Environmental Engineering Reference
In-Depth Information
Smith studied morphological divergence in another Central African species, the
black-bellied seedcracker (
Pyrenestes ostrinus
). Seedcrackers show polymorphism
in bill size. A megabilled morph was found in the ecotone that specializes on a very
hard-seeded sedge found only in ecotonal areas [
27
]. This ecotonal megamorph was
maintained in the population, despite high levels of gene flow with rain forest
populations that had only smaller-billed morphs, owing to its selective advantage
for feeding on the hard seeds. Morphological divergence between habitats across an
ecotone was also found in leaf-litter skinks (
Carlia rubigularis
) in the wet tropical
rain forest of Australia [
34
]. Adult skinks occurring across sharp ecotones from
open (wet sclerophyll) forests to adjacent rain forests showed large morphological
and life history differences over short distances despite moderate to high levels of
mitochondrial gene flow [
34
]. Populations occurring across the ecotone had larger
size differences than populations located dozens to hundreds of kilometers away
that were geographically isolated millions of years ago, suggesting that in this case,
and perhaps in others that await research, speciation with gene flow may have
importance.
If ecotones regions harbor unique and endemic species and alleles, this may
provide support for the notion that these regions may also have importance in
speciation. If this is the case, ecotonal regions are expected to contain
a preponderance of recently derived species that are yet to expand their ranges
(neoendemics). Fjelds
˚
and Rahbek [
7
] suggested that more recently evolved
species are concentrated in transitional ecotones surrounding the main central
African rain forest. This reasoning is consistent with the finding that terrestrial
ecotones sustain high morphological divergence, providing evidence that current
speciation processes may indeed be taking place in these regions [
27
,
33
,
37
].
Ecotones, Ecological Gradients, and Species Ranges
One of the important questions that arise is what are the processes that lead to
higher species richness in ecotonal regions compared with adjacent areas? Evolu-
tionary processes, as discussed above, may be one explanation. Ecological factors
may also have importance in shaping this pattern. Several authors, such as Gosz
[
11
] and Risser [
32
], have suggested that transitional areas not only share the two
types of environments of the habitats that coincide in the ecotone, but also have
a unique ecotonal environment. Indeed, Odum [
30
] proposed that transition zones
often support a unique community with characteristics additional to those of the
communities that adjoin the ecotone, although also commenting this is by no means
a universal phenomenon. Studies testing these predictions show mixed results,
some pointing toward the occurrence of ecotonal species, while others not finding
evidence for species unique and highly abundant in ecotones. The inconsistency
among studies is complicated by the fact that different species, systems, scales, and
regions were used in different studies or due to methodological factors, such as
differences in sampling and analysis approaches.
