Database Reference
In-Depth Information
1: N
2: K
3: ACORN MAST INDEX
1:
60.00
2:
3:
200.00
1:
30.00
2:
3:
100.00
2
1
1
2
1
1
2
2
1:
2:
0.00
3
3
3
3
3:
0.00
0.00
208.00
416.00
624.00
832.00
Fig. 7.4
two litters per year, but when the acorn mast is substantial some may have three.
Because of this additional litter event, the natural rate of increase (R) for the pop-
ulation can vary based on the acorn mast production. Thus, we have assumed R to
vary with changes in K. Since K is mast index-derived, R will also be driven by
mast index due to its relationship with K. Nevertheless, the R value when K
=
20 is
based on Wolff's work.
Though we have made many assumptions that may not reflect the actual con-
ditions in the mouse population, we have modeled a population that reaches the
predicted density the summer following a mast event. Figure 7.4 shows the time-
varying behavior of the mouse population size N, the carrying capacity, and the
acorn mast index using Wolff's summer mouse population correlation equation.
Figure 7.5 displays the relationship of R and the acorn mast index.
Changes in deer population dynamics are handled differently in the model from
the ways in which we specified changes in mouse population (Figure 7.6). Based
on the work by McShea and Schwede
6
, we assumed deer respond to the variation
in acorn crops by spending more or less time in particular parts of their habitats.
The authors showed that deer are more likely to use the oak forest habitat if acorn
production is good, and the percentage of habitat use is correlated with acorn pro-
duction. Based on McShea and Schwede's data, we assumed an average 3.5 deer/ha.
Their study area was approximately 60% nonforested and 40% forested. Thus, we
could assume for the purpose of this model that if our model was placed in 1 kmˆ2
of their study area, 4 ha would be forested and one of those hectares would be our
model. Assume a deer had 50% habitat use in the forest. Since our model is 1/4
of the total forest, we assumed that the percentage of habitat used by deer in our
model is 12.5% (1/2 * 1/4 = 1/8). We were able to roughly correlate the increases
6
McShea, William J., and Georg Schwede. 1993. Variable acorn crops: Responses of white-tailed
deer and other mast consumers.
Journal of Mammology
74, 4: 999-1006.
