GRAZERS, PRODUCER STOICHIOMETRY, AND THE LIGHT : NUTRIENT HYPOTHESIS REVISITED

Regulation of open populations of a stream insect through larval density-dependence

In organisms with complex life cycles, the various stages occupy different habitats creating demographically open populations. The dynamics of these populations will depend on the occurrence and timing of stochastic influences relative to demographic density dependence, but understanding of these fundamentals, especially in the face of climate warming, has been hampered by the difficulty of empirical studies.

Using a logically feasible organism, we conducted a replicated density‐perturbation experiment to manipulate late‐instar larvae of nine populations of a stream caddisfly, Zelandopsyche ingens, and measured the resulting abundance over 2 years covering the complete life cycle of one cohort to evaluate influences on dynamics.

Negative density feedback occurred in the larval stage, and was sufficiently strong to counteract variation in abundance due to manipulation of larval density, adult caddis dispersal in the terrestrial environment as well as downstream drift of newly hatched and older larvae in the current. This supports theory indicating regulation of open populations must involve density dependence in local populations sufficient to offset variability associated with dispersal, especially during recruitment, and pinpoints the occurrence to late in the larval life cycle and driven by food resource abundance.

There were large variations in adult, egg mass and early instar abundance that were not related to abundance in the previous stage, or the manipulation, pointing to large stochastic influences. Thus, the results also highlight the complementary nature of stochastic and deterministic influences on open populations. Such density dependence will enhance population persistence in situations where variable dispersal and transitioning between life stages frequently creates mismatches between abundance and the local availability of resources, such as might become more common with climate warming.

Crucian Carp (Carassius carassius) Strongly Affect C/N/P Stoichiometry of Suspended Particulate Matter in Shallow Warm Water Eutrophic Lakes

Crucian carp (Carassius carassius) is a key fish species in most Chinese subtropical and tropical shallow lakes. Through sediment feeding, crucian carp could greatly change water turbidity and nutrient levels, as well as the abundance of herbivorous consumers, which may have important influences on seston element stoichiometry. However, so far, experimental studies on this topic are lacking. We conducted a 36-day mesocosm experiment to explore the effects of crucian carp on water physicochemical and biological properties, and C/N/P ratios in suspended particulate matter (SPM) under eutrophic conditions. Our results provided three major findings: (1) Crucian carp resuspended sediments and along with them, reduced light penetration and lower light/total phosphorus (TP) ratios. (2) Crucian carp reduced biomasses of both zooplankton and macrozoobenthos, whereas their effect on phytoplankton was weak, potentially because of resuspension-induced light limitation. (3) Both C/P and N/P ratios in SPM were significant lower in mesocosms with crucian carp than in fish-free controls, which may be attributed to the high contribution of P-rich sediments and low light to nutrient supply caused by fish-induced resuspension. Our results suggest that besides planktivorous fish, benthivore (e.g., crucian carp) in warm shallow waters could also affect pelagic C/N/P stoichiometry via sediment feeding, which may further influence energy transfer efficiency in lake food chain.

Effects of nutrient supplementation on host-pathogen dynamics of the amphibian chytrid fungus: a community approach

Anthropogenic stressors may influence hosts and their pathogens directly or may alter host-pathogen dynamics indirectly through interactions with other species. For example, in aquatic ecosystems, eutrophication may be associated with increased or decreased disease risk. Conversely, pathogens can influence community structure and function and are increasingly recognised as important members of the ecological communities in which they exist.In outdoor mesocosms, we experimentally manipulated nutrients (nitrogen and phosphorus) and the presence of a fungal pathogen, Batrachochytrium dendrobatidis (Bd), and examined the effects on Bd abundance on larval amphibian hosts (Pseudacris regilla: Hylidae), amphibian traits and community dynamics. We predicted that resource supplementation would mitigate negative effects of Bd on tadpole growth and development and that indirect effects of treatments would propagate through the community.Nutrient additions caused changes in algal growth, which benefitted tadpoles through increased mass, development and survival. Bd-exposed tadpoles metamorphosed sooner than unexposed individuals, but their mass at metamorphosis was not affected by Bd exposure. We detected additive rather than interactive effects of nutrient supplementation and Bd in this experiment.Nutrient supplementation was not a significant predictor of infection load of larval amphibians. However, a structural equation model revealed that resource supplementation and exposure of amphibians to Bd altered the structure of the aquatic community. This is the first demonstration that sublethal effects of Bd on amphibians can alter aquatic community dynamics.

