Predator-driven elemental cycling: the impact of predation and risk effects on ecosystem stoichiometry

Predator-driven behavioural shifts in a common lizard shape resource-flow from marine to terrestrial ecosystems

Foraging decisions shape the structure of food webs. Therefore, a behavioural shift in a single species can potentially modify resource-flow dynamics of entire ecosystems. To examine this, we conducted a field experiment to assess foraging niche dynamics of semi-arboreal brown anole lizards in the presence/absence of predatory ground-dwelling curly-tailed lizards in a replicated set of island ecosystems. One year after experimental translocation, brown anoles exposed to these predators had drastically increased perch height and reduced consumption of marine-derived food resources. This foraging niche shift altered marine-to-terrestrial resource-flow dynamics and persisted in the diets of the first-generation offspring. Furthermore, female lizards that displayed more risk-taking behaviours consumed more marine prey on islands with predators present. Our results show how predator-driven rapid behavioural shifts can alter food-web connectivity between oceanic and terrestrial ecosystems and underscore the importance of studying behaviour-mediated niche shifts to understand ecosystem functioning in rapidly changing environments.

Prey defense phenotype mediates multiple-predator effects in tri-trophic food-webs

1. The emphasis on mechanisms governing the interaction among predators (e.g., cooperation, competition, or intraguild predation) has driven the understanding of multiple-predator effects on prey survival and dynamics. However, overwhelming evidence shows that prey can adaptively respond to predators, exhibiting multiple defensive phenotypes to cope with predation. Nevertheless, there is still a relatively scarce theory connecting the emergence of prey defenses in complex multi-predator scenarios and their ecological consequences. 2. Using a mathematical approach, we evaluated the prevalence of defended prey phenotypes as a function of predator-induced mortality in a two-predator system, and how prey and phenotype dynamics affect trophic cascades. We also evaluated such responses when prey manifests a general defense against both predators (i.e., risk-reducing) or a specialized defense against one predator at the expense of defense against the other predator (i.e., risk trade-off), and when such phenotypes induce fitness and foraging costs. 3. We showed that the emergence of defended phenotypes under multiple predators depends on predator-induced mortality rates, the magnitude of phenotype costs, and the effect of the defensive phenotype on the performance of all predators. 4. Risk-reducing phenotypes enhance prioritized responses to predators with high killing rates, but prioritized responses are diminished when prey manifest risk trade-off phenotypes. Finally, we showed that resource abundance across the predation gradient directly depends on the prevalence of certain prey phenotypes and their effect on foraging costs. 5. Ultimately, our results depict the implications of prey defenses on prey and basal resources abundance in a multiple predators' environment, highlighting the role of the identity of defensive strategies in mediating the strength and nature of trophic cascades, via consumptive or non-consumptive effects.

Organic carbon and eukaryotic predation synergistically change resistance and resilience of aquatic microbial communities

With rapid global urbanization, anthropogenic activities alter aquatic biota in urban rivers through inputs of dissolved organic carbon (DOC) and nutrients. Microorganisms-mediated global element cycles provide functions in maintaining microbial ecology stability. The DOC (bottom-up control) and microbial predation (top-down control) may synergistically drive the competition and evolution of aquatic microbial communities, as well as their resistance and resilience, for which experimental evidences remain scarce. In this study, laboratory sediment-water column experiments were employed to mimic the organic carbon-driven water blackening and odorization process in urban rivers and to elucidate the impact of DOC on microbial ecology stability. Results showed that low (25-75 mg/L) and high DOC (100-150 mg/L) changed the aquatic microbial community assemblies in different patterns: (1) the low DOC enriched K-selection microorganisms (e.g., C39, Tolumonas and CR08G) with low biomass and low resilience, as well as high resistance to perturbations in changing microbial community assemblies; (2) the high DOC was associated with r-selection microorganisms (e.g., PSB-M-3 and Clostridium) with high biomass and improved resilience, together with low resistance detrimental to microbial ecology stability. Overall, this study provided new insight into the impact of DOC on aquatic microbial community stability, which may help guide sustainable urban river management.

Sublethal effects of parasitism on ruminants can have cascading consequences for ecosystems

Parasitic infections are common, but how they shape ecosystem-level processes is understudied. Using a mathematical model and meta-analysis, we explored the potential for helminth parasites to trigger trophic cascades through lethal and sublethal effects imposed on herbivorous ruminant hosts after infection. First, using the model, we linked negative effects of parasitic infection on host survival, fecundity, and feeding rate to host and producer biomass. Our model, parameterized with data from a well-documented producer–caribou–helminth system, reveals that even moderate impacts of parasites on host survival, fecundity, or feeding rate can have cascading effects on ruminant host and producer biomass. Second, using meta-analysis, we investigated the links between helminth infections and traits of free-living ruminant hosts in nature. We found that helminth infections tend to exert negative but sublethal effects on ruminant hosts. Specifically, infection significantly reduces host feeding rates, body mass, and body condition but has weak and highly variable effects on survival and fecundity. Together, these findings suggest that while helminth parasites can trigger trophic cascades through multiple mechanisms, overlooked sublethal effects on nonreproductive traits likely dominate their impacts on ecosystems. In particular, by reducing ruminant herbivory, pervasive helminth infections may contribute to a greener world.

