Exploitative Competition Research Articles

When carnivores collide: a review of studies exploring the competitive interactions between bobcatsLynx rufusand coyotesCanis latrans

AbstractBobcatsLynx rufusand coyotesCanis latransare two widespread mesopredators with a complex history of sympatry. The competitive interactions between these species are of interest to biologists due to the furbearer status of bobcats, the recent range expansion of coyotes, and the recolonisation of several parts of North America by bobcats following their extirpation. Although studies exploring the dynamics and competition between bobcats and coyotes span decades, there is a lack of understanding regarding what factors influence exploitative or interference competition, and what methodologies are conducive to identifying these types of competition.We gathered a comprehensive list of research papers (n = 41) exploring bobcat–coyote competitive interactions in North America. From them, we collected the following: study site characteristics, number and types of research methods, number and types of metrics explored, history of sympatry of the two species at the study location, presence of apex predators, and documentation of interference and/or exploitative competition.Using generalised linear models, we determined that interference competition between bobcats and coyotes was observed primarily in open habitat. However, habitat heterogeneity, the number of research methods and metrics used, presence of an apex predator, and history of sympatry could not be used to predict the occurrence of interference competition. Studies that included diet overlap were less likely to observe interference competition than studies that used other metrics to infer competition.Competitive interactions between coyotes and bobcats are largely a function of prey availability. Our findings suggest that habitat type may be a surrogate for prey availability, which many researchers mention, but do not explicitly measure. Future studies investigating bobcat–coyote interactions should include the quantification of prey densities to gain a more comprehensive understanding of the system at large, and should avoid using solely diet or habitat overlap as metrics to assess competition.

Size-asymmetric competition among snails disrupts production of human-infectious Schistosoma mansoni cercariae.

Parasites can harm hosts and influence populations, communities, and ecosystems. However, parasites are reciprocally affected by population- and community-level dynamics. Understanding feedbacks between infection dynamics and larger-scale epidemiological and ecological processes could improve predictions and reveal novel control methods. We evaluated how exploitative resource competition among hosts, a fundamental aspect of population biology, influences within-host infection dynamics of the widespread human parasite Schistosoma mansoni in its intermediate host, Biomphalaria glabrata. We added size-dependent consumption of shared resources to a parameterized bioenergetics model to predict a priori the growth, parasite production, and survival of an infected focal host coexisting with an uninfected conspecific competitor in an experiment that varied competitor size. The model quantitatively anticipated that competitors disrupt growth and parasite production and that these effects increase with competitor size. Fitting the model to these data improved its match to host survivorship. Thus, resource competition alters infection dynamics, there are strong size asymmetries in these effects, and size-asymmetric resource competition effects on infection dynamics can be accurately predicted by bioenergetics theory. More broadly, this framework can assess parasite transmission and control in other contexts, such as in resource competitive host communities, or in response to eutrophication, food supplementation, or culling.

Bene"fit" Assessment in Pollination Coevolution: Mechanistic Perspectives on Hummingbird Bill-Flower Matching.

One of the reasons why flowering plants became the most diverse group of land plants is their association with animals to reproduce. The earliest examples of this mutualism involved insects foraging for food from plants and, in the process, pollinating them. Vertebrates are latecomers to these mutualisms, but birds, in particular, present a wide variety of nectar-feeding clades that have adapted to solve similar challenges. Such challenges include surviving on small caloric rewards widely scattered across the landscape, matching their foraging strategy to nectar replenishment rate, and efficiently collecting this liquid food from well-protected chambers deep inside flowers. One particular set of convergent traits among plants and their bird pollinators has been especially well studied: the match between the shape and size of bird bills and ornithophilous flowers. Focusing on a highly specialized group, hummingbirds, we examine the expected benefits from bill-flower matching, with a strong focus on the benefits to the hummingbird and how to quantify them. Explanations for the coevolution of bill-flower matching include (1) that the evolution of traits by bird-pollinated plants, such as long and thin corollas, prevents less efficient pollinators (e.g., insects) from accessing the nectar and (2) that increased matching, as a result of reciprocal adaptation, benefits both the bird (nectar extraction efficiency) and the plant (pollen transfer). In addition to nectar-feeding, we discuss how interference and exploitative competition also play a significant role in the evolution and maintenance of trait matching. We present hummingbird-plant interactions as a model system to understand how trait matching evolves and how pollinator behavior can modify expectations based solely on morphological matching, and discuss the implications of this behavioral modulation for the maintenance of specialization. While this perspective piece directly concerns hummingbird-plant interactions, the implications are much broader. Functional trait matching is likely common in coevolutionary interactions (e.g., in predator-prey interactions), yet the physical mechanisms underlying trait matching are understudied and rarely quantified. We summarize existing methods and present novel approaches that can be used to quantify key benefits to interacting partners in a variety of ecological systems.

