Understanding demographic responses of ectothermic organisms to increased temperatures due to climate change is necessary to plan management and conservation actions for the future. Here we present field estimates of overwinter larval survival as well as adult survival for a locally rare wetland butterfly that overwinters as an early-instar larva. In order to better understand the phenology of the butterfly, we estimated timing of the critical photoperiod triggering the onset of diapause. We carried out manipulative warming experiments at field sites at Fort Bragg, NC. We measured larval survival during winter 2016-17 by raising butterflies from eggs to adults under increased temperature scenarios using open-top warming chambers at field sites. We estimated adult survival by using a multi-state mark and recapture survey in plots that varied in temperature due to experimental restoration. Restoration actions included hardwood removal resulting in decreased shade. We carried out a model selection process using Program Mark to identify the most relevant variables affecting survival. To estimate critical photoperiod, we raised offspring of wild caught female butterflies over the course of several consecutive weeks in July-August 2017 and evaluated the proportion of each clutch that entered diapause versus developed directly over time.
Results/Conclusions
Our warming chambers resulted in an increase in average temperature of up to 2 degrees Celsius. We compared larval survival to temperature variables using a general linear model with a binomial regression. We found that larval survival was not affected by increased temperatures over the time period and range of temperatures tested. However, when we divided the winter time period into ‘seasons’ with ‘Fall’ represented by September-November temperatures, ‘Winter’ as December-February, and ‘Spring’ as March-May, we found that larval survival significantly decreased with ‘Winter’ maximum temperatures (N= 282, z=-3.031, p=0.0024). From our mark-recapture surveys, we found that the highest ranked model had adult butterfly survival constant across sites and time. Our phenology experiment revealed a window of time during which larvae were susceptible to a critical photoperiod, between July 23rd and August 15th. This suggested a critical day length of 13.5-14 hours. Our findings suggest that management strategies must take into account the future demographic effects of increased temperatures on cryptic life stages of ectotherms.