Johann Walker

Predicting Risk of Habitat Conversion in Native Temperate Grasslands

Native grasslands that support diverse populations of birds are being converted to cropland at an increasing rate in the Prairie Pothole Region of North America. Although limited funding is currently available to mitigate losses, accurate predictions of probability of conversion would increase the efficiency of conservation measures. We studied conversion of native grassland to cropland in the Missouri Coteau region of North and South Dakota (U.S.A.) during 1989-2003. We estimated the probability of conversion of native grassland to cropland with satellite imagery and logistic regression models that predicted risk of conversion and by comparing the overlap between areas of high biological value and areas most vulnerable to conversion. Annualized probability of conversion was 0.004, and 36,540 ha of native grassland were converted to cropland during the period of our study. Our predictive models fit the data and correctly predicted 70% of observed conversions of grassland. Probability of conversion varied spatially and was correlated with landscape features like amount of surrounding grassland, slope, and soil productivity. Tracts of high biological value were not always at high risk of conversion. We concluded the most biologically valuable areas that are most vulnerable to conversion should be prioritized for conservation. This approach can be applied broadly to other systems and offers great utility for implementing conservation in areas with spatially variable biological value and probability of conversion.

Economics of grassland conversion to cropland in the prairie pothole region.

Much of the remaining grassland, particularly in North America, is privately owned, and its conversion to cultivated cropland is largely driven by economics. An understanding of why landowners convert grassland to cropland could facilitate more effective design of grassland-conservation programs. We built an empirical model of land-use change in the Prairie Pothole Region (north-central United States) to estimate the probability of grassland conversion to alternative agricultural land uses, including cultivated crops. Conversion was largely driven by landscape characteristics and the economic returns of alternative uses. Our estimate of the probability of grassland conversion to cultivated crops (1.33% on average from 1979 to 1997) was higher than past estimates (0.4%). Our model also predicted that grassland-conversion probabilities will increase if agricultural commodity prices continue to follow the trends observed from 2001 to 2006 (0.93% probability of grassland conversion to cultivated crops in 2006 to 1.5% in 2011). Thus, nearly 121,000 ha (300,000 acres) of grassland could be converted to cropland annually from 2006 to [corrected] 2011. Conversion probabilities, however, are spatially heterogeneous (range 0.2% to 3%), depending on characteristics of a parcel (e.g., soil quality and economic returns). Grassland parcels with relatively high-quality land for agricultural production are more likely to be converted to cultivated crops than lower-quality parcels and are more responsive to changes in the economic returns on alternative agricultural land uses (i.e., conversion probability increases by a larger magnitude for high-quality parcels when economics returns to alternative uses increase). Our results suggest that grassland conservation programs could be proactively targeted toward high-risk parcels by anticipating changes in economic returns, such as could occur if a new biofuel processing plant were to be built in an area.

Satellite Telemetry in Avian Research and Management: Sample Size Considerations

Abstract Satellite tracking is currently used to make inferences to avian populations. Cost of transmitters and logistical challenges of working with some species can limit sample size and strength of inferences. Therefore, careful study design including consideration of sample size is important. We used simulations to examine how sample size, population size, and population variance affected probability of making reliable inferences from a sample and the precision of estimates of population parameters. For populations of >100 individuals, a sample >20 birds was needed to make reliable inferences about questions with simple outcomes (i.e., 2 possible outcomes). Sample size demands increased rapidly for more complex problems. For example, in a problem with 3 outcomes, a sample of >75 individuals will be needed for proper inference to the population. Combining data from satellite telemetry studies with data from surveys or other types of sampling may improve inference strength.

