Optimal restoration for pollination services increases forest cover while doubling agricultural profits.

Pollinators are currently facing dramatic declines in abundance and richness across the globe. This can have profound impacts on agriculture, as 75% of globally common food crops benefit from pollination services. As many native bee species require natural areas for nesting, restoration efforts within croplands may be beneficial to support pollinators and enhance agricultural yields. Yet, restoration can be challenging to implement due to large upfront costs and the removal of land from production. Designing sustainable landscapes will require planning approaches that include the complex spatiotemporal dynamics of pollination services flowing from (restored) vegetation into crops. We present a novel planning framework to determine the best spatial arrangement for restoration in agricultural landscapes while accounting for yield improvements over 40 years following restoration. We explored a range of production and conservation goals using a coffee production landscape in Costa Rica as a case study. Our results show that strategic restoration can increase forest cover by approximately 20% while doubling collective landholder profits over 40 years, even when accounting for land taken out of production. We show that restoration can provide immense economic benefits in the long run, which may be pivotal to motivating local landholders to undertake conservation endeavours in pollinator-dependent croplands.

Plant interaction networks reveal the limits of our understanding of diversity maintenance.

Species interactions are key drivers of biodiversity and ecosystem stability. Current theoretical frameworks for understanding the role of interactions make many assumptions which unfortunately, do not always hold in natural, diverse communities. This mismatch extends to annual plants, a common model system for studying coexistence, where interactions are typically averaged across environmental conditions and transitive competitive hierarchies are assumed to dominate. We quantify interaction networks for a community of annual wildflowers in Western Australia across a natural shade gradient at local scales. Whilst competition dominated, intraspecific and interspecific facilitation were widespread in all shade categories. Interaction strengths and directions varied substantially despite close spatial proximity and similar levels of local species richness, with most species interacting in different ways under different environmental conditions. Contrary to expectations, all networks were predominantly intransitive. These findings encourage us to rethink how we conceive of and categorize the mechanisms driving biodiversity in plant systems.

What Can Evolutionary History Tell Us about the Functioning of Ecological Communities? The ASN Presidential Debate.

AbstractIn January 2018, Sharon Strauss, then president of the American Society of Naturalists, organized a debate on the following topic: does evolutionary history inform the current functioning of ecological communities? The debaters-Ives, Lau, Mayfield, and Tobias-presented pro and con arguments, caricatured in standard debating format. Numerous examples show that both recent microevolutionary and longer-term macroevolutionary history are important to the ecological functioning of communities. On the other hand, many other examples illustrate that the evolutionary history of communities or community members does not influence ecological function, or at least not very much. This article aims to provide a provocative discussion of the consistent and conflicting patterns that emerge in the study of contemporary and historical evolutionary influences on community function, as well as to identify questions for further study. It is intended as a thought-provoking exercise to explore this complex field, specifically addressing (1) key assumptions and how they can lead us astray and (2) issues that need additional study. The debaters all agree that evolutionary history can inform us about at least some aspects of community function. The underlying question at the root of the debate, however, is how the fields of ecology and evolution can most profitably collaborate to provide a deeper and broader understanding of ecological communities.

Weak evidence of trade-offs modulated by seed mass among a guild of closely related winter annuals.

Plant-plant interactions are integral to the establishment and persistence of diversity in plant communities. For annual plant species that depend on seeds to regenerate, seed characteristics that confer fitness advantages may mediate processes such as plant-plant interactions. Seed mass is known to vary widely and has been shown to associate with species' differences in stress tolerance and competitive effects. However, understanding of how seed mass influences species' responses to competition is less well understood. Using natural assemblages of six closely related annual plant species in Western Australia, we implemented a thinning study to assess how seed mass influences the outcomes of plant-plant interactions. We found relatively weak evidence for competition or facilitation among species. Our strongest results indicated that heavy-seeded species had lower survivorship than light-seeded species when interacting with heterospecifics. Seed mass was also negatively related to overall survival, counter to expectations. These findings indicate some evidence for trade-offs mediated by seed mass in this system. However, we acknowledge that other factors may have influenced our results, such as the use of natural assemblages (rather than using sowing experiments) and the presence of important small-scale environmental variation not captured with our choice of abiotic variables. Further research is required to clarify the role of seed mass in this diverse annual system, ideally including many focal species, and using sowing experiments.

Accounting for demographic uncertainty increases predictions for species coexistence: A case study with annual plants.

