Extending the parasitic day

Kehoe, R., Sanders, D., Cruse, D., Silk, M., Gaston, K.J., Bridle, J.R. & van Veen, F. 2020. Longer photoperiods through range shifts and artificial light lead to a destabilising increase in host-parasitoid interaction strength. Journal of Animal Ecology [online early].

  1. Many organisms are experiencing changing daily light regimes due to latitudinal range shifts driven by climate change and increased artificial light at night (ALAN). Activity patterns are often driven by light cycles, which will have important consequences for species interactions.
  2. We tested whether longer photoperiods lead to higher parasitism rates by a day-active parasitoid on its host using a laboratory experiment in which we independently varied day length and the presence of ALAN. We then tested whether reduced nighttime temperature tempers the effect of ALAN.
  3. We found that parasitism rate increased with day length, with ALAN intensifying this effect only when the temperature was not reduced at night. The impact of ALAN was more pronounced under short day length. Increased parasitoid activity was not compensated for by reduced lifespan, indicating that increased day length leads to an increase in total parasitism effects on fitness.
  4. To test the significance of increased parasitism rate for population dynamics, we developed a host-parasitoid model. The results of the model predicted an increase in time-to-equilibrium with increased day length and, crucially, a threshold day length above which interactions are unstable, leading to local extinctions.
  5. Here we demonstrate that ALAN impact interacts with day length and temperature by changing the interaction strength between a common day-active consumer species and its host in a predictable way. Our results further suggest that range expansion or ALAN induced changes in light regimes experienced by insects and their natural enemies will result in unstable dynamics beyond key tipping points in day length.