Chapter 1 - Núria BONADA

Chapter 3
intermittency, flood frequency and predictability and flow predictability suggesting highest
richness under high flow predictability by low in high flood and drought frequency.
Mediterranean rivers, are characterized by predictable disturbances (floods and droughts)
(Gasith & Resh, 1999) what would imply a higher regional richness compare with other
climatic regions where floods and droughts are more extreme and less predictible.
Consequently, although the intensity and frequency of this discharge disturbance is
impredectible (Vidal-Abarca, 1990; Gasith & Resh, 1999) regional richness found in med-
regions (as number of family taxa) appears is very high, except for SWAustralia, where
historical events constrained a major number of taxa (Bunn & Davies, 1990). Values of
richness obtained from this study are difficult to compare with other climatic areas because no
comparative studies between climates have been performed. However, some extensive studies
performed elsewhere may be useful. Thus, for example, an extensive study in NWAustralia,
under two climates, 90 macroinvertebrate families were collected, 77 in the tropical wet-dry
climate and 73 and 61 in the desert one (Kay et al., 1999), values lower than the found in
med-regions. Studies in plant biology agree with this high biodiversity in med-regions (Raven
1973; Deacon, 1983), what exhibit that under intermediate stress conditions a high
biodiversity can be hold. The explanation may be found in the predictability of events (floods
and drougths) in the mediterranean climate (Gasith & Resh, 1999) and the presence of
resilient and resistant adaptations to these disturbances by evolutionary forces acting since
the begining of the earth history.
According to Ricklefs (1987) the relationship between regional and local richness indicate the
relative importance of regional and local processess influencing local richness. Thus, if med-
regions with different regional richness have similar local richness, local processes are
important, whereas if a positive relationship between regional and local richness is present,
regional factors dominate. Figure 14 displays the relationship between regional and local
richness in the sampled med-regions. The relationship is positive between SWAustralia, South
Africa and California. Regional processes are important to determine local diversity in these
areas because their different biogeographic history. However, although MedBasin presents the
highest regional richness, it displays a highly variable intermediate local richness, indicating
that there is a regulation of local richness by local factors. This phenomena does not agree
with Shmida (1981) who comparing vegetation of chaparral and coastal sage between Israel
and southern California found four time more regional richness and two times local richness
in Israel than in California. MedBasin region present a changing topography, landscapes and
microclimates (see Plate 2) that originate numerous river types (e.g., ramblas, short and nival-
influence rivers, karsts,…). In this situation, different local factors (biotic or abiotic) can act
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