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In-Depth Information
the MTC summer drought since this is a drought-deciduous taxon. More recent
Quaternary speciation is also supported by molecular studies for the largely
herbaceous South African genera such as
Heliophila
and
Ehrharta
(Verboom
et al.
2003
; Linder
2005
), also perhaps in response to intensification of the MTC
summer drought. The herbaceous Australian Restionaceae in contrast appear to
have had a constant rate of speciation since their early Tertiary origin (Linder
et al.
2003
). These include both facultative and obligate postfire seeders, suggest-
ing a long history with fire and possibly a factor driving diversification.
Life History and Community Structure
At a localized scale the distribution of species reflects abiotic filters, such as
climate, fire and soils, as well as biotic interactions such as competition and
facilitation, and the resultant community composition is thought to be con-
strained in predictable ways. Community assembly rules are based on the concept
that there are not only restrictions arising from species-specific responses to
the environment but from the presence or abundance of other species as well
(Diamond
1975
; Weiher & Keddy
1995
). The rules are modified by assembly
histories and multiple stable equilibria are possible (Chase
2003
). We are only
beginning to unravel the rules for fire-prone ecosystems but it appears that life
history is an important determinant.
Ultimately adaptive evolution is the mechanism by which organisms make the
appropriate fit to a given environment and may be the primary factor accounting
for presence or absence in a given community (e.g. Cody & Mooney
1978
).
Equally important are ecological sorting processes, whereby functional traits are
influenced by the trait combinations present in other taxa to the extent that non-
random combinations are common (Ackerly
2003
). This is illustrated in the two
lineages of the Californian genus
Ceanothus
with different postfire regeneration
modes, as well as different morphologies and physiologies, which are more likely
to co-occur than by chance alone (
Fig. 9.12
). The genus dates to the Eocene and
the divergence in the two lineages dates to a basal split 18-39 Ma (Hardig
et al.
2000
). The
Cerastes
exhibits extraordinary niche conservatism in that all species are
obligate seeders, whereas the other subgenus
Ceanothus
has both obligate and
facultative seeders. Niche conservatism also is quite evident with respect to serotiny
and resprouting in lineages from MTC ecosystems of both hemispheres (
Fig. 9.8
).
Other examples of non-random sorting are found in MTC crown fire shrub-
lands. Because seeders are favored by predictable crown fires, the proportion of
seeder species in a woody community is expected to increase with the frequency
of fires (assuming frequency does not exceed time to reproductive maturity).
One comparison in the Mediterranean Basin communities under frequent crown
fires showed an overrepresentation of postfire seeders relative to what would be
predicted by the regional species pool (Verdu´ & Pausas
2007
). This is also evident
in timing of postfire recruitment in the semi-deciduous sage scrub shrubland in
