Resource Gradients Research Articles

Optimal foraging and the information theory of gambling.

At a macroscopic level, part of the ant colony life cycle is simple: a colony collects resources; these resources are converted into more ants, and these ants in turn collect more resources. Because more ants collect more resources, this is a multiplicative process, and the expected logarithm of the amount of resources determines how successful the colony will be in the long run. Over 60 years ago, Kelly showed, using information theoretic techniques, that the rate of growth of resources for such a situation is optimized by a strategy of betting in proportion to the probability of pay-off. Thus, in the case of ants, the fraction of the colony foraging at a given location should be proportional to the probability that resources will be found there, a result widely applied in the mathematics of gambling. This theoretical optimum leads to predictions as to which collective ant movement strategies might have evolved. Here, we show how colony-level optimal foraging behaviour can be achieved by mapping movement to Markov chain Monte Carlo (MCMC) methods, specifically Hamiltonian Monte Carlo (HMC). This can be done by the ants following a (noisy) local measurement of the (logarithm of) resource probability gradient (possibly supplemented with momentum, i.e. a propensity to move in the same direction). This maps the problem of foraging (via the information theory of gambling, stochastic dynamics and techniques employed within Bayesian statistics to efficiently sample from probability distributions) to simple models of ant foraging behaviour. This identification has broad applicability, facilitates the application of information theory approaches to understand movement ecology and unifies insights from existing biomechanical, cognitive, random and optimality movement paradigms. At the cost of requiring ants to obtain (noisy) resource gradient information, we show that this model is both efficient and matches a number of characteristics of real ant exploration.

A unified framework for plant life-history strategies shaped by fire and herbivory.

Fire and herbivory both remove aboveground biomass. Environmental factors determine the type and intensity of these consumers globally, but the traits of plants can also alter their propensity to burn and the degree to which they are eaten. To understand plant life-history strategies associated with fire and herbivory we need to describe both response and effect functional traits, and how they sort within communities, along resource gradients, and across evolutionary timescales. Fire and herbivore functional traits are generally considered separately, but there are advances made in understanding fire that relate to herbivory, and vice versa. Moreover, fire and herbivory interact: the presence of one consumer affects the type and intensity of the other. Here, we present a unifying conceptual framework to understand plant strategies that enable tolerance and persistence to fire and herbivory. Using grasses as an example, we discuss how flammability and fire tolerance, palatability, and grazing tolerance traits might organize themselves in ecosystems exposed to these consumers, and how these traits might have evolved with reference to other strong selective processes, like aridity. Our framework can be used to predict both the diversity of life-history strategies and plant species diversity under different consumer regimes.

Does a tradeoff between trait plasticity and resource conservatism contribute to the maintenance of alternative stable states?

Phenotypic plasticity facilitates species persistence across resource gradients but may be limited in low-resource environments requiring resource conservation. We investigated the tradeoff between trait plasticity and resource conservatism across a biome boundary characterized by high turnover in nutrient and light availability, and whether this contributes to the maintenance of alternative stable states. Differences in plasticity were determined by comparing species' leaf and foliar nutritional trait responses to light, represented by leaf area index (LAI), and soil nutrient availability across forest-shrubland boundaries in South Africa. Although forest had higher LAI and soil nutrient availability than shrubland, forest species experienced greater resource variation. With increasing LAI and nutrient availability, forest species increased their leaf size, specific leaf area and leaf area/stem length, and decreased their foliar [N] and [K]. Although these responses are indicative of plasticity, shrubland species appeared to lack plasticity as evidenced by limited trait variation with environmental heterogeneity. Inhabiting diverse light environments imposed by forests probably selects for plasticity, whereas light-saturated, fire-prone, nutrient-poor environments that select for conservative leaf traits and below-ground investments compromise plasticity in shrubland species. This pattern suggests a tradeoff between trait plasticity and resource conservatism, which may support the stability of alternative vegetation states.

