Functional Ecology

O_LIGlobal warming is reducing prey availability in many aquatic systems, raising questions about the combined effects of higher temperatures and lower food availability on fish life histories and reproductive output. C_LIO_LIIn ectotherms, higher temperatures accelerate growth and promote an earlier onset of reproduction. However, when fish have less food during development, resource depletion might constrain these temperature-driven processes. C_LIO_LIWe manipulated water temperature (24 or 28{degrees}C) and early-life food availability (control or restricted) for female guppies (Poecilia reticulata). We measured how both factors affected key life history traits (growth, reproduction, survival, self-maintenance). C_LIO_LIHigher temperature significantly affected female life histories. Females at 28{degrees}C matured at a larger size, but then grew more slowly and produced fewer, smaller offspring than females at 24{degrees}C. The effect of temperature on reproduction persisted even after controlling for body size, suggesting there was a shift in the fecundity-size relationship. C_LIO_LIAdult mortality was greater at 28{degrees}C. Higher temperature also resulted in a longer gut, potentially enhancing resource acquisition, but a higher temperature did not affect immunity or telomere length of the surviving females. C_LIO_LIEarly-life food shortage affected very few traits, except for a weak interaction with temperature that affected total fecundity. At 28{degrees}C, females that experienced early-life food restriction produced fewer offspring than females with continual food supply. No such diet effect occurred at 24{degrees}C. C_LIO_LIOur results suggest that tropical fish may be severely impacted by increased temperatures (i.e., decreased reproduction with increased morality), but are likely to be resilient to brief periods of food limitations during early development. C_LIO_LIInterestingly, early-life food shortage caused a reduction in total offspring number but only at 28{degrees}C, suggesting that global prey decline might exacerbate the negative effects of a warming climate on stock recruitment of tropical fish. C_LI

Species expand their spatial distribution when environmental conditions are favorable or when mutations arise that allow them to live in previously unfavorable conditions. The European wasp spider, Argiope bruennichi, is known to have expanded its range poleward faster than climate change would predict. Northern edge populations show higher cold tolerance and are genetically differentiated from core populations, suggesting local adaptation to colder winter conditions. To investigate the degree and limits of plasticity in a cold-adapted population, we exposed overwintering juveniles (spiderlings) from Estonia - the northern edge of the distribution - to three winter regimes: two with a strong difference in day/night temperatures and an overall 10 degrees difference (warm and cold treatment) and one with moderate temperatures and less difference between day and night (moderate). We investigated if survival, lipid content, metabolites, and gene expression patterns differ depending on these temperature regimes. The survival probability of the spiderlings and their overall lipid content decreased over winter, with no difference between treatments, suggesting high resilience of the spiderlings towards very different temperature regimes at the edge of the distribution. At the end of winter, the content of saturated and monounsaturated fatty acids per spiderling also did not differ between treatments. However, omega-3 polyunsaturated fatty acids (PUFAs) levels were significantly lower in spiders exposed to the warm winter suggesting increased metabolic activity. We identified 4096 significant differentially expressed genes (DEGs) across the treatments, of which 1389 were specific for the moderate treatment, and 832 specific for the warm treatment, while 69 were unique for the cold treatment, showing a stronger temperature stress response to the moderate and warmer than to the cold treatment. Taken together, our results show that A. bruennichi has physiological plasticity and the ability to cope with very different winter temperature regimes despite being cold adapted. However, warmer winters might come with metabolic costs that could impact on the spiderling[s] survival and foraging success when they emerge from the egg sac in spring.

Extreme weather events, such as heatwaves, pose a concerning threat to global biodiversity, particularly to terrestrial ectotherms like insects. While lethal effects of thermal stress are widely studied, very little is known about sublethal carry-over effects of heatwaves experienced during development on adult fitness. Addressing this fundamental knowledge gap is crucial for key pollinators such as bumblebees, which face alarming population declines. Therefore, we used a highly controlled in vitro approach to expose Bombus terrestris pupae to ecologically relevant cyclic thermal stress, and studied their emergence success, subsequent adult longevity, and morphological traits. We found that cyclic thermal stress caused significant acute pupal mortality (up to 30% reduction in emergence). Most notably, this developmental stress resulted in deferred mortality, significantly reducing adult longevity (HR = 1.81) in both workers and males. Furthermore, we identified antennae and wing deformations as a powerful hazard indicator (HR = 2.50) that strongly predicts premature adult mortality. Our findings reveal that cyclic thermal stress during pupation imposes irreversible developmental damage that undermines adult physiological resilience. We argue that life-stage specific carry-over effects should be considered more in the future to avoid underestimating the long-term impact of climate change on crucial insect pollinator populations.

