Climate change is increasing the incidence and severity of extreme events such as heatwaves, which is a possible cause for observed increases in emergent diseases. Infectious diseases threaten a variety of species particularly in freshwater ecosystems. One emergent disease in freshwater communities is caused by the oomycete, Saprolegnia spp. and infection is observed in fish, frogs, crustaceans, and aquatic insects.
Saprolegnia infection prevalence in caddisfly populations increased dramatically in recent years and causes high mortality in eggs, most prominently in Ulmerochorema rubiconum caddisflies. We investigated the effects of experimental heatwaves (short-term [12 hour], longer [21 day]) on Saprolegnia infection, the duration of the egg period, and the survival of U. rubiconum eggs. Population modelling put our experimental results into context, by exploring the impact on population growth rates.
Increases in temperature (short-term and sustained) increased Saprolegnia infection of eggs, causing higher mortality rates. Furthermore longer, but not short-term, increases in temperature resulted in faster hatching than sustained low temperatures. Longer hatching times in short-term temperature increases likely lengthen the time available for infection to spread across the egg mass, yielding similar infection rates as longer increased temperatures, which had more rapid infection spread. As high temperatures yielded greater mortality than low temperatures, and infection levels between short and long-duration heatwaves were similar, short-term heatwaves may pose the greater threat to caddisfly populations.
Population modelling supports this conclusion, predicting populations with lower per capita growth rates when exposed to short temperature increases compared to longer increases or low temperatures. Further, short-term heatwaves occur more often than sustained periods of high temperature, increasing the likelihood that eggs experience conditions favourable for infection. With climate change forecasts predicting an increase in heatwave frequency and magnitude, the resulting changes are likely to affect caddisfly populations, as well as ecosystem functioning.