Environmental incomes sustained as provisioning ecosystem service availability declines along a woodland resource gradient in Zimbabwe

Abstract Forest and woodland resources can play a key role in rural livelihoods in the Global South, making it critical to understand what forest change could mean for rural wellbeing. Calculating environmental income has become a popular method of highlighting the importance of environmental resources in livelihoods, but few studies have quantified both provisioning ecosystem service availability and environmental income in the same landscape, or disaggregated environmental income by source land cover. This limits our ability to anticipate how forest change could impact rural livelihoods and could result in management interventions detrimental to vulnerable groups. The objective of this study was therefore to explore links between woodland cover, provisioning ecosystem service availability and household environmental income by applying a novel interdisciplinary approach in six villages on a gradient of woodland resource availability in Zimbabwe. We firstly use techniques from quantitative ethnobotany to score the species underpinning six locally important provisioning ecosystem services, and combine these scores with data from 80 tree survey plots to establish provisioning service availability. We then use income data from 91 households to explore relationships between provisioning service availability and household income portfolios. We find that villages with less woodland have lower availability of all studied ecosystem services and also a lower diversity of species underpinning service provision, but that there are no significant relationships between woodland resource availability and environmental income, livelihood diversity or intra-community income inequality in the case study area. We suggest that income portfolios are very resilient to woodland loss because households can still derive significant resources from woodlands which would be considered degraded in ecological terms and can draw upon kin networks which facilitate access to resources beyond village boundaries. The novel combination of approaches used in this study, particularly if applied at greater spatial and temporal scales, can provide valuable insight into the complexities of resource use in forest-agriculture mosaics.

Improved foraging by switching between diffusion and advection: benefits from movement that depends on spatial context

Animals use different modes of movement at different times, in different locations, and on different scales. Incorporating such context dependence in mathematical models represents a substantial increase in complexity, but creates an opportunity to more fully integrate key biological features. Here, we consider the spatial dynamics of a population of foragers with two subunits. In one subunit, foragers move via diffusion (random search) whereas in the other, foragers move via advection (gradient-following search). Foragers switch back and forth between the subunits as functions of their spatial context (i.e., depending on whether they are inside or outside of a patch, or depending on whether or not they can detect a gradient in resource density). We consider a one-dimensional binary landscape of resource patches and non-habitat and gauge success in terms of how well the mobile foragers overlap with the distribution of resources. Actively switching between dispersal modes can sometimes greatly enhance this spatial overlap relative to the spatial overlap possible when foragers merely blend advection and diffusion modes at all times. Switching between movement modes is most beneficial when organism’s gradient-following abilities are weak compared to its overall capacity for movement, but switching can actually be quite detrimental for organisms that can rapidly follow resource gradients. An organism’s perceptual range plays a critical role in determining the conditions under which switching movement modes benefits versus disadvantages foragers as they seek out resources.

Unimodal productivity-diversity relationships among bacterial communities in a simple polar soil ecosystem.

Unlike other macroecological principles, relationships between productivity and diversity have not been effectively tested for microbial communities. Here we describe an experiment in which the availability of resources to soil bacterial communities was manipulated in a model system, the McMurdo Dry Valleys of Antarctica. Mannitol additions were used to simulate a productivity gradient such that a response in bacterial biomass production, taxonomic diversity and functioning (e.g., enzyme activity) were induced. Resource amendment induced a positive linear response in microbial productivity (P < 0.001) but a unimodal (hump-shaped) response in microbial diversity at multiple taxonomic scales (P = 0.035). Putative oligotrophic (e.g., phyla Nitrospirae and Cyanobacteria) and copiotrophic (e.g., phylum Proteobacteria) taxa were apparent through substantial community turnover along the resource gradient. Soil enzyme activity was inversely related to bacterial biomass but positively related to diversity, suggesting the latter may be a stronger control over enzyme-mediated decomposition. The mechanisms behind this pattern are consistent with macroecological theory of a shift from environmental (e.g., stress tolerance) to biotic (e.g., competition) drivers with increasing resource availability. This evidence is among the first of its kind to document a significant unimodal productivity-diversity relationship for soil bacteria.

