Environmental Engineering Reference
In-Depth Information
It's the Population Dynamics
In 1991, I established two large (2.25-ha) live-trapping plots within typical second-
growth, eastern deciduous forest on the grounds of the Cary Institute of Ecosystem Studies
in Millbrook, New York. Healthy populations of ticks had been discovered at Cary Institute
several years earlier, and Lyme disease was an established problem for local residents.
I had been trained as a population and community ecologist with a focus on the causes and
consequences of fluctuating populations of small mammals. I knew that white-footed mice
were likely to be the most abundant small mammal in this type of habitat, that these mice
were known to be the most competent reservoir for
B. burgdorferi
, and that population
size of these mice fluctuated dramatically through time. My purpose in establishing these
trapping plots was to provide a monitoring baseline to pursue the impacts of fluctuating
mice on the dynamics of their prey—particularly gypsy moths (
Lymantria dispar
) and tree
seeds—and of their parasites—specifically blacklegged ticks and
B. burgdorferi
.
Because most of my prior studies of small mammals had been undertaken in grasslands,
I was unprepared for the forest phenomenon I experienced during the fall of 1991, namely
an extraordinary mast seeding event courtesy of red oaks (
Quercus rubrum
)andblackoaks
(
Q. velutina
). Personal observations indicated that the masting event extended throughout the
eastern New York/western New England region. This impressive phenomenon (
100 acorns
per square meter in some places) prompted my colleague, Charles Canham, and me to estab-
lish seed traps to quantify the magnitude of acorn masting at our sites. It was well known
that mice and other wildlife were voracious consumers of acorns, and it seemed possible that
consumer populations could ebb and flow with variable acorn production. Indeed, live-
trapping the following spring on these plots revealed abundant mouse populations, which
then grew to extraordinary densities (
.
100 individual per hectare) by late summer of 1992,
after which they crashed to very low numbers (
.
10 per hectare).
Another striking discovery my colleagues and I made the summer following the 1991
mast was that larval tick abundance had exploded in oak-dominated forest stands but not in
maple-dominated stands that we were also monitoring. At about this time, I saw a presenta-
tion at a scientific meeting by William J. McShea showing that deer gravitate toward oak-
dominated stands in the fall of a mast year to eat fallen acorns, but outside of mast years
tend to avoid oak-dominated stands. Knowledge of this work (later published in
McShea
and Schwede 1993
) led me to hypothesize that the burgeoning larval tick populations in 1992
in oak stands were a consequence of the heavy acorn crop in 1991, which attracted deer and
their attached adult ticks to these sites and away from other habitat types. If so, we would
expect that fed adult ticks would drop off deer largely in oak stands, ovipositing there, and
leading to eruptions of larvae the following summer. The fact that very few acorns were pro-
duced in 1992 (heavy mast production is often followed by one or a few years of acorn fail-
ure) allowed us to assess this hypothesis. We expected that deer would avoid oak stands in
the fall of 1992, that few fed adult ticks would occur there, and that consequently larvae
would be sparse in oak stands in the summer of 1993 but abundant in maple-dominated
sites. This pattern was exactly what we found (
Ostfeld et al. 1996; Ostfeld 1997
).
As described earlier, larval ticks hatch uninfected with Lyme disease spirochetes, so
booms and busts in abundance of this life stage have no immediate impact on human
health. But, larvae that feed on mice are highly likely to acquire a spirochete infection and
,
