Ecological Causes of Life History Variation Tested by Meta-analysis, Comparison, and Experimental Approaches
The ecological causes of life history variation among taxa and the arrangement of such variation along geographic gradients is enigmatic despite the proximity of life history traits to fitness and implications for understanding basic and applied population ecology. One classic explanation for the arrangement of avian life histories along a `slow-fast' gradient, where species at low latitudes have `slow' life history traits (low fecundity and mortality) and species at high latitudes have `fast' life history traits (high fecundity and mortality), is the increase in seasonality of resources with increasing latitude (Ashmole's hypothesis). Despite broad acceptance, this hypothesis has been supported only indirectly. I tested two key predictions of this hypothesis - that most mortality occurs in winter and that most mortality is caused by starvation - using meta-analysis. Surprisingly, in many populations, the season of greatest mortality was summer, and most mortality was caused by predation. These results suggest alternative explanations for life history variation should remain under consideration despite support for Ashmole's hypothesis. The relationship between provisioning behavior and offspring number was long recognized to integrate key life history tradeoffs between number and quality of offspring and between current and future reproductive success. Studies of the response of parental provisioning behavior to brood size variation played a formative role in the development of life history theory. Yet, the inference of such experiments for explaining among-species differences has always been limited by lack of comparative context. I expanded predictions of alternative ecological explanations (food limitation, nest predation, adult mortality) for life history variation to an among-species context and test these predictions using a comparative-experimental design across a broad range of bird species from three continents. I found resource limitation and adult mortality risk interact to explain variation among species in responses to natural and experimental variation in brood size, with the degree of food limitation appearing to vary across a gradient of adult mortality risk. This result helps to explain the potentially conflicting results of previous studies and suggests a pluralistic approach to understanding what factors explain life history variation may be fruitful. Understanding variation among species in mortality rates may thus be pivotal to understanding ecological causes of life history variation. To this end, I compared differences in spatiotemporal variance in survival among three temperate-breeding species with differing migratory strategy. I found that migratory behavior may be associated with reduced spatial variance in annual survival because resident species disperse less, reducing population connectivity. I also found that migratory behavior is associated with increased temporal variance in survival, counter to expectations of general theory. Given the potential importance of mortality risk in life history evolution, expanded geographic comparisons of annual and within-year patterns of variance in survival rates is likely key to understanding variation among species in life history traits.
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