NEST SURVIVAL OF SCAUP AND OTHER DUCKS IN THE BOREAL FOREST OF ALASKA

Abstract We estimated variation in nest survival of lesser scaup (Aythya affinis), greater scaup (A. marila), and other common duck species at Minto Flats, Alaska, USA, during 1989–1993 and 2002–2003. Daily survival probability of scaup nests, as well as nests of all other duck species, varied with year, date, and nest habitat. Daily survival probability was unrelated to nest age and distance from the nest to water. Average, year-specific nest survival of all ducks at Minto Flats was 0.11 (95% CI: 0.05 to 0.22), comparable to nest survival of ducks breeding in mid-continent regions (i.e., the prairie pothole region and the Canadian prairie-parklands). Nest survival of scaup was variable among years, ranging from 0.01 (95% CI: 0.00 to 0.06) in 1992 to 0.61 (95% CI: 0.50 to 0.74) in 1993 and was probably related to variation in predation risk and water levels. Scaup production could have been limited by low nest survival during most years of this study; nest survival exceeded 20% only in 1993 and 2002. Because of the high densities of breeding scaup and other waterfowl species at Minto Flats, we recommend management to maintain existing habitat for breeding scaup and other duck species. This management could be most effectively informed by yearly monitoring of production to more accurately understand spatial and temporal variation in recruitment and to identify potential effects of proposed oil and gas exploration on recruitment of ducks at Minto Flats.

A General Bayesian Hierarchical Model for Estimating Survival of Nests and Young

ABSTRACT. Models for estimating survival probability of nests and young have changed dramatically since the development of the Mayfield method. Improvements in software and a steady increase in computing power have allowed more complexity and realism in these models, allowing researchers to provide better estimates of survival and to relate survival rates to relevant covariates. However, many current analysis methods utilize fixed-effects models with the implicit assumption that the covariates explain all of the variation in the data, other than random variation within a specified family of distributions. This is generally a strong assumption, and, in the presence of heterogeneity and lack of independence, these estimates have been shown to be negatively biased. Others have begun to explore random-effects models for these situations, but a readily applicable Bayesian approach has been lacking. We present a general Bayesian modeling framework appropriate for survival of both nests and young that simultaneously allows for the inclusion of individual covariates and random effects and provides a measure of goodness-of-fit. We used previously published data on survival of Common Goldeneye (Bucephala clangula) ducklings in interior Alaska and on nest survival in three species of prairie-nesting ducks that nested in the Missouri Coteau region of North Dakota to demonstrate this approach. The inclusion of a brood-level random effect in the Common Goldeneye example increased point estimates and credible interval [CI] coverage from 0.62 (95% CI: 0.49–0.73) and 0.66 (95% CI: 0.58–0.74) for 2002 and 2003, respectively, to 0.69 (95% CI: 0.42–0.88) and 0.74 (95% CI: 0.57–0.88) for 2002 and 2003, respectively.

SURVIVAL OF SCAUP DUCKLINGS IN THE BOREAL FOREST OF ALASKA

Abstract We studied duckling survival of lesser (Aythya affinis) and greater (A. marila) scaup at Minto Flats, Alaska during 2002–2003. We captured and marked 42 female scaup during the study, and we subsequently monitored survival of 32 scaup broods with 284 ducklings. Daily Survival Rate (DSR) of ducklings varied between years and was positively related to female body condition and duckling age. Estimated duckling survival to 30 days for both species of scaup was 0.24 (95% CI: 0.16 to 0.36) in 2002 and 0.03 (95% CI: 0.00 to 0.19) in 2003. Because we obtained little support for relationships between environmental covariates and duckling survival, we speculated that most of the variation between years could have been related to high rates of predation by gulls (Larus spp.) during 2003. Estimated recruitment of female scaup to 30 days (clutch size * nest survival * duckling survival * 0.5) was 0.30 (SE = 0.19) in 2002 and 0.01 (SE = 0.01) in 2003. Given the low recruitment we observed at Minto Flats and the current population status of scaup, restriction of harvest of adult female scaup could be useful to support scaup populations at Minto Flats.

Time-lagged variation in pond density and primary productivity affects duck nest survival in the Prairie Pothole Region

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.