Sublethal concentration of insecticide amplifies interference competition in a tortrix moth

Insecticides are extensively used worldwide to kill insect pests, yet organisms are most often exposed to insecticides at sublethal concentrations. Our understanding of sublethal effects on life histories is needed to predict the impact of insecticides on population dynamics and improve insecticide use and pest control. Sublethal concentrations can impact life histories directly and indirectly through changes in the intraspecific competition. Yet, few studies have evaluated the sublethal effects on intraspecific competition and these do not disentangle the insecticide effects on interference competition versus exploitative competition. As such, sublethal effects on the relative contribution of each pathways in shaping life histories are largely unknown, despite the fact that this can impact population dynamics. In this study, we focused on the neurotoxic insecticide spinosad and investigated its sublethal effects on interference among the aggressive larvae of the tortrix moth Adoxophyes honmai and the consequences for life histories. We conducted a set of paired experiments to disentangle the insecticide effects on interference from the ones on exploitation. Spinosad was found to amplify interference with most effects on mortality which lets us suggest that the insecticide likely increases the level of aggressive interactions resulting in more conspecific killings (e.g. cannibalism). Spinosad exposure was found to impair movement ability. Less movements may increase susceptibility to conspecific attacks and or increase aggresivity for better defence, two plausible mechanisms that could explain the increase in interference with insecticide. This study shows that insecticide at sublethal concentration can impact life histories by altering the strength of interference competition. Many organisms (pest and non-target species) compete through interference and theory predicts that a change in interference can substantially change dynamics. Our finding therefore reveals the importance of assessing the effect of insecticides on the mechanisms of competition when predicting their impact on populations.

Herbivory of a biocontrol agent on a native plant causes an indirect trait‐mediated non‐target effect on a native insect

Abstract Identifying food web linkages between biocontrol agents of invasive plants and native species is crucial for predicting indirect non‐target effects. Biocontrol insects can integrate into food webs within recipient habitats and influence native insects through apparent competition (altering shared natural enemies) or density‐mediated exploitation competition (changing density of native plants). However, whether and how trait‐mediated exploitation competition (modifying native plant chemicals and volatiles profiles) can produce indirect non‐target effects remains largely overlooked, despite plant phenotypic responses to insect herbivory being common and widely documented. The flea beetle Agasicles hygrophila was introduced into China for management of alligator weed Alternanthera philoxeroides, but it also attacks the native congener sessile joyweed Alternanthera sessilis, which may cause indirect non‐target effects on the native tortoise beetle Cassida piperata that also feeds on sessile joyweed. Here, we examined the relationships among abundances of the flea beetle and tortoise beetle, and coverage of sessile joyweed in the field. Then, we investigated the impact of flea beetle herbivory on tortoise beetle development and oviposition, as well as on sessile joyweed primary metabolites and leaf volatiles. Tortoise beetle abundance was not related to sessile joyweed coverage, but they were less abundant on plants with more flea beetles in the field survey, and produced fewer offspring on plants with more prior flea beetle damage in the field cage experiment. Tortoise beetle development was inhibited by prior flea beetle herbivory in bioassays in enemy‐free conditions and they preferred to oviposit on sessile joyweed that had experienced little or no flea beetle damage. Flea beetle herbivory decreased sessile joyweed foliar glucose and protein, and substantially changed its leaf volatile blend. Synthesis. Our results show that the flea beetle has major indirect non‐target effects on the tortoise beetle through trait‐mediated exploitation competition, rather than apparent competition or density‐mediated exploitation competition. Our results demonstrate a new example for indirect non‐target effects of biocontrol agents. Furthermore, our results indicate that minor brief negative impacts of biocontrol agents on non‐target plants might propagate to higher trophic levels and such negative impacts can strengthen with increasing intensity of the direct non‐target effect.

Effect of scavenging on predation in a food web.

