. Integrating layer-specific structural and trophic indicators into forest biodiversity assessments and management strategies is therefore essential to fully evaluate biodiversity dynamics and multifunctionality in structurally complex forest ecosystems. C_LI

Global biodiversity is declining at rates faster than at any other point in human history. Experimental manipulations of biodiversity at small spatial scales have demonstrated that communities with fewer species consistently produce less biomass than higher diversity communities. However, understanding how the global extinction crisis is likely to impact global ecosystem functioning will require applying these local and largely experimental findings to natural systems at substantially larger spatial and temporal scales. Here we propose that we can use two simple macroecological patterns - the species area curve and the biomass-area curve - to upscale the species richness-biomass relationship. We demonstrate that at local spatial scales, each additional species will contribute more to biomass production with increasing area sampled because the species-area curve saturates and the biomass-area curve increases monotonically. We use species-area and biomass-area curves from a Minnesota grassland and a Panamanian tropical dry forest to examine the species richness - biomass relationship at three and ten sampling extents, respectively. In both datasets, the observed relationship between biodiversity and biomass production at every sampling extent was predicted from simple species-area and biomass-area relationships. These findings suggest that macroecological patterns like the species-area curve underpin the scaling of biodiversity-ecosystem functioning research and can be used to predict these relationships at the global scales where they are relevant for species loss.

Understanding the mechanisms that link biodiversity to ecological stability is crucial for predicting ecosystem responses to global change. Using three decades of standardized monitoring data from eight subtropical estuaries, we analyze diversity components (richness, evenness, dissimilarity, and variance-mean scaling) and interpret stability patterns through synchrony or portfolio mechanisms. Regional {gamma}-diversity increased steadily over time, reflecting sustained gains in fish and invertebrate assemblages. Community stability, defined here as community invariability, was strongly and consistently predicted by Shannon diversity index but not by species richness, underscoring the stabilizing role of evenness. Portfolio effects were robust, with community stability averaging approximately threefold higher than mean population stability, and the strength of this effect more than doubled with each unit increase in Shannon diversity. Structural equation models paired with a null model revealed that shared environmental forcing synchronizes estuarine populations. Shannon diversity generated a large portfolio effect that stabilized the community despite this environmental forcing, whereas richness effects were weak or absent. Taylors law scaling confirmed that abundant, persistent taxa such as blue crab (Callinectes sapidus), brown shrimp (Farfantepenaeus aztecus), and pinfish (Lagodon rhomboides) exhibited higher baseline invariability, contributing to community buffering, while rarer, more variable taxa introduced volatility. In contrast, compositional turnover strongly eroded stability, with high Bray-Curtis dissimilarity predicting reductions in community stability. Together, these results show that long-term estuarine community stability emerges from the interplay of portfolio averaging, demographic variance scaling of dominant species, and the persistence of community composition, highlighting the central role of evenness in biodiversity-stability relationships.

Determining the drivers of ecological stability amid accelerating global environmental change is a critical goal of contemporary ecology. Various candidate drivers have been suggested, with recent attention turning to response diversity--the variation among organism-environment responses. However, despite conceptual interest in response diversity as a driver of stability, there remain few field tests of this relationship. Using multi-species competitive communities of floating aquatic macrophytes as an experimental model for measuring temporal stability and response diversity to nutrient loading, we show that response diversity does not promote temporal stability of total macrophyte cover, but that communities with an uneven distribution of species responses were more resistant to an exogenous shock. To quantify macrophyte composition and growth dynamics from photographic time series of our experimental communities, we developed an open-source, scalable, machine learning workflow (LeafMosaic) capable of classifying four species from noisy field data including variable lighting, resolution, and plant morphology. We measured response diversity as the balance of positive and negative biomass growth responses to dissolved nitrate concentration, weighted by species relative contributions to biomass, and tested its effect on temporal stability and resistance to an unexpected pulse disturbance (a large typhoon that disrupted our outdoor mesocosms). Response imbalance predicted typhoon resistance, but species asynchrony and mean population stability best predicted community stability, with no direct or indirect effect of species responses. Overall, our results provide new experimental evidence for how the structure of species responses promotes stability, and we aim our LeafMosaic workflow to empower future field experiments using floating macrophytes to study response diversity and ecological stability.

Introduced species are a common feature of modern plant communities and experience environmental challenges alongside native species. Changes to the environment may reveal distinct species-environment relationships for native and introduced components of plant communities. Extreme environmental change, such as drought, is predicted to result in declines in native species and increased opportunities for invasion, but empirical support for these ideas remains mixed. We tested for differences in the response of native and invaded species to environmental changes by analyzing a longterm dataset of species abundance in California grasslands collected during a period of severe drought. Sampling sites included a combination of stressful serpentine soils, which are resilient against invasion and maintain diverse native species assemblages, and more benign nonserpentine soils, which are heavily invaded and harbor low levels of native species cover. We found a significant correlation between sampling year and species composition for nonserpentine sites, but not for serpentine sites. These patterns were repeated when only introduced species were included in the analysis but no pattern of change was found for native species. The species most strongly associated with directional change on nonserpentine soils were three invasive Eurasian grasses, Bromus hordaceus, Taeniatherium caput-medusae, and Avena fatua. Differences in species composition on both serpentine and nonserpentine soils were significantly correlated with specific leaf area, a trait which has been linked to drought tolerance in these communities, although changes in abundance for the three Eurasian grasses most strongly associated with change did not consistently follow this pattern. Our analyses indicate relatively stable native community composition and strong directional change in introduced species composition, contradicting predictions for how native and introduced species will respond to environmental shifts, but supporting the hypothesis that native and invading species groups have important functional differences that shape their relationships to the environment.