Natural systems contain more complexity than is accounted for in models of modern coexistence theory. Coexistence modelling often disregards variation arising from stochasticity in biological processes, heterogeneity among individuals and plasticity in trait values. However, these unaccounted-for sources of uncertainty are likely to be ecologically important and have the potential to impact estimates of coexistence. We applied a Bayesian modelling framework to data from an annual plant community in Western Australia to propagate uncertainty in coexistence outcomes using the invasion criterion and ratio of niche to fitness differences. We found accounting for this uncertainty altered predictions of coexistence versus competitive exclusion for 3 out of 14 species pairs and yielded a probability of priority effects for an additional species pair. The propagation of uncertainty arising from sources of biological complexity improves our ability to predict coexistence more accurately in natural systems.

Disentangling key species interactions in diverse and heterogeneous communities: A Bayesian sparse modelling approach.

Modelling species interactions in diverse communities traditionally requires a prohibitively large number of species-interaction coefficients, especially when considering environmental dependence of parameters. We implemented Bayesian variable selection via sparsity-inducing priors on non-linear species abundance models to determine which species interactions should be retained and which can be represented as an average heterospecific interaction term, reducing the number of model parameters. We evaluated model performance using simulated communities, computing out-of-sample predictive accuracy and parameter recovery across different input sample sizes. We applied our method to a diverse empirical community, allowing us to disentangle the direct role of environmental gradients on species' intrinsic growth rates from indirect effects via competitive interactions. We also identified a few neighbouring species from the diverse community that had non-generic interactions with our focal species. This sparse modelling approach facilitates exploration of species interactions in diverse communities while maintaining a manageable number of parameters.

Drivers of Acacia and Eucalyptus growth rate differ in strength and direction in restoration plantings across Australia.

Functional traits are proxies for a species' ecology and physiology and are often correlated with plant vital rates. As such they have the potential to guide species selection for restoration projects. However, predictive trait-based models often only explain a small proportion of plant performance, suggesting that commonly measured traits do not capture all important ecological differences between species. Some residual variation in vital rates may be evolutionarily conserved and captured using taxonomic groupings alongside common functional traits. We tested this hypothesis using growth rate data for 17,299 trees and shrubs from 80 species of Eucalyptus and 43 species of Acacia, two hyper-diverse and co-occurring genera, collected from 497 neighborhood plots in 137 Australian mixed-species revegetation plantings. We modeled relative growth rates of individual plants as a function of environmental conditions, species-mean functional traits, and neighbor density and diversity, across a moisture availability gradient. We then assessed whether the strength and direction of these relationships differed between the two genera. We found that the inclusion of genus-specific relationships offered a significant but modest improvement to model fit (1.6%-1.7% greater R2 than simpler models). More importantly, almost all correlates of growth rate differed between Eucalyptus and Acacia in strength, direction, or how they changed along the moisture gradient. These differences mapped onto physiological differences between the genera that were not captured solely by measured functional traits. Our findings suggest taxonomic groupings can capture or mediate variation in plant performance missed by common functional traits. The inclusion of taxonomy can provide a more nuanced understanding of how functional traits interact with abiotic and biotic conditions to drive plant performance, which may be important for constructing trait-based frameworks to improve restoration outcomes.

Variable seed bed microsite conditions and light influence germination in Australian winter annuals.

Environmentally cued germination may play an important role in promoting coexistence in Mediterranean annual plant systems if it causes niche differentiation across heterogeneous microsite conditions. In this study, we tested how microsite conditions experienced by seeds in the field and light conditions in the laboratory influenced germination in 12 common annual plant species occurring in the understorey of the York gum-jam woodlands in southwest Western Australia. Specifically, we hypothesized that if germination promotes spatial niche differentiation, then we should observe species-specific germination responses to light. In addition, we hypothesized that species' laboratory germination response may depend on the microsite conditions experienced by seeds while buried. We tested the laboratory germination response of seeds under diurnally fluctuating light and complete darkness, which were collected from microsites spanning local-scale environmental gradients known to influence community structure in this system. We found that seeds of 6 out of the 12 focal species exhibited significant positive germination responses to light, but that the magnitude of these responses varied greatly with the relative light requirement for germination ranging from 0.51 to 0.86 for these species. In addition, germination increased significantly across a gradient of canopy cover for two species, but we found little evidence to suggest that species' relative light requirement for germination varied depending on seed bank microsite conditions. Our results suggest that variability in light availability may promote coexistence in this system and that the microsite conditions seeds experience in the intra-growing season period can further nuance species germination behaviour.

