Carry-over effects negatively impact per capita breeding output in an experimental seasonal population

Background/Question/Methods

Our ability to understand why populations fluctuate relies on our knowledge of the mechanisms that affect individual performance. In seasonal environments, where density dependence can operate in more than one season, vital rates are considered a function of the number of individuals that move between seasons. However, density at the beginning of a season could also cause an individual that survives to the end of the season to be in poor physiological condition, which could carry-over to influence individual success the following season (‘carry-over effects’). We examine this hypothesis using replicated populations of Drosophila melanogaster over 23 non-overlapping generations with distinct breeding and non-breeding. If density at the beginning of the non-breeding season influences the physiological condition of surviving individuals, then high density at that time of the season should result in lower condition among those that survive and decrease per capita breeding output in the following season. Moreover, the decrease in breeding output should be stronger when breeding density is low, which would weaken the strength of density dependence in the breeding season. Thus, we predict that both non-breeding density and an interaction between non-breeding density and breeding density should best explain variation in per capita breeding output.

 

Results/Conclusions

We found that density at the beginning of the non-breeding season negatively impacted fresh weight of individuals that survived the non-breeding season and resulted in a 25% decrease in per capita breeding output among those that moved to the next season to breed. At the population level, per capita breeding output was best explained by a model that incorporated density at the beginning of the previous non-breeding season (carry-over effect) and density at the beginning of the breeding season. Our results support the idea that density-mediated carry-over effects are critical for understanding population dynamics in seasonal environments. In addition, by including density at the beginning of the previous non-breeding season in a model to explain per capita breeding output we provided a simple way to test for density-mediated COEs in a time series. Such a test only requires observations on seasonal abundance and information on breeding output and, therefore, coupled with information on how density influences individual condition, could be applied to a wide range of field data.