Interspecific homeostatic regulation and growth across aquatic invertebrate detritivores: a test of ecological stoichiometry theory.

Across resource quality gradients, primary consumers must regulate homeostasis and release of nutrients to optimize growth and fitness. Based primarily on internal body composition, the ecological stoichiometry theory (EST) offers a framework to generalize interspecific patterns of these responses, yet the predictions and underlying assumptions of EST remain poorly tested across many species. We used controlled laboratory feeding experiments to measure homeostasis, nutrient release, and growth across seven field-collected aquatic invertebrate detritivore taxa fed wide resource carbon:nitrogen (C:N) and carbon:phosphorus (C:P) gradients. We found that most invertebrates exhibited strict stoichiometric homeostasis (average 1/H = - 0.018 and 0.026 for C:N and C:P, respectively), supporting assumptions of EST. However, the stoichiometry of new tissue production during growth intervals (growth stoichiometry) deviated - 30 to + 54% and - 145 to + 74% from initial body C:N and C:P, respectively, and across species, growth stoichiometry was not correlated with initial body stoichiometry. Notably, smaller non- and hemimetabolous invertebrates exhibited low, decreasing growth C:N and C:P, whereas larger holometabolous invertebrates exhibited high, often increasing growth C:N and C:P. Despite predictions of EST, interspecific sensitivity of egestion stoichiometry and growth rates to the resource gradient were weakly related to internal body composition across species. While the sensitivity of these patterns differed across taxa, such differences carried a weak phylogenetic signal and were not well predicted by EST. Our findings suggest that traits beyond internal body composition, such as feeding behavior, selective assimilation, and ontogeny, are needed to generalize interspecific patterns in consumer growth and nutrient release across resource quality gradients.

Migration triggers in a large herbivore: Galápagos giant tortoises navigating resource gradients on volcanoes.

To understand how migratory behavior evolved and to predict how migratory species will respond to global environmental change it is important to quantify the fitness consequences of intra- and inter-individual variation in migratory behavior. Intra-individual variation includes behavioral responses to changing environmental conditions and hence behavioral plasticity in the context of novel or variable conditions. Inter-individual variation determines the degree of variation on which selection can act and the rate of evolutionary responses to changes in average and extreme environmental conditions. Here we focus on variation in the partial migratory behavior of giant Galápagos tortoises (Chelonoidis spp.) and its energetic consequences. We evaluate the extent and mechanisms by which tortoises adjust migration timing in response to varying annual environmental conditions, and integrate movement data within a bioenergetic model of tortoise migration to quantify the fitness consequences of migration timing. We find strong inter-individual variation in the timing of migration, which was not affected by environmental conditions prevailing at the time of migration but rather byaverage expectations estimated from multi-annual averaged conditions. This variation is associated with an average annual loss in efficiency of ~15% relative to optimal timing based on year-specific conditions. These results point towards a limited ability of tortoises to adjust the timing of their migrations based on prevailing (and, by extension, future) conditions, suggesting that the adaptability of tortoise migratory behavior to changing conditions is predicated more by past "normal" conditions than responses to prevailing, changing conditions. Our work offers insights into the level of environmental-tuning in migratory behavior and a general framework for future research across taxa.