Both temperature and community complexity are known to affect trophic interaction strength (TIS), but whether their effect is additive or not is as yet an open question. Here we used a 2-predator/3-prey system consistently occurring in avocado orchards to explore the effects of increasing warming and community complexity on the strength of predator:prey trophic interactions. The two predator species differed in their diet breath (a carnivore and an omnivore) and the prey types included a herbivore, heterospecific juvenile predators, and/or pollen. Overall, analyses revealed multiplicative effects of community complexity and both temperature and predator diet breath on the net predator:prey(s) TIS. Indeed, warming led to increased TIS in the community with omnivore as top predator, but only in absence of its preferred food source. When the carnivore was the top predator, in contrast, higher temperatures led to TIS being more negative, but only for the IGprey. We conclude that sources of context dependence in specific systems need to be identified to unveil effects of warming on communities.

Winter survival in dormant animals depends on conserving finite energy reserves, yet winter temperatures fluctuate around shifting means. In ectotherms, metabolic rate increases exponentially with temperature, so thermal variability is expected to accelerate energy loss, with important consequences for overwinter survival and population persistence under climate change. However, it remains unclear whether dormant ectotherms can compensate physiologically for thermal variability. We overwintered Bombus impatiens queens under constant (2, 3, 4{degrees}C) or variable (2 {+/-} 6{degrees}C or 4 {+/-} 6{degrees}C) regimes for six weeks, then measured metabolic rates across a range of temperatures. The temperature dependence of metabolic rate shifted in response to thermal experience, but the direction of compensation depended on mean temperature: variability centered on 2{degrees}C elevated metabolic rate and increased thermal sensitivity relative to all other conditions, whereas variability centered on 4{degrees}C reduced metabolic rate and dampened thermal sensitivity relative to constant 4{degrees}C. We used these metabolic responses to simulate rates of lipid depletion and found that survival trajectories echoed physiological shifts: experiencing variability around 2{degrees}C would reduce subsequent survival time, whereas experiencing variability around 4{degrees}C would preserve subsequent survival even under variable future conditions. Thus, identical thermal variance produced opposite energetic outcomes depending on the mean temperature around which fluctuations occurred. Integrating both temperature means and variability is, therefore, essential for predicting overwintering survival in a changing world.

Small juvenile and larval teleosts are typically more susceptible to starvation and predation, so the largest individuals tend to survive. However, most support for this bigger is better hypothesis is experimental and does not account for the variation in food availability, competition, and other conditions that alter how starvation and predation affect small fish. To assess how natural populations experience size-selective mortality, we reviewed the past 30 years of literature on the subject while compiling 76 effect sizes of longitudinal survival data, which evaluates selection against size classes, to test for evidence that bigger is better. Our meta-analysis shows that the effect of body size on survival is consistently weak across species, populations, times, and locations. We discuss several reasons why larger young teleosts may not have a distinct survival advantage. We argue that they: 1) may be more profitable or noticeable to predators, 2) are unable to outgrow all their predators, and 3) require more resources, increasing their predator exposure out of necessity to forage. We recommend that mortality not be treated as constant across young teleost size classes, as different ecological conditions may favour smaller, larger, or neither size class. We suggest that the relationship between metabolic scope and mortality, geographic gradients in the importance of predation and starvation, and the interactive effects of alternative sources of mortality with predation and starvation in warming oceans are underexplored and could change how we think about the relationship between young teleost size and survival. Investigating these drivers will be important as temperatures rapidly rise, altering growth in wild fish.

Earth is currently facing unprecedented global changes, hurrying scientists to provide predictive tools to explore the futures responses of ecosystems. Among those changes, temperature increase and alterations of nutrient availabilities largely drive consumer performances, yet their interactive effect remains poorly understood. Here we investigate how the dietary C:P ratio that optimizes consumer growth (TERC:P: Threshold Elemental Ratio) changes along temperature gradients by combining a TERC:P model and growth experiments on the model organism Daphnia magna. Both lines of evidence show that the TERC:P responds to temperature in an U-shaped fashion. This shape reconciles previous contradictive observations into a common framework, thereby improving our capacity to forecast the combined effects of nutrient cycle and climatic alterations on ectotherms.