In defense of elemental currencies: can ecological stoichiometry stand as a framework for terrestrial herbivore nutritional ecology?

Nutritional ecologists aim to predict population or landscape-level effects of food availability, but the tools to extrapolate nutrition from small to large extents are often lacking. The appropriate nutritional ecology currencies should be able to represent consumer responses to food while simultaneously be simple enough to expand such responses to large spatial extents and link them to ecosystem functioning. Ecological stoichiometry (ES), a framework of nutritional ecology, can meet these demands, but it is typically associated with ecosystem ecology and nutrient cycling, and less often used to study wildlife nutrition. Despite the emerging zoogeochemical evidence that animals, and thus their diets, play critical roles in nutrient movement, wildlife nutritional ecology has not fully embraced ES, and ES has not incorporated nutrition in many wildlife studies. Here, we discuss how elemental currencies are "nutritionally, organismally, and ecologically explicit" in the context of terrestrial herbivore nutritional ecology. We add that ES and elemental currencies offer a means to measure resource quality across landscapes and compare nutrient availability among regions. Further, we discuss ES shortcomings and solutions, and list future directions to advance the field. As ecological studies increasingly grow in spatial extent, and attempt to link multiple levels of biological organization, integrating more simple and unifying currencies into nutritional studies, like elements, is necessary for nutritional ecology to predict herbivore occurrences and abundances across regions.

Foxes fertilize the subarctic forest and modify vegetation through denning

Ecosystem engineers modify habitats through processes other than trophic interactions, such as by regulating soil nutrients, and can influence resource availability and quality for other organisms. Predator-mediated elemental cycling may be especially important in determining plant diversity and growth in ecosystems where soil fertility and primary productivity are low. Red foxes (Vulpes vulpes L.), top predators in the Subarctic, could engineer local ecosystems through denning, which could create biogeochemical hotspots of nutrients due to continual input of feces, urine and prey remains. We examined soil and vegetation characteristics on red fox dens and paired control sites in woodland habitats near the Arctic treeline in Manitoba, Canada. The organic soil layer on den sites had 81% more inorganic nitrogen and 250% more extractable phosphorus than in control areas. Denning also increased soil respiration and pH in the organic layer, suggesting improved soil quality and nutrient availability for plants. By enriching nutrients and disturbing soils through digging, den sites had a higher plant species ß-diversity and a greater cover of erect woody shrubs (Salix spp.), grasses (Leymus mollis (Trinius) Pilger) and weedy ephemerals compared to control sites, resulting in a regional increase in plant species richness. Our research highlights the importance of considering impacts of predators other than through their consumption of prey, and provides insight into the role of red foxes in modifying plant diversity and productivity in the Subarctic.

Food Webs and Ecosystems: Linking Species Interactions to the Carbon Cycle

All species within ecosystems contribute to regulating carbon cycling because of their functional integration into food webs. Yet carbon modeling and accounting still assumes that only plants, microbes, and invertebrate decomposer species are relevant to the carbon cycle. Our multifaceted review develops a case for considering a wider range of species, especially herbivorous and carnivorous wild animals. Animal control over carbon cycling is shaped by the animals’ stoichiometric needs and functional traits in relation to the stoichiometry and functional traits of their resources. Quantitative synthesis reveals that failing to consider these mechanisms can lead to serious inaccuracies in the carbon budget. Newer carbon-cycle models that consider food-web structure based on organismal functional traits and stoichiometry can offer mechanistically informed predictions about the magnitudes of animal effects that will help guide new empirical research aimed at developing a coherent understanding of the interactions and importance of all species within food webs.