Identifying "Useful" Fitness Models: Balancing the Benefits of Added Complexity with Realistic Data Requirements in Models of Individual Plant Fitness.

AbstractDirect species interactions are commonly included in individual fitness models used for coexistence and local diversity modeling. Though widely considered important for such models, direct interactions alone are often insufficient for accurately predicting fitness, coexistence, or diversity outcomes. Incorporating higher-order interactions (HOIs) can lead to more accurate individual fitness models but also adds many model terms, which can quickly result in model overfitting. We explore approaches for balancing the trade-off between tractability and model accuracy that occurs when HOIs are added to individual fitness models. To do this, we compare models parameterized with data from annual plant communities in Australia and Spain, varying in the extent of information included about the focal and neighbor species. The best-performing models for both data sets were those that grouped neighbors based on origin status and life form, a grouping approach that reduced the number of model parameters substantially while retaining important ecological information about direct interactions and HOIs. Results suggest that the specific identity of focal or neighbor species is not necessary for building well-performing fitness models that include HOIs. In fact, grouping neighbors by even basic functional information seems sufficient to maximize model accuracy, an important outcome for the practical use of HOI-inclusive fitness models.

Aridity drives coordinated trait shifts but not decreased trait variance across the geographic range of eight Australian trees.

Large intraspecific functional trait variation strongly impacts many aspects of communities and ecosystems, and is the medium upon which evolution works. Yet intraspecific trait variation is inconsistent and hard to predict across traits, species and locations. We measured within-species variation in leaf mass per area (LMA), leaf dry matter content (LDMC), branch wood density (WD), and allocation to stem area vs leaf area in branches (branch Huber value (HV)) across the aridity range of seven Australian eucalypts and a co-occurring Acacia species to explore how traits and their variances change with aridity. Within species, we found consistent increases in LMA, LDMC and WD and HV with increasing aridity, resulting in consistent trait coordination across leaves and branches. However, this coordination only emerged across sites with large climate differences. Unlike trait means, patterns of trait variance with aridity were mixed across populations and species. Only LDMC showed constrained trait variation in more xeric species and drier populations that could indicate limits to plasticity or heritable trait variation. Our results highlight that climate can drive consistent within-species trait patterns, but that patterns might often be obscured by the complex nature of morphological traits, sampling incomplete species ranges or sampling confounded stress gradients.

Looks can be deceiving: ecologically similar exotics have different impacts on a native competitor.

Exotic species are often predicted to successfully invade when their functional traits differ from species in recipient communities. Many studies have related trait differences among natives and invaders to competitive outcomes. Few studies, however, have tested whether functionally similar invaders have similar competitive impacts on natives. We investigated interactions in communities of a native annual forb Waitzia acuminata (Asteraceae) and two invasive annual grasses that are ecologically similar and co-occur in southwestern Australia. Using a combination of field and laboratory experiments and several performance measures, we assessed impacts of these grasses on W. acuminata. We also examined differences among species in their responses to intraspecific versus interspecific competition, including their frequency dependence. The two similar exotic grasses differed in interaction impacts, with one facilitating and the other suppressing the native. In general, intraspecific competition was stronger than interspecific competition for the native, while evidence of competition was weak for the exotics. These patterns may reflect that W. acuminata does well in these communities due to the combined impacts of stabilization and facilitation, whereas the exotics benefit from limited stabilization (mediated by their weak intraspecific competition) or weak interspecific competition with W. acuminata. We found divergent impacts of the exotic species despite their similar functional traits. We demonstrate that a native species may benefit from interactions with an exotic "benefactor" species, highlighting the potential importance of positive interactions in invaded communities. Our findings underscore the necessity of considering neutral and positive interactions in addition to competition in understanding invasion dynamics in real plant communities.

Non-bee insects are important contributors to global crop pollination.

