Effects of Temperature Different for Host and Parasite
Consistent with previous work (Burns 1969, Kirk et al. 2018), we found that D. magna time to hatching and time to maturation were slowed, though not prevented, at lower temperatures and decreased with higher temperatures. However, we also found that P. ramosainfections are slowed and/or limited at low temperatures and accelerated at higher temperatures to a greater degree than the host’s relationship to temperature. For both ephippia and live D. magna treatments, final prevalence increased with increasing temperature, indicating that the onset of an epidemic in Spring is significantly influenced by the effect of temperature on the parasite, and not only the presence of hosts. Additionally, time to visible infection decreased with increasing temperature for both ephippia-hatched and live D. magna . Importantly, differing reaction norms obtained for our host and parasite suggests that increases in temperature due to climate change and earlier Springs may accelerate infection rates more severely than host growth is accelerated, which could have implications for epidemic and long-term population dynamics. Since P. ramosa is a castrating parasite, our findings imply that as temperature increases, it is possible that infected hosts might lose their ability to reproduce shortly after or even before reaching maturity. Overall, this finding supports our hypothesis that outbreak timing is driven by temperature rather than other seasonal forces, and highlights the importance of considering the impacts of environmental factors on host and parasite separately, especially for those that spend part of their life cycles apart (Gethings et al. 2015, Gehman et al. 2018, McDevitt-Galles et al. 2020).
The finding that live D. magna became visibly infected at 10 and 12.5 oC (confirmed by dissection at the end of the experiment) suggests that warming temperatures may not “trigger” outbreaks in the manner previously assumed, but rather accelerate them. To our knowledge infections at these temperatures had not been observed before, and previous experiments suggested that infection cannot occur below about 13 oC (Vale et al. 2008, Vale et al. 2011), despite D. magna being able to hatch and reproduce at these temperatures. This contrast may be explained by the fact that we ran our experiment for much longer than previous ones to allow for different exposure rates due to slower D. magna metabolism at lower temperatures. These results may also clarify why infection at these low temperatures has not been observed in the wild at our field site, as our findings indicate that the time it would take for infection to occur at these low temperatures is much longer than the time the pond typically spends at these temperatures in Spring (Ameline et al. 2020).