Book Description

Populations living in natural environments experience fluctuations in environmental conditions that drive variability in demographic rates. This dissertation develops new and existing mathematical methods for studying environmental stochasticity and uses these tools to investigate the role of environmental stochasticity in driving observed population dynamics and plant life history evolution. In the first two chapters I develop new approaches to a classic method in population biology, the life table response experiment (LTRE). Whereas existing methods used time-averaged demographic rates and deterministic sensitivities to decompose observed differences in population growth rates, this new method allows estimation of the contributions to those differences made by variances in demographic rates as well as by mean rate values. I use this stochastic LTRE to show how differential variability in the vital rates of Anthyllis vulneraria (kidney vetch) contribute to differences in the population growth rates of nine populations growing in southwest Belgium; we also show how the effects of demographic rate variability depend on soil depth, where the greater moisture retention of deeper soils buffers populations against the otherwise negative effects of demographic variability. The second chapter provides a different approach to LTRE that uses an iterated two-factor decomposition of the small noise approximation of the stochastic population growth rate to quantify contributions to that growth rate made by: (i) mean vital rates, (ii) temporal variability in vital rates, (iii) elasticities of the population growth rate to individual vital rates, and (iv) correlations between vital rates across the study period. Contributions of elasticities tell us about differences in local selection pressures acting on distinct populations and contributions of correlations tell us about differences in the phenotypic tradeoffs associated with vital rates. I use this new method to show how these differences drive dynamics in two species: Anthyllis vulneraria (the same populations studied in the first chapter) and Cypripedium calceolus (lady's slipper orchid). In Anthyllis vulneraria, variability in large adult fertility and seedling survival made the largest contributions; there were also effects of differences in elasticities of large adult fertility and survival, as well as differences in the correlations between rapid growth and survival in seedlings (a survival cost of rapid early development), between large adult fertility and survival (a survival cost of reproduction) and between large adult fertility and seedling survival. In Cypripedium calceolus, population growth rates were driven most by differences in the elasticities to the probabilities of adult stasis vs. entering dormancy, as well as by differences in the variability and tradeoffs associated with adult dormancy; correlation played a role through differences in the survival payoff of dormancy vs. the complimentary fertility cost of dormancy in terms of lost opportunity for reproduction. The third and final chapter investigates the role of fire disturbance in driving the life histories and population-level dynamics of five woody plant species growing in the Brazilian cerrado, a savannah-forest mosaic in which woody vegetation cover is primarily mediated by fire disturbance. This study presents a set of diagnostics that use demographic responses to recurring disturbance to categorize species along a continuum of adaptation: on one end we find 'resistant' species that must weather disturbance in order to attain large sizes that are buffered against fire-induced mortality; on the other end we find 'resilient' species that are relatively indifferent to disturbance and harness transient opportunities afforded by early post-fire successional habitats in order to take advantage of increased nutrient availability and reduced competition. Each of these chapters uses stochastic demographic analysis to extend theory describing the dynamics of populations in variable environments; together, these studies present a variegated perspective on the role of environmental stochasticity that provides new methods and novel perspectives that should be useful in the study of population biology and life history evolution.