Agriculture Reference
In-Depth Information
universal, and the concept of MTC ecosystem convergence has been criticized
(Barbour & Minnich
1990
).
To be sure, one of the main limitations to the study of convergence in MTC
regions is the inordinate attention to comparing landscapes with broad climatic
similarities, but often with marked differences in other environmental factors: for
example, soil texture and nutrients (
Fig. 1.5
), topography, fire, or subtle variations
in rainfall distribution, all of which contribute to different selective pressures in
the five MTC regions. These other factors lower the expectation of convergence in
community structure and function (Blondel
1991
). Thus, it is important to stress
that ecosystem convergence is to be expected for organisms in similar environ-
ments, and this includes a multitude of environmental factors beyond just climate.
Ecosystem or community convergence may be the result of evolutionary or
ecological processes. If phylogenetically unrelated organisms that occupy similar
ecological niches in similar environments become more similar than their ances-
tors, this is termed evolutionary convergence. Evolutionary convergence is one
form of
homoplasy
, a term for the generation of similar structures or functions in
organisms that does not imply the mechanism of origin. Homoplasy may arise
through multiple evolutionary mechanisms, including selection (both convergent
and parallel evolution), genetic drift, and migration (Wake
1991
;Leroi
et al
.
1994
).
Homoplasy may also arise through purely competitive interactions in
ecological communities, a process known as ecological sorting (Wilson
1999
).
In the former case convergence is viewed as an indicator of the efficacy of natural
selection, whereas in the latter case it is a measure of competitive displacement,
evident in the overdispersion of plant traits in a community (Pausas & Verdu´
2010
). Regardless of the origin, convergence phenomena may provide useful tests
of the predictability of functional types (Reich
et al
.
1997
), community assembly
rules (Schluter & Ricklefs
1993
; Wilson
1999
) and development of the ecological
niche concept (Harmon
et al
.
2005
).
Demonstrating evolutionary convergence requires that one have some infor-
mation on the ancestral condition and evidence that it has changed in response
to a particular environment. This is generally not possible at the community
scale, but with a combination of phylogenetics and fossils, demonstrating
changes in individual species' traits is possible. However, results of such com-
parisons must recognize that limitations to similarity may be imposed by differ-
ent genetic or developmental constraints within lineages (Wake
1991
)and
historical events not replicated in the different environments (Peet
1978
). The
role of phylogeny and other historical factors has been addressed by testing
the null hypothesis that communities in similar environments do not differ from
control communities in very dissimilar environments (Crowder
1980
). With this
approach it is possible to partition the variance among communities into phylo-
genetic effects and habitat effects in order to compare assumed convergent
communities (Schluter
1986
).
Ackerly (
2004a
) investigated the relative importance of phylogeny in determin-
ing specific leaf area (leaf area per unit mass) in California chaparral shrubs by
