Environmental Engineering Reference
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as hosts for ticks and could be involved in determining Lyme disease risk in humans.
Recently, Dustin Brisson and I have found that two species of shrews ( Blarina brevicauda
and Sorex cinereus ) together feed more larval ticks than do mice and chipmunks combined
(Brisson et al. 2008). Even though the shrews are somewhat poorer reservoirs for the spiro-
chete than are mice or chipmunks, the huge number of larval ticks they feed results in
shrews being responsible for infecting about half of all the infected nymphs in the forest.
Although it is difficult to monitor population dynamics of some shrews due to poor cap-
ture probabilities in standard live-traps, these shrews apparently do not fluctuate synchro-
nously with mice and chipmunks. Whether the shrews and small rodents interact in other
ways, for instance, via shared predators, is unknown.
It's Biodiversity
Well over 100 years ago, medical entomologists suggested a connection between species
diversity and transmission of vector-borne diseases of humans (reviewed in Service 1991 ).
Researchers argued that malaria transmission might be reduced if alternative hosts for
mosquito vectors (e.g., livestock) were placed around areas of human habitation, diverting
vector meals away from humans, an idea termed zooprophylaxis . Recently, there has been
renewed interest in the potential effects of biological diversity on disease risk, in large part
because of interest in identifying and evaluating utilitarian functions of biodiversity
( Loreau et al. 2001 ). Given the large number of host species involved in the Lyme disease
system, with each host species potentially playing a distinct role as a tick host and patho-
gen reservoir, we might expect that variation in species diversity would strongly influence
Lyme-disease transmission and risk. For instance, a low-diversity community composed
largely of white-footed mice and deer might be expected to produce large numbers of
infected nymphal ticks, owing to the high quality of mice as hosts for larval ticks and
reservoirs for B. burgdorferi . In fact, such low-diversity communities exist and appear to be
facilitated by human disturbances such as forest fragmentation and degradation. Under
these conditions, mammalian and avian predators appear to be reduced or lost and white-
footed mice thrive (references in Ostfeld and Keesing 2000 ). As vertebrate diversity
increases, both the numbers of nymphal ticks and their infection prevalence with B. burg-
dorferi might decrease.
This response would be expected if the added species were poorer hosts for larval ticks,
were to deflect tick meals away from mice, and/or were poorer reservoirs for B. burgdor-
feri . Evidence supports all of these expectations ( LoGiudice et al. 2003; Keesing et al. 2006 ).
We have used two general approaches to test these expectations. In one ( LoGiudice et al.
2003; Ostfeld and LoGiudice 2003 ), we determined for each reasonably common host spe-
cies the average population density, the average number of larvae fed, and the
proportion of larvae that molt into infected nymphs. Together, these values allow us to
estimate each species' individual contribution to producing infected nymphs. We then
used computer simulation to create realistic “virtual” communities composed of different
species compositions and to determine the expected proportion of nymphs that should
become infected. We assessed these simulations by sampling natural nymph populations
and determining whether observed values for infection prevalence matched those
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