[Show abstract][Hide abstract] ABSTRACT: The paradox of enrichment has been studied almost exclusively within communities or metacommunities, without ex- plicit nutrient dynamics. Yet local recycling of materials from en- riched ecosystems may affect the stability of connected ecosystems. Here we study the effect of nutrient, detritus, producer, andconsumer spatial flows—combined with changes in regional enrichment—on the stability of a metaecosystem model.We considered both spatially homogeneous and heterogeneous enrichment. We found that nutri- ent and detritus spatial flows are destabilizing, whereas producer or consumer spatial flows are either neutral or stabilizing. We noticed that detritus spatial flows have only a weak impact on stability. Our study reveals that heterogeneity no longer stabilizes well-connected systems when accounting for explicit representation of nutrient dy- namics. We also found that intermediate consumer diffusion could lead to multiple equilibria in strongly enriched metaecosystems. Sta- bility can emerge from a top-down control allowing the storage of materials into inorganic form, a mechanism never documented be- fore. In conclusion, local enrichment can be stabilized if spatial flows are strong enough to efficiently redistribute the local excess of en- richment to unfertile ecosystems. However, high regional enrichment can be dampened only by intermediate consumer diffusion rates.
Full-text · Article · Dec 2014 · The American Naturalist
[Show abstract][Hide abstract] ABSTRACT: We are interested in the impact of natural selection in a prey-predator
community. We introduce an individual-based model of the community that takes
into account both prey and predator phenotypes. Our aim is to understand the
phenotypic coevolution of prey and predators. The community evolves as a
multi-type birth and death process with mutations. We first consider the
infinite particle approximation of the process without mutation. In this limit,
the process can be approximated by a system of differential equations. We prove
the existence of a unique globally asymptotically stable equilibrium under
specific conditions on the interaction among prey individuals. When mutations
are rare, the community evolves on the mutational scale according to a
Markovian jump process. This process describes the successive equilibria of the
prey-predator community and extends the Polymorphic Evolutionary Sequence to a
coevolutionary framework. We then assume that mutations have a small impact on
phenotypes and consider the evolution of monomorphic prey and predator
populations. The limit of small mutation steps leads to a system of two
differential equations which is a version of the canonical equation of adaptive
dynamics for the prey-predator coevolution. We illustrate these results with an
example including different prey defense mechanisms.
[Show abstract][Hide abstract] ABSTRACT: Resource enrichment can potentially destabilize predator-prey dynamics. This phenomenon historically referred as the "paradox of enrichment" has mostly been explored in spatially homogenous environments. However, many predator-prey communities exchange organisms within spatially heterogeneous networks called metacommunities. This heterogeneity can result from uneven distribution of resources among communities and thus can lead to the spreading of local enrichment within metacommunities. Here, we adapted the original Rosenzweig-MacArthur predator-prey model, built to study the paradox of enrichment, to investigate the effect of regional enrichment and of its spatial distribution on predator-prey dynamics in metacommunities. We found that the potential for destabilization was depending on the connectivity among communities and the spatial distribution of enrichment. In one hand, we found that at low dispersal regional enrichment led to the destabilization of predator-prey dynamics. This destabilizing effect was more pronounced when the enrichment was uneven among communities. In the other hand, we found that high dispersal could stabilize the predator-prey dynamics when the enrichment was spatially heterogeneous. Our results illustrate that the destabilizing effect of enrichment can be dampened when the spatial scale of resource enrichment is lower than that of organismss movements (heterogeneous enrichment). From a conservation perspective, our results illustrate that spatial heterogeneity could decrease the regional extinction risk of species involved in specialized trophic interactions. From the perspective of biological control, our results show that the heterogeneous distribution of pest resource could favor or dampen outbreaks of pests and of their natural enemies, depending on the spatial scale of heterogeneity.
[Show abstract][Hide abstract] ABSTRACT: The paradox of enrichment has been studied almost exclusively within communities or metacommunities, without explicit nutrient dynamics. Yet local recycling of materials from enriched ecosystems may affect the stability of connected ecosystems. Here we study the effect of nutrient, detritus, producer, and consumer spatial flowscombined with changes in regional enrichmenton the stability of a metaecosystem model. We considered both spatially homogeneous and heterogeneous enrichment. We found that nutrient and detritus spatial flows are destabilizing, whereas producer or consumer spatial flows are either neutral or stabilizing. We noticed that detritus spatial flows have only a weak impact on stability. Our study reveals that heterogeneity no longer stabilizes well-connected systems when accounting for explicit representation of nutrient dynamics. We also found that intermediate consumer diffusion could lead to multiple equilibria in strongly enriched metaecosystems. Stability can emerge from a top-down control allowing the storage of materials into inorganic form, a mechanism never documented before. In conclusion, local enrichment can be stabilized if spatial flows are strong enough to efficiently redistribute the local excess of enrichment to unfertile ecosystems. However, high regional enrichment can be dampened only by intermediate consumer diffusion rates.
[Show abstract][Hide abstract] ABSTRACT: Global warming leads to increased intensity and frequency of weather extremes. Such increased environmental variability might in turn result in increased variation in the demographic rates of interacting species with potentially important consequences for the dynamics of food webs. Using a theoretical approach, we here explore the response of food webs to a highly variable environment. We investigate how species richness and correlation in the responses of species to environmental fluctuations affect the risk of extinction cascades. We find that the risk of extinction cascades increases with increasing species richness, especially when correlation among species is low. Initial extinctions of primary producer species unleash bottom-up extinction cascades, especially in webs with specialist consumers. In this sense, species-rich ecosystems are less robust to increasing levels of environmental variability than species-poor ones. Our study thus suggests that highly species-rich ecosystems such as coral reefs and tropical rainforests might be particularly vulnerable to increased climate variability.
