Mark C. Drever

Norway rats as predators of burrow-nesting seabirds : Insights from stable isotope analyses

Introduced species to oceanic islands can cause tremendous declines and extinctions of native avifauna. On Langara Island. British Columbia, Canada, the burrow-nesting ancient murrelet (Synthliboramphus antiquus) has declined from all estimated original population of 200,000 pairs to 14,600 pairs ill 1993. Previously, causes of this decline were unknown, but the introduction of Norway rats (Rattus norvegicus) has been implicated as a major factor and has resulted in a recent rat eradication program. A major obstacle in the investigation of the effects of rats on seabirds, here and elsewhere, has been the inability to accurately assess the importance of seabirds to the diets of rats that also consume a variety of plants and invertebrates. We used stable-carbon (δ 13 C), nitrogen (δ 15 N), and sulphur (δ 34 S) isotope analyses of muscle and liver tissues of rats and prey organisms from 3 regions of Langara Island to evaluate evidence for marine foods, including seabirds, in the diets of rats. Rats were segregated into 3 isotopic groups corresponding to upland, littoral, and seabird nesting areas on the island. We interpret these groups to represent individuals consuming predominantly C-3 terrestrial foods (x δ 15 N = 5.4‰; x δ 13 C = -24.9‰; n = 24), intertidal invertebrates (x δ 15 N = 8.9‰; x δ 13 C = -14.3‰; n = 21), and ancient murrelet adults, chicks, or eggs (x egg δ 15 N = 13.2‰; x δ 13 C = -17.6‰; n = 8). We found strong correlations between liver and muscle isotope values for both 13 C and 15 N, suggesting that dietary preferences within individuals remained relatively constant. Stable-sulphur isotope values of rat liver were less useful in segregating marine and terrestrial dietary inputs, possibly because sources of sulphur to tile terrestrial food web were of marine origin pooled x δ 34 S value = 17.8‰; n = 20). Our measurement of liver and muscle tissues gave dietary estimates based on relatively short- and long-tern integrations and revealed tile 3 groups of rats remained isotopically segregated at least over the 2-month period of ancient murrelet breeding on Langara Island. Our results have important ramifications for dietary investigations of introduced fauna and their effect on native scabirds on oceanic slands.

Geographical gradients in the population dynamics of North American prairie ducks

1. Geographic gradients in population dynamics may occur because of spatial variation in resources that affect the deterministic components of the dynamics (i.e. carrying capacity, the specific growth rate at small densities or the strength of density regulation) or because of spatial variation in the effects of environmental stochasticity. To evaluate these, we used a hierarchical Bayesian approach to estimate parameters characterizing deterministic components and stochastic influences on population dynamics of eight species of ducks (mallard, northern pintail, blue-winged teal, gadwall, northern shoveler, American wigeon, canvasback and redhead (Anas platyrhynchos, A. acuta, A. discors, A. strepera, A. clypeata, A. americana, Aythya valisineria and Ay. americana, respectively) breeding in the North American prairies, and then tested whether these parameters varied latitudinally. 2. We also examined the influence of temporal variation in the availability of wetlands, spring temperature and winter precipitation on population dynamics to determine whether geographical gradients in population dynamics were related to large-scale variation in environmental effects. Population variability, as measured by the variance of the population fluctuations around the carrying capacity K, decreased with latitude for all species except canvasback. This decrease in population variability was caused by a combination of latitudinal gradients in the strength of density dependence, carrying capacity and process variance, for which details varied by species. 3. The effects of environmental covariates on population dynamics also varied latitudinally, particularly for mallard, northern pintail and northern shoveler. However, the proportion of the process variance explained by environmental covariates, with the exception of mallard, tended to be small. 4. Thus, geographical gradients in population dynamics of prairie ducks resulted from latitudinal gradients in both deterministic and stochastic components, and were likely influenced by spatial differences in the distribution of wetland types and shapes, agricultural practices and dispersal processes. 5. These results suggest that future management of these species could be improved by implementing harvest models that account explicitly for spatial variation in density effects and environmental stochasticity on population abundance.

