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).