Ecological impacts of human-induced animal behaviour change

A growing body of literature has documented myriad effects of human activities on animal behaviour, yet the ultimate ecological consequences of these behavioural shifts remain largely uninvestigated. While it is understood that, in the absence of humans, variation in animal behaviour can have cascading effects on species interactions, community structure and ecosystem function, we know little about whether the type or magnitude of human-induced behavioural shifts translate into detectable ecological change. Here we synthesise empirical literature and theory to create a novel framework for examining the range of behaviourally mediated pathways through which human activities may affect different ecosystem functions. We highlight the few empirical studies that show the potential realisation of some of these pathways, but also identify numerous factors that can dampen or prevent ultimate ecosystem consequences. Without a deeper understanding of these pathways, we risk wasting valuable resources on mitigating behavioural effects with little ecological relevance, or conversely mismanaging situations in which behavioural effects do drive ecosystem change. The framework presented here can be used to anticipate the nature and likelihood of ecological outcomes and prioritise management among widespread human-induced behavioural shifts, while also suggesting key priorities for future research linking humans, animal behaviour and ecology.

Patterns and potential drivers of intraspecific variability in the body C, N, and P composition of a terrestrial consumer, the snowshoe hare (Lepus americanus)

Intraspecific variability in ecological traits is widespread in nature. Recent evidence, mostly from aquatic ecosystems, shows individuals differing at the most fundamental level, that of their chemical composition. Age, sex, or body size and condition may be key drivers of intraspecific variability in the body concentrations of carbon (C), nitrogen (N), and phosphorus (P). However, we still have a rudimentary understanding of the patterns and drivers of intraspecific variability in chemical composition of terrestrial consumers, particularly vertebrates.Here, we investigate the elemental composition of the snowshoe hare Lepus americanus. Based on snowshoe hare ecology, we predicted older, larger individuals to have higher concentration of N or P and lower C content compared with younger, smaller individuals. We also predicted females to have higher concentrations of N, P, and lower C than males due to the higher reproductive costs they incur. Finally, we predicted that individuals in better body condition would have higher N and P than those in worse condition, irrespective of age.We obtained C, N, and P concentrations and ratios from a sample of 50 snowshoe hares. We then used general linear models to test our predictions on the relationship between age, sex, body size or condition and stoichiometric variability in hares.We found considerable variation in C, N, and P stoichiometry within our sample. Contrary to our predictions, we found weak evidence of N content decreasing with age. As well, sex appeared to have no relationship with hare body elemental composition. Conversely, as expected, P content increased with body size and condition. Finally, we found no relationship between variability in C content and any of our predictor variables.Snowshoe hare stoichiometry does not appear to vary with individual age, sex, body size, or condition. However, the weak relationship between body N concentration and age may suggest varying nutritional requirements of individuals at different ages. Conversely, body P's weak relationship to body size and condition appears in line with this limiting element's importance in terrestrial ecosystems. Snowshoe hares are keystone herbivores in the boreal forest of North America, and the substantial stoichiometric variability we find in our sample could have important implications for nutrient dynamics, in both boreal and adjacent ecosystems.

Microbial and animal nutrient limitation change the distribution of nitrogen within coupled green and brown food chains

Numerous biotic mechanisms can control ecosystem nutrient cycling, but their full incorporation into ecological models or experimental designs can result in inordinate complexity. Including organismal nutrient limitation in models of highly dimensional systems (i.e., those with many nutrient pools/species) presents a critical challenge. We evaluate the importance of explicitly considering microbial and animal nutrient limitation to predict ecosystem nitrogen cycling across plant-based and detritus-based food chains. We investigate how eight factorial scenarios of microbial, herbivore, and microbi-detritivore (i.e., omnivores consuming microbes and detritus) nitrogen or carbon limitation alter the stocks and flows of nitrogen in an ecosystem model. We used a combination of partial derivatives of model equilibrium solutions and numerical simulations using randomly drawn parameter sets to explore the impact of each nutrient limitation scenario on nutrient stocks and flows. We show that switching microbes, herbivores, or microbi-detritivores from nitrogen to carbon limitation consistently altered the ecosystem response to changes in inorganic nitrogen supply, plant C:N ratio, and microbial C:N ratio. Organism nutrient limitation changed ecosystem nitrogen flows by altering the feedbacks between the abiotic and biotic pools. For example, microbi-detritivore nutrient limitation determined whether the microbial response to changes in inorganic nitrogen supply and C:N ratios was dependent on the size of detrital carbon or detrital nitrogen pool. Such correlated responses among biotic and abiotic pools set up a network of predictable changes in ecosystem properties sensitive to organism nutrient limitation. Scenarios with microbial limitation were generally sufficient to capture the suite of ecosystem responses to increasing inorganic nitrogen supply, while scenarios with animal limitation added new behavior whenever C:N ratios changed. We make the case for explicitly considering both microbial and animal nutrient limitation when predicting the flow and distribution of nitrogen across green and brown food chains.

