Individual and Population Responses to Environmental Change: Allee Effects, Phenotypic Plasticity, and Life History in Fishes

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Life-history theory offers explanatory and predictive frameworks for understanding individual and population responses to natural and anthropogenic environmental change. By affecting reproductive success, life histories affect fitness; by affecting fitness, life histories affect population demography. Comprehensive understanding of the causes and consequences of life-history variation is undermined by uncertainties in our ability to predict physiological, life history, demographic, and evolutionary responses to environmental change. It is these uncertainties that stimulate my long-term objective to explore the adaptive significance of within- and among-population variability in life history, by determining how environmental change influences fitness. My short-term objective is to gain insight into the adaptive significance of individual and population responses to environmental change in light of: i) Allee effects; (ii) reaction norms; (iii) regime shifts; and (iv) ecotypic variation. Integral to the dynamics of small populations are Allee effects - positive relationships between abundance (or density) and realized per capita population growth. The greater the magnitude of decline, the greater the likelihood of Allee effects, negatively affecting fitness and recovery. Recent models of density-dependent selection emphasize the importance of rapid evolution, a form of `evolutionary rescue'. However, direct evidence is limited. Using laboratory-based selection experiments on Japanese medaka, I will examine whether demographic and emergent Allee effects generate different selection responses in small populations and different recovery trajectories. Life histories evolve in response to changes in the mean and variance of environmental conditions. However, research has focused almost exclusively on shifts in average temperature rather than shifts in thermal variability, limiting understanding of how thermal variability affects fitness independently of directional changes in mean temperature. Based on experiments on zebrafish, I ask how plasticity (developmental & transgenerational) and thermal fluctuations affect fitness, resistance to heat shock, and tolerance to low-oxygen environments. Environmental change affects biodiversity. Numerous models have generated predictive analyses of how species will respond to global change but few predictions have been ground-truthed. Using a Bayesian change-point detection algorithm, I will determine how anthropogenic and natural `stressors' have affected coastal fish biodiversity, by applying regime-shift analyses to exceptionally long (up to 100 years) datasets. At the intra-specific level of biodiversity, I will explore how environmental change influences the proportional representation of genetically different ecotypes in Atlantic cod, and test the hypothesis that ecotypes are maintained by factors substantive enough to negatively affect the probability of interbreeding or the survival of hybrid offspring.