Environmental Engineering Reference
In-Depth Information
Table 5.1 Values used
as inputs for simula-
tions of koala popula-
tions at Oakey
(declining) and
Springsure (secure).
Values in brackets are
standard deviations due
to environmental
variation; the model
procedure involves the
selection of values at
random from the range.
Catastrophes are
assumed to occur with a
certain probability; in
years when the model
'selects' a catastrophe,
reproduction and
survival are reduced by
the multipliers shown
(e.g. in a year with a
catastrophe, reproduc-
tion is reduced to 55%
of what it would
otherwise have been).
(After Penn et al., 2000.)
Variable
Oakey
Springsure
Maximum age
12
12
Sex ratio (proportion male)
0.575
0.533
Litter size of 0 (%)
57.00 (
±
17.85)
31.00 (
±
15.61)
Litter size of 1 (%)
43.00 (
±
17.85)
69.00 (
±
15.61)
Female mortality at age 0
32.50 (
±
3.25)
30.00 (
±
3.00)
Female mortality at age 1
17.27 (
±
1.73)
15.94 (
±
1.59)
Adult female mortality
9.17 (
±
0.92)
8.47 (
±
0.85)
Male mortality at age 0
20.00 (
±
2.00)
20.00 (
±
2.00)
Male mortality at age 1
22.96 (
±
2.30)
22.96 (
±
2.30)
Male mortality at age 2
22.96 (
±
2.30)
22.96 (
±
2.30)
Adult male mortality
26.36 (
±
2.64)
26.36 (
±
2.64)
Probability of catastrophe
0.05
0.05
Multiplier for reproduction
0.55
0.55
Multiplier for survival
0.63
0.63
% of males in breeding pool
50
50
Initial population size
46
20
Carrying capacity, K
70 (
±
7)
60 (
±
6)
in the current time period. The program is run many times, each giving a different
population trajectory because of the random elements involved. The outputs include
estimates of population size each year and the probability of extinction during the
modeled period (the proportion of simulated populations that go extinct).
I will present three case studies to illustrate how simulation modeling can be
applied to species at risk. The account of Australian koalas serves to highlight that
different populations of a single species can have signifi cantly different demographic
features (Section 5.4.1). Plants have their own problems when it comes to simulation
modeling, as you will see for the royal catchfl y (Section 5.4.2). Finally, disease can
be a particular risk factor, and I explore how the epidemiology of rabies has been
introduced into simulation modeling of Ethiopian wolf populations (Section
5.4.3).
5.4.1 An Australian
icon at risk
Koalas ( Phascolarctos cinereus ) are regarded as potentially threatened nationally,
with populations in different parts of Australia varying from secure to vulnerable
or extinct. The primary aim of the nation's national management strategy is to retain
viable populations throughout their natural range (ANZECC, 1998). Penn et al.
(2000) used a widely available simulation modeling package, known as VORTEX
(Lacey, 1993), to model two populations in Queensland, one thought to be declining
(at Oakey), the other secure (at Springsure).
Koala breeding commences at 2 years old in females and 3 years old in males;
these values were used for both population models. The other demographic values
used in the two analyses were derived from extensive databases from the two popu-
lations and are shown in Table 5.1. Note how the Oakey population has somewhat
higher annual female mortality and fewer females producing young each year. The
Oakey population was modeled from 1971 and the Springsure population from 1976
(when fi rst estimates of density were available) and the trends in the modeled popu-
lations were indeed declining and stable, respectively. Over the modeled period
(shown in Figure 5.5), the probability of extinction of the Oakey population is 0.380
 
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