Scavenging can have important consequences for food web dynamics, for example, it may support additional consumer species and affect predation on live prey. Still, few food web models include scavenging. We develop a dynamic model that includes two facultative scavenger species, which we refer to as the predator or scavenger species according to their natural scavenging propensity, as well as live prey, and a carrion pool to show ramifications of scavenging for predation in simple food webs. Our modeling suggests that the presence of scavengers can both increase and decrease predator kill rates and overall predation in model food webs and the impact varies (in magnitude and direction) with context. In particular, we explore the impact of the amount of dynamics (exploitative competition) allowed in the predator, scavenger, and prey populations as well as the direction and magnitude of interference competition between predators and scavengers. One fundamental prediction is that scavengers most likely increase predator kill rates, especially if there are exploitative feedback effects on the prey or carrion resources like is normally observed in natural systems. Scavengers only have minimal effects on predator kill rate when predator, scavenger, and prey abundances are kept constant by management. In such controlled systems, interference competition can greatly affect the interactions in contrast to more natural systems, with an increase in interference competition leading to a decrease in predator kill rate. Our study adds to studies that show that the presence of predators affects scavenger behavior, vital rates, and food web structure, by showing that scavengers impact predator kill rates through multiple mechanisms, and therefore indicating that scavenging and predation patterns are tightly intertwined. We provide a road map to the different theoretical outcomes and their support from different empirical studies on vertebrate guilds to provide guidance in wildlife management.

The evolution of the irreversible transition from a free-swimming state to an immobile sessile state in aquatic invertebrates modelled in a chemostat

To better understand the environmental factors and ecological processes underlying the evolution of the irreversible transition from a free-swimming state to an immobile sessile state as seen in many aquatic invertebrates, we study the adaptive dynamics of the settling rate of a hypothetical microorganism onto the wall of a chemostat. The two states, floating or settled, differ in their nutrient ingestion, reproduction and death rate. We consider three different settling mechanisms involving competition for space on the wall: (i) purely exploitative competition where free-swimming individuals settle in vacant space only, (ii) mixed exploitative and interference competition where individuals attempt to settle in any place but fail and die if the space is already occupied, and (iii) mixed exploitative and interference competition, but now settling in occupied space is successful and the former occupant dies. In the simplified environment of the chemostat, the input concentration of nutrients and the dilution rate of the tank are the main environmental control variables. Using the theory of adaptive dynamics, we find that the settling mechanisms and environmental control variables have qualitatively different effects on the evolution of the settling rate in terms of the direction of evolution as well as on species diversity. In the case of purely exploitative competition a small change in the settings of the environmental control variables can lead to an abrupt reversal of the direction of evolution, while in the case of mixed exploitative and interference competition the effect is gradual. For all three settling mechanisms, periodic fluctuations in the nutrient input open the possibility of evolutionary branching leading to the long-term coexistence of an intermediate and an infinitely high settling rates (in the case of low-frequency fluctuations), and an intermediate and a zero settling rates (in the case of high-frequency fluctuations).

Big in the tropics: Ecogeographical clines in peccary size reveal the converse of Bergmann’s rule

AbstractAimBergmann's rule is an ecogeographical rule that describes a negative relationship between body size and temperature. Here, we used a multivariate measure of skull size (centroid size) as a proxy for body size to test the influence of temperature, precipitation, elevation, human influence and competition on size in Dicotyles tajacu and Tayassu pecari.LocationAmerican tropics and sub‐tropics.TaxonTayassuidae.MethodsUsing geometric morphometric methods, we measured 426 adult skulls of both peccary species from 174 different localities south of the equator and 83 in the north. We explored the effects of temperature, precipitation, altitude and human influence, controlling for spatial autocorrelation, using ordinary least squares models and the Akaike information criterion. Size differences where the species occur allopatrically or sympatrically, where competition could be a factor, were investigated using ANCOVA models.ResultsWe found that peccaries were larger in the tropics. Temperature and precipitation explained the size variation of both peccaries in the southern hemisphere, whereas human influence is a predictor of size only for T. pecari. Size variation of northern D. tajacu is mainly explained by elevation and temperature. Tayassu pecari is consistently larger than D. tajacu in both hemispheres. Peccaries did not exhibit significant shifts in either rate of change in size or size–latitude relationships between allopatric and sympatric areas.Main conclusionsGeographical variation in size of these peccaries does not support Bergmann's rule. In the tropics, precipitation and human influence appear to exert strong selective pressures on body size of peccaries, whose resource utilization patterns are shaped by group living. Size differences of the two species across the continents may contribute to their stable coexistence via interference rather than exploitative competition.