Animals and the zoogeochemistry of the carbon cycle

Predicting and managing the global carbon cycle requires scientific understanding of ecosystem processes that control carbon uptake and storage. It is generally assumed that carbon cycling is sufficiently characterized in terms of uptake and exchange between ecosystem plant and soil pools and the atmosphere. We show that animals also play an important role by mediating carbon exchange between ecosystems and the atmosphere, at times turning ecosystem carbon sources into sinks, or vice versa. Animals also move across landscapes, creating a dynamism that shapes landscape-scale variation in carbon exchange and storage. Predicting and measuring carbon cycling under such dynamism is an important scientific challenge. We explain how to link analyses of spatial ecosystem functioning, animal movement, and remote sensing of animal habitats with carbon dynamics across landscapes.

Caught in the web: Spider web architecture affects prey specialization and spider-prey stoichiometric relationships

Quantitative approaches to predator-prey interactions are central to understanding the structure of food webs and their dynamics. Different predatory strategies may influence the occurrence and strength of trophic interactions likely affecting the rates and magnitudes of energy and nutrient transfer between trophic levels and stoichiometry of predator-prey interactions. Here, we used spider-prey interactions as a model system to investigate whether different spider web architectures-orb, tangle, and sheet-tangle-affect the composition and diet breadth of spiders and whether these, in turn, influence stoichiometric relationships between spiders and their prey. Our results showed that web architecture partially affects the richness and composition of the prey captured by spiders. Tangle-web spiders were specialists, capturing a restricted subset of the prey community (primarily Diptera), whereas orb and sheet-tangle web spiders were generalists, capturing a broader range of prey types. We also observed elemental imbalances between spiders and their prey. In general, spiders had higher requirements for both nitrogen (N) and phosphorus (P) than those provided by their prey even after accounting for prey biomass. Larger P imbalances for tangle-web spiders than for orb and sheet-tangle web spiders suggest that trophic specialization may impose strong elemental constraints for these predators unless they display behavioral or physiological mechanisms to cope with nutrient limitation. Our findings suggest that integrating quantitative analysis of species interactions with elemental stoichiometry can help to better understand the occurrence of stoichiometric imbalances in predator-prey interactions.

Predator community composition is linked to soil carbon retention across a human land use gradient

Soil carbon (C) storage is a major component of the carbon cycle. Consensus holds that soil C uptake and storage is regulated by plant-microbe-soil interactions. However, the contribution of animals in aboveground food webs to this process has been overlooked. Using insights from prior long-term experimentation in an old-field ecosystem and mathematical modeling, we predicted that the amount of soil C retention within a field should increase with the proportion of active hunting predators comprising the aboveground community of active hunting and sit-and-wait predators. This comes about because predators with different hunting modes have different cascading effects on plants. Our test of the prediction revealed that the composition of the arthropod predator community and associated cascading effects on the plant community explained 41% of variation in soil C retention among 15 old fields across a human land use gradient. We also evaluated the potential for several other candidate factors to explain variation in soil C retention among fields, independent of among-field variation in the predator community. These included live plant biomass, insect herbivore community composition, soil arthropod decomposer community composition, degree of land use development around the fields, field age, and soil texture. None of these candidate variables significantly explained soil C retention among the fields. The study offers a generalizable understanding of the pathways through which arthropod predator community composition can contribute to old-field ecosystem carbon storage. This insight helps support ongoing efforts to understand and manage the effects of anthropogenic land use change on soil C storage.

Predator-driven elemental cycling: the impact of predation and risk effects on ecosystem stoichiometry

Empirical evidence is beginning to show that predators can be important drivers of elemental cycling within ecosystems by propagating indirect effects that determine the distribution of elements among trophic levels as well as determine the chemical content of organic matter that becomes decomposed by microbes. These indirect effects can be propagated by predator consumptive effects on prey, nonconsumptive (risk) effects, or a combination of both. Currently, there is insufficient theory to predict how such predator effects should propagate throughout ecosystems. We present here a theoretical framework for exploring predator effects on ecosystem elemental cycling to encourage further empirical quantification. We use a classic ecosystem trophic compartment model as a basis for our analyses but infuse principles from ecological stoichiometry into the analyses of elemental cycling. Using a combined analytical-numerical approach, we compare how predators affect cycling through consumptive effects in which they control the flux of nutrients up trophic chains; through risk effects in which they change the homeostatic elemental balance of herbivore prey which accordingly changes the element ratio herbivores select from plants; and through a combination of both effects. Our analysis reveals that predators can have quantitatively important effects on elemental cycling, relative to a model formalism that excludes predator effects. Furthermore, the feedbacks due to predator nonconsumptive effects often have the quantitatively strongest impact on whole ecosystem elemental stocks, production and efficiency rates, and recycling fluxes by changing the stoichiometric balance of all trophic levels. Our modeling framework predictably shows how bottom-up control by microbes and top-down control by predators on ecosystems become interdependent when top predator effects permeate ecosystems.