Graduate Thesis Or Dissertation
 

The Influence of Alternate Life History Strategies and Natal Conditions on the Reproductive Performance of Adélie Penguins Breeding on Ross Island, Antarctica

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  • A fundamental objective of ecology and population biology is to identify factors that drive population dynamics and determine the population-level consequences of their interaction with the environment. Studies of reproductive performance can illuminate population dynamic processes, including the links between organismal biology, the environment, and life history theory. A central tenent of life history theory is that organisms face trade-offs when partitioning limited resources among growth, maintenance, survival, and lifetime reproductive potential. For long-lived species that have multiple breeding attempts during their lifetime (i.e., iteroparous species), these trade-offs can be observed in the age-related variation in reproductive performance associated with alternate breeding strategies, namely, variation in the age first reproduction/recruitment into the breeding population and lifetime reproductive potential. Understanding the consequences of this variation can provide insights into population dynamics and life history theory and is critical to predicting how individuals and thus populations are likely to respond to anthropogenic and natural changes in their environment. In Chapter 2, I used generalized linear mixed models to disentangle population and within-individual processes influencing observed patterns in age-specific reproductive performance. I tested the following five distinct patterns in breeding success as predicted by various life history hypotheses: 1) linearly increasing with breeding experience; 2) increasing with low levels of experience to a plateau; 3)increasing with low levels of experience to a plateau or peak near optimum performance with intermediate levels of experience, then increasing again in older age classes; 4) increasing with low levels of experience to a plateau or peak near optimum performance with intermediate levels of experience, then decreasing; or (5) remains constant. Overall patterns of reproductive performance were similar at all three colonies. Regardless of recruitment age, breeding success improved with post-recruitment experience at all three colonies, reaching a maximum at intermediate experience (4-8 attempts), which occurred roughly 4 years earlier for younger recruits than for older recruits. At all three colonies, improvement in breeding success was most dramatic for the youngest recruits, while the oldest recruits showed the lowest overall improvement in breeding success with additional post-recruitment experience. Together, these results support the “constraint hypothesis” and suggest that individuals face trade-offs late in life based on recruitment decisions. When controlling for selective disappearance of lower quality phenotypes, I documented pronounced declines (i.e., senescence) in breeding success for the oldest recruitment cohorts at all three colonies. In Chapter 3 I decomposed age-related reproductive performance into its constituent processes at each colony. The selective appearance of new phenotypes contributed negatively (range: 20% - 54%) to overall age-related reproductive performance at all colonies. At the small- and medium-sized colonies improvements in age-related reproductive performance was driven by a combination of within-individual maturation (15% at both colonies) and selective disappearance (41% and 32%, respectively). At the largest colony, the majority of the improvement was due to within-individual maturation (67%). Comparisons of the relative contribution of each of the processes in early life versus late life stages at all three colonies revealed similar contributions early in life, but important differences at advanced ages. At the two smaller colonies, observed changes late in life were primarily driven by population level changes resulting from selective disappearance of individuals of lower phenotypic quality. However, at the largest colony, changes in performance were driven by a combination of decline due to within-individual maturation and the selective disappearance of individuals of higher phenotypic quality. This unexpected result may be due to a combination of potential trade-offs associated with differences between colonies of different sizes and associated differences between individual life history strategies. These results highlight the importance of accounting for the different processes contributing to reproductive performance and of incorporating replication into studies of age-related performance. In Chapter 4 I tested the prediction from the internal predictive adaptive response (internal PAR) hypothesis that individuals raised under poor natal predictions can mitigate those conditions and have equally productive reproductive life spans as individuals raised under better natal conditions. For five years, two giant icebergs substantially altered sea ice conditions throughout the annual cycle, including preferred penguin habitat, sea ice concentrations on the foraging grounds near breeding colonies, and increased the extent of fast ice near breeding colonies. As a result, 5 iceberg-affected cohorts experienced significantly reduced early-life conditions from other non-iceberg-affected cohorts. In accordance with predictions from the internal PAR hypothesis, iceberg cohorts entered the breeding population earlier, exhibited higher levels of breeding success, and had shorter lifespans than non-iceberg-affected cohorts. Non-iceberg-affected cohorts recruited to the breeding population later, never achieved levels of breeding success documented for iceberg-affected cohorts, and lived longer than icebergs cohorts. At the end of their lifespan iceberg-affected cohorts were nearly able to completely mitigate the fitness costs of poor natal conditions. These results support the internal PAR hypothesis and highlight the importance of accounting for early life conditions when studying life history strategies and population dynamics. Taken together, these results highlight the importance of long-term studies that incorporate sufficient spatial and temporal variability to understand how varying reproductive strategies drive population dynamics. Without the temporal scale of 20 years and spatial scale across three colonies, the quantitative analysis of age-related reproductive strategies (Chapter 1), the decomposition of reproductive success into constituent processes (Chapter 2), or the testing of hypotheses relating natal environment to life-time reproductive success (Chapter 3) would have been impossible. The results of these three studies will further guide and refine the next 20 years of research and contribute to our knowledge of life history theory and an ice-obligate species facing dramatic changes in sea ice conditions in the coming decades.
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