A white noise approach to evolutionary ecology

Although the evolutionary response to random genetic drift is classically modelled as a sampling process for populations with fixed abundance, the abundances of populations in the wild fluctuate over time. Furthermore, since wild populations exhibit demographic stochasticity and since random genetic drift is in part due to demographic stochasticity, theoretical approaches are needed to understand the role of demographic stochasticity in eco-evolutionary dynamics. Here we close this gap for quantitative characters evolving in continuously reproducing populations by providing a framework to track the stochastic dynamics of abundance density across phenotypic space using stochastic partial differential equations. In the process we develop a set of heuristics to operationalize the powerful, but abstract theory of white noise and diffusion-limits of individual-based models. Applying these heuristics, we obtain stochastic ordinary differential equations that generalize classical expressions of ecological quantitative genetics. In particular, by supplying growth rate and reproductive variance as functions of abundance densities and trait values, these equations track population size, mean trait and additive genetic variance responding to mutation, demographic stochasticity, random genetic drift, deterministic selection and noise-induced selection. We demonstrate the utility of our approach by formulating a model of diffuse coevolution mediated by exploitative competition for a continuum of resources. In addition to trait and abundance distributions, this model predicts interaction networks defined by niche-overlap, competition coefficients, or selection gradients. Using a high-richness approximation, we find linear selection gradients and competition coefficients are uncorrelated, but magnitudes of linear selection gradients and quadratic selection gradients are both positively correlated with competition coefficients. Hence, competing species that strongly affect each other’s abundance tend to also impose selection on one another, but the directionality is not predicted. This approach contributes to the development of a synthetic theory of evolutionary ecology by formalizing first principle derivations of stochastic models tracking feedbacks of biological processes and the patterns of diversity they produce.

Competition between a native and introduced pollinator in unmanaged urban meadows

There is little understanding of the role of exotic bees in the observed pollinator decline. Recent models of species invasions predict that aggressive species have more impact; applying this theory to pollinators suggests that aggressive territorial Anthidium (Megachilidae) bees, whose range is expanding globally, could have high impacts on native bees. We examined exploitation and interference competition for floral resources between Anthidium and native bees in unmanaged urban meadows in Montreal, Canada. First, we conducted a survey of floral visits by Anthidium and native bee genera. We found high niche overlap with Megachile (Megachilidae), one of the most common genera of native pollinators in the area, suggesting high potential for exploitation competition. Second, we tested for interference competition between Anthidium and Megachile with behavioural observations of foraging on a common introduced host plant (Lotus corniculatus, Fabaceae). This presence of Anthidium decreased patch residence and number of floral visits by Megachile bees, suggesting that Anthidium has a significant per capita effect on foraging behavior of Megachile.

High honeybee abundances reduce wild bee abundances on flowers in the city of Munich

The increase in managed honeybees (Apis mellifera) in many European cities has unknown effects on the densities of wild bees through competition. To investigate this, we monitored honeybees and non-honeybees from 01 April to 31 July 2019 and 2020 at 29 species of plants representing diverse taxonomic and floral-functional types in a large urban garden in the city of Munich in which the same plant species were cultivated in both years. No bee hives were present in the focal garden, and all bee hives in the adjacent area were closely monitored by interviewing the relevant bee keepers in both 2019 and 2020. Honeybee numbers were similar in April of both years, but increased from May to July 2020 compared to 2019. The higher densities correlated with a significant increase in shifts from wild bee to honeybee visits in May/June/July, while visitor spectra in April 2019 and 2020 remained the same. Most of the species that experienced a shift to honeybee visits in 2020 were visited mostly or exclusively for their nectar. There were no shifts towards increased wild bee visits in any species. These results from a flower-rich garden have implications for the discussion of whether urban bee keeping might negatively impact wild bees. We found clear support that high honeybee densities result in exploitative competition at numerous types of flowers.

Rich and abundant spider communities result from enhanced web capture breadth and reduced overlap in urban greenspaces.

