Impacts of cohort and maternal effects mediated by body size on population growth and fitness in Windermere pike: An integral projection model
Maternal effects and cohort effects of early life conditions can have lasting impacts on phenotypic traits and vital rates of individuals, affecting fitness and population growth. While both maternal and cohort effects have been demonstrated for a range of organisms, few studies have used demographic approaches to evaluate their relative importance. A currently relevant hypothesis for fish is that large and old females are important because of positive maternal effects of body size on offspring survival, and may therefore warrant increased protection by management actions.
We extended and analyzed an integral projection model (IPM) parameterized for a population of pike (Esox lucius) from Windermere, UK, to evaluate the impacts of maternal effects of body size on egg weight, and cohort effects of temperature conditions during the first year of life. The model included heterogeneity in growth and survival mediated by offspring length (at age 1). Of the seven vital rate functions, offspring survival could not be estimated from data, and we developed four scenarios for how this rate depends on egg size and temperature. Impacts of vital rate-specific effects of each state variable on the population growth rate lambda (average fitness) were evaluated with a sensitivity analysis.
Egg weight increased with maternal length but leveled off for large lengths, following a typical pattern for many fish species. However, this maternal effect had only minor impacts on lambda, even when we assumed very strong positive effects of egg weight on offspring survival, and a stronger, linear effect of maternal length on egg weight. In contrast, lambda was sensitive to temperature conditions during the first year, affecting offspring length and survival. The indirect effects of offspring length on fitness via growth were larger than direct impacts on survival and growth, although these were also substantial.
This study suggests that old / large females are important to population growth through their high survival rate allowing repeated reproductive events, and not because of maternal effects on offspring survival. The cohort effect of temperature shows that early life stages can be important determinants of population responses to climate change, through lasting direct and indirect effects on the subsequent life history. Although certain parts of the model are uncertain due to lack of data, most results were robust to the assumptions, highlighting that demographic approaches like IPMs can help disentangle the underlying mechanisms of population dynamics of structured organisms.