It is well understood that global climate change is having effects on various species, but there are few studies that have quantified the costs of sustained directional selection in response to global climate change. Reed et al. conducted a study to test whether the population growth of Parus major (great tit) was negatively affected by climate change. Specifically, to see if climate change induced a phenological mismatch. They took four decades of individual level life history data from a population of great tits in the Netherlands whose breeding season is closely tied with the development of caterpillars as a food source to feed the young birds. Caterpillar growth is closely linked to warm temperatures which are critical to the breeding success of the great tit. Due to warmer springs there has been a mismatch of the breeding time and the food peak creating an intensification of direction selection to earlier laying dates. This mismatch has not affected the population growth. Reed et al. demonstrate a mechanism that contributes to the decoupling; that fitness losses due to the mismatch are countered by fitness gains due to less competition. It implies that populations may be able to tolerate maladaptation from climate without immediately declining. —Cameron Lukos
Reed, T.E., Grøtan, V., Jenouvrier, S., Sæther, B.-E., Visser, M.E., 2013. Population Growth in a Wild Bird Is Buffered Against Phenological Mismatch. Science 340, 488-491.
To conduct their study, Reed et al. studied a population of Parus major in relation to a food source of caterpillars. The focus area has experienced spring warming in recent decades due to climate change. The birds rely on caterpillars as a food source for the fledglings and so match their breeding patterns with the seasonal peak of caterpillars. The experimenters ran a statistical analysis to test how strong their connection is with the mismatch affecting population growth. When their results showed no statistical significance, the experimenters created a fitness variable to further understand the decoupling of population growth.
Their results show that warmer temperatures create a larger mismatch between caterpillars and the tits. But there was no statistical significance with either analysis. The mismatch had no statistically significant connection between the population growth and directional selection. The addition of fitness also did not yield any statistical significance. The lack of statistical significance reveals that there is something further that has not been accounted for. The question becomes why is population growth not lower in years with a large fraction of females lay too late? Reed et al. discuss two reasons for this. One is that the food peak is much narrower than the distribution of breeding dates and so reproductive fitness cannot be high for all females every year. The experimenters show that in an early reproductive year relative to food peaks, females who start early produced fewer fledglings than females that started later. Those females that started in the middle had the highest reproductive output. In a relatively late reproductive year late females had lower reproductive output than intermediates. The ones who do the best in this situation are those that reproduce early. This pattern is most likely due to the fact that some pay a higher energetic costs when feeding their broods. The second reason is that fledgling production is counterbalanced by improved independent survival of offspring due to relaxed competition.