Urbanization is a key contributor to biodiversity loss, but evidence is mounting that cities can support rich arthropod communities, including rare and threatened species. Furthermore, greenspace is growing within hundreds of "shrinking cities" that have lost population resulting in a need to demolish an overabundance of infrastructure creating vacant land. Efforts are underway to transform vacant lots, often viewed as blighted areas, into habitats that promote biodiversity and generate ecosystem services, such as urban agroecosystems. To understand how reconfiguring these greenspaces might influence species conservation, elucidation of the factors that drive the distribution of an urban species pool is needed. In particular, the importance of species interactions in structuring urban communities is poorly understood. We tested hypotheses that (1) greater breadth of prey captured by web-building spiders and reduced overlap of prey capture among individuals facilitates the conservation of genera richness and abundance and (2) heterogeneity within a greenspace patch facilitates enhanced dietary niche breadth and greater resource partitioning. In 2013 and 2014, the abundance, breadth and degree of overlap in prey capture of sheet web spiders (Linyphiidae) was measured using web mimic traps at 160 microsites (0.25m2 ) situated in four urban vacant lots and four urban farms in the city of Cleveland, Ohio, USA. Within a subset of 40 microsites, we used vacuum sampling and hand collection to measure the abundance and genera richness of Linyphiidae. Spider richness and abundance were significantly reduced within urban farms relative to vacant lots. The distribution of spiders and prey was explained by habitat structure, with microsites dominated by tall grasses and flowering plants, with a high bloom abundance and richness, supporting greater prey capture and a higher genera richness and abundance of spiders. In 2014, web capture overlap was significantly greater within microsites dominated by bare ground. These findings illustrate that urban greenspace conservation efforts that focus on reducing bare ground and incorporating a diversity of grasses and flowering plant species can promote linyphiid spiders, potentially by relaxing exploitative competition for shared prey.

Prey preference of top predators manipulates the functioning and stability of multi-trophic ecosystems

The relationship between biodiversity and ecosystem functioning, and the mechanisms underpinning the food web stability, have been intensively investigated in ecological research. The ubiquities of generalists in natural food webs and its important role in dictating these ecosystem properties have been generally recognized. However, how competition between multiple top predators shape these ecosystem properties and determine the success of invasive predators remain largely unexplored. Here, we use a well-developed food web model to investigate the effects of prey preference of top predators on ecosystem functioning and food web stability in both local and invasive conditions. We design several modeling scenarios to mimic combinations of different types of top predators (specialist/generalist) and their origins (local/invasive). Our model theoretically shows that lower exploitation competition for prey between top predators (with distinct prey preferences featured by higher attack rates) would be beneficial for the ecosystem functioning and food web stability. We also demonstrate that the success of top predator invasion depends on the prey preference of both local and invasive top predators. Sensitivity analysis on the model further supports our findings. Our results highlight the importance of prey preference of multiple top predators in manipulating the properties of multi-trophic ecosystems. Our findings may have important implications because the current ongoing global changes profoundly change the phenology of many biological systems and create trophic mismatch, which may manipulate prey preference of top predators and in turn deteriorate ecosystem functioning and food web stability.

Does the invasive plague skink (Lampropholis delicata) compete with native skink species in New Zealand?

AbstractInvasive species are a major threat to biodiversity worldwide, causing many of the recent declines and extinctions of native species. Competition is a common mechanism through which invasive species impact the native biota. In particular, exploitative competition results when the invader outcompetes native species for essential resources, such as food and shelter sites. Despite being pervasive invaders, the ecological impacts of invasive lizards are understudied and it is often unknown whether these invaders have detrimental impacts. The plague skink (Lampropholis delicata), native to eastern Australia, has invaded several locations throughout the Pacific region (Hawaiian Islands, New Zealand, Lord Howe Island). However, no studies have documented the plague skink as having a negative impact on native species anywhere throughout its introduced range. Here, we conducted a mark‐recapture study in northern New Zealand (Shakespear Regional Park) to investigate whether the plague skink competes with native skink species (Oligosoma aeneum, O. ornatum, O. moco). Though the plague skink has a broader distribution within the study region and occurred across a broader range of habitats, there was substantial range and niche overlap between the plague skink and the three native skink species. Both the absolute capture rates and estimated population size of the plague skink were significantly higher than the three native skink species. Although plague skinks have smaller body sizes well as shorter and narrower heads than the native skink species, there was still considerable scope for dietary overlap among the species. While our study does not provide definitive evidence for competition between the plague skink and native skink species in New Zealand, we clearly demonstrate the presence of the core components required for exploitative competition to occur. Further field‐ and laboratory‐based studies will be required to confirm the existence of exploitative or interference competition by the plague skink in its introduced range.