Thermal adaptation in ectotherms is a critical response to global warming. Resting metabolic rate (RMR), reflecting the energy required for body maintenance, is a key factor of thermal adaptation due to its fundamental role in regulating energy acquisition and allocation within other biological functions. Yet, the importance of thermal adaptation of RMR in controlling individual response to warming still requires to be elucidated. Here, we investigated physiological mechanisms underlying thermal adaptation in two Daphnia magna lines with contrasting thermal preferences, focusing on RMR but also energy use efficiency (EUE) over a large thermal gradient. Using structural equation modelling we showed that adaptation of EUE between the cold and warm lines was able to mitigate the consequences of RMR thermal variability on individuals growth rate. These findings highlight the pivotal role of EUE in ectotherm thermal adaptation, offering key insights to predict the metabolic response of these ectotherms to climate change. Under a context of global warming, ectotherms metabolic response is critical as energy balance and allocation within individuals have repercussions on energy transfers within food web. Accurate predictions of metabolic thermal response of ectotherms may thus constitute an important step toward predicting ecosystem functioning under warming.

O_LIAnimals ranging from mosquitoes to humans often vary their feeding behavior when infected or merely exposed to pathogens. For example, some individuals drastically reduce their food intake ( illness-mediated anorexia) while others increase food intake ( hyperphagia). While these so-called sickness behaviors are well documented, their functional consequences remain poorly resolved. C_LIO_LIHere, we examine links between natural genetic variation in susceptibility to infection, feeding behaviors, multiple traits of the host, and within-host pathogen production. Using a zooplankton host (Daphnia dentifera) and a fungal pathogen (Metschnikowia bicuspidata) as a case study, we show that genotypic and dose-dependent variation in feeding behaviors are associated with both resistance and tolerance mechanisms. C_LIO_LIIn one genotype, immune-mediated anorexia was associated with increased tolerance to infection; unlike other genotypes, these individuals did not upregulate phenoloxidase activity, but lived longer, had the highest overall fecundity, and produced higher pathogen loads, despite their reduced growth rates and resultant smaller body sizes. In these hosts, peak parasite load remained unchanged, suggesting a tolerance mechanism that offset fecundity costs. C_LIO_LIIn other genotypes, feeding behaviors followed either a flat or hump-shaped pattern with pathogen dose, exhibiting hyperphagia at intermediate doses and anorexia at higher doses. In these cases, anorexia functioned primarily in resistance. C_LIO_LIOur results suggest that infection-mediated changes in host feeding behavior -- which are traditionally interpreted as immunopathology -- may in fact serve as crucial components of host defense strategies. Moreover, these phenomena vary across host genotypes, and were associated with apparent trade-offs with another melanization component of immune defense. Together, these results underscore that while resistance and tolerance are typically viewed as alternative and fixed defense strategies, the immense genetic diversity for immune defense may result in more of a plastic spectrum spanning a gradient from resistance to tolerance. C_LI

Body size is a key trait that influences ecological processes such as metabolism, abundance, and species interactions. While the metabolic theory of ecology (MTE) proposes a universal scaling of metabolic rate with body mass, recent evidence shows that this relationship is not fixed. Environmental factors like temperature and predation can alter the metabolic scaling exponent, potentially reshaping size distribution. However, most research has examined these patterns within individual species, leaving open questions about how environmental drivers affect scaling at the community level. To address this, we performed a mesocosm experiment manipulating both temperature and fish predator presence in freshwater macroinvertebrate communities. We found that metabolic scaling at the community level is highly responsive to environmental context: warming steepened the scaling exponent in predator-free tanks but flattened it when predators were present. This suggests that larger individuals reduce their baseline metabolic rates under predation risk, especially at higher temperatures. Interestingly, the slope of the community size distribution remained stable across treatments, indicating that shifts in metabolic scaling occurred independently of changes in size structure. Together, these findings highlight the environmental sensitivity of metabolic scaling and suggest that links between metabolism scaling and size distribution may be more complex than MTE predicts.

