Plant Diversity and Fire - Fire in Mediterranean Ecosystems - page 310

11 Plant Diversity and Fire

Mediterranean-type climate (MTC) regions are some of the most botanically
diverse landscapes in the world ( Table 11.1 ). They are among the 25 global hot-
spots of diversity in both richness of species and endemics (Myers et al. 2000 ).
Occupying a bit more than 2% of the Earth's surface these landscapes hold
15-20% of the world's total vascular plants (Cowling et al. 1996 ; Rundel 2004 ).
Between the five regions there is extraordinary variation in temporal and spatial
patterns of vascular plant diversity and the relationship between fire and diversity
is quite different across the five MTC ecosystems.
Differences between MTC regions are evident at many scales but one of the
frequently noted differences is the regional species density or number of species per
unit area. To put this in perspective we need to recognize that one of the commonly
held generalizations about species diversity is that it increases with area ( Fig. 11.1a , b ).
This species-area relationship is understandable since there are constraints on the
number of individuals that can sustainably occupy a given area. Thus, as area
increases, the probability of encountering more species increases. However, despite
the observation that the number of species increases with increasing area is one of
the few “laws” in ecology (Lomolino 2001 ), there are exceptions. Dissimilar environ-
ments often have very different species richness. Thus, this species-area relationship
only approaches the status of a “law” when describing patterns in nested samples
(Dunn & Loehl 1988 ); that is, samples of different size taken from within the bound-
aries of larger samples so that species from the smallest sample unit share environ-
mental features with larger sample units ( Box 11.1 ). There is no clearer demonstration
of this than the species-area relationship observed for total regional diversity between
the five MTC regions ( Fig. 11.1c ). The glaring lack of fit to an idealized species-area
relationship ( Fig. 11.1a ) points up some of the important differences in diversity
between these MTC regions. These patterns are the result of complex responses to
subtle variations in climate, not so subtle variations in geology, and to their interaction
with fire, as well as to phylogenetic and biogeographic histories.

Fire and Community Diversity

One of the key differences in diversity between MTC plant communities is tied to
differences in vegetation structure, which reflect differences in soil fertility, and to