Exploitative competition and displacement mediated by eusocial bees: experimental evidence in a wild pollinator community

Eusocial bees are likely to be ecologically important competitors for floral resources, although competitive effects can be difficult to quantify in wild pollinator communities. To investigate this, we excluded honeybees (HBE treatment), bumblebees (BBE) or both (HB&BBE) from wild-growing patches of bramble, Rubus fruticosus L. agg., flowers in two eight-day field trials at separate locations, with complementary mapping of per-site local floral resource availability. Exclusions increased per-flower volume of nectar and visitation rates of non-excluded bees, compared to control patches with no bee exclusions (CON). There was a large increase in average nectar standing crop volume both at Site 1 (+ 172%) and Site 2 (+ 137%) in HB&BBE patch flowers, and no significant change in HBE or BBE, compared to CON patches. Foraging bee responses to exclusion treatments were more pronounced at Site 2, which may be due to lower local floral resource availability, since this is likely to increase the degree of exploitative competition present. Notably, at Site 2, there was a 447% increase in larger-bodied solitary (non-Apis/Bombus) bees visiting HB&BBE patches, suggesting ecological release from competition. Hoverflies showed no response to bee removals. Numbers of other non-bee insect groups were very small and also showed no clear response to exclusions. Our findings reveal patterns of competitive exclusion between pollinator groups, mediated by resource depletion by eusocial bees. Possible long-term implications of displacement from preferred flowers, particularly where alternative forage is reduced, are discussed.Significance statementUnderstanding patterns of exploitative competition and displacement is necessary for pollinator conservation, particularly for vulnerable or threatened species. In this research, experimental methods reveal underlying patterns of resource competition exerted by eusocial bees in a wild pollinator community. We show that honeybees and bumblebees competitively displace each other and particularly solitary (non-Apis/Bombus) bees from bramble, an important native nectar and pollen source. Effects were stronger where local floral resource availability was identified to be limited. Notably, following experimental exclusion of both honey- and bumblebees from flowers, visitation by solitary bees increased by up to 447%, strongly suggesting ecological release from competition. These results highlight the need for informed landscape management for pollinator wellbeing, including appropriate honeybee stocking densities and improved floral resource availability.

Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charr in Hokkaido, Japan.

Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density-dependent growth and survival. Agonistic interactions (n=761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length. Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks. These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference. Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr. The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade-offs in ecological performance can contribute to a mechanistic understanding of species coexistence.

A Generalist Lifestyle Allows Rare Gardnerella spp. to Persist at Low Levels in the Vaginal Microbiome.

Gardnerella spp. are considered a hallmark of bacterial vaginosis, a dysbiosis of the vaginal microbiome. There are four cpn60 sequence-based subgroups within the genus (A, B, C and D), and thirteen genome species have been defined recently. Gardnerella spp. co-occur in the vaginal microbiome with varying abundance, and these patterns are shaped by a resource-dependent, exploitative competition, which affects the growth rate of subgroups A, B and C negatively. The growth rate of rarely abundant subgroup D, however, increases with the increasing number of competitors, negatively affecting the growth rate of others. We hypothesized that a nutritional generalist lifestyle and minimal niche overlap with the other more abundant Gardnerella spp. facilitate the maintenance of subgroup D in the vaginal microbiome through negative frequency-dependent selection. Using 40 whole-genome sequences from isolates representing all four subgroups, we found that they could be distinguished based on the content of their predicted proteomes. Proteins associated with carbohydrate and amino acid uptake and metabolism were significant contributors to the separation of subgroups. Subgroup D isolates had significantly more of their proteins assigned to amino acid metabolism than the other subgroups. Subgroup D isolates were also significantly different from others in terms of number and type of carbon sources utilized in a phenotypic assay, while the other three could not be distinguished. Overall, the results suggest that a generalist lifestyle and lack of niche overlap with other Gardnerella spp. leads to subgroup D being favoured by negative frequency-dependent selection in the vaginal microbiome.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00248-020-01643-1.