Wild and managed bees are well documented as effective pollinators of global crops of economic importance. However, the contributions by pollinators other than bees have been little explored despite their potential to contribute to crop production and stability in the face of environmental change. Non-bee pollinators include flies, beetles, moths, butterflies, wasps, ants, birds, and bats, among others. Here we focus on non-bee insects and synthesize 39 field studies from five continents that directly measured the crop pollination services provided by non-bees, honey bees, and other bees to compare the relative contributions of these taxa. Non-bees performed 25-50% of the total number of flower visits. Although non-bees were less effective pollinators than bees per flower visit, they made more visits; thus these two factors compensated for each other, resulting in pollination services rendered by non-bees that were similar to those provided by bees. In the subset of studies that measured fruit set, fruit set increased with non-bee insect visits independently of bee visitation rates, indicating that non-bee insects provide a unique benefit that is not provided by bees. We also show that non-bee insects are not as reliant as bees on the presence of remnant natural or seminatural habitat in the surrounding landscape. These results strongly suggest that non-bee insect pollinators play a significant role in global crop production and respond differently than bees to landscape structure, probably making their crop pollination services more robust to changes in land use. Non-bee insects provide a valuable service and provide potential insurance against bee population declines.

Native turncoats and indirect facilitation of species invasions.

At local scales, native species can resist invasion by feeding on and competing with would-be invasive species. However, this relationship tends to break down or reverse at larger scales. Here, we consider the role of native species as indirect facilitators of invasion and their potential role in this diversity-driven 'invasion paradox'. We coin the term 'native turncoats' to describe native facilitators of non-native species and identify eight ways they may indirectly facilitate species invasion. Some are commonly documented, while others, such as indirect interactions within competitive communities, are largely undocumented in an invasion context. Therefore, we use models to evaluate the likelihood that these competitive interactions influence invasions. We find that native turncoat effects increase with the number of resources and native species. Furthermore, our findings suggest the existence, abundance and effectiveness of native turncoats in a community could greatly influence invasion success at large scales.

The 'filtering' metaphor revisited: competition and environment jointly structure invasibility and coexistence.

'Filtering', or the reduction in species diversity that occurs because not all species can persist in all locations, is thought to unfold hierarchically, controlled by the environment at large scales and competition at small scales. However, the ecological effects of competition and the environment are not independent, and observational approaches preclude investigation into their interplay. We use a demographic approach with 30 plant species to experimentally test: (i) the effect of competition on species persistence in two soil moisture environments, and (ii) the effect of environmental conditions on mechanisms underlying competitive coexistence. We find that competitors cause differential species persistence across environments even when effects are lacking in the absence of competition, and that the traits which determine persistence depend on the competitive environment. If our study had been observational and trait-based, we would have erroneously concluded that the environment filters species with low biomass, shallow roots and small seeds. Changing environmental conditions generated idiosyncratic effects on coexistence outcomes, increasing competitive exclusion of some species while promoting coexistence of others. Our results highlight the importance of considering environmental filtering in the light of, rather than in isolation from, competition, and challenge community assembly models and approaches to projecting future species distributions.

Distinct invasion strategies operating within a natural annual plant system.

Alien plant species are known to have a wide range of impacts on recipient communities, from resident species' exclusions to coexistence with resident species. It remains unclear; however, if this variety of impacts is due to different invader strategies, features of recipient communities or both. To test this, we examined multiple plant invasions of a single ecosystem in southwestern Australia. We used extensive community data to calculate pairwise segregation between target alien species and many co-occurring species. We related segregation to species' positions along community trait hierarchies and identified at least two distinct invasion strategies: 'exploiters' which occupy high positions along key trait hierarchies and reduce local native species diversity (particularly in nutrient-enriched situations), and 'coexisters' who occupy intermediate trait positions and have no discernable impact on native diversity. We conclude that trait hierarchies, linked to measures of competition, can provide valuable insights about the processes driving different invasion outcomes.

Specific leaf area responses to environmental gradients through space and time.

Plant communities can respond to environmental changes by altering their species composition and by individuals (within species) adjusting their physiology. These responses can be captured by measuring key functional traits among and within species along important environmental gradients. Some anthropogenic changes (such as fertilizer runoff) are known to induce distinct community responses, but rarely have responses across natural and anthropogenic gradients been compared in the same system. In this study, we used comprehensive specific leaf area (SLA) data from a diverse Australian annual plant system to examine how individual species and whole communities respond to natural and anthropogenic gradients, and to climatically different growing seasons. We also investigated the influence of different leaf-sampling strategies on community-level results. Many species had similar mean SLA values but differed in SLA responses to spatial and temporal environmental variation. At the community scale, we identified distinct SLA responses to natural and anthropogenic gradients. Along anthropogenic gradients, increased mean SLA, coupled with SLA convergence, revealed evidence of competitive exclusion. This was further supported by the dominance of species turnover (vs. intraspecific variation) along these gradients. We also revealed strong temporal changes in SLA distributions in response to increasing growing-season precipitation. These climate-driven changes highlight differences among co-occurring species in their adaptive capacity to exploit abundant water resources during favorable seasons, differences that are likely to be important for species coexistence in this system. In relation to leaf-sampling strategies, we found that using leaves from a climatically different growing season can lead to misleading conclusions at the community scale.

