Chapter 1 - Núria BONADA

Regional amd local scale: convergences and divergences
and permanent streams washing out downstream algae and macroinvertebrate communities.
After the flood, communities may be reestablished relatively fast (Badri et al., 1987; Vidal-
Abarca et al., 1992; Pires et al., 2000). According to the variability of the discharge along the
year the rivers and streams can be classified into three groups (Bonada et al., Chapter 5):
permanents with running water in summer, intermittents with isolated pools in summer and
ephemerals that are totally dried up after few months (even weeks) of the flow cessation.
This variety in characteristics in mediterranean rivers, provided by natural disturbances, is
the origin of a high floristic and faunistic diversity (Prat et al., 2000a) and endemism (Gasith &
Resh, 1999). However, these ecosystems are very sensitive to a loss of diversity, because the
seasonal variability of the mediterranean climate makes the fluvial systems more vulnerable to
pollution (Gasith, 1992; Davies et al., 1993; Sabater et al., 1995; Gasith & Resh, 1999).
Consequently, Mediterranean rivers are highly affected by human disturbances yielding a very
poor ecological status (Prat, 1993; Prat & Munné, 2000). Moreover, in nowhere else in the
world the aquatic fauna is declining as rapidly than in mediterranean regions (Moyle & Leidy,
1992) because of the non-native introductions, high population density, limited availability of
water and unsatisfactory management.
Johnson (1973) postulated that although similar biota between isolated areas is possible
because of a common ancestor, usually the environmental conditions have actuated more or
less equally over the evolution of the biota, and so it is not likely that the convergence
characteristics are reached by chance. Therefore, because the mediterranean climate shares
similar kind of environments, macroinvertebrate communities should also be similar (Gasith &
Resh, 1999), as the vegetation (Mooney and Dunn, 1970) or lizard communities are (Fuentes,
1976). Several ecological freshwater studies have been done comparing different biomes,
climates or continents (Vinson & Hawkins, 1998) looking for convergences and divergences
(Moyle & Herbold, 1987; Lamoroux et al., 2002), but few of them are focused on comparisons
between mediterranean rivers (see King et al., 1988), although the numerous local studies
performed in these areas (see Gasith & Resh, 1999). Studies about ecological convergences in
mediterranean ecosystems are required to determine the applicability to another area of the
ecological principles derived from local studies (Kruger et al., 1983).
Because communities have different structure, composition and responses may change at
different scales of observation (Kotliar & Wiens, 1990; Tonn et al., 1990; Wiens et al., 1993;
Peckarsky et al., 1997). Choosing the appropriate scale of study have been a problem for many
stream ecologists (Peckarsky et al., 1997) and thereby, several authors recommend performing
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