Environmental Engineering Reference
In-Depth Information
in Scotland were confined within a particular social
group, but several female voles dispersed long distances
and joined new social groups. Populations or colonies
of blacktail prairie dog (
Cynomis ludovicianus
) con-
sist of several family groups called coteries. These
populations remain spatially distinct from each other
because of a rigid social structure. The regular influx
of male immigrants should reduce inbreeding within
local populations and elevate levels of heterozygosity,
potentially rescuing populations from inbreeding
depression and possible extinction. White-footed
mice (
Peromyscus leucopus
) have persisted in a rem-
nant network of woodlot patches in North America,
connected by migration routes. Populations linked by
these high levels of migration had higher growth
rates than populations linked by lower levels of
migration. Since winter populations can be as small
as two individuals, rapid population growth in the
spring could be critical for the persistence of local popu-
lations. There is clear evidence for a similar rescue
effect in several other small mammal species. For
amphibians, such as red-spotted newts (
Notophtalmus
viridescens
) in the eastern United States, there is
evidence for a relatively high migration rate between
ponds, and no local extinctions were observed.
Natterjack toad (
Bufo calamita
) populations in Ger-
many showed low genetic distances between local
populations. Most adult female toads and some juven-
iles migrate between breeding ponds within seasons,
while most males leave their first breeding sites. Most
sites are occupied continuously, and local populations
persist due to immigration and the rescue effect.
Along the Baltic coast of Sweden, pool frogs (
Rana
lessonae
) occur in natural metapopulations, repro-
ducing only in distinct water bodies. Over a 6-year
period, populations isolated by greater than 1 km
became extinct, whereas less-isolated populations
tended to persist.
(2000) has shown that the degree of inbreeding
depression in populations of
Silene alba
varied with
the degree of isolation from other established popu-
lations. Crossing experiments confirmed that isol-
ated populations are inbred and had probably been
founded by related individuals. If seed movement is
rare and pollen movement is common, then the
potential for inbreeding to occur soon after estab-
lishment is offset by genetic rescue through pollen-
mediated gene flow from another population. Crosses
among isolated sites restored germination rates to
levels comparable to outcrossed lines.
The question now remains as to the relative import-
ance of genetic variation compared to other aspects
of habitat fragmentation, such as habitat quality and
demographic factors, in determining the fate of local
populations. Vergeer
et al.
(2003) applied path ana-
lysis to separate and quantify the relatively direct
and indirect contributions of genetic variation and soil
conditions on population size and plant performance
in
Succisa pratensis
(Fig. 6.3). The number of seeds
per flowerhead, germination percentage, germination
rate, seed weight and seedling survival were all pos-
itively correlated with population size. Reduced plant
performance was better explained by genetic erosion
and habitat deterioration (eutrophication) than by
population size in itself.
6.2.2 Effective (meta)population size
Colonization and extinction are determined by the
effective population size rather than by the overall popu-
lation size. The effective population is composed of
only those individuals that contribute genes to the next
generation. The effective population size is classically
defined as the size of an idealized population that
results in a given variance in allele frequency or amount
of inbreeding (Hedrick & Gilpin 1997). The amount
of genetic variation in a population is generally
determined using heterozygosity as a measure. In
metapopulations, the average levels of heterozygosity
of the component populations and the spatial dis-
tribution of heterozygosity are important parameters.
The loss of genetic variation in a metapopulation
can be dramatically lower than that expected from a
population the size of an average census number in
the system (Fig. 6.4). Both the steady-state levels of
Plants
The relationship between genetic diversity and survival
in plant populations has been reviewed by Booy
et al.
(2000), who concluded that there is no general
relationship between genetic diversity and various
fitness components. If a lower level of heterozygosity
represents an increased level of inbreeding, a reduc-
tion of fitness can be expected. Indeed, Richards