Plant size is related to biomass partitioning and stress resistance in water-limited annual plant communities

Plant functional traits are used increasingly for linking environmental conditions, community structure and ecosystem function. Traits associated with rapid resource capture may come at the expense of those related to stress resistance. In annual plants, such a tradeoff may be reflected through plant size (biomass) vs. biomass partitioning patterns, as the former represents resource capture rate while the latter represents reorganization to cope with resource stress. In a water-limited annual plant community we investigated: 1) the effects of water and nitrogen availability on plant size and biomass partitioning, i.e., biomass partitioning between organs (root to shoot ratio, R/S, and reproductive effort, Rep/Veg), and biomass partitioning in space (specific leaf area, SLA), 2) the relationship between plant size and biomass partitioning at both the interspecific and intraspecific level, and 3) the relationships between plant size and biomass partitioning strategy and stress resistance. Eight key species varying in their natural size range were grown under variable water and nitrogen availability and their size and biomass partitioning were monitored throughout their growth and at fixed phenological phases. In all species, reduced water and/or nitrogen availability was associated with smaller size, increased R/S, and decreased SLA while Rep/Veg remained constant. At the intraspecific level, plant size was negatively related to R/S and positively related to SLA. At the interspecific level, no relationship was found between size or stress resistance and any of the biomass-partitioning traits or their plasticity. However, species size was negatively related to stress resistance. This tradeoff between species size and stress resistance emerges as a consistent property of fundamental importance in shaping annual plant communities along resource gradients.

Variation in Acer saccharum Marshall (Sugar Maple) Bark and Stemflow Characteristics: Implications for Epiphytic Bryophyte Communities

Past studies have demonstrated successions of epiphyte communities on trees of progressively larger diameters, suggesting the existence of temporal resource-gradients associated with aging/growing phorophyte hosts. The objective of this study was to identify possible resource gradients associated with the diameter/age of Acer saccharum (Sugar Maple) in northern New York. We determined epiphytic bryophyte cover by species on 102 Sugar Maples (min–max = 11–84 cm diameter at breast height) from 12 Adirondack northern hardwood forest stands. We extracted a 12.6-cm2 bark sample from each tree to analyze in the laboratory for moisture-holding capacity, surface-moisture availability, drying rates, and leachate cation and nitrogen concentrations. We collected throughfall and stemflow from 15 trees (19–79 cm dbh) at a separate site over the course of a growing season and analyzed samples for cation and nitrogen concentrations. Bark mass per unit surface area (g cm-2) was positively correlated with tree diameter, reflecting increasing bark thickness with age. Bark moisture-holding capacity (H2O as % dry mass) was independent of tree diameter, but bark surface-moisture availability (g H2O cm-2 bark) increased with diameter as a result of thickening bark. Bark drying rates were negatively correlated with bark mass (thickness). Cation (Ca2+, Mg2+, K+) concentrations in bark leachate were all positively correlated with tree diameter, but NH4+ and DON concentrations varied independently of tree diameter, and NO3- concentrations were typically below detection limits. Stemflow became enriched 10- to 20-fold with dissolved cations but not with dissolved nitrogen. Percent cover of several mesophytic and calciphilic epiphytes (e.g., Anomodon rugelii, Brachythecium laetum, Neckera pennata, and Porella platyphylla) were positively correlated with cation concentration in bark leachate, bark thickness, and moisture availability, and negatively correlated with bark drying rate. The results of this study are consistent with hypotheses that increased moisture and nutrient availability and slower drying rates of bark on large-diameter trees may account for increasing total cover and species richness of bryophtyes and increasing dominance of mesophytic and calciphilic bryophytes on larger trees. We extend McCune's similar gradient hypothesis with an analogous set of nutrient-based gradients, and offer an alternative mechanism for McCune's original time-based moisture gradient.

