Browsing by Author "Walker, Johann"

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Estimating nest abundance while accounting for time-to-event processes and imperfect detection
(Ecological Society of America, 2014) Peron, Guillaume; Walker, Johann; Rotella, Jay J.; Hines, James; Nichols, James
Birds and their population dynamics are often used to understand and document anthropogenic effects on biodiversity. Nest success is a critical component of the breeding output of birds in different environments; but to obtain the complete picture of how bird populations respond to perturbations, we also need an estimate of nest abundance or density. The problem is that raw counts generally underestimate actual nest numbers because detection is imperfect and because some nests may fail or fledge before being subjected to detection efforts. Here we develop a state-space superpopulation capture–recapture approach in which inference about detection probability is based on the age at first detection, as opposed to the sequence of re-detections in standard capture–recapture models. We apply the method to ducks in which (1) the age of the nests and their initiation dates can be determined upon detection and (2) the duration of the different stages of the breeding cycle is a priori known. We fit three model variants with or without assumptions about the phenology of nest initiation dates, and use simulations to evaluate the performance of the approach in challenging situations. In an application to Blue-winged Teal Anas discors breeding at study sites in North and South Dakota, USA, nesting stage (egg-laying or incubation) markedly influenced nest survival and detection probabilities. Two individual covariates, one binary covariate (presence of grazing cattle at the nest site), and one continuous covariate (Robel index of vegetation), had only weak effects. We estimated that 5–10% of the total number of nests were available for detection but were missed by field crews. An additional 6–15% were never available for detection. These percentages are expected to be larger in less intense, more typical sampling designs. User-friendly software nestAbund is provided to assist users in implementing the method.
An integrated strategy for grassland easement acquisition in the Prairie Pothole Region, USA
(2013-12) Walker, Johann; Rotella, Jay J.; Loesch, C.; Renner, R. W.; Ringelman, J. K.; Lindberg, M. S.; Dell, R.; Doherty, K. E.
Acquisition of perpetual grassland easements is a principal tactic used by the United States Fish and Wildlife Service (USFWS) and its partners to protect nesting habitat for migratory birds in the Prairie Pothole Region of North and South Dakota, USA. This public–private partnership resulted in the conservation of >344,000 ha of grassland during 1998–2012. Past easement acquisition has been targeted to landscapes with greatest expected accessibility to breeding duck pairs without active consideration of probability of conversion or cost of protection. The rising cost of easement acquisition in recent years indicates that re-evaluation and refinement of the easement acquisition strategy could help to improve programmatic outcomes. We assessed regional patterns of easement acquisition during 1998–2012, evaluated the current targeting strategy, and used a combination of publicly available and proprietary geospatial data to develop an easement-targeting Geographic Information System that integrated information about conversion probability and protection cost with current targeting criteria. Our assessment indicated that grassland protection was negatively affected by rising land prices during 1998–2012. In the 5 y between 2008 and 2012, about 100,000 ha of grassland were protected at a cost of $83 million (U.S. dollars). The 2008–2012 acquisitions represented 30% of total protection during 1998–2012 but composed 47% of the total expenditure. We observed strong evidence that easements were targeted to priority landscapes both before and after formalization of the USFWS conservation strategy in 2004. We also found evidence of an opportunity to increase efficiency of future acquisitions. We identified 0.9 million ha of currently unprotected priority grassland in the region with greater than expected conversion risk and smaller than expected protection cost. We suggest that future grassland easement acquisition be refocused on this refined priority area and that an adaptive approach to future easement acquisition (including targeted acquisitions, directed monitoring, and data-based decisions) provides a logical framework for implementation of this new strategy and will facilitate continued conservation success.
Surface-attached cells, biofilms and biocide susceptibility: Implications for hospital cleaning and disinfection
(2015-01) Otter, J. A.; Vickery, Karen; Walker, Johann; Pulcini, Elinor D.; Goldenberg, S. D.; Salkeld, J. A. G.; Chewins, J.; Yezli, S.; Edgeworth, J. D.
Microbes tend to attach to available surfaces and readily form biofilms, which is problematic in healthcare settings. Biofilms are traditionally associated with wet or damp surfaces such as indwelling medical devices and tubing on medical equipment. However, microbes can survive for extended periods in a desiccated state on dry hospital surfaces, and biofilms have recently been discovered on dry hospital surfaces. Microbes attached to surfaces and in biofilms are less susceptible to biocides, antibiotics and physical stress. Thus, surface attachment and/or biofilm formation may explain how vegetative bacteria can survive on surfaces for weeks to months (or more), interfere with attempts to recover microbes through environmental sampling, and provide a mixed bacterial population for the horizontal transfer of resistance genes. The capacity of existing detergent formulations and disinfectants to disrupt biofilms may have an important and previously unrecognized role in determining their effectiveness in the field, which should be reflected in testing standards. There is a need for further research to elucidate the nature and physiology of microbes on dry hospital surfaces, specifically the prevalence and composition of biofilms. This will inform new approaches to hospital cleaning and disinfection, including novel surfaces that reduce microbial attachment and improve microbial detachment, and methods to augment the activity of biocides against surface-attached microbes such as bacteriophages and antimicrobial peptides. Future strategies to address environmental contamination on hospital surfaces should consider the presence of microbes attached to surfaces, including biofilms.
