Environmental Engineering Reference
In-Depth Information
Population
size
0.22
Germination
0.60
Productive plants (%)
0.45
*
Seed weight
0.54
Soil conditions
(NH
4ex
)
*
Performance
Seedling mortality
0.62
*
Flower buds
0.25
0.70
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Seeds /flower bud
*
0.58
Genetic
variation
(
F
is
)
Germination rate
0.96
0.41
Fig. 6.3
Path model depicting the hypothesized causal relationships between population size, genetic variation,
soil conditions and plant performance in 17 populations of
Succisa pratensis
in the Netherlands. Arrow width is
proportional to the standardized path coefficients, and dashed lines indicate negative paths. Asterisks indicate
values significantly different from 0 (
P
< 0.05). Numbers in bold are estimates of the proportion of the total
variance explained (squared multiple correlations) for each independent variable (i.e. all except soil conditions).
NH
4ex
, exchangeable ammonium;
F
is
, inbreeding coefficient. After Vergeer
et al
. (2003). Reproduced by permission
of Blackwell Publishing.
approach, Hedrick and Gilpin (1997) concluded that
the rate of extinction of patches and the number and
type of founders recolonizing empty patches are the
main factors determining the minimum effective
metapopulation size. Overall, the effective metapopula-
tion size increased if the rate of extinction (or the
rate of turnover, extinction and recolonization) was
reduced, while the number of local populations, the
numbers of founders and the rate of gene flow
increased. In cases where many human impacts are
fairly recent, fragmented populations might be in the
initial phase of heterozygosity loss, which may pro-
ceed much faster than predicted from either census
numbers or traditional estimates of effective popula-
tion size.
These results encourage genetic approaches to con-
servation and support the importance of preserving
genetic variability as a way of increasing the variability
of wild populations. However, there does not yet seem
to be a general rule as to what the minimum size or
the minimum genetic variation of populations should
be, in order to survive and safeguard the potential for
evolutionary adaptation (see section 6.5).
High
H
Low
H
H
= 0
1
High
H
Low
H
H
= 0
2
High
H
H
= 0
3
0
20
40
60
80
100
Generation
Fig. 6.4
The level of heterozygosity (
H
) over time in
a simulation of a population existing in three patches.
The short vertical bars on the right-hand ends of
horizontal lines indicate extinctions in a patch and the
vertical arrows indicate recolonization. After Hedrick &
Gilpin (1997). Reproduced by permission of Elsevier.
heterozygosity and changes in heterozygosity are
governed by the effective size of the population. Meta-
population structure might affect variance-effective size
by reducing total population size, and by increasing
or reducing the variance in the reproductive success
of the individuals (Hanski 1999). From their modelling
