Environmental Engineering Reference
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Temperature Annual Range) and four precipitation variables (Precipitation Seasonal-
ity, Precipitation of Wettest Quarter, Precipitation of Driest Quarter, and Precipitation
of Warmest Quarter). We also found that plague presence exhibits a quadratic response
to temperature increases. These results are consistent with other studies [12-14] that
have examined the role of temperature and precipitation variables on plague outbreaks
in human and animal populations. In addition to having a positive effect on rodent
population dynamics, certain soil moisture, humidity and temperature variables may
influence flea ecology and the transmission of the plague pathogen [53]. Specifically,
while warmer temperatures may in general stimulate plague activity, temperatures
above 35°C are associated with a negative effect on flea fecundity, survival, and be-
havior [13, 18, 54].
Under future emissions scenarios, our models indicate that climate conditions will
drive (a) an overall decrease in the probability of plague the state, (b) a subtle shift
to higher elevations as well as (c) a subtle shift to higher latitudes. These results are
generally consistent with other climate modeling studies that show species movement
to higher latitudes and elevations in response to warming [55], and with studies that
have examined the historical record of plague response to climate and show a shift to
higher latitudes [16, 22]. Several other recent studies have also projected a potential
decrease in plague activity in certain areas of the US in response to more frequent hot
days 19, 23, 24].
In addition, these results provide insight into the relationship between plague
maintenance in carnivore and rodent populations. Carnivores, and particularly coy-
otes, have been implicated in plague transmission and serve as sentinel species for the
disease [7, 56]. Recent studies [57, 58] conducted on the Central Plains Experimental
Range and Pawnee National Grasslands (which collectively cover a ~80,000 ha area)
link the prevalence of carnivores and rodent hosts in a spatially explicit manner. Our
results expand these analyses to a larger scale, by exploring the overlap in predicted
plague-positive rodent distributions with positive and negative coyote samples derived
through an independent sampling program. Model results demonstrate a link between
positive coyote samples and areas of predicted rodent infection, providing additional
support for rodent surveillance and follow-up in areas where the carnivore surveil-
lance program identifi es plague-positive animals.
California ground squirrels are the rodents that have been the most frequently
sampled for plague in California. However, six other species (Douglas' squirrel,
Lodgepole chipmunk, Merriam's chipmunk, Shadow chipmunk, Yellow-pine chip-
munk, and Belding's ground squirrel) often had higher serum titers than California
ground squirrels. This suggests these species may be of interest for further sampling
and surveillance, and that additional modeling of these species' distributions could
be conducted to explore the spatial heterogeneity of plague foci in California [59].
Maxent models of California ground squirrels fi t better than models that used all rodent
specimens as training points. Because California ground squirrels occupy a narrower
ecologic zone than all rodents collectively, with less variable climatic conditions, these
models described a more precise climatic niche for plague.
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