An ecosystem is a very complex entity with many interactive components. It can be defined as "a system of complex interactions of populations between themselves and with their environment" or as "the joint functioning and interaction of these two compartments (populations and environment) in a functional unit of variable size" (Odum, 1975; Ellenberg, 1973; Nybakken, 1982; Scialabba, 1998). In this review, and in EAF, we will consider "populations" as including people, and especially people involved in fisheries, with their technology and institutions (see Figure 1).
Ecosystems may be considered at different geographical scales, from a grain of sand with its rich microfauna, to a whole beach, a coastal area or estuary, a semi-enclosed sea and, eventually, the whole Earth. As stated by Lackey (1999), ecosystems are defined at a wide range of scales of observation "from a drop of morning dew to an ocean,... from a pebble to a planet". Ecosystems defined at a given geographical and functional scale are therefore nested within larger ones and contain smaller ones with which they exchange matter and information. Fisheries stocks, operations and management (including EAF) are relevant at intermediate geographical scale (from a few to thousands of kilometres).
While mapping ecosystems is one of the prerequisites of their management, their geographical boundaries are not easy to determine. They depend on the scale considered and can never encompass all the relevant processes. Boundaries may also be variable as the ecosystem's extension and location change seasonally or from year to year under changing climatic conditions (see below). This variability is higher in the pelagic than in the demersal domain.
Ecosystems are dynamic, composite entities within which large quantities of matter, energy and information flow, within and between components, in a way that is not yet completely understood. These flows depend on the ecosystem structure and determine it. There is not yet agreement as to whether the flows are controlled primarily by: (1) top predators' feeding behaviour (top-down control); (2) primary producers (bottom-up control); (3) some numerically abundant species somewhere in the middle of the food chain (wasp-waist control); or (4) some combination of some or all of these, depending on systems and their possible states (Cury et al., 2003).
Ecosystems' structure, species composition and functioning change seasonally (a type of change well understood by fisheries) as well as between years (a major source of uncertainty). Those changes can appear as quasi-cyclic (e.g. 11-12 years' cycles related to sunspot activity) with multiple frequencies (Klyashtorin, 1998) or as sudden shifts between apparently alternative "stable" states. Some well-known environmental fluctuations are those causing El Niño events, which change the patterns of Eastern Pacific ocean currents and affect global weather every few years. These long- and medium-term natural fluctuations result in changes in distribution, abundance and physiology of marine organisms, associated with changes in the extension, localization, structure, productivity and other characteristics of the ecosystems in which they live.
Environmental changes can produce effects similar to those of fishing, and it is often difficult to distinguish between them. Although they cannot be controlled directly, they exert a fundamental influence on the stability and resilience of marine ecosystems and their resources and they must be taken into account by managers. As most of these changes are not predictable with the available knowledge, they create uncertainty as to the future states of the system or its reaction to exploitation and management, leading to potential errors and consequent risk for the people and the resources (Cury et al., 2003; Christensen et al., 1996).
The functioning of an ecosystem results from the organization of its species communities, consisting of species populations having their own dynamics in terms of abundance, survival, growth, production, reproductive and other strategies. The community's resilience depends on its capacity to adapt to the physical environment and on its relations with the other communities, e.g. through competition or predation. Communities are interdependent and interconnected as trophic networks (resulting from predator-prey relationships) depending on environmental variables. Food-web analysis and estimates of consumption are essential for understanding possible reactions of the ecosystem to exploitation regimes as well as rebuilding strategies and, during the last decade, the information on this matter has greatly improved (Trites, 2003). However, uncertainty about the structure and functioning of food webs remains a major concern in EAF. The interactions between populations and the environment in which they live lead to a hierarchy of abundance and to the organization of the community. This organization is one of the measurable elements of biodiversity, related to the health or integrity of the ecosystems.
The description of the fishers' interaction within the ecosystem requires identification of four main ecosystem compartments: (1) a biotic compartment, including target fish resources, associated and dependent species and the living habitat (seagrass, algal beds, corals); (2) an abiotic compartment, characterized by its topography, bottom types, water quality and local weather/climate; (3) a fishery compartment, in which harvesting and processing activities take place, with a strong technological character, and (4) an institutional compartment, comprising laws, regulations and organizations needed for fisheries governance. Humans are part of the biotic component of the ecosystem from which they draw resources, food, services and livelihood as well as part of the fishery component which they drive. These components interact and are affected by: (i) non-fishing activities; (ii) the global climate; (iii) other ecosystems, usually adjacent, with which they exchange matter and information; and (iv) the socio-economic environment as reflected in the market, relevant policies and societal values. A simplified diagram of the interactions involved in an exploited ecosystem is given in Figure 1 below.
Figure 1. Simplified diagram of an ecosystem and its components