Environmental Engineering Reference
In-Depth Information
the parasite. Care has to be taken to thouroughly
check relocation candidates for deseases and other
parasites.
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8.3 FOUNDING NUMBERS, DIVERSITY
AND POPULATION STRUCTURE
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bh
eb
gz
mf
mg
mx
te
rd
wb
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In general, the number of individuals that are released
in reintroduction attempts is small. This means that
founding groups are susceptible to the same dangers of
increased extinction risks as small, natural popula-
tions: environmental fl uctuations, demographic sto-
chasticity and inbreeding. Therefore, to achieve the
highest possible success, a primary goal of reintroduc-
tion should be to maximize the initial rate of popula-
tion increase in order to shorten the period during
which the introduced population is exposed to these
risks. This can be brought about by releasing a high
number of individuals in a high-quality habitat.
Komers and Curman (2000) investigated how the rate
of increase of more than 30 newly reintroduced popu-
lations was affected by various population characteris-
tics such as population size, sex and age structure in
Artiodactyla (even - toed ungulates). Their results were
in line with the general notion that reintroduction
success increases with the number of animals released
(Figure 8.1). The function became asymptotic at about
20 animals. When fewer than 20 animals were
released, the variance in growth rate increased sub-
stantially and, of a number of factors, only age struc-
ture explained a signifi cant portion of this variance.
The population growth increased with the proportion
of mature animals in the population (Figure 8.2). This
fi nding could be explained by a higher fecundity of
mature females.
Loss of genetic variability, due to genetic drift and/or
inbreeding, is especially likely when an effectively small
number of individuals is used in founder populations.
Because of its importance, many conservation plans
call for the maintenance of genetic variability in trans-
located populations. Stockwell et al . (1996) examined
10 translocations in mosquito fi sh ( Gambusia affi nis
and G. holbrooki ). These populations had signifi cantly
lower levels of heterozygosity than their respective
parental source populations. The reduction in the
diversity of relatively rare alleles varied from 24 to
40%, while the initial translocations involved hun-
dreds of mosquito fi sh, which have various reproduc-
tive traits that appear to minimize the effects of
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10
20
30
40
50
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Number of animals
Figure 8.1 The rate of population increase in relation to
the number of animals in reintroduced ungulate populations.
Source species are shown in the key: mf, moufl on; mx,
muskox; te, tule elk; rd, red deer; mg, mountain goat; eb,
European bison, wb, wood bison; bh, bighorn sheep; and gz,
mountain gazelle. (Modifi ed from Komers and Curman 2000.
Reproduced by permission of Elsevier.)
te
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wb
wb
te
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mx
eb
mg
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eb
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Proportion of mature animals
Figure 8.2 The rate of population increase in
reintroduced ungulate populations in relation to the
proportion of socially mature animals in populations of
fewer than 20 animals. Source species: mx, muskox; te, tule
elk; mg, mountain goat; eb, European bison; wb, wood
bison. (Modifi ed from Komers and Curman 2000.
Reproduced by permission of Elsevier.)
bottlenecks on genetic diversity. Similar effects have
been found in other introduced populations (e.g. sea-
bream, trout, salmon, Anolis lizards, house sparrow,
common myna, reindeer and ibex), as reviewed by
Stockwell et al . (1996). Reductions in allelic diversity
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