Species-energy theory, pulsed resources, and regulation of avian richness during a mountain pine beetle outbreak.

Species-energy theory provides a framework through which to link two features commonly noted in local communities: episodic production of resources (i.e., resource pulses) and the regulation of local species richness through time. We examined the pathways through which a resource pulse, a large-scale outbreak of mountain pine beetles, was borne out in six foraging guilds comprising a forest bird community in British Columbia, Canada, 1997-2007. We investigated statistical relationships between richness and abundance in each guild to evaluate the prediction that the outbreak should be manifested in species most reliant upon bark beetles (i.e., the bark insectivore guild). We then employed randomization methods to evaluate whether the beetle outbreak obscured evidence for local regulation for the six foraging guilds. Density and richness of bark insectivores increased over the course of the outbreak. More species of bark insectivores were detected for a given number of individuals following the outbreak, consistent with an increase in the number and types of resources. Richness of bark insectivores showed no evidence of regulation. In contrast, densities of most other foraging guilds were not strongly correlated with the habitat changes resulting from the beetle outbreak and displayed only weak evidence of regulation of richness. We suggest that such weak regulation of richness may be a general feature of forest bird communities. Coupled with long-term data, resource pulses provide exceptional opportunities through which to test predictions of species-energy theory.

Fire and the relative roles of weather, climate and landscape characteristics in the Great Lakes-St. Lawrence forest of Canada

Abstract Question: In deciduous-dominated forest landscapes, what are the relative roles of fire weather, climate, human and biophysical landscape characteristics for explaining variation in large fire occurrence and area burned? Location: The Great Lakes-St. Lawrence forest of Canada. Methods: We characterized the recent (1959–1999) regime of large (≥ 200 ha) fires in 26 deciduous-dominated landscapes and analysed these data in an information-theoretic framework to compare six hypotheses that related fire occurrence and area burned to fire weather severity, climate normals, population and road densities, and enduring landscape characteristics such as surficial deposits and large lakes. Results: 392 large fires burned 833 698 ha during the study period, annually burning on average 0.07% ± 0.42% of forested area in each landscape. Fire activity was strongly seasonal, with most fires and area burned occurring in May and June. A combination of antecedent-winter precipitation, fire season precipitation deficit/surplus and percent of landscape covered by well-drained surficial deposits best explained fire occurrence and area burned. Fire occurrence varied only as a function of fire weather and climate variables, whereas area burned was also explained by percent cover of aspen and pine stands, human population density and two enduring characteristics: percent cover of large water bodies and glaciofluvial deposits. Conclusion: Understanding the relative role of these variables may help design adaptation strategies for forecasted increases in fire weather severity by allowing (1) prioritization of landscapes according to enduring characteristics and (2) management of their composition so that substantially increased fire activity would be necessary to transform landscape structure and composition.