Global biodiversity, stoichiometry and ecosystem function responses to human-induced C-N-P imbalances

Global change analyses usually consider biodiversity as a global asset that needs to be preserved. Biodiversity is frequently analysed mainly as a response variable affected by diverse environmental drivers. However, recent studies highlight that gradients of biodiversity are associated with gradual changes in the distribution of key dominant functional groups characterized by distinctive traits and stoichiometry, which in turn often define the rates of ecosystem processes and nutrient cycling. Moreover, pervasive links have been reported between biodiversity, food web structure, ecosystem function and species stoichiometry. Here we review current global stoichiometric gradients and how future distributional shifts in key functional groups may in turn influence basic ecosystem functions (production, nutrient cycling, decomposition) and therefore could exert a feedback effect on stoichiometric gradients. The C-N-P stoichiometry of most primary producers (phytoplankton, algae, plants) has been linked to functional trait continua (i.e. to major axes of phenotypic variation observed in inter-specific analyses of multiple traits). In contrast, the C-N-P stoichiometry of higher-level consumers remains less precisely quantified in many taxonomic groups. We show that significant links are observed between trait continua across trophic levels. In spite of recent advances, the future reciprocal feedbacks between key functional groups, biodiversity and ecosystem functions remain largely uncertain. The reported evidence, however, highlights the key role of stoichiometric traits and suggests the need of a progressive shift towards an ecosystemic and stoichiometric perspective in global biodiversity analyses.

Quasi-equilibrium reduction in a general class of stoichiometric producer-consumer models

This article compares a general closed nutrient, stoichiometric producer-consumer model to a two-dimensional 'quasi-equilibrium' approximation. We demonstrate that the quasi-equilibrium system can be rigorously analysed, resulting in nullcline-based criteria for the local stability of system equilibria and for the non-existence of periodic orbits. These results are applied to a study of the dependence of the reduced system on nutrient and energy enrichment. When energy and nutrient enrichment are considered together, the associated bifurcation structures of the two models are seen to share the same essential qualitative characteristics. However, numerical simulations of the three-dimensional parent model show highly complex domains of the persistence and extinction that by Poincare-Bendixson theory are not possible for the two-dimensional reduction. This complexity demonstrates a major difference between the two models, and suggests potential challenges in the use of either model for predicting the long-term behaviour of real-world systems at specific nutrient and energy levels.

Food web architecture and basal resources interact to determine biomass and stoichiometric cascades along a benthic food web

Understanding the effects of predators and resources on primary producers has been a major focus of interest in ecology. Within this context, the trophic cascade concept especially concerning the pelagic zone of lakes has been the focus of the majority of these studies. However, littoral food webs could be especially interesting because base trophic levels may be strongly regulated by consumers and prone to be light limited. In this study, the availability of nutrients and light and the presence of an omnivorous fish (Hyphessobrycon bifasciatus) were manipulated in enclosures placed in a humic coastal lagoon (Cabiúnas Lagoon, Macaé - RJ) to evaluate the individual and interactive effects of resource availability (nutrients and light) and food web configuration on the biomass and stoichiometry of periphyton and benthic grazers. Our findings suggest that light and nutrients interact to determine periphyton biomass and stoichiometry, which propagates to the consumer level. We observed a positive effect of the availability of nutrients on periphytic biomass and grazers' biomass, as well as a reduction of periphytic C∶N∶P ratios and an increase of grazers' N and P content. Low light availability constrained the propagation of nutrient effects on periphyton biomass and induced higher periphytic C∶N∶P ratios. The effects of fish presence strongly interacted with resource availability. In general, a positive effect of fish presence was observed for the total biomass of periphyton and grazer's biomass, especially with high resource availability, but the opposite was found for periphytic autotrophic biomass. Fish also had a significant effect on periphyton stoichiometry, but no effect was observed on grazers' stoichiometric ratios. In summary, we observed that the indirect effect of fish predation on periphyton biomass might be dependent on multiple resources and periphyton nutrient stoichiometric variation can affect consumers' stoichiometry.

Prey dispersal rate affects prey species composition and trait diversity in response to multiple predators in metacommunities

1. Recent studies indicate that large-scale spatial processes can alter local community structuring mechanisms to determine local and regional assemblages of predators and their prey. In metacommunities, this may occur when the functional diversity represented in the regional predator species pool interacts with the rate of prey dispersal among local communities to affect prey species diversity and trait composition at multiple scales. 2. Here, we test for effects of prey dispersal rate and spatially and temporally heterogeneous predation from functionally dissimilar predators on prey structure in pond mesocosm metacommunities. An experimental metacommunity consisted of three pond mesocosm communities supporting two differentially size-selective invertebrate predators and their zooplankton prey. In each metacommunity, two communities maintained constant predation and supported either Gyrinus sp. (Coleoptera) or Notonecta ungulata (Hemiptera) predators generating a spatial prey refuge while the third community supported alternating predation from Gyrinus sp. and N. ungulata generating a temporal prey refuge. Mesocosm metacommunities were connected at either low (0.7% day(-1)) or high (10% day(-1)) planktonic prey dispersal. The diversity, composition and body size of zooplankton prey were measured at local and regional (metacommunity) scales. 3. Metacommunities experiencing the low prey dispersal rate supported the greatest regional prey species diversity (H') and evenness (J'). Neither dispersal rate nor predation regime affected local prey diversity or evenness. The spatial prey refuge at low dispersal maintained the largest difference in species composition and body size diversity between communities under Gyrinus and Notonecta predation, suggesting that species sorting was operating at the low dispersal rate. There was no effect of dispersal rate on species diversity or body size distribution in the temporal prey refuge. 4. The frequency distribution, but not the range, of prey body sizes within communities depended upon prey dispersal rate and predator identity. Taken together, these results demonstrate that prey dispersal rate can moderate the strength of predation to influence prey species diversity and the local frequency distribution of prey traits in metacommunities supporting ecologically different predators.

