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Final Report Summary - MARINECOSYSTABILITY (Complexity, stability and chaos in marine model ecosystems for present day and global warming conditions)

Article 1: In the face of stochastic climatic perturbations, the overall stability of an ecosystem will be determined by the balance between its resilience and its resistance, but their relative importance is still unknown. Using aquatic food web models we study ecosystem stability as a function of food web complexity. We measured three dynamical stability properties: resilience, resistance, and variability. Specifically, we evaluate how a decrease in the strength of predator-prey interactions with food web complexity, reflecting a decrease in predation efficiency with the number of prey per predator, affects the overall stability of the ecosystem. We find that in mass conservative ecosystems, a lower interaction strength slows down the mass cycling rate in the system and this increases its resistance to perturbations of the growth rate of primary producers. Furthermore, we show that the overall stability of the food webs is mostly given by their resistance, and not by their resilience. Resilience and resistance display opposite trends, although they are shown not to be simply opposite concepts but rather independent properties. The ecological implication is that weaker predator-prey interactions in closed ecosystems can stabilize food web dynamics by increasing its resistance to climatic perturbations.

Article 2: Predators' switching towards the most abundant prey is a behaviour that allows for a higher degree of species co-existence in food webs. Active preyswitching is a mechanism that can stabilize population dynamics and overcome competitive exclusion. However, current parametrizations of prey-switching suffer from the problems known as antagonistic and sub-optimal feeding, in which predators are unable to maximize the ingestion of the total food available. We analyse three previously published multi-species functional responses which have either active prey-switching or maximal feeding, but not both. We identify the cause of this apparent incompatibility and describe a kill-the-winner (KTW) parametrization that reconciles active prey-switching with maximal feeding. Global simulations using a marine ecosystem model with 64 phytoplankton species belonging to 4 major functional groups show that the species richness and biogeography of phytoplankton are very sensitive to the choice of the functional response for grazing. The combination of active prey-switching with maximal feeding results in the highest level of diversity as well as the most plausible phytoplankton functional-group biogeography of the four functional responses evaluated.

Article 3: There is an ongoing debate about what should be the shape (if any) of the relationship between productivity and diversity or about the mechanisms behind it. Does productivity control diversity? Does diversity control productivity? Or do productivity and diversity affect each other in a convoluted way? Our global ocean simulations using killing-the-winner predation give support to a well stablished hypothesis which suggests that diversity should respond to primary production following a "hump-shaped" relationship, with diversity peaking at intermediate levels of productivity. The theory supporting the "hump-shaped" relationship is essentially based on the idea that a moderate increase in the nutrient supply will help sustain more diversity by allowing more species to exploit a fraction of the total potential production of the system. We show that diversity increases with the nutrient supply because KTW selective grazing helps down-regulate the most dominant species, which allows other species to invade and persist.

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