It is increasingly common for studies of animal ecology to use model-based predictions of environmental variables as explanatory or predictor variables, even though model prediction uncertainty is typically unknown. To demonstrate the potential for misleading inferences when model predictions with error are used in place of direct measurements, we compared snow water equivalent (SWE) and snow depth as predicted by the Snow Data Assimilation System (SNODAS) to field measurements of SWE and snow depth. We examined locations on elk (Cervus canadensis) winter ranges in western Wyoming, because modeled data such as SNODAS output are often used for inferences on elk ecology. Overall, SNODAS predictions tended to overestimate field measurements, prediction uncertainty was high, and the difference between SNODAS predictions and field measurements was greater in snow shadows for both snow variables compared to non-snow shadow areas. We used a simple simulation of snow effects on the probability of an elk being killed by a predator to show that, if SNODAS prediction uncertainty was ignored, we might have mistakenly concluded that SWE was not an important factor in where elk were killed in predatory attacks during the winter. In this simulation, we were interested in the effects of snow at finer scales (< 1 km2) than the resolution of SNODAS. If bias were to decrease when SNODAS predictions are averaged over coarser scales, SNODAS would be applicable to population-level ecology studies. In our study, however, averaging predictions over moderate to broad spatial scales (9-2200 km2) did not reduce the differences between SNODAS predictions and field measurements. This study highlights the need to carefully evaluate two issues when using model output as an explanatory variable in subsequent analysis: (1) the models resolution relative to the scale of the ecological question of interest and (2) the implications of prediction uncertainty on inferences when using model predictions as explanatory or predictor variables.
Brucella abortus is the causative agent of brucellosis, a disease enzootic in populations of free-ranging elk (Cervus elaphus) and bison (Bison bison) in the Greater Yellowstone Ecosystem, USA. We define the distribution of the disease in elk throughout Wyoming, USA, using three epidemiologic/geographic classifications based on winter-foraging opportunity. Antibody prevalence for >3,300 yearling and adult, female elk, sampled from supplemental feedgrounds in western Wyoming, USA, since 1985, was 21.9%. Surveillance of nearly 3,800 winter free-ranging, adult, female elk, sampled from herd units adjacent to feedgrounds from 1991 to 2008, indicated a total antibody prevalence of 3.7%. In contrast, none of the 1,930 male and female elk sampled from 1991 to 2008 in herd units distant from feedgrounds in Wyoming, USA, demonstrated exposure to B. abortus. In comparison to elk, bison had a higher incidence of brucellosis. Surveillance of 321 cow bison harvested within the Absaroka and Jackson bison herds in western Wyoming, USA, from 1992 to 2008 yielded an antibody prevalence of 63.9%.
The relationship between host density and parasite transmission is central to the effectiveness of many disease management strategies. Few studies, however, have empirically estimated this relationship particularly in large mammals. We applied hierarchical Bayesian methods to a 19-year dataset of over 6400 brucellosis tests of adult female elk (Cervus elaphus) in northwestern Wyoming. Management captures that occurred from January to March were over two times more likely to be seropositive than hunted elk that were killed in September to December, while accounting for site and year effects. Areas with supplemental feeding grounds for elk had higher seroprevalence in 1991 than other regions, but by 2009 many areas distant from the feeding grounds were of comparable seroprevalence. The increases in brucellosis seroprevalence were correlated with elk densities at the elk management unit, or hunt area, scale (mean 2070 km(2); range = [95-10237]). The data, however, could not differentiate among linear and non-linear effects of host density. Therefore, control efforts that focus on reducing elk densities at a broad spatial scale were only weakly supported. Additional research on how a few, large groups within a region may be driving disease dynamics is needed for more targeted and effective management interventions. Brucellosis appears to be expanding its range into new regions and elk populations, which is likely to further complicate the United States brucellosis eradication program. This study is an example of how the dynamics of host populations can affect their ability to serve as disease reservoirs.
Brucellosis is endemic in elk (Cervus elaphus nelsoni) using winter feedgrounds of western Wyoming, USA presumably because of increased animal density, duration of attendance, and subsequent contact with aborted fetuses. However, previous research addressed antibody prevalence rather than more direct measures of transmission and did not account for elk behavior or scavenging in transmission risk. Throughout March and early April 2005-07, we monitored 48 sets of culture-negative, pseudoaborted elk fetuses, placentas, and fluids (fetal units, FUs) on one winter free-ranging (WFR) location and four sites (Feedline, High Traffic, Low Traffic, Adjacent) associated with four feedgrounds. "At-risk" elk (total elk within 5 m of FU) and proportions of elk sniffing and contacting FUs were highest on Feedlines and decreased toward Low Traffic sites. We did not observe elk investigating FUs Adjacent to feedgrounds or on the WFR location. At-risk elk on Feedline and High Traffic sites decreased throughout the sampling period, whereas proportions of elk investigating FUs were correlated positively to at-risk elk among all sites within feedgrounds. At-risk elk and proportions of elk investigating FUs were correlated with total feedground elk density and population only on High Traffic and Low Traffic sites. Proportions of sex/age groups (female, juvenile, male) investigating FUs did not differ from background populations. Females, however, spent more time (mean [SE], 21.07 [3.47] sec) investigating FUs than juveniles (14.73 [3.53] sec) and males (10.12 [1.45] sec), with positive correlation between total investigations and time spent investigating per female. Eight species of scavengers consumed FUs, removing FUs faster on feedgrounds than WFR locations and reducing proportions of elk that investigated FUs. Our results suggest that 1) reduction of elk density and time attending feedgrounds, particularly on Feedlines; and 2) protection of scavengers on and adjacent to feedgrounds would likely reduce intraspecific transmission risk of brucellosis.
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