Environmental Engineering Reference
In-Depth Information
In addition, ecotones tend to shift in space and time over several spatial scales
[
11
,
19
], as a response to climatic variation, other environmental changes [
6
,
19
,
29
], and human activity [
10
]. Ecotones show high spatial and temporal heterogene-
ity, which may serve as important factors contributing to their high genetic and
species diversity [
32
]. For example, multiple ecotones can be defined within and
around the African Sahel (
Fig. 9.1a
), depending on the scale of interest and on the
definitions used [
1
]. The different transitions (e.g., that between the Sahel and the
desert to the north) experience shifts in time and space, showing high spatiotempo-
ral variability. Another, simpler, process shaping this pattern is that ecotones,
comprising meeting areas between adjoining communities, include a combination
of species from two or more community types [
32
]. Ecotonal areas often comprise
the edge of the range for species on both sides and are where many peripheral
populations occur [
17
,
35
].
An important question is whether populations occurring in ecotonal areas are
viable populations that exist over time within ecotones, or are rather present
temporarily due to the constant flow of individuals from other parts of their
range into the ecotone areas, are not self-sustainable over time, and will disappear
if this flow is stopped. Shmida and Wilson [
35
] proposed that the high number of
species in transitional areas could be due to a process they called the mass effect,
which is the flow of individuals from favorable to unfavorable areas. For species
that reach the margins of their range at the ecotone, this effect may result in some
individuals of a given species establishing in ecotonal areas where they cannot
maintain viable populations, existing in sinks adjacent to larger source
populations [
35
]. This may lead to increased species richness in ecotonal areas,
which is maintained by constant immigration of individuals from more favorable
environments. Some evidence for the existence of mass effects can be found in the
literature, yet these effects seem to be rather weak, and it is currently unclear
whether they can indeed act as a major factor generating high diversity in
ecotones [
21
].
The mass effect, however, does not predict the occurrence of unique or endemic
ecotonal species or genotypes. If some species or genotypes are characteristic of an
ecotone or occur at the ecotone in higher abundances than in the neighboring
habitats, as proposed above, this could suggest that some ecotone populations are
ecologically viable. Several recent studies have found peak genetic and morpho-
logical diversity within species in ecotone regions, with populations in these
regions harboring unique and rare alleles not found elsewhere [
17
]. For example,
a study examining allozyme (protein level) diversity within chukar partridge
(
Alectoris chukar
) populations across a rainfall gradient from northern to southern
Israel found that the highest levels of diversity occur in the sharp ecotone area
between the Mediterranean region and the desert, in the northern Negev area.
Populations in this area not only showed higher genetic diversity based on 32 loci
(proteins) examined, but also had unique and rare alleles that did not occur
elsewhere across the range [
14
]. The same species also had peak levels of morpho-
logical diversity in the ecotone region based on 35 traits and 23 ratios between traits
[
15
]. More studies on genetic and species uniqueness in ecotonal areas are needed
to enable generalizations.