Resource fluctuation patterns influence emergent properties of phytoplankton assemblages and their resistance to harmful algal blooms

Recent advances in phytoplankton modelling have used species-rich, self-organising assemblages. These models have shown that phytoplankton with complementary life-history traits related to resource exploitation assemble into stable states of lumpy coexistence when resources fluctuate where species’ niches occur in clusters along resource gradients. They have also shown that a high degree of competitive dissimilarity between clusters arises, and that this relates to the incidence of monospecific blooms of allelochemical-producing taxa, i.e. some harmful algal bloom (HAB) species. These findings further suggest that the mode (sudden v. gradual changes) under which limiting resources fluctuate plays an important role in determining the emergent properties of the assemblage. For example, productivity, biodiversity and the number of species clusters (and, therefore, resistance to HABs) are all enhanced when switches in resource supplies are gradual, compared with when they are sudden. These theoretical findings, as well as others discussed herein, are of particular interest in watersheds where human activities, such as dam construction, have the capacity to dramatically alter natural-resource fluctuation patterns.

Functional traits of acquisitive invasive woody species differ from conservative invasive and native species

One of the most important sources of invasiveness is species’ functional traits and their variability. However there are still few studies on invasive tree species traits conducted along resource gradients that allow for a comparison of acquisitive and conservative strategies. We aimed to assess the differences in trait variation among native alien conservative and alien acquisitive tree species along resource availability gradients (soil fertility and light availability) and to assess the traits variability of the species studied along resources availability gradients. Our study compared invasive tree species in Europe (Prunusserotina Ehrh. Quercusrubra L. and Robiniapseudoacacia L.) with their native competitors (Acerpseudoplatanus L. A.platanoides L. Quercuspetraea (Matt.) Liebl. and Fagussylvatica L.). The study was conducted on 1329 seedlings and saplings collected in a system of 372 study plots in W Poland. For each individual we assessed leaf stem and root mass ratios total biomass leaf area ratio specific leaf area and projected leaf area. Two invasive species (P.serotina and R.pseudoacacia) represented a more acquisitive strategy than native species – along litter pH and light availability gradients these species had higher leaf mass fraction specific leaf area and leaf area ratio. In contrast Q.rubra had the highest total biomass and root mass fraction. Alien species usually had higher coefficients of variation of studied traits. This suggests that relatively high projected leaf area as a way of filling space and outcompeting native species may be reached in two ways – biomass allocation to leaves and control of leaf morphology or by overall growth rate. High variability of invasive species traits also suggests randomness in seedling survival which similarly to the neutral theory of invasion highlights the necessity of including randomness in modelling biological invasions.

Sounding the alarm: Defining thresholds to trigger a public health response to monkeypox.

Endemic to the Democratic Republic of the Congo (DRC), monkeypox is a zoonotic disease that causes smallpox-like illness in humans. Observed fluctuations in reported cases over time raises questions about when it is appropriate to mount a public health response, and what specific actions should be taken. We evaluated three different thresholds to differentiate between baseline and heightened disease incidence, and propose a novel, tiered algorithm for public health action. Monkeypox surveillance data from Tshuapa Province, 2011–2013, were used to calculate three different statistical thresholds: Cullen, c-sum, and a World Health Organization (WHO) method based on monthly incidence. When the observed cases exceeded the threshold for a given month, that month was considered to be ‘aberrant’. For each approach, the number of aberrant months detected was summed by year—each method produced vastly different results. The Cullen approach generated a number of aberrant signals over the period of consideration (9/36 months). The c-sum method was the most sensitive (30/36 months), followed by the WHO method (12/24 months). We conclude that triggering public health action based on signals detected by a single method may be inefficient and overly simplistic for monkeypox. We propose instead a response algorithm that integrates an objective threshold (WHO method) with contextual information about epidemiological and spatiotemporal links between suspected cases to determine whether a response should be operating under i) routine surveillance ii) alert status, or iii) outbreak status. This framework could be modified and adopted by national and zone level health workers in monkeypox-endemic countries. Lastly, we discuss considerations for selecting thresholds for monkeypox outbreaks across gradients of endemicity and public health resources.