Full-text · Article · Apr 2012 · Ecology and Evolution
[Show abstract][Hide abstract] ABSTRACT: 1. The interaction between mutualism, facilitation or interference and exploitation competition is of major interest as it may govern species coexistence. However, the interplay of these mechanisms has received little attention. This issue dates back to Gause, who experimentally explored competition using protists as a model [Gause, G.F. (1935) Vérifications expérimentales de la théorie mathématique de la lutte pour la vie. Actualités Scientifiques et Industrielles, 277]. He showed the coexistence of Paramecium caudatum with a potentially allelopathic species, Paramecium bursaria.
2. Paramecium bursaria hosts the green algae Chlorella vulgaris. Therefore, P. bursaria may benefit from carbohydrates synthesised by the algae. Studying endosymbiosis with P. bursaria is possible as it can be freed of its endosymbiont. In addition, C. vulgaris is known to produce allelochemicals, and P. bursaria may benefit also from allelopathic compounds.
3. We designed an experiment to separate the effects of resource exploitation, endosymbiosis and allelopathy and to assess their relative importance for the coexistence of P. bursaria with a competitor that exploits the same resource, bacteria. The experiment was repeated with two competitors, Colpidium striatum or Tetrahymena pyriformis.
4. Results show that the presence of the endosymbiont enables the coexistence of competitors, while its loss leads to competitive exclusion. These results are in agreement with predictions based on resource equilibrium density of monocultures (R*) supporting the idea that P. bursaria’s endosymbiont is a resource provider for its host. When P. bursaria and T. pyriformis coexist, the density of the latter shows large variation that match the effects of culture medium of P. bursaria. Our experiment suggests these effects are because of biochemicals produced in P. bursaria culture.
5. Our results expose the hidden diversity of mechanisms that underlie competitive interactions. They thus support Gauses’s speculation (1935) that allelopathic effects might have been involved in his competition experiments. We discuss how a species engaged both in competition for a resource and in costly interference such as allelopathy may counterbalance these costs with a resource-provider endosymbiont.
Full-text · Article · Aug 2011 · Journal of Animal Ecology
[Show abstract][Hide abstract] ABSTRACT: Although density-dependent dispersal and relative dispersal (the difference in dispersal rates between species) have been documented in natural systems, their effects on the stability of metacommunities are poorly understood. Here we investigate the effects of intra- and interspecific density-dependent dispersal on the regional stability in a predator-prey metacommunity model. We show that, when the dynamics of the populations reach equilibrium, the stability of the metacommunity is not affected by density-dependent dispersal. However, the regional stability, measured as the regional variability or the persistence, can be modified by density-dependent dispersal when local populations fluctuate over time. Moreover these effects depend on the relative dispersal of the predator and the prey. Regional stability is modified through changes in spatial synchrony. Interspecific density-dependent dispersal always desynchronizses local dynamics, whereas intraspecific density-dependent dispersal may either synchronize or desynchronize it depending on dispersal rates. Moreover, intra- and interspecific density-dependent dispersal strengthen the top-down control of the prey by the predator at intermediate dispersal rates. As a consequence the regional stability of the metacommunity is increased at intermediate dispersal rates. Our results show that density-dependent dispersal and relative dispersal of species are keys to understanding the response of ecosystems to fragmentation.
Full-text · Article · Oct 2010 · Journal of Theoretical Biology
[Show abstract][Hide abstract] ABSTRACT: Spatial heterogeneity of the environment has long been recognized as a major factor in ecological dynamics. Its role in predator-prey systems has been of particular interest, where it can affect interactions in two qualitatively different ways: by providing (1) refuges for the prey or (2) obstacles that interfere with the movements of both prey and predators. There have been relatively fewer studies of obstacles than refuges, especially studies on their effect on functional responses. By analogy with reaction-diffusion models for chemical systems in heterogeneous environments, we predict that obstacles are likely to reduce the encounter rate between individuals, leading to a lower attack rate (predator-prey encounters) and a lower interference rate (predator-predator encounters). Here, we test these predictions under controlled conditions using collembolans (springtails) as prey and mites as predators in microcosms. The effect of obstacle density on the functional response was investigated at the scales of individual behavior and of the population. As expected, we found that increasing obstacle density reduces the attack rate and predator interference. Our results show that obstacles, like refuges, can reduce the predation rate because obstacles decrease the attack rate. However, while refuges can increase predator dependence, we suggest that obstacles can decrease it by reducing the rate of encounters between predators. Because of their opposite effect on predator dependence, obstacles and refuges could modify in different ways the stability of predator-prey communities.
[Show abstract][Hide abstract] ABSTRACT: 1.
Dispersal intensity is a key process for the persistence of prey–predator metacommunities.
Consequently, knowledge of the ecological mechanisms of dispersal is
fundamental to understanding the dynamics of these communities. Dispersal is often
considered to occur at a constant
rate; however, some experiments
demonstrated that dispersal may be a function of local species density.
Here we use aquatic experimental microcosms under controlled conditions to explore
intra- and interspecific density-dependent dispersal in two protists, a prey
and its predator
We observed intraspecific density-dependent dispersal for the prey and interspecific
density-dependent dispersal for both the prey and the predator. Decreased prey density
lead to an increase in predator dispersal, while prey dispersal increased with predator
Additional experiments suggest that the prey is able to detect its predator through
chemical cues and to modify its dispersal behaviour accordingly.
Density-dependent dispersal suggests that regional processes depend on local
community dynamics. We discuss the potential consequences of density-dependent
dispersal on metacommunity dynamics and stability.
Full-text · Article · May 2007 · Journal of Animal Ecology