178
Lloyd Goldwasser, Scott Ferson, and Lev Ginzburg
of breeding pairs over a period of 100 years. All calculations were done by using
RAMAS/stage (Ferson 1990).
Effects of Windstorms and Forest Maturation
The possibility of catastrophic windstorms does not appear to have a dramatic
effect on the mean population size (Table 11.2) or on the extinction risks (Table
11.3) over 100 years. Considering that, on average, over the century-long simulation there will be three events that remove some of the available habitat, the
influence of these catastrophes on the demography of owls is perhaps surprisingly
low. The explanation for this small effect hinges on the fact that the impact only
affects the ceiling carrying capacity. Because the population is declining anyway,
the ceiling is reached only rarely, and so the impact on the actual owl population is
negligible.
However, the maturation of young forests into habitat suitable for Spotted Owl
breeding territories increases the mean population size (Table 11.2) by about 15%.
Although we might expect it consequently to decrease the probability of extinction, we found negligible differences in the probability values as a result of
including or excluding young forests (Table 11.3). In fact, our result is an overestimate, because we treated the removal of habitat as permanent, yet some
regrowth may occur. Again, the reason for this is that a gradual addition of
potential habitat would only be consequential if the population were a growing
one. Because it is generally declining, it does not get an opportunity to take
advantage of the improving situation.
Table 11.2. Mean number of breeding pairs over 1,000 replicates after 100 years to
two significant digits, under various assumptions about habitat concerning the initial
number of territories, the occurrence of windstorms, and the effect of maturation of
young forests.
a
Initial territories
No windstorms
Windstorms
No regrowth
200
73
72
300
110
110
With regrowth
200
85
82
300
130
130
a
“Initial territories” refers to the number of available breeding territories present at the beginning of the
simulation. Both 200 and 300 territories were used for this value. “No regrowth” means there will be
no increase in number of available owl territories resulting from maturation of young forests. “With
regrowth” means breeding habitat increases at a rate of 0.35% per year as a result of maturation of
young forests. “Windstorms” means that blowdowns occur with annual frequency 0.033. The first
windstorm removes 12 territories, the second removes 6, the third removes 3, the fourth removes 1, the
fifth and thereafter remove 0. “No windstorms” means that no territories are lost to blowdowns.
Lloyd Goldwasser, Scott Ferson, and Lev Ginzburg
of breeding pairs over a period of 100 years. All calculations were done by using
RAMAS/stage (Ferson 1990).
Effects of Windstorms and Forest Maturation
The possibility of catastrophic windstorms does not appear to have a dramatic
effect on the mean population size (Table 11.2) or on the extinction risks (Table
11.3) over 100 years. Considering that, on average, over the century-long simulation there will be three events that remove some of the available habitat, the
influence of these catastrophes on the demography of owls is perhaps surprisingly
low. The explanation for this small effect hinges on the fact that the impact only
affects the ceiling carrying capacity. Because the population is declining anyway,
the ceiling is reached only rarely, and so the impact on the actual owl population is
negligible.
However, the maturation of young forests into habitat suitable for Spotted Owl
breeding territories increases the mean population size (Table 11.2) by about 15%.
Although we might expect it consequently to decrease the probability of extinction, we found negligible differences in the probability values as a result of
including or excluding young forests (Table 11.3). In fact, our result is an overestimate, because we treated the removal of habitat as permanent, yet some
regrowth may occur. Again, the reason for this is that a gradual addition of
potential habitat would only be consequential if the population were a growing
one. Because it is generally declining, it does not get an opportunity to take
advantage of the improving situation.
Table 11.2. Mean number of breeding pairs over 1,000 replicates after 100 years to
two significant digits, under various assumptions about habitat concerning the initial
number of territories, the occurrence of windstorms, and the effect of maturation of
young forests.
a
Initial territories
No windstorms
Windstorms
No regrowth
200
73
72
300
110
110
With regrowth
200
85
82
300
130
130
a
“Initial territories” refers to the number of available breeding territories present at the beginning of the
simulation. Both 200 and 300 territories were used for this value. “No regrowth” means there will be
no increase in number of available owl territories resulting from maturation of young forests. “With
regrowth” means breeding habitat increases at a rate of 0.35% per year as a result of maturation of
young forests. “Windstorms” means that blowdowns occur with annual frequency 0.033. The first
windstorm removes 12 territories, the second removes 6, the third removes 3, the fourth removes 1, the
fifth and thereafter remove 0. “No windstorms” means that no territories are lost to blowdowns.