Veijo Kaitala

Is the impact of environmental noise visible in the dynamics of age-structured populations?

Climate change has ignited lively research into its impact on various population–level processes. The research agenda in ecology says that some of the fluctuations in population size are accountable for by the external noise (e.g. weather) modulating the dynamics of populations. We obeyed the agenda by assuming population growth after a resource–limited Leslie matrix model in an age–structured population. The renewal process was disturbed by superimposing noise on the development of numbers in one or several age groups. We constructed models for iteroparous and semelparous breeders so that, for both categories, the population growth rate was matching. We analysed how the modulated population dynamics correlates with the noise signal with different time–lags. No significant correlations were observed for semelparous breeders, whereas for iteroparous breeders high correlations were frequently observed with time–lags of −1 year or longer. However, the latter occurs under red–coloured noise and for low growth rates when the disturbance is on the youngest age group only. It is laborious to find any clear signs of the (red) noise– and age group–specific fluctuations if the disturbance influences older age groups only. These results cast doubts on the possibility of detecting the signature of external disturbance after it has modulated temporal fluctuations in age–structured populations.

Sex in space: population dynamic consequences

Sex, so important in the reproduction of bigametic species, is nonetheless often ignored in explorations of the dynamics of populations. Using a growth model of dispersal–coupled populations we can keep track of fluctuations in numbers of females and males. The sexes may differ from each other in their ability to disperse and their sensitivity to population density. As a further complication, the breeding system is either monogamous or polygamous. We use the harmonic mean birth function to account for sex–ratio–dependent population growth in a Moran–Ricker population renewal process. Incorporating the spatial dimension stabilizes the dynamics of populations with monogamy as the breeding system, but does not stabilize the population dynamics of polygamous species. Most notably, in populations coupled with dispersal, where the sexes differ in their dispersal ability there are rarely stable and equal sex ratios. Rather, a two–point cycle, four–point cycle and eventually complex behaviour of sex–ratio dynamics will emerge with increasing birth rates. Monogamy often leads to less noisy sex–ratio dynamics than polygamy. In our model, the sex–ratio dynamics of coupled populations differ from those of an isolated population system, where a stable 50:50 sex ratio is achievable with equal density–dependence costs for females and males. When sexes match in their dispersal ability, population dynamics and sex–ratio dynamics of coupled populations collapse to those of isolated populations.

Foraging, vigilance and risk of predation in birds—a dynamic game study of ESS

A dynamic multi-period game theory model is developed to study evolutionary stable scanning strategies of birds foraging in flocks. Scanning, while foraging, is carried out to detect attacking predators. In our model there is a chance that birds are killed by predators during foraging days. After each loss of a flock mate the remaining birds face the following foraging day in a smaller flock. The model has non-unique solutions. The evolutionary stable scanning rates may be constant over the foraging season, or they may show a monotonic decreasing trend with time. The former ESS solution does not depend on the future survival probabilities, and is independent of the future flock dynamics. Hence, the scanning rates depend on the current flock size only. The latter ESS solution is interpreted as follows. A loss of a flock mate decreases the future survival of each remaining bird. Hence, the birds increase their scanning rates to take care of their flock mates and to face the future predator attacks in a bigger flock. The choice between the ESS solutions depends on the behavior of the birds at the end of the foraging season. The scanning rates depend on the behavior of the predators.

Punishment of polygyny

We investigated the evolution of monogamy (one male, one female) and polygyny (one male, more than one female). In particular, we studied whether it is possible for a mutant polygynous mating strategy to invade a resident population of monogamous breeders and, alternatively, whether a mutant monogamy can invade resident polygyny. Our population obeys discrete–time Ricker dynamics. The role of males and females in the breeding system is incorporated via the harmonic birth function. The results of the invasability analysis are straightforward. Polygyny is an evolutionarily stable strategy mating system; this holds throughout the examined range of numbers of offspring produced per female. So that the two strategies can coexist, polygyny has to be punished. The coexistence of monogamy and polygyny is achieved by reducing the offspring number for polygyny relative to monogamy. This yields long–term persistence of the strategies for all offspring numbers studied. An alternative punishment is to increase the sensitivity of polygynous breeders to population density. The coexistence is possible only with a limited range of offspring produced. The third way to achieve coexistence of the two mating strategies is to assume that individuals live in a spatially structured population, where dispersal links population subunits to a network. Reducing the dispersal rate of polygynous breeders relative to that of monogamous individuals makes the coexistence feasible. However, for monogamy to persist, the number of offspring produced has to be relatively high.

Ecology of Populations: Index

The theme of the book is the distribution and abundance of organisms in space and time. The core of the book lies in how local births and deaths are tied to emigration and immigration processes, and how environmental variability at different scales affects population dynamics with stochastic processes and spatial structure and shows how elementary analytical tools can be used to understand population fluctuations, synchrony, processes underlying range distributions and community structure and species coexistence. The book also shows how spatial population dynamics models can be used to understand life history evolution and aspects of evolutionary game theory. Although primarily based on analytical and numerical analyses of spatial population processes, data from several study systems are also dealt with. • Details the explicit spatial extension of the analyses of population and community processes • This book illustrates how theory and data analysis are close together and how data can be used to illustrate fundamental processes and vice versa • Through viewing population dynamics as a spatial process allows us to approach population ecology from the perspective of self-organised processes (Less)