ecological effects of marine protected areas empafish project ...

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.
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