Functional and phylogenetic diversity of bird assemblages are filtered by different biotic factors on tropical mountains

AbstractAimIt is not yet clear whether similar mechanisms influence the assembly of ecological communities across different continents. Here, we investigated the functional and phylogenetic diversity of bird assemblages along elevational gradients in two biogeographic regions in order to identify how these are driven by biotic factors, such as food resources and vegetation structure, and abiotic factors, such as ambient temperature and precipitation.LocationTwo 2,000‐m elevational gradients in the Ecuadorian Andes and on Mount Kilimanjaro, Tanzania.TaxonForest‐dwelling bird species.MethodsWe recorded bird species abundances on 18 and 30 plots in the Ecuadorian Andes and on Mount Kilimanjaro respectively. We measured 10 functional morphological traits, related to bird feeding and movement, and utilized bird phylogenies to compare observed values and null‐model corrected effect sizes of functional and phylogenetic diversity along elevational gradients and between biogeographic regions. Furthermore, we assessed how observed values and effect sizes of functional and phylogenetic diversity were associated with the underlying gradients in available food resources, vegetation structure, temperature and precipitation.ResultsFunctional and phylogenetic diversity were generally higher in species assemblages in the Ecuadorian Andes than on Mount Kilimanjaro. Both observed values and effect sizes of functional and phylogenetic diversity decreased significantly with increasing elevation in both biogeographic regions. Functional diversity consistently increased with increasing resource availability, whereas phylogenetic diversity increased with increasing vegetation heterogeneity and canopy closure in both biogeographic regions. Temperature and precipitation were not significantly associated with functional and phylogenetic diversity.Main conclusionsOur results suggest that in both mountain systems the diversity of functional traits in bird species assemblages is the result of environmental filtering by available food resources, whereas phylogenetic diversity is primarily limited by vegetation structure. These findings suggest important differences in the main drivers of functional and phylogenetic diversity. We conclude that biotic factors might be more important for driving bird diversity patterns than abiotic factors and that a loss of resource availability and vegetation structure, e.g., through human impacts, is likely to trigger changes in community assembly on tropical mountains.

Metaecosystem Dynamics of Marine Phytoplankton Alters Resource Use Efficiency along Stoichiometric Gradients.

Metaecosystem theory addresses the link between local (within habitats) and regional (between habitats) dynamics by simultaneously analyzing spatial community ecology and abiotic matter flow. Here we experimentally address how spatial resource gradients and connectivity affect resource use efficiency (RUE) and stoichiometry in marine phytoplankton as well as the community composition at local and regional scales. We created gradostat metaecosystems consisting of five linearly interconnected patches, which were arranged either in countercurrent gradients of nitrogen (N) and phosphorus (P) supply or with a uniform spatial distribution of nutrients and which had either low or high connectivity. Gradient metaecosystems were characterized by higher remaining N and P concentrations (and N∶P ratios) than uniform ones, a difference reduced by higher connectivity. The position of the patch in the gradient strongly constrained elemental stoichiometry, local biovolume production, and RUE. As expected, algal carbon (C)∶N, biovolume, and N-specific RUE decreased toward the N-rich end of the gradient metaecosystem, whereas the opposite was observed for most of the gradient for C∶P, N∶P, and P-specific RUE. However, at highest N∶P supply, unexpectedly low C∶P, N∶P, and P-specific RUE values were found, indicating that the low availability of P inhibited efficient use of N and biovolume production. Consequently, gradient metaecosystems had lower overall biovolume at the regional scale. Whereas treatment effects on local richness were weak, gradients were characterized by higher dissimilarity in species composition. Thus, the stoichiometry of resource supply and spatial connectivity between patches appeared as decisive elements constraining phytoplankton composition and functioning in metaecosystems.

Vertical patterns of photosynthesis and related leaf traits in two contrasting agricultural crops.

