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EMPAFISH Booklet no. 1 Ecological effects of Atlanto-Mediterranean MPAs in the EU dispersal distance and can help set the appropriate geographic scales on which marine reserve systems will function well (Pérez Ruzafa et al. 2006). When the density of a population is higher inside a reserve than in adjacent non-reserve areas, random movements are expected to produce a net emigration from the reserve (Rakitin & Kramer 1996). Furthermore, frequency-dependent models of animal distribution such as the Ideal Free Distribution Model predict that, when the species fitness is affected by the relationship of population density and resource availability, animals will tend to move from areas where their density is high (Rakitin & Kramer 1996). Emigration of individuals from MPAs has been proposed as a potential benefit of reserves for fisheries management and population replenishment (Russ & Alcala 1996), and is expected to produce a gradient of abundance and mean size across reserve boundaries. Kramer & Chapman (1999) examined the implications of fish home range size and relocation on marine reserve function and ability to increase abundance outside reserve boundaries. They predict that species with intermediate levels of mobility and density-dependence of space use will provide the greatest spillover benefits to nearby fisheries. Potential emigration could thus be important for demersal? fishes and some invertebrates such as lobsters or shrimps. These species may spend enough time inside the reserve to experience a significant reduction in fishing mortality while having the ability to move outside the protected area. Even though this spillover effect is widely assumed and expected, there is remarkably little evidence of this effect so far (Sanchez-Lizaso et al. 2000). Spillover is the main focus of a EU research project in progress – BIOMEX (http://biomex.univ-perp.fr) – that will provide significant input to EMPAFISH. 6 2.3 Other biological effects The expected higher densities of previously-exploited species in MPAs may produce an augmentation of intraspecific and interspecific competition as biomass of the populations approaches the carrying capacity of the area (Sánchez Lizaso et al. 2000). Life history parameters (such as life-span, growth, natural mortality, age and size at maturity and reproproductive patterns) are thought of as plastic or adaptive (Stearns & Crandall 1984) and could conceivably be easily affected by changes in population density in protected areas after fishing restrictions are put in place if resources become limiting (Sánchez Lizaso et al. 2000). Finally, since protected areas are not closed systems, resource limitation brought about by increased competition in dense protected populations could induce density-dependent emigration from MPAs, leading to spillover. 2.4 Indirect effects The trophic cascade is a major ecosystem process based on predation interactions, involving at least three or four trophic levels of an ecosystem (Pinnegar et al. 2000). Variations in food consumption by one trophic level, usually top-carnivores, cascade down the food web, each trophic level influencing the one below. Whereas the energy transfer propagates upwards
EMPAFISH Booklet no. 1 Ecological effects of Atlanto-Mediterranean MPAs in the EU through the food web (from primary producers to top-predators, i.e. a 'bottom-up' process), true trophic cascades are generally viewed from the 'top-down' pathway (from top-predators to primary producers). The "true trophic cascades imply keystone species, with such top-down dominance that their removal causes precipitous change in the system" (ref). Nevertheless, the concept of 'keystone' species has been given different meanings and definitions, and has been applied to organisms with very different effects on their communities. Demonstrating cascading impacts is difficult as reactions down through the food webs progressively decrease. Depending on the complexity of the system and the specialization of the species concerned, changes down the food web could be highly variable and diffuse. The effects of removal of top-predators would be weakened down a species-rich system and/or down a system with a high abundance of omnivorous species[or just “a system with abundant omnivores”]. In an ecosystem, species are interconnected by different types of direct and indirect interactions, the effects of which are intermingled in the ecosystem functioning. Direct predator effectiveness is mediated by various indirect effects (shelter availability, heterogeneity of habitats, variability of recruitment, size refuge, patchiness of resources, disease, stochasticity of abiotic events) which obscure the pure trophic cascading effects. Trophic cascades generally occur when the runaway consumption flow is unified, i.e. specifically directed to some kind of food. According to the numerous papers published about trophic cascades in fresh waters ecosystems, we could hypothesize that a true cascade effect is evident if the number of links in the trophic web is not too large, e.g. when the trophic web is damaged. However, if the number of trophic levels and the number of trophic links increase, a clear trophic cascade effect may probably be impossible to detect. Consequently, are MPA’s the best places to study cascade effects? A strict banning of all fishing pressures in an MPA is likely to allow recovery, not to pristine conditions, but to a greater species richness and/or trophic diversity than outside, in unprotected areas. There may therefore be a lower chance of detecting effects of trophic cascades in MPAs, where trophic diversity has increased. 2.5 The interference of habitat structure In assessing the effects of MPAs, there is a potential source of confounding in the fact that, usually, marine reserves are established in zones that already harbour structurally complex habitats, which form favourable habitats for the development of a rich and abundant reef fish fauna (García Charton & Pérez- Ruzafa 1999; García Charton et al. 2000, 2004). Part of the observed variability in fish assemblage structure could be due to selection of areas to be protected that are particularly favourable to high fish abundance and diversity, because they offer predominantly rocky, complex habitats. Nevertheless, the influence of habitat structure seems to be exerted mainly at small-to-intermediate spatial scales, since, isolating the part of variation due to habitat from the variables chosen as indicators of the ‘‘reserve effect’’ produces mainly the loss of heterogeneity among sectors and/or zones (García Charton et al. 2004), although in some cases (e.g. Ordines et al. 7
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