Early and late life environments can interact in complex ways to influence the fitness of individuals. Most studies investigating effects of the environment on fitness focus on environments experienced and traits expressed at a single point in an organisms life. However, environments vary with time, thus the environments organisms experience at different ages may interact to affect how traits change throughout life. Here, we test whether thermal stress experienced during development leads individuals to cope better with thermal stress as adults. We manipulated temperature during both development and adulthood and measured a range of life-history traits, including senescence, in male and female seed beetles, Callosobruchus maculatus. We found that favourable developmental conditions increased reproductive performance of females (i.e. silver-spoon effects). In contrast, non-reproductive traits such as lifespan and survival senescence were only affected by adult environments- high adult temperatures decreased longevity and survival. Additionally, developmental and adult environments interacted to affect age-dependent changes in male weight. Overall, our results show that effects of early and late environments can be both sex- and trait- specific, and that a full understanding of how environments interact to affect fitness and ageing requires the integrated study of conditions experienced during different stages of ontogeny.

Alternative reproductive tactics (ARTs) are discrete polymorphisms that help maximise reproductive success. Although flexible ARTs are ubiquitous, theoretical predictions for why flexible ARTs persist over evolutionary time have rarely been empirically tested. We hypothesised that flexible ARTs will persist if they have equal fitness benefits under a range of ecological contexts, or, there are trade-offs between ARTs in different ecological contexts and individuals display the most optimal phenotype in a context-dependent manner. Specifically, we investigated predation risk effects on the expression and fitness consequences of two flexible ARTs: acoustic signalling and being silent, expressed by tree cricket Oecanthus henryi males. In large outdoor enclosures, we exposed natural populations of O. henryi to three different abundances of their predator, the green lynx spider Peucetia viridans. Behavioural observations across successive nights revealed that higher predation risk did not alter the expression levels of the male ARTs, despite crickets experiencing differential risk and survival across treatments. Male crickets demonstrated an equal likelihood of calling or remaining silent on a night. ARTs resulted in similar mating success across differential predation risk, supporting the hypothesis that equal fitness benefits of flexible ARTs under a range of ecological contexts explain their persistence.

The nutritional traits of herbivores affect demographic rates and regulate nutrient and energy fluxes among trophic levels. Herbivore nutritional requirements, and the nutrient contents of herbivore biomass, depend on temperature - at temperature extremes, more nutritious food is required to maximize growth rate and herbivore biomass contains fewer nutrients. Yet, the consequences of these thermal responses for the population dynamics of herbivore-autotroph systems have not been explored. Here, we develop and analyze a stoichiometrically-explicit, temperature-dependent model of herbivore-autotroph systems to answer the question: How does the thermal response of herbivore nutritional traits affect population responses to temperature and nutrient (phosphorus) supply? We find that temperature-dependent herbivore nutritional traits restrict the range of temperatures at which herbivore populations persist, reduce the stability of population dynamics at high phosphorus supplies, and limit the herbivores capacity to control autotroph population density. These results reflect temperature-dependent changes in the herbivores sensitivity to nutrient-poor autotroph biomass and ability to retain nutrients in biomass (and thereby dilute autotroph nutrient contents). The thermal response of herbivore nutritional traits may therefore be an important factor influencing population and community responses to warming and nutrient enrichment.

Microbial populations play a pivotal role in ecosystem-level responses to rising temperatures and both their ecology and evolution can be directly influenced by warming. However, predicting microbial evolution and its ecological consequences is challenging because different genotypes within a population might respond uniquely to shifts in the abiotic and biotic environment. To understand how, we quantified evolutionary and ecological responses across temperatures in a protist of wide geographic distribution in the presence and absence of other microbial species with whom they interact (i.e., heterospecifics). In the absence of heterospecifics, we found that intraspecific interactions and warming selected in favor of a particular genotype, reducing genotypic diversity. In the presence of heterospecifics, 1) genetic diversity was further reduced under warming, resulting in temperature-dependent selection; but, 2) the magnitude of this change depended on the sign (+, 0, -) of the net ecological effect on the focal species by the heterospecifics, and this effect was itself temperature-dependent. Together, our results demonstrate that both intra- and interspecific interactions can mediate how temperature shapes microbial population rapid evolutionary responses, underscoring the importance of the ecological context in predicting evolutionary outcomes under climate change.

