Spatial Analysis of Plague in California - Recent Advances and Issues in Environmental Science - page 130

Current model results matched areas with historical and recent plague activity,
including the San Francisco peninsula and San Bruno Mountain, the San Jacinto
Mountains, and the Los Padres National Forest area [32]. Models did not yield high
prediction values for the Modoc plateau region, which has historically been a focus
of plague [10]. Because the low population density and rural nature of this area does
not readily lend itself to observation of epizootic events, it is not surprising that no
positive rodents were collected in the Modoc plateau during the study period. Thus, no
model input points were used from this area, which can present a challenge to niche
modeling techniques in terms of extrapolating results to new conditions in geographic
and ecologic space [60]. In addition, the low prediction values for the Modoc plateau
may be related to the extreme climate profi le and characteristics of the plague system
in this area, where plague maintenance and transmission is driven by a climate regime
and rodent-host complex that differ from the rest of California. Many areas of the
Modoc plateau experience plague, but in wood rats ( Neotoma spp.), as well as in yel-
low pine chipmunks ( Tamias amoenus ) and their associated fl eas.
It is important to keep in mind that by modeling the climatic niche for plague in
California, we have modeled a potential distribution for plague that is not the actual
or realized distribution. Other important factors, including landscape confi guration,
biotic variables, and barriers to dispersal likely limit the actual distribution of plague
to smaller areas than those predicted using a climatic niche modeling approach [40,
59, 61]. Second, a number of studies have demonstrated that different modeling ap-
proaches can yield substantially different predictions [42, 62]. Thus, future work could
include modeling plague potential distributions under a suite of different modeling
approaches. Additionally, using niche models to predict distributions into expanded
temporal and/or spatial domains can result in signifi cant variance infl ation [62]. We
have attempted to dampen this variability by averaging future model outputs based
on three different global climate models. However, the use of global climate models
(as opposed to local or regional climate models) may itself be another source of error
in niche modeling studies, and thus a potential area of research could explore the ef-
fects of different modeling datasets on disease distributions (for example, see [63]).
Finally, averaged climate variables dampen seasonal effects and do not capture cli-
matic anomalies, which may be important drivers of plague epizootics [11-13]. Thus
multi-temporal modeling is required to elucidate the effects that increased climatic
variability will have on vector-borne disease dynamics.
CONCLUSION
Because different modeling approaches can yield substantially different results, care
should be taken when interpreting future model predictions. Nonetheless, niche mod-
eling can provide general trends in response to climate conditions. Models of plague
activity in California ground squirrels, based on recent climate data, accurately identi-
fied plague-positive rodent locations, as well as areas of historical and recent plague
activity. Maxent model results were significantly correlated with coyote samples, and
suggest that carnivore and rodent plague surveillance programs should be more tightly
coupled in California. The final models were used to identify potential plague risk