Molecular Ecological Network Analyses: An Effective Conservation Tool for the Assessment of Biodiversity, Trophic Interactions, and Community Structure

Global biodiversity is threatened by the anthropogenic restructuring of animal communities, rewiring species interaction networks in real-time as individuals are extirpated or introduced. Conservation science and adaptive ecosystem management demand more rapid, quantitative, and non-invasive technologies for robustly capturing changing biodiversity and quantifying species interactions. Here we develop molecular ecological network analyses (MENA) as an ecosystem assessment tool to address these needs. To construct the ecological network, we used environmental DNA from feces to identify the plant and mammal diet of two carnivores: puma (Puma concolor) and bobcat (Lynx rufus); two omnivores: coyote (Canis latrans) and gray fox (Urocyon cinereoargenteus); and two herbivores: black-tailed deer (Odocoileus hemionus) and black-tailed jackrabbit (Lepus californicus) in a well-studied Californian reserve. To evaluate MENA as a comprehensive biodiversity tool, we applied our framework to identify the structure of the network, patterns of trophic interactions, key species, and to assess its utility in capturing the biodiversity of the area. The high dietary taxonomic resolution enabled the assessment of species diversity, niche breadth and overlap. The network analysis revealed a dense ecological network with a high diversity of weakly connected species and a community that is highly modular and non-nested. The significant prevalence of tri-trophic chain and exploitative competition patterns indicates (i) the removal or reintroduction of a top predator would trigger a trophic cascade within this community, directly affecting their prey and indirectly the plant communities, and (ii) the potential impact of indirect effects between two predators that consume the same prey. These results suggest that the recent resurgence of puma in the study area may impact the herbaceous and woody vegetation and the population size of other predators. This effect of fluctuating predator populations and plant communities could be predicted through MENA’s fine-scale assessment of the diet selection and the identified keystone species. Although just using a subset of species, MENA more rapidly, accurately, and effectively captured the broader biodiversity of the area in comparison to other methodologies. MENA reconstructed and unveiled the hidden complexity in trophic structure and interaction networks within the community, providing a promising toolkit for biodiversity and ecosystem management.

Hyaenas and lions: how the largest African carnivores interact at carcasses

The study of the competitive interactions between predators has traditionally been approached within the context of predatory behavior. By using a quasi‐experimental, non‐intrusive approach, we go beyond the classical view that predators only compete for living prey and disentangle the mechanisms of exploitative and interference competition between two charismatic apex predators, lions Panthera leo and spotted hyaenas Crocuta crocuta, at carcasses other than their own kills. Carcasses of different sizes were monitored in two study areas of South Africa: one with both lions and hyaenas and the other with hyaenas only. We found evidence of both symmetric exploitative competition and asymmetric interference competition. Interactions were strongly conditioned by carcass size, and co‐occurrence was mostly recorded at large ungulate carcasses. Interference competition seemed to dominate interspecific interactions, with lions having preferential access. However, interference while feeding did not exert any cost on the subordinate hyaenas, in terms of amount of ingested food and consumption rate, probably thanks to the reward provided by large carcasses in the form of abundant carrion, the use of lions to detect carcasses, and spatiotemporal segregation at the carcass‐scale. Securing a diverse carrion supply in terms of carcass size, including megaherbivore carcasses, may favor the coexistence of the largest African carnivores, especially in small protected areas. Overall, our study reveals mechanisms related to both the food resource heterogeneity and the competitor behavior that allow the coexistence of strongly interacting species.

Intraspecific competition in size-structured populations: ontogenetic shift in the importance of interference competition in a key marine herbivore

Individuals rarely have equal competitive abilities, with body size being one of the most important attributes affecting the mechanism (i.e. exploitative and interference) and consequences of competition. Competitive interactions within size-structured populations are complex and can have major implications for population dynamics, community structure and evolutionary processes. Destructive grazing of kelp beds by the green urchin Strongylocentrotus droebachiensis creates barrens where high-quality food is scarce and intraspecific competition may have an important role in structuring populations. In this study, we experimentally identified the mechanisms underlying size-asymmetric competition between small, medium, and large size classes of the green urchin. A field-based mesocosm experiment showed that small and medium sea urchins grew less and produced smaller gonads when competing for food with large conspecifics. Surprisingly, when food was provided ad libitum but large urchins were present, small individuals’ growth and foraging behavior were reduced, providing strong evidence for interference competition between small and large sea urchins. Interactions between medium and large sea urchins were, however, more influenced by exploitative competition, suggesting that sea urchins shift ontogenetically from a situation of intense interference competition to one dominated by exploitative competition. The size structure of the population can thus determine the relative importance of interference and exploitative competition. In turn, the importance of interference competition may influence size structure by inhibiting the growth of smaller urchins, a pattern consistent with the prediction of theoretical models. The consideration of size-asymmetric competitive interactions can lead to a better understanding of population size structure and dynamics.