Agriculture Reference
In-Depth Information
asking the question “which came first: the trait or the environment?” His phylo-
genetic analysis concluded that in most cases low specific leaf area predated the
origins of the MTC; consequently it was presumed that this trait was not the result
of adaptation to the current environment but rather due to ecological sorting
processes. The early origin for such sclerophyllous leaves is consistent with the
fossil record (Axelrod
1973
), but the conclusion that this leaf type is not an
adaptation to the current environment needs some qualification. As discussed in
Chapter 10
, the timing of the origin of the MTC was quite possibly much earlier
than is often assumed. Also, demonstrating that a particular leaf type predates the
current MTC is not equivalent to demonstrating it predates the present environ-
ment. Such a conclusion assumes that the MTC summer drought is the primary
driver of the evolution of specific leaf mass, when in fact it may be some other
contemporary environmental factor, for example drought at any time of the year
or shallow substrates or even the winter rainfall pattern, all of which predate the
MTC and argue against categorically excluding an adaptive origin for small
sclerophyllous leaves. Similar caveats apply to other MTC studies that have
dismissed adaptive origins for contemporary traits (Herrera
1992
).
We view convergence as most usefully measured at the level of communities and
recognized when similar environments generate communities comprising species
with similar structures and functions. Quantifying this convergence requires
testing with null models against other communities under different environments.
Separating evolutionary convergence from ecological convergence requires some
information on the phylogenetic changes observed in lineages of the component
species of the communities, although such comparative methods have been chal-
lenged (Leroi
et al
.
1994
; Westoby
et al
.
1995
). Most important is the need to
closely evaluate the critical environmental factors contributing to producing simi-
lar environments. One example of matching similar environments was the search
for convergence in MTC sites in southwestern Australia and South Africa by
Cowling & Witkowski (
1994
). They matched substrate characteristics between the
two regions, ensuring a more precise comparison of similar environments. Add-
itional factors that need to be considered in comparing MTC regions include
variation in the timing of rainfall and severity of summer drought (
Box 1.1
).
With respect to convergence in MTC ecosystems we see that there are varying
levels of similarity in environments and that similarity varies with the organism
and scale of focus and consequently differing expectations of convergence. For
example, with respect to soil animals, humus development appears to be a critical
factor determining “similar” environments, perhaps more so than the climate (di
Castri
1973
). Also, convergence should not be expected at all scales. For example,
postfire species diversity at the point scale of 1 m
2
is strikingly similar between
MTC regions, but at the community scale of 0.1 ha, it is remarkably different (see
Chapter 11
).
Many studies have focused on convergent aspects of vegetation response to fire
in MTC ecosystems. As discussed in more detail in
Chapters 3
and
9
these have
largely focused on modes of postfire regeneration. In this topic we will examine