A global quantitative synthesis of local and landscape effects on wild bee pollinators in agroecosystems.

Bees provide essential pollination services that are potentially affected both by local farm management and the surrounding landscape. To better understand these different factors, we modelled the relative effects of landscape composition (nesting and floral resources within foraging distances), landscape configuration (patch shape, interpatch connectivity and habitat aggregation) and farm management (organic vs. conventional and local-scale field diversity), and their interactions, on wild bee abundance and richness for 39 crop systems globally. Bee abundance and richness were higher in diversified and organic fields and in landscapes comprising more high-quality habitats; bee richness on conventional fields with low diversity benefited most from high-quality surrounding land cover. Landscape configuration effects were weak. Bee responses varied slightly by biome. Our synthesis reveals that pollinator persistence will depend on both the maintenance of high-quality habitats around farms and on local management practices that may offset impacts of intensive monoculture agriculture.

Differences in forest plant functional trait distributions across land-use and productivity gradients.

PREMISE OF STUDY: Plant functional traits are commonly used as proxies for plant responses to environmental challenges, yet few studies have explored how functional trait distributions differ across gradients of land-use change. By comparing trait distributions in intact forests with those across land-use change gradients, we can improve our understanding of the ways land-use change alters the diversity and functioning of plant communities. METHODS: We examined how the variation and distribution of trait values for seven plant functional traits differ between reference natural forest and three types of land-use conversion (pasture, old-field, or "legacy" sites-regrowth following logging), landscape productivity (NPP) and vegetation strata (tree or non-tree "understory"), in a meta-analysis of studies from 15 landscapes across five continents. KEY RESULTS: Although trait variation often differed between land-uses within a landscape, these patterns were rarely consistent across landscapes. The variance and distribution of traits were more likely to differ consistently between natural forest and land-use conversion categories for understory (non-tree) plants than for trees. Landscape productivity did not significantly alter the difference in trait variance between natural forest and land-use conversion categories for any trait except dispersal. CONCLUSIONS: Our results suggest that even for traits well linked to plant environmental response strategies, broad classes of land-use change and landscape productivity are not generally useful indicators of the mechanisms driving compositional changes in human-modified forest systems.

Ecology: Lifting the curtain on higher-order interactions.

Higher-order interactions - the modification of interactions between a species pair by a third - remain poorly understood in nature. A new study manipulates pairwise and higher-order interactions in the field, offering exciting new insights into how higher-order interactions contribute to coexistence.

Patterns of frequency and density dependence are highly variable in diverse annual flowering plant communities.

Applications of ecological theory to natural communities often assume that competitive, negative density-dependent processes are the only type of interaction important for diversity maintenance. Recent advances suggest that positive interactions within trophic levels (e.g., plant-plant) may also affect plant coexistence. Though positive plant-plant interactions theoretically might result in positive or nonmonotonic frequency or density dependence (FD/DD), less is known about how commonly these patterns occur or which ecological processes might result in such patterns in natural plant communities. In this study we tested for signals of variable frequency and density dependence in annual flowering plant communities in Western Australia and searched for evidence that interactions among plants during flowering might induce positive or nonmonotonic FD/DD in flowering plants. Using four common annual wildflower species, we ask if plant fecundity exhibited positive or nonmonotonic FD/DD and if pollinator-mediated plant-plant interactions during flowering change patterns of FD/DD relative to pollinator-independent plant interactions. Three species exhibited nonmonotonic (hump-shaped) density dependence, and only one species experienced strictly negative density dependence. Each species exhibited a different pattern of frequency dependence (positive, negative, weakly nonmonotonic, and no detectable frequency dependence). Pollinator-mediated plant-plant interactions during flowering induced both nonmonotonic density dependence and negative frequency dependence in one species. Importantly, the extent of variation in FD/DD observed in our study brings into question the dominance of negative density and frequency dependence in theory, suggesting instead that demographic responses of plants to their communities fall along a continuum of possible density- and frequency-dependent patterns.