Traits affecting nutrient recycling by mobile consumers can explain coexistence and spatially heterogeneous trophic regulation across a meta-ecosystem.

Ecosystems are linked through spatial flows of organisms and nutrients that impact their biodiversity and regulation. Theory has predominantly studied passive nutrient flows that occur independently of organism movement. Mobile organisms, however, commonly drive nutrient flows across ecosystems through nutrient recycling. Using a meta-ecosystem model where consumers move between ecosystems, we study how consumer recycling and traits related to feeding and sheltering preferences affect species diversity and trophic regulation. We show local effects of recycling can cascade across space, yielding spatially heterogeneous top-down and bottom-up effects. Consumer traits impact the direction and magnitude of these effects by enabling recycling to favour a single ecosystem. Recycling further modifies outcomes of competition between consumer species by creating a positive feedback on the production of one competitor. Our findings suggest spatial interactions between feeding and recycling activities of organisms are key to predicting biodiversity and ecosystem functioning across spatial scales.

Converting Ecological Currencies: Energy, Material, and Information Flows.

Understanding how the three currencies of life - energy, material, and information - interact is a key step towards synthesis in ecology and evolution. However, current theory focuses on the role of matter as a resource and energy, and typically ignores how the same matter can have other important effects as a carrier of information or modifier of the environment. Here we present the hypothesis that the dynamic conversion of matter by organisms among its three currencies mediates the structure and function of ecosystems, and that these effects can even supersede the effects of matter as a resource. Humans are changing the information in the environment and this is altering species interactions and flows of matter within and among ecosystems.

Meta-ecosystem processes alter ecosystem function and can promote herbivore-mediated coexistence.

Herbivory and dispersal play roles in the coexistence of primary producers with shared resource limitation by imposing trade-offs either through apparent competition or dispersal limitation. These mechanisms of coexistence can further interact with meta-ecosystem effects, which results in spatial heterogeneity through the movement of herbivores and nutrients. Here, we investigate how herbivores influence autotroph coexistence through a meta-ecosystem effect, and how this effect couples mechanisms of coexistence to ecosystem structure and functioning. We articulate this framework through a parameterized one resource-k producer-one herbivore meta-ecosystem model. The results show that herbivore movement with nutrient recycling can generate spatial heterogeneity to allow coexistence where the well-mixed system predicts competitive exclusion. Furthermore, the presence of movement alters local and regional ecosystem functioning even when coexistence would occur without movement. These results highlight how meta-ecosystem theory can provide a mechanistic context for the observed complexity of biodiversity-ecosystem function relationships.

Nonhierarchical Dispersal Promotes Stability and Resilience in a Tritrophic Metacommunity.

Community interactions (e.g., predation, competition) can be characterized by two factors: their strengths and how they are structured between and within species. Both factors play a role in determining community dynamics. In addition to trophic interactions, dispersal acts as an interaction between separate populations. As with other interactions, the structure of dispersal can affect the stability of a system. However, the primary structure that has been studied in consumer-resource models has been hierarchical dispersal, where between-patch dispersal rates increase with trophic level. Here we use analytical, numerical, and simulation approaches on a two-patch, three-species metacommunity model to investigate the relationship between structure and community stability and resilience. We show that metacommunity stability is greater in systems with both weak and strong dispersal rates. Our system is stabilized by the formation of patterns when predators disperse frequently and herbivores disperse rarely, and via asynchrony when both predators and herbivores disperse infrequently. Our results show how interaction strengths within both trophic and spatial networks shape metacommunity stability.

Emergence of nutrient co-limitation through movement in stoichiometric meta-ecosystems.

Evidence that ecosystems and primary producers are limited in their productivity by multiple nutrients has caused the traditional nutrient limitation framework to include multiple limiting nutrients. The models built to mimic these responses have invoked local mechanisms at the level of the primary producers. In this paper, we explore an alternative explanation for the emergence of co-limitation by developing a simple, stoichiometrically explicit meta-ecosystem model with two limiting nutrients, autotrophs and herbivores. Our results show that differences in movement rates for the nutrients, autotrophs and herbivores can allow for nutrient co-limitation in biomass response to emerge despite no local mechanisms of nutrient co-limitation. Furthermore, our results provide an explanation to why autotrophs show positive growth responses to nutrients despite 'nominal' top-down control by herbivores. These results suggest that spatial processes can be mechanisms for nutrient co-limitation at local and regional scales, and can help explain anomalous results in the co-limitation literature.

Meta-ecosystem dynamics and functioning on finite spatial networks.

The addition of spatial structure to ecological concepts and theories has spurred integration between sub-disciplines within ecology, including community and ecosystem ecology. However, the complexity of spatial models limits their implementation to idealized, regular landscapes. We present a model meta-ecosystem with finite and irregular spatial structure consisting of local nutrient-autotrophs-herbivores ecosystems connected through spatial flows of materials and organisms. We study the effect of spatial flows on stability and ecosystem functions, and provide simple metrics of connectivity that can predict these effects. Our results show that high rates of nutrient and herbivore movement can destabilize local ecosystem dynamics, leading to spatially heterogeneous equilibria or oscillations across the meta-ecosystem, with generally increased meta-ecosystem primary and secondary production. However, the onset and the spatial scale of these emergent dynamics depend heavily on the spatial structure of the meta-ecosystem and on the relative movement rate of the autotrophs. We show how this strong dependence on finite spatial structure eludes commonly used metrics of connectivity, but can be predicted by the eigenvalues and eigenvectors of the connectivity matrix that describe the spatial structure and scale. Our study indicates the need to consider finite-size ecosystems in meta-ecosystem theory.

A stoichiometric model of early plant primary succession.

The relative importance of plant facilitation and competition during primary succession depends on the development of ecosystem nutrient pools, yet the interaction of these processes remains poorly understood. To explore how these mechanisms interact to drive successional dynamics, we devised a stoichiometric ecosystem-level model that considers the role of nitrogen and phosphorus limitation in plant primary succession. We applied this model to the primary plant community on Mount St. Helens, Washington State, to check the validity of the proposed mechanisms. Our results show that the plant community is colimited by nitrogen and phosphorus, and they confirm previous suggestions that the presence of a nitrogen-fixing legume, Lupinus lepidus, can enhance community biomass. In addition, the observed nutrient supply rates may promote alternative successional trajectories that depend on the initial plant abundances, which may explain the observed heterogeneity in community development. The model further indicates the importance of mineralization rates and other ecosystem parameters to successional rates. We conclude that a model framework based on ecological stoichiometry allows integration of key biotic processes that interact nonlinearly with biogeochemical aspects of succession. Extension of this approach will improve the understanding of the process of primary succession and its application to ecosystem rehabilitation.

Nutrient flows between ecosystems can destabilize simple food chains.

Dispersal of organisms has large effects on the dynamics and stability of populations and communities. However, current metacommunity theory largely ignores how the flows of limiting nutrients across ecosystems can influence communities. We studied a meta-ecosystem model where two autotroph-consumer communities are spatially coupled through the diffusion of the limiting nutrient. We analyzed regional and local stability, as well as spatial and temporal synchrony to elucidate the impacts of nutrient recycling and diffusion on trophic dynamics. We show that nutrient diffusion is capable of inducing asynchronous local destabilization of biotic compartments through a diffusion-induced spatiotemporal bifurcation. Nutrient recycling interacts with nutrient diffusion and influences the susceptibility of the meta-ecosystem to diffusion-induced instabilities. This interaction between nutrient recycling and transport is further shown to depend on ecosystem enrichment. It more generally emphasizes the importance of meta-ecosystem theory for predicting species persistence and distribution in managed ecosystems.