To include within-canopy leaf acclimation responses to light and other resource gradients in photosynthesis modelling, it is imperative to understand the variation of leaf structural, biochemical and physiological traits from canopy top to bottom. In the present study, leaf photosynthetic traits for top and bottom canopy leaves, canopy structure and light profiles, were measured over one growing season for two contrasting crop types, winter barley (Hordeum vulgare L.) and rape seed (Brassica napus L.). With the exception of quantum yield, other traits such as maximum photosynthetic capacity (Amax), dark respiration, leaf nitrogen and chlorophyll contents, and leaf mass per area, showed consistently higher (P<0.05) values for top leaves throughout the growing season and for both crop types. Even though Amax was higher for top leaves, the bottom half of the canopy intercepted more light and thus contributed the most to total canopy photosynthesis up until senescence set in. Incorporating this knowledge into a simple top/bottom-leaf upscaling scheme, separating top and bottom leaves, resulted in a better match between estimated and measured total canopy photosynthesis, compared with a one-leaf upscaling scheme. Moreover, aggregating to daily and weekly temporal resolutions progressively increased the linearity of the leaf photosynthetic responses to light for top leaves.

Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands

Despite growing recognition of the conservation values of grassy biomes, our understanding of how to maintain and restore biodiverse tropical grasslands (including savannas and open-canopy grassy woodlands) remains limited. To incorporate grasslands into large-scale restoration efforts, we synthesised existing ecological knowledge of tropical grassland resilience and approaches to plant community restoration. Tropical grassland plant communities are resilient to, and often dependent on, the endogenous disturbances with which they evolved-frequent fires and native megafaunal herbivory. In stark contrast, tropical grasslands are extremely vulnerable to human-caused exogenous disturbances, particularly those that alter soils and destroy belowground biomass (e.g. tillage agriculture, surface mining); tropical grassland restoration after severe soil disturbances is expensive and rarely achieves management targets. Where grasslands have been degraded by altered disturbance regimes (e.g. fire exclusion), exotic plant invasions, or afforestation, restoration efforts can recreate vegetation structure (i.e. historical tree density and herbaceous ground cover), but species-diverse plant communities, including endemic species, are slow to recover. Complicating plant-community restoration efforts, many tropical grassland species, particularly those that invest in underground storage organs, are difficult to propagate and re-establish. To guide restoration decisions, we draw on the old-growth grassland concept, the novel ecosystem concept, and theory regarding tree cover along resource gradients in savannas to propose a conceptual framework that classifies tropical grasslands into three broad ecosystem states. These states are: (1) old-growth grasslands (i.e. ancient, biodiverse grassy ecosystems), where management should focus on the maintenance of disturbance regimes; (2) hybrid grasslands, where restoration should emphasise a return towards the old-growth state; and (3) novel ecosystems, where the magnitude of environmental change (i.e. a shift to an alternative ecosystem state) or the socioecological context preclude a return to historical conditions.

Bacteria and Competing Herbivores Weaken Top-Down and Bottom-Up Aphid Suppression.

Herbivore suppression is mediated by both plant defenses and predators. In turn, plant defenses are impacted by soil fertility and interactions with soil bacteria. Measuring the relative importance of nutritional and microbial drivers of herbivore resistance has proven problematic, in part because it is difficult to manipulate soil-bacterial community composition. Here, we exploit variation in soil fertility and microbial biodiversity across 20 farms to untangle suppression of aphids (Brevicoryne brassicae) through bottom–up and top–down channels. We planted Brassica oleracea plants in soil from each farm, manipulated single and dual infestations of aphids alone or with caterpillars (Pieris rapae), and exposed aphids to parasitoid wasps (Diaeretiella rapae) in the open field. We then used multi-model inference to identify the strongest soil-based predictors of herbivore growth and parasitism. We found that densities of Bacillus spp., a genus known to include plant-growth-promoting rhizobacteria, negatively correlated with aphid suppression by specialist parasitoids. Aphid parasitism also was disrupted on plants that had caterpillar damage, compared to plants attacked only by aphids. Relative abundance of Pseudomonas spp. bacteria correlated with higher aphid growth, although this appeared to be a direct effect, as aphid parasitism was not associated with this group of bacteria. Non-pathogenic soil bacteria are often shown to deliver benefits to plants, improving plant nutrition and the deployment of anti-herbivore defenses. However, our results suggest that these plant growth-promoting bacteria may also indirectly weaken top–down aphid suppression by parasitoids and directly improve aphid performance. Against a background of varying soil fertility, microbial biodiversity, competing herbivores, and natural enemies, we found that effects of non-pathogenic soil microbes on aphid growth outweighed those of nutritional factors. Therefore, predictions about the strength of plant defenses along resource gradients must be expanded to include microbial associates.