Survival of Ducks Banded in the Boreal Forest of Alaska

Abstract Limited information is available about the population dynamics of ducks in the boreal forest. We conducted an analysis of recovery data from 5 species of ducks banded in the boreal forest of Alaska, USA, during 1959–1966, and records of 3 species of ducks banded during 1989–2000, with the objective of examining sources of variation in survival and sampling probability and to complement recent studies of the breeding ecology of ducks in the boreal forest. Survival of ducks during 1959–1966 was species- and year-specific. During 1989–2000, survival of northern pintail (Anas acuta) was age-, sex-, and year-specific. Age-class by sex interaction, without year-specificity, however, best-described survival of mallard (A. platyrhynchos) and green-winged teal (A. crecca). Annual survival of male ducks from the Alaska boreal forest was generally similar to annual survival of the same species banded in the midcontinent. Survival probability of female ducks from the Alaska boreal forest, however, was generally higher than survival of female ducks from midcontinent regions. Sampling probability during 1989–2000 was lower for females than males, and increased after 1996, concurrent with the initiation of electronic band reporting. Our results suggest that patterns of duck survival differ between the boreal forest and the midcontinent, especially for after-hatch-year females. Regional variation in survival and reproduction and the factors affecting these parameters should continue to be monitored and considered in continental management plans.

Influence of Wind Turbines on Presence of Willet, Marbled Godwit, Wilson's Phalarope and Black Tern on Wetlands in the Prairie Pothole Region of North Dakota and South Dakota

Abstract. The millions of wetlands that define the Prairie Pothole Region (PPR) harbor large proportions of continental populations of several species of North American waterfowl, waterbirds, and shorebirds. The PPR also has some of the highest wind energy potential in the United States. Thousands of wind turbines are being erected in the PPR to produce electricity and have the potential to affect migratory bird populations through collisions, displacement, barriers to movement, habitat fragmentation, and habitat loss. We assessed occurrence of waterbirds and shorebirds from 2008 through 2010 on wetlands in two wind energy development sites, defined as wetlands within 805 m of a wind turbine, and two reference sites in the PPR of North and South Dakota. We conducted 10,321 wetland visits on 3,542 individual wetland basins and related bird occurrence to wetland characteristics, upland characteristics, survey type (roadside vs. off-road), seasonal timing of sampling, year of sampling, and site type (wind energy development vs. reference). Models characterizing occurrence of Willet (Catoptrophorus semipalmatus), Marbled Godwit (Limosa fedoa), Wilsons Phalarope (Phalaropus tricolor) and Black Tern (Chlidonias niger) indicated that occurrence varied with wetland characteristics and among sites and years, was not substantially reduced on either wind energy site, but was slightly and consistently lower on one of the wind energy sites for the three shorebird species. Our results suggest that wetlands have conservation value for these species when wind turbines are present, but additional sampling across time and space will be necessary to understand the effects of wind turbines on shorebird and waterbird presence, density, survival, and reproductive success.

Habitat use and population status of Yellow-billed and Pacific loons in western Alaska, USA

ABSTRACT Effective conservation of sympatric avian populations depends on unbiased estimates of population size, distribution, and habitat use. For populations of Yellow-billed Loons (Gavia adamsii) and Pacific Loons (G. pacifica) co-occurring in Arctic wetland communities in Alaska, USA, such data are limited and difficult to obtain, hindering population assessments and decision making. The Yellow-billed Loon is also under consideration for additional protections under the Endangered Species Act due to small global population size, specific habitat requirements, and low fecundity, further increasing the need for information at the landscape scale. To help evaluate the population status and habitat use of both species, we used repeated aerial surveys and a dynamic multistate occupancy modeling approach to jointly estimate 1) probability of lake use and 2) probability of use for nesting for Yellow-billed and Pacific loon populations at the landscape scale on the Seward Peninsula and Cape Krusenstern, Alaska, in 2011 and 2013. We also estimated state-specific transition probabilities and degree of interspecific competition to assess population stability and degree of species interactions. We found that probability of site reuse (φYellow-billed = 0.73 [0.44–0.94]; φPacific = 0.86 [0.72–0.98]) or reuse for nesting (φYellow-billed = 0.72 [0.46–0.97]; φPacific = 0.59 [0.38–0.85]) in 2013 was high, as was overall use of lakes >7 ha by loons (>80%). These results suggested that lake habitats may have been saturated, and that populations of both species were stable over the two-year interval between surveys. Our estimates indicated that nesting populations in western Alaska were much larger than previously thought for both Yellow-billed (∼2.5 times larger) and Pacific loons (∼1.5–2.0 times larger). Together our results indicate that Arctic wetlands in western Alaska are important for both species and that loon populations in this area warrant additional consideration for conservation.