DECLINE OF DUCK NEST SUCCESS REVISITED: RELATIONSHIPS WITH PREDATORS AND WETLANDS IN DYNAMIC PRAIRIE ENVIRONMENTS

Abstract Covariation among factors that may affect nest success of dabbling ducks in the Prairie Pothole Region of North America (e.g. productivity of upland and wetland habitat related to climate variation, and duck and predator densities) often confounds efforts to interpret the effect of any individual factor. A comparison of nest success of dabbling ducks at sites with and without predator management provided an opportunity to separate the effect of predation pressure from other factors because predator management has occurred over a range of climatic conditions. We updated an existing study on temporal trends of nest success for prairie ducks in the Prairie Pothole Region of North America by compiling recent estimates of nest success for five species of dabbling ducks (Mallard [Anas platyhrynchos], Northern Pintail [A. acuta], Northern Shoveler [A. clypeata], Blue-winged Teal [A. discors], and Gadwall [A. strepera]). In addition, we compared trends of nest success at unmanaged sites and sites where nest predators were excluded or removed. We used pond density calculated from annual surveys for breeding waterfowl as an index of upland and wetland productivity and a correlate of predator and duck density. At unmanaged sites, the best approximating local regression model suggested that, rather than having undergone a monotonic decline, average nest success has fluctuated through time, although those changes do not appear to be associated with changes in pond density. At sites where predators were excluded, nest success did not vary with time but varied positively with pond density in the previous year, although that effect was tempered by high pond density in the year of observation. At sites where predators were removed but could emigrate back into study plots, nest success varied widely over time and we found no evidence of an effect of pond density. We show that nest success of dabbling ducks is higher under predator management than at sites without predator management, and that this relationship varies with climatic conditions, possibly related to complex interactions within and among duck species, their predators, and their prey.

Agricultural Policy and Nest Success of Prairie Ducks in Canada and the United States

The Prairie Pothole Region of North America has been modified by agriculture during the past 100 yr, resulting in habitat loss, fragmentation, and degradation that have reduced the abundance and productivity of many wildlife species. The 1985 U.S. Farm Bill provided economic incentives to agriculture that are considered by many to be beneficial to nesting waterfowl and other wildlife. Canada has not experienced an equally comprehensive legislative initiative, which would seem to indicate that benefits to waterfowl in Canada should lag behind those in the United States. However, with the removal of some agricultural subsidies in Canada during the 1990s, the amount of perennial cover in the Canadian prairies increased to levels similar to those of the 1970s. Therefore, it is unclear whether and how the U.S. and Canadian prairies might differ with regard to habitat quality for nesting waterfowl. We used historical and contemporary data to compare temporal trends in duck nest success between the United States and Canada and to assess how mean nest success varied with proportion of cropland and wetland density. The data best supported models with nonlinear temporal trends that varied between the two countries and suggested that mean nest success in Canada declined from its high point in 1930s and remained below the long-term value of 0.16 until the end of the time series in 2005. Mean nest success in the United States also declined from its high point in the 1930s, but increased to above the long-term value of 0.25 during the early 2000s. Mean nest success varied negatively with proportion of cropland in both the United States and Canada. Mean nest success was positively correlated with pond density at Canadian sites, but showed only a weak association with pond density at U.S. sites. All models explained the low proportions of the variation in nest success, suggesting that unmeasured factors such as the abundance and identity of nest predators may have strong effects on nest success. Nonetheless, these results support earlier suggestions that agricultural policy that encourages permanent cover positively influences duck reproductive success. We also found that, for reasons that are not entirely clear, nest success for the same intensity of row cropping was generally higher in the United States than in Canada. Further research is required to elucidate the exact nature of the composition, size, and distribution of permanent cover that coincides with greater average nest success by dabbling ducks in the United States. In addition, the data suggest that the benefits that might accrue from increases in the amount of perennial cover in Canada would be better realized if these efforts are accompanied by strong measures to conserve wetlands. RESUME. La region des cuvettes des Prairies nord-americaines a ete alteree par l’agriculture au cours des 100 dernieres annees, menant a la perte, a la fragmentation et a la degradation d’habitats, lesquelles ont contribue a diminuer l’abondance et la productivite de nombreuses especes sauvages. Le Farm Bill etatsunien de 1985 a fourni des incitatifs financiers aux agriculteurs qui auraient ete benefiques a la nidification de la sauvagine et a d’autres especes sauvages. Le Canada n’a pas adopte une politique aussi elaboree, ce qui suggere que les benefices pour la sauvagine devraient y etre observes plus tard qu’aux Etats-Unis. University of British Columbia, University of Guelph, Environment Canada Avian Conservation and Ecology Ecologie et conservation des oiseaux 2(2): 5 http://www.ace-eco.org/vol2/iss2/art5/ Cependant, avec l’abolition de certaines subventions a l’agriculture durant les annees 1990, le couvert de plantes vivaces dans les Prairies canadiennes a atteint des niveaux semblables a ceux des annees 1970. Il n’est donc pas certain que la qualite de l’habitat de nidification de la sauvagine devrait differer entre les Prairies canadiennes et etats-uniennes, ni dans quelle mesure elle pourrait differer, Nous avons utilise des donnees historiques et contemporaines afin de comparer l’evolution temporelle des tendances du succes de nidification des canards entre les Etats-Unis et le Canada et de determiner comment le succes de nidification moyen a varie en fonction de la proportion de terres cultivees et de milieux humides. Les donnees s’ajustaient le mieux aux modeles avec des tendances non-lineaires qui variaient entre les deux pays, et elles indiquaient que le succes de nidification moyen au Canada a decline apres son sommet dans les annees 1930 et qu’il est demeure en-deca de la valeur a long terme de 0,16 jusqu’a la fin de la serie temporelle en 2005. Le succes de nidification moyen aux Etats-Unis a egalement decline a partir de son sommet dans les annees 1930, mais il a augmente, puis depasse la valeur a long terme de 0,25 durant les annees 2000. Le succes de nidification moyen etait relie negativement a la proportion de terres cultivees, tant aux Etats-Unis qu’au Canada. Au Canada, cette variable etait reliee positivement a la densite des etangs, mais cette correlation etait faible aux Etats-Unis. Tous les modeles ont explique la faible proportion de la variation du succes de nidification, ce qui suggere que des facteurs non quantifies, comme l’abondance et la composition des assemblages de predateurs de couvees, peuvent avoir des effets importants sur le succes de nidification. Neanmoins, ces resultats appuient les suggestions selon lesquelles les politiques agricoles qui encouragent un couvert vegetal permanent influencent positivement le succes reproducteur des canards. Nous avons aussi trouve, pour des raisons qui ne sont pas entierement claires, que le succes de nidification etait generalement plus eleve aux Etats-Unis qu’au Canada pour une meme intensite agricole. Des travaux futurs devront etre entrepris afin de determiner quelles composition, etendue et repartition du couvert permanent correspondent au succes de nidification moyen le plus eleve des canards barboteurs aux EtatsUnis. De plus, nos donnees indiquent que les effets benefiques associes a l’augmentation de la quantite de plantes vivaces pourraient augmenter au Canada s’ils etaient accompagnes par une politique efficace de protection des milieux humides.

