Life History Theory: Basics

There is a deep connection between population dynamics and life history, since populations are comprised of individuals. ‘Population dynamics’ refers to a time series of population size (and structure). ‘Life history’ refers to the timing of ‘key events’ across the life cycle of individuals. Population projection matrices can be decomposed into a sum of two matrices, one for single time-step fates of already existing individuals and another for single time-step production of new individuals. Matrices are clearly related to age-specific survivorship and fertility schedules of life tables, directly for age-structured populations and through Markov chain analysis for stage-structured populations

The contributions of age and sex to variation in common tern population growth rate

The decomposition of population growth rate into contributions from different demographic rates has many applications, ranging from evolutionary biology to conservation and management. Demographic rates with low variance may be pivotal for population persistence, but variable rates can have a dramatic influence on population growth rate.In this study, the mean and variance in population growth rate (?) is decomposed into contributions from different ages and demographic rates using prospective and retrospective matrix analyses for male and female components of an increasing common tern (Sterna hirundo) population.Three main results emerged: (1) subadult return was highly influential in prospective and retrospective analyses; (2) different age-classes made different contributions to variation in ?: older age classes consistently produced offspring whereas young adults performed well only in high quality years; and (3) demographic rate covariation explained a significant proportion of variation in both sexes. A large contribution to ? did not imply a large contribution to its variation.This decomposition strengthens the argument that the relationship between variation in demographic rates and variation in ? is complex. Understanding this relationship and its consequences for population persistence and evolutionary change demands closer examination of the lives, and deaths, of the individuals within populations within species

Analysis of interspecific competition in perennial plants using life table response experiments

The impact of interspecific competition is usually measured by its effect upon plant growth, neglecting impacts upon other stages of the life cycle such as fecundity which have a direct influence upon individual fitness and the asymptotic population growth rate of a population (lambda). We used parameterized matrix models for three perennial plant species grown with and without interspecific competition to illustrate how the methodology of Life Table Response Experiments (LTRE) can be used to link any change in population dynamics to changes in any part of the life cycle. Plants were herbaceous grassland species grown for two years in a field experiment at Rothamsted Experimental Station, England. Interspecific competition reduced X by over 90% in all species. Survival and growth were slightly affected by competition whereas plant fecundity was greatly reduced. Nearly all of the observed difference in X between the competition treatments was explained by the fecundity terms, and more precisely by a large difference in the number of seeds, and a high sensitivity of X to the germination rate. Whereas most competition studies focus on the measurement of change in individual fitness, our study illustrates how informative it is to take account not only of the effect of competition upon vital rates but also of how different vital rates affect population growth rate