Population ecology of wood ducks in Nevada under experimental manipulation of harvest
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Wildlife populations are regulated by vital rates that are influenced by environmental conditions, demographic stochasticity and increasingly, anthropogenic effects. Management directed at specific vital rates, like per capita births or deaths, is often used as a tool to influence change in wildlife populations. I experimentally manipulated harvest regulations for wood ducks in Nevada, to study how harvest affects population level change in abundance and the mortality process. Additionally, I assessed how variation among individuals affects the mortality and recruitment processes. Harvest regulations are used to minimize detrimental effects of hunting on wild populations. In the second chapter I used capture-mark-reencounter data, from wood ducks that live along the Carson River near Fallon, NV, to estimate total population size of females during the spring (breeding) and juvenile females during autumn (juvenile production) to assess the effects of habitat conditions and harvest on change in the breeding population. Using spring snowpack, at the headwaters of the Carson River as a measure of habitat conditions, my results suggest habitat conditions are positively related to autumn age ratios and change in breeding population size the following year but not by direct recovery (harvest) rate. Based on these findings, I suggest future efforts to conserve waterfowl should focus the effects of climate change, which will influence precipitation patterns in the future. Harvest mortality occurs during autumn and winter months when waterfowl populations are at peak abundance, and has been shown to disproportionately select against lower quality individuals. Results from chapter two suggest harvest is does not affect population level change in abundance, which suggests that harvest mortality is compensated by natural mortality. This compensation can occur in the mortality process, the recruitment process, or both. In the third chapter I use seasonal capture-mark-reencounter data to investigate how harvest affects the mortality process. I also measured tarsus length during autumn and spring to investigate population level differences in structural body size before and after the hunting season to understand selection on body size during the hunting season. My results support the compensatory mortality hypothesis in wood ducks; with additional support for the individual heterogeneity hypotheses as a causal mechanism. I detected negative relationships between preseason mortality and total mortality during the hunting season for all age and sex classes, between harvest and non-harvest mortality during the hunting season, and between total mortality during the hunting season and mortality in the season following hunting mortality. Finally, I found a significant increase in tarsus length between summer and the following spring, indicating that smaller individuals had suffered disproportionate mortality during the fall and winter months. Management aimed at reducing anthropogenic influence on the mortality process relies on models to inform population level response to exploitation. If anthropogenic harvest is compensatory, then relationships between harvest management and population trends will not be linear, as mortality selects against ‘low quality’ individuals.Compensation in the recruitment process occurs when there is a positive effect of harvest rate on reproduction. Individual heterogeneity provides a mechanism for compensation in the recruitment process (e.g. more ‘high quality’ individuals breeding will increase reproductive rates). In duck species, the probability of duckling survival is an important component of recruitment into the breeding population. In chapter four I investigated how variation in maternal investment in offspring by nesting female wood ducks affected duckling mortality (1-survival) to 60 days of age using capture-mark-recapture techniques. I used CMR data from 3,035 ducklings that were marked with uniquely coded webtags on the day of hatch over nine nesting seasons (2008-2016). Duckling mortality was greatest during the first week of life and approached zero by the second week after hatching. I observed a quadratic effect of hatch date on duckling mortality of hatch date, suggesting that successful females maximize fitness by nesting early in the year, but not too early (15 days before mean hatch). Increased egg volume and duckling mass reduced the probability of ducking mortality during the duckling period. These results suggest that risk of mortality for ducklings is influenced, at least in part, by maternal effects, which are manifested through energy allocation and behavioral effects during the breeding season.