Stoichiometrically Explicit Food Webs: Feedbacks between Resource Supply, Elemental Constraints, and Species Diversity

A stoichiometrically explicit approach to food web ecology yields new insight into promotion and degradation of diversity, changes in species composition along environmental gradients, biomass partitioning among trophic levels, and limitation of primary production. These revelations emerge from food web modules that incorporate fundamental constraints imposed by mass balance and a key trait, stoichiometric body composition, into a species’ niche. These niche components involve a species’ requirements from its environment and its own impacts on its environment. More specifically, stoichiometric composition influences minimal nutrient requirements of consumers (perhaps especially grazers); this component becomes pertinent because large imbalances often arise between nutrient:carbon content of consumers relative to prey. Furthermore, these imbalances then modulate the impact of consumers on their own resources through nutrient recycling. Once these niche components become synthesized, their implications in shaping food webs provide powerful mechanisms linking changes in environmental gradients with community structure and ecosystem function.

Macroevolutionary turnover through the Ediacaran transition: ecological and biogeochemical implications

Ecological and evolutionary principles are often context-dependent, particularly where the context is biologically defined. Organ-grade animals (eumetazoans) are particularly powerful contextual agents, with a unique capacity to drive escalatory co-evolution and build multi-tiered food-webs. The evolution of eumetazoans through the Ediacaran and early Cambrian fundamentally altered macroecological and macroevolutionary dynamics, including the structure and function of the marine carbon cycle. Pelagic eumetazoans can be held responsible for driving the evolution of relatively large eukaryotic phytoplankton, thereby shifting the system from a turbid, stratified, cyanobacteria-dominated stable state to the clear-water, well-oxygenated, algae-dominated condition typical of the Phanerozoic. Intermittent return to the pre-Ediacaran state during Phanerozoic extinctions and oceanic anoxic events suggests that the widespread anoxia detected in pre-Ediacaran deep-marine sequences may be a consequence of this alternate biological pump rather than a reflection of fundamentally lower levels of atmospheric oxygen. The transition between the pre- and post-Ediacaran states is also associated with the oldest commercially exploitable hydrocarbons, a possible by-product of invading animals and their top-down impact on the biological pump.

Light, nutrients, and food-chain length constrain planktonic energy transfer efficiency across multiple trophic levels

The efficiency of energy transfer through food chains [food chain efficiency (FCE)] is an important ecosystem function. It has been hypothesized that FCE across multiple trophic levels is constrained by the efficiency at which herbivores use plant energy, which depends on plant nutritional quality. Furthermore, the number of trophic levels may also constrain FCE, because herbivores are less efficient in using plant production when they are constrained by carnivores. These hypotheses have not been tested experimentally in food chains with 3 or more trophic levels. In a field experiment manipulating light, nutrients, and food-chain length, we show that FCE is constrained by algal food quality and food-chain length. FCE across 3 trophic levels (phytoplankton to carnivorous fish) was highest under low light and high nutrients, where algal quality was best as indicated by taxonomic composition and nutrient stoichiometry. In 3-level systems, FCE was constrained by the efficiency at which both herbivores and carnivores converted food into production; a strong nutrient effect on carnivore efficiency suggests a carryover effect of algal quality across 3 trophic levels. Energy transfer efficiency from algae to herbivores was also higher in 2-level systems (without carnivores) than in 3-level systems. Our results support the hypothesis that FCE is strongly constrained by light, nutrients, and food-chain length and suggest that carryover effects across multiple trophic levels are important. Because many environmental perturbations affect light, nutrients, and food-chain length, and many ecological services are mediated by FCE, it will be important to apply these findings to various ecosystem types.

A cross-system synthesis of consumer and nutrient resource control on producer biomass

Nutrient availability and herbivory control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that (i) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems and (ii) bottom-up and top-down forces are interdependent, with statistical interactions that synergize or dampen relative influences on producer biomass. We used simple dynamic models to review ecological mechanisms that generate independent vs. interactive responses of community-level biomass. We calibrated these mechanistic predictions with the metrics of factorial meta-analysis and tested their prevalence across freshwater, marine and terrestrial ecosystems with a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients. Our analysis showed that producer community biomass increased with fertilization across all systems, although increases were greatest in freshwater habitats. Herbivore removal generally increased producer biomass in both freshwater and marine systems, but effects were inconsistent on land. With the exception of marine temperate rocky reef systems that showed positive synergism of nutrient enrichment and herbivore removal, experimental studies showed limited support for statistical interactions between nutrient and herbivory treatments on producer biomass. Top-down control of herbivores, compensatory behaviour of multiple herbivore guilds, spatial and temporal heterogeneity of interactions, and herbivore-mediated nutrient recycling may lower the probability of consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently multiple producer resources (e.g. nitrogen, phosphorus, light), multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates) and multiple trophic levels; and - in addition to measuring producer biomass - assess the responses of species diversity, community composition and nutrient status.