Spatial synchrony of prairie ducks: roles of wetland abundance, distance, and agricultural cover

Populations exhibit spatial synchrony when their numbers rise and fall in concert at several sites over their distribution. I examined the relationship between synchrony, abundance of wetlands (ponds), distance, and agricultural cover using count data of ten duck species counted in 23 aerial survey strata on the mid-continental prairies of North America. Expansion of agriculture may have resulted in increased synchrony of duck populations through increased foraging efficiency of nomadic predators and/or if the homogenization of nesting habitat has removed habitat features that allow differential local responses to large-scale population drivers such as precipitation. As a measure of synchrony, I calculated all pair-wise cross-correlation coefficients based on population growth rates (rt) at each survey stratum, and then regressed these correlation coefficients against measures of cross-correlation of pond (wetland) counts, distance between strata, and mean percent area of strata seeded to row crops. Synchrony for most species was most strongly related to synchrony of wetland availability among sites, and decreased with distance between sites. Synchrony of ducks that nest over water showed little effect of agricultural cover, whereas the effect of agricultural cover on synchrony of upland nesting ducks differed by species. Mobile large-bodied species showed evidence of increased synchrony due to agricultural cover, whereas smaller-bodied, more philopatric species showed evidence of decreased synchrony due to agricultural cover.

