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This hypothesis was developed based on the white-footed mouse population re-
search performed by Wolff,
4
showing that summer mouse populations correlate with
acorn production. Accordingly, if the summer mouse density increases following a
large masting event, then larval ticks should not have any difficulty finding mice
on which to obtain a blood meal—the authors seem to assume that the mouse den-
sity establishes the carrying capacity for larval ticks. Thus, many larvae will be
able to complete molting into the nymphal stage and reappear the following spring
(1.5 years after the masting event).
Unfed larvae generally do not carry the Lyme disease bacterial organism
B
.
and so the active larvae season (summer into fall) is not correlated with human
cases of Lyme disease. Larvae generally get the infection from mice that had been
infected earlier in the year by the other generation's active nymphal stage (spring
into summer). After winter stasis and early spring molting, the unfed nymph is ready
for a blood meal and is likely to transmit the Lyme disease agent to an intermediate
host or to a dead-end host, such as humans. Additionally, the unfed nymphal stage is
the most dangerous, for it is the infected stage most likely to feed on human blood.
Based on these facts, we should expect that if the population density of mice is
high and many larvae can acquire a blood meal, the following spring should see a
significant increase in active nymphs and an increase in human Lyme disease cases.
One key component of the ecology of Lyme disease is the production of acorns
(Figure 7.1). We assume that acorns can be produced by two different species of oak
trees, the white oak (
Quercus alba
) and the black oak (
Quercus velutina
). Based on
theworkbySorketal.
5
in east-central Missouri, we have been able to simulate the
average yearly acorn production for each tree species, as well as simulate the pro-
duction of the large acorn mast event associated with the two species. The acorn
production is normally distributed and nonnegative. The white oak has large acorn
production about every three years, whereas the black oak has a large mast event
every two years.
Based on the amount of total acorns produced, we have developed an assump-
tion about ACORN MAST INDEX—an index that is ideally calculated on the basis
of field observations by counting the number of acorns on a series of randomly se-
lected branches on a series of randomly selected trees. However, we did not want to
extrapolate how many branches a tree has. Instead, we assumed that if acorn pro-
duction is zero, then the mast index is zero. Additionally, acorn mast indices above
200 seem to be rare, so we set the highest acorn production possible in our model to
correlate with a mast index of 200. We assumed a linear relationship between acorn
production and mast index. Because of this relationship and because the mast index
is not a measure of a particular tree but a measure applied to several branches of sev-
eral trees, we do not require a real value for the number of trees in our model, but a
proportion of trees. Figure 7.2 shows an example model run for a 50% composition
of both oak species in our forest patch.
.
d
4
Wolff, Jerry O. 1996. Population fluctuations of mast-eating rodents are correlated with produc-
tion of acorns.
Journal of Mammology
77, 3: 850-856.
5
Sork, Victoria L., Judy Bramble, and Owen Sexton. 1993. Ecology of mast-fruiting in three
species of North American deciduous oaks.
Ecology
74, 2: 528-541.
