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Incorporating metapopulation dynamics of greater gliders into reserve design in disturbed landscapes
Ecology, March, 1999 by Michael A. McCarthy, David B. Lindenmayer
Sensitivity analysis
The results of the sensitivity analysis (Table 2) reveal that changing the mean migration distance had a limited effect on the predicted risk of extinction. When the mean migration distance of greater gliders was increased to 8 kin, the risk of extinction decreased to 0.40. When the mean migration distance was 0.5 km, the risk of extinction was 0.51. Thus, large (16-fold) changes in the dispersal ability of greater gliders did not have correspondingly large effects on the results of the model. Changing the carrying capacity of patches had a limited influence on the predicted risk of extinction, indicating that populations rarely attained densities in excess of 1.5 females/ha (Table 2). Reducing the fecundity or survival rates such that the deterministic growth rate under ideal conditions was 1.0 (fecundity f = 0.176, or survival s = 0.80) made extinction within 300 yr almost certain (risk [greater than] 0.93). Increasing the rates had a proportionally smaller effect on the predicted risk of extinction, reducing it by at most 0.26. Increasing the strength of spatial correlation increased the risk of metapopulation extinction because populations in patches were more likely to go extinct at the same time and to experience more complete burning of patches. Smaller fires (weaker spatial correlation) reduced the chance of extinction. The predicted risk of extinction within 300 yr was sensitive to the mean fire interval, with persistence favored by longer intervals (Table 2). Suitable habitat would be more likely to develop with longer fire intervals, and mortality associated with fires would be less frequent.
Although the predicted risk of extinction in the Aria Forest Block was quite sensitive to variation in parameters such as fecundity, survivorship, and mean fire interval (Table 2), the optimum reserve configuration did not change as much in response to changes in the input parameters. In most cases examined in the sensitivity analysis, the size of patches required to minimize the risk of extinction within 300 yr remained at 32 ha (Table 3). The optimum size of patches increased in three cases, and only decreased when the mean fire interval was increased to 200 yr. The number of reserves to minimize risk of extinction within 1000 yr only changed (from one to two) when the fire interval increased. Variation in the parameters resulted in the estimated value of c (Eqs. 2 and 3) varying between 0.65 and 1.24. A single reserve maximized the mean time to extinction (Eq. 4) over this range of values for c.
TABLE 2. Results of a sensitivity analysis to determine effects of
parameters on the predicted risk of extinction of greater gliders
within 300 yr in old-growth mountain ash forest of the Aria Forest
Block. The predicted risk of extinction is shown in parentheses
below each parameter value. The risk of extinction for the unchanged
set of parameters was 0.44.
Parameter Low value High value
Annual fecundity (no. juvenile 0.176 0.324
females per adult female) (0.94) (0.31)
Annual survival rate 0.80 0.90
(0.98) (0.18)
Mean migration distance (km) 0.5 8.0
(0.51) (0.40)
Maximum population density 1.5 3.0
(no./ha) (0.48) (0.45)
Spatial correlation strength halved doubled
(32) (54)
Mean fire interval (yr) 75 200
(0.64) (0.12)