ESTIMATION OF POPULATION PARAMETERS FROM AERIAL COUNTS OF NORTH AMERICAN MALLARDS: A CAUTIONARY TALE

The accuracy of population estimates strongly interferes with our ability to obtain unbiased estimates of population parameters based on analyses of time series of population fluctuations. Here we use long-term data on fluctuations in the size of Mallard populations collected as part of the May Breeding Waterfowl Survey covering a large section of North America. We assume a log-linear model of density dependence and use a hierarchical Bayesian state-space approach in which all parameters are assumed to be realizations from a common underlying distribution. Thus, parameters for different populations are not allowed to vary independently of each other. We then simulated independent time series of aerial counts, using the estimated parameters and adding various levels of observation error. These simulations showed that the estimates of stochastic population growth rate and strength of density dependence were biased even when moderate sampling errors were present. In contrast, the estimates of the environmental stochasticity and the carrying capacity were unbiased even for short time series and large observation error. Our results underline the importance of reducing the magnitude of sampling error in the design of large-scale monitoring programs of population fluctuations.

The efficacy of common species as indicators: avian responses to disturbance in British Columbia, Canada

Common species can be major drivers of species richness patterns and make major contributions to biomass and ecosystem function, and thus should be important targets for conservation efforts. However, it is unclear how common species respond to disturbance, because the underlying reasons for their commonness may buffer or amplify their responses to disturbance. To assess how well common species reflect changes in their community (and thus function as indicator species), we studied 58 bird species in 19 mixed conifer patches in northern British Columbia, Canada, between 1998 and 2010. During this time period two disturbance events occurred, stand level timber harvest and a regional-scale bark beetle outbreak. We examined relationships among densities of individual species, total bird density and overall species richness, correlations in abundance among species, and responses to disturbance events. We found three broad patterns. First, densities of common species corresponded more strongly with changes in total bird density and overall species richness than rare species. These patterns were non-linear and species with intermediate-high commonness showed similar or better correspondence than the most common species. Second, common species tended to be more strongly correlated with abundances of all other species in the community than less-common species, although on average correlations among species were weak. Third, ecological traits (foraging guild, migratory status) were better predictors of responses to disturbance than species commonness. These results suggest that common species can collectively be used to reflect changes in the overall community, but that whenever possible monitoring programs should be extended to include species of intermediate-high commonness and representatives from different ecological guilds.

A century of change in Glaucous-winged Gull (Larus glaucescens) populations in a dynamic coastal environment

ABSTRACT As conspicuous midtrophic omnivores, gulls can serve as useful indicators to characterize long-term ecological changes in marine ecosystems. Glaucous-winged Gulls (Larus glaucescens) have been studied in the Georgia Basin of British Columbia, Canada, an urbanized coastal zone, since the late 1800s. We collated all available information to develop a (noncontinuous) 111-year time series of counts at breeding colonies, and combined these counts with demographic vital rates to assess how changes in historical gull egg harvesting practices, forage fish abundance, and Bald Eagle (Haliaeetus leucocephalus) numbers affected gull population trajectories from 1900 to 2010. Mean counts at 87 breeding colonies in the Georgia Basin showed a nonlinear trend, increasing from historical low counts in the early part of the twentieth century to peak values in the 1980s, and declining thereafter to the end of the time series. Demographic models that integrated temporal trends in clutch size and nesting success, and which also included a food-related decline in first-year survival or a further reduction in nesting success as a function of eagle abundance, successfully reproduced trajectories of gull population growth rates over the study period. Glaucous-winged Gulls have thus responded to a series of changes in the Georgia Basin. These patterns are consistent with population release following cessation of egg harvesting; growing reliance by gulls on nonfish foods and resulting declines in clutch size, productivity, and first-year survival; and the effects of recovering Bald Eagle populations. These results highlight the value of compiling data from multiple retrospective studies to better understand the complex factors affecting long-term trends in animal populations.