Global warming is reshaping food webs globally. Rapid evolution has been proposed as a buffer against climate change, but how simultaneous shifts in biotic and abiotic environments may influence evolution is unknown. Using experimental evolution and mathematical modeling in microbial food webs of prey algae and ciliate predators, we tested 1) how temperature affects prey evolution and 2) how the food-web context--i.e., predator identity, abundance, and competition among predators-- mediates prey evolutionary dynamics. We found that temperature alone does not drive prey evolution unless predators are present, and food-web context determines ensuing evolutionary dynamics. These seemingly complex evolutionary responses are predictable from the joint effects of temperature-dependent, predator-specific predation rates, and the emergence of temperature-dependent prey plasticity. We reveal that evolutionary outcomes under warming are shaped by the broader food web context of species, suggesting that the same species may exhibit different eco-evolutionary responses in different food webs under novel climates. SIGNIFICANCEPredicting how species evolve under climate change is critical for understanding future changes in food webs. Evolutionary responses have long been known to be driven by environmental change--like temperature--but whether and how ecological interactions influence this process is unknown. Using experimental evolution and mathematical modeling, we show that temperature alone does not drive prey evolution. Instead, the broader food webs context--predator identity, abundance, and competition--mediates how species evolve under warming. Additionally, we demonstrate that prey evolution depends on temperature-dependent predator-specific predation rates and prey plasticity. Our findings highlight that the same species may evolve differently within different food webs, urging the need to integrate ecological interactions when forecasting evolutionary responses to climate change.

Animal stoichiometry affects fundamental processes ranging from organismal physiology to global element cycles. However, it is unknown whether animal stoichiometry follows predictable scaling relationships with body mass and whether adaptation to life on land or water constrains patterns of elemental allocation. To test both interspecific and intraspecific body-size scaling relationships of the nitrogen (N), phosphorus (P), and N:P content of animals, we used a subset of the StoichLife database encompassing 9,933 individual animals (vertebrates and invertebrates) belonging to 1,543 species spanning 10 orders of magnitude of body size from terrestrial, freshwater, and marine realms. Across species, body mass did not explain much variation in %N and %P composition, although the %P of invertebrates decreased with size. The effects of body size on species elemental content were small in comparison to the effects of taxonomy. Body size was a better predictor of intraspecific than interspecific elemental patterns. Between 42 to 45% in intraspecific stoichiometric variation was explained by body size for 27% of vertebrate species and 35% of invertebrate species. Further, differences between organisms inhabiting aquatic and terrestrial realms were observed only in invertebrate interspecific %N, suggesting that the realm does not play an important role in determining elemental allocation of animals. Based on our analysis of the most comprehensive animal stoichiometry database, we conclude that (i) both body size and realm are relatively weak predictors of animal stoichiometry across taxa, and (ii) body size is a good predictor of intraspecific variation in animal elemental content, which is consistent with tissue-scaling relationships that hold broadly across large groups of animals. This research reveals a lack of general scaling patterns in the elemental content across animals and instead points to a large variation in scaling relationships within and among lineages.

O_LIChill-susceptible insects such as Drosophila are vulnerable to progressive disruption of ion and water homeostasis during cold stress, and low temperature exposure is a key factor affecting their physiology and distribution. Comparative studies of cold tolerance traditionally use simple or single-condition assays for interspecific comparisons, but the emergence of thermal death time (TDT) models offers a comprehensive framework to assess cold tolerance across different stress intensities and durations. C_LIO_LIHere we construct TDT curves for six Drosophila species, spanning boreal to tropical habitats, using Lt50 estimates across a range of stressful low temperatures (Lt50 ranging from [~] 20 min to 2 days). For all species, the TDT curves provided good fits to the log(Lt50) vs. temperature data (R2 = 0.87 - 0.99). C_LIO_LITDT curves from all species had steep slopes demonstrating that cold injury rate has a high thermal sensitivity such that small changes in temperature have profound effects on survival duration. The interspecific similarity of TDT slopes indicates that a conserved physiological dysfunction underlies cold injury across species. Further, additive accumulation of cold-induced injury in split-dose experiments suggests that acute and moderate cold damage represent the same underlying physiological dysfunction occurring at different rates. C_LIO_LIThe TDT curve intercepts (species-specific tolerance thresholds) differed markedly between boreal, temperate, and tropical species and correlated strongly with their habitat temperature. Data from the present study and meta-analysis of published data find that the inherent species cold tolerance decreases by [~] 0.45 {degrees}C for each {degrees}C colder the winter environment of the species is. When also considering the cold acclimation cues in cold climates we argue the experienced level of cold stress intensity is similar across environments inhabited by the Drosophila genus. This suggests that cold tolerance is important in shaping the fundamental niche of both boreal and tropical species. C_LIO_LIOverall, the TDT analysis of Drosophila at low temperature provides a powerful and predictive tool for quantifying insect cold tolerance. This approach enables detailed cross-species comparisons that allows for both ecological and physiological inference. Thus, TDT curves offer relevant approximations of insect cold resistance that could help predict insect responses to climatic change. C_LI

