Chapter 1 - Núria BONADA

Chapter 3
In spite of these observed divergences between macroinvertebrate assemblage in med-region,
numerous ubiquitous taxa are present, what implies the existence of similar evolutionary
mechanisms of dispersion, extinction and adaptation of the taxa along time (Deacon, 1983).
Because of the dispersion and colonization of one organism depends on the environmental
conditions and life cycles (Cox & Moore, 1993), organisms with short life cycles, continuous
reproductions and aerial phases should be easily dispersed, and therefore they will have a
wider distribution (Williams & Feltmate, 1993). Baetidae, Caenidae, Leptophlebiidae,
Leptoceridae, Hydropsychidae, Oligochaeta and almost all dipterans are ubiquitous taxa, and
most of them have been considered as organisms easily to disperse because their
morphological and reproductive traits (for example see Gray, 1981; Gray & Fisher, 1981;
Fisher et al., 1982). Moreover, most of these taxa are characterized by having a very old origin
(e.g., Baetidae, Caenidae and Leptophlebiidae) in contrast with others with a local distribution
(e.g., Teloganodidae — Edmunds, 1972).
Historical factors are important to understand taxonomical convergences and divergences, but
the viability and success of one taxa in one new region will depend on the local and
environmental conditions (Resh & Solem, 1996). In that sense, Ball (1975) distinguishes
between an analytical (Historical Biogeography) and an empirical biogeography (Ecological
Biogeography). Thus, the abundance of EPT in Northern hemisphere and Chile respect other
areas could be interpreted by the mountain river typology of some rivers flowing from high
mountains near the coast (Sierra Nevada in California, Andes in Chile and Sierra Nevada,
Pyrenees, Apennines, Atlas,... in Med-Basin), that let the establishment of species adapted to
steep, cold and fast flowing rivers. On the other hand, even though some mountainous and
high gradient rivers with cold waters are present in South Africa and SAustralia, the low
EPT/OCH is explained by the poor contribution of Plecoptera in these regions because of
biogeographical factors (Zwick, 2000), instead of environmental ones. This difficulty to discern
between ecological and historical factors has been emphasized by Endler (1982), but both have
to be present to understand community structure and composition (Ricklefs, 1987; Menge &
Olson, 1990).
Local scale: Ecological factors and the spatio-temporal variability
In our study and according to the measured factors in reference conditions (without human
disturbance), pH is a key variable to diferenciate med-regions. Calcareous geology in
mediterranean basin (di Castri, 1981) is the responsible of a high pH in its rivers and streams
(Toro et al., (in press)), whereas heavily washed soils in South Africa and SWAustralia (Specht
& Moll, 1983), provide a high acidity in reference conditions. In South Africa, fynbos vegetation
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