Time-lagged variation in pond density and primary productivity affects duck nest survival in the Prairie Pothole Region
(2013-07) Walker, Johann; Rotella, Jay J.; Stephens, Scott E.; Lindberg, M. S.; Ringelman, J. K.; Hunter, C. M.; Smith, A. J.
The Prairie Pothole Region (PPR) is the primary breeding region for most species of North American dabbling ducks (Anas spp.). Conservation of these species is guided in part by knowledge of relationships between nest survival probability and habitat features. Positive relationships between duck nest survival and amount and configuration of herbaceous perennial vegetation have been observed in previous studies, but these 2- to 4-year studies might not have adequately characterized the temporal effect of wet–dry episodes on nest survival. Over an eight-year period, we studied nest survival of five species of ducks in the PPR relative to spatial and temporal variation in pond density, primary productivity, and hydrologic status of wetlands, soil, and vegetation on 52 study sites selected to span a gradient of spatial variation in proportion of herbaceous perennial vegetation and in number of wetland basins. We observed the fate of 12 754 nests. Consistent with past studies, 90% of nests that failed to hatch were destroyed by predators. Nest survival probability was positively related to current-year pond density and primary productivity, negatively related to pond density and primary productivity during the previous two years, and positively related to the number of wetland basins on the study site. Predicted relationships between nest survival and proportion or configuration of herbaceous perennial vegetation in the surrounding landscape were not supported. For mallard (Anas platyrhynchos), median estimated nest survival probability ranged from 0.02 (SE = 0.01) to 0.22 (SE = 0.02). Estimated nest survival was greatest on sites with numerous wetland basins that had transitioned from dry, unproductive conditions to wet, productive conditions in the previous 1–2 years. Our results were consistent with time-lagged responses of food webs to resource pulses in a broad array of ecosystems. Our study highlighted the importance of wetland basins and wet–dry episodes to duck nest survival in the PPR. Current habitat conservation efforts focus on landscapes with numerous wetland basins and a high proportion of herbaceous perennial vegetation. Our results suggest that future conservation efforts should focus on preserving high-density wetland complexes across as large a geographic extent as possible even in cropland-dominated landscapes.
Unusual types of pitting corrosion of copper tubes in potable water systems
(1993) Geesey, Gill G.; Bremer, Philip J.; Fischer, W. R.; Wagner, D.; Keevil, C. W.; Walker, Johann; Chamberlain, A. H. L.; Angell, P.
Waterfowl production from winter wheat fields in North & South Dakota
(2015-09) Skone, Brandi R.; Rotella, Jay J.; Walker, Johann
The Prairie Pothole Region (PPR) of North America produces more than 50% of North America's upland-nesting ducks. With the recent increase in economic value of some cash-crops and the potential to lose productive nesting habitat enrolled in the Conservation Reserve Program (CRP), there has been interest in evaluating the efficacy of alternative farming practices to provide additional breeding habitat for waterfowl. We evaluated and compared daily survival rates (DSR) of duck nests (Anas spp.) in winter wheat with those in perennial cover. We also examined the number of hatched nests/ha in each habitat and compared them to estimates in spring wheat to put habitat-specific estimates of nest survival in perspective. We monitored 1,195 nests in winter wheat and 3,147 in perennial cover in North and South Dakota on 13–19, 10.36-km2 sites each year from 2010 to 2012. In 2010, we also monitored 75 nests in spring wheat. We used an information-theoretic approach to develop and evaluate a set of competing models based on plausible and previously established covariates affecting nest survival. Across all species, nest survival was at least as high in winter wheat as in perennial cover, and for northern pintails and mallards, estimated nest survival rates were greater in winter wheat. Nest survival also varied by year and study area, was positively related to nest age, and was negatively related to the number of wetland basins, the proportion of cropland in the landscape, and vegetation density. Density of hatched nests in perennial cover (0.14/ha) was on average twice as high as nests in winter wheat fields (0.07/ha), which was in turn 4 times higher than estimates for spring wheat fields (0.02/ha). Our results provide evidence that winter wheat could be a useful tool for wildlife managers seeking to add productive nesting habitat in landscapes under intensive crop production.