The temperature-dependencies of life history traits are increasingly being used to predict how climatic warming will affect vector-borne disease dynamics, partially by affecting the abundance dynamics of the vector population. Such predictions generally arise from mathematical models that incorporate the temperature dependence of traits measured under laboratory conditions. These temperature-trait relationships are typically estimated from juvenile populations reared under optimal resource conditions, even though natural populations experience intermittent resource depletion. Using laboratory experiments on the mosquito Aedes aegypti, combined with a stage-structured population model, we show that resource depletion in the juvenile habitat can significantly depress the vectors maximal population growth rate (rm) across the entire temperature range, cause it to peak at a lower temperature, and narrow its thermal niche width. Our results provide compelling evidence for future studies to consider resource depletion when predicting the effects of global change on vector-borne disease transmission, disease vectors and other arthropods.

Urbanization creates novel environments that can drive phenotypic and behavioural responses, yet how multiple traits respond across spatial scales remains poorly understood. In particular, elevated ambient temperatures via the urban heat island effect may drive morphological and behavioural responses. We investigated body size, abdominal colouration, microhabitat use, behavioural thermoregulation and thermal offset relative to ambient air in the orb-weaving spider Araneus diadematus across rural-urban gradients in northern Belgium. Contrary to predictions from the temperature-size rule, body size increased with urbanization at large spatial scales, whereas size-corrected abdomen area--reflecting body condition and reproductive investment--declined with urbanization, with strongest support at local spatial scales. Abdominal colouration showed no response to urbanization despite evidence for both carotenoid-like pigments and melanin-associated structures. Nevertheless, body size and colouration covaried, with sites containing larger spiders tending to harbour darker individuals, whereas within sites larger individuals were slightly brighter than smaller conspecifics. Thermal responses showed little variation along the urbanization gradient. Retreats were consistently warmer than web hubs, and spiders maintained body temperatures above both their immediate microhabitat and ambient air. Only retreat-associated behavioural thermoregulation showed a weak decline with urbanization at local spatial scales. Our results reveal contrasting trait responses to urbanization across spatial scales and demonstrate that size-colour covariation can persist despite divergent responses of individual traits. These findings highlight the importance of considering multiple traits, their covariation and spatial scale to accurately understand and predict ecological responses of ectotherms to urban environments.

A major challenge in community ecology is to understand the mechanisms promoting stable local coexistence. A necessary feature of local coexistence is that species show negative frequency dependence, rescuing rare species from exclusion. However, most studies have focused on ecological differences driving negative frequency dependence, ignoring non-ecological mechanisms such as reproductive interactions. Here, we combined field studies with behavioural and mesocosm experiments to investigate how reproductive interactions within and between species promote coexistence. Our results indicate that the intensity of male mating harassment and sexual conflict increases as species become more common, reducing female productivity and leading to negative frequency dependence. Moreover, field surveys reveal that negative frequency dependence operates in natural settings, consistent with our experimental results. These results suggest that sexual conflict can promote local coexistence and highlights the importance of studying reproductive interactions together with ecological differences to better understand the mechanisms promoting species coexistence. Significance statementResearch on the mechanisms promoting local species coexistence have focused on canonical ecological differences that increase intraspecific over interspecific competition. However, one intrinsic factor of species that can promote coexistence are the reproductive interactions. We performed a series of behavioural and mesocosm experiments manipulating species frequencies together with field observations and show that sexual conflict can decrease female fitness when species are common and promote local coexistence. Our results suggest that reproductive interactions are an understudied mechanism that can promote species coexistence even when species are ecologically equivalent.