Seed mass diversity along resource gradients: the role of allometric growth rate and size-asymmetric competition.

The large variation in seed mass among species inspired a vast array of theoretical and empirical research attempting to explain this variation. So far, seed mass variation was investigated by two classes of studies. One class focuses on species varying in seed mass within communities, while the second focuses on variation between communities, most often with respect to resource gradients. Here, we develop a model capable of simultaneously explaining variation in seed mass within and between communities. The model describes resource competition (for both soil and light resources) in annual communities and incorporates two fundamental aspects: light asymmetry (higher light acquisition per unit biomass for larger individuals) and growth allometry (negative dependency of relative growth rate on plant biomass). Results show that both factors are critical in determining patterns of seed mass variation. In general, growth allometry increases the reproductive success of small-seeded species while light asymmetry increases the reproductive success of large-seeded species. Increasing availability of soil resources increases light competition, thereby increasing the reproductive success of large-seeded species and ultimately the community (weighted) mean seed mass. An unexpected prediction of the model is that maximum variation in community seed mass (a measure of functional diversity) occurs under intermediate levels of soil resources. Extensions of the model incorporating size-dependent seed survival and disturbance also show patterns consistent with empirical observations. These overall results suggest that the mechanisms captured by the model are important in determining patterns of species and functional diversity.

Direct and indirect effects of interspecific competition in a highly partitioned guild of reef fishes

AbstractCompetitive interactions and resource partitioning facilitate species coexistence in complex ecosystems. However, while pairwise interactions between ecologically similar species have been well studied, multi‐species competitive networks have received less attention. When interference competition between two species results in partitioning of resources, this may have indirect consequences for other species distributed along the same resource gradient. Here, we tested whether interference competition between two territorial damselfish influenced the fine‐scale species distributions of five other territorial damselfish in Kimbe Bay, Papua New Guinea. These species partition habitat across three reef zones—the flat, crest, and slope, with distinct patterns of distribution within these zones. We predicted the two species with similar distributions and microhabitat use, Pomacentrus adelus and Pomacentrus bankanensis, would display the greatest level of aggression toward one another. This was tested through an intruder experiment where stimulus fish were introduced into a resident's territory, which confirmed disproportionately high levels of interspecific aggression between these two species. We also predicted that the fine‐scale differences in the distribution of each species were maintained through multi‐species interference competition among neighboring species, with further indirect effects on species that did not directly interact. To test this, we conducted a large‐scale (22 × 10 m) experimental removal of the most abundant species, Po. adelus, and quantified the abundance and distribution of all territorial damselfish species for 6 months to a 25 cm resolution. The main direct competitor, Po. bankanensis, exhibited a marked increase in abundance and expanded its distribution (+1.33 m) to acquire the space previously occupied by Po. adelus. This competitive release triggered indirect effects on the distribution of other neighboring species further back on the reef flat, with Chrysiptera unimaculata moving into the zone formerly occupied by Po. bankanensis. This study indicates that the distinct distribution patterns among the reef crest species are linked to levels of interspecific agonistic behavior. We argue that the competitive release following the removal of a superior competitor resulted in both direct and indirect effects, with the immediate neighbor shifting into the newly available space, followed by successive shifts in species responding to the change in the distributions of their immediate neighbors.