Simulation results: changes in host-parasitoid persistence with phenological shifts, warming, and community context
The persistence of host-parasitoid interactions was less probable if parasitoids were perfectly synchronized with their hosts or if the shifts in relative emergence times became too delayed (e.g., delay 6-days) (Fig. 3). However, the effects of phenological delays on host-parasitoid persistence varied significantly with warming; that is, the probability of parasitoid persistence was significantly greater at the 4-day delay window in ambient temperatures (OR = 2093.8, P < 0.001), while a delay of 2-days resulted in the highest chance of persistence under warming (OR = 0.49, P = 0.0013). We also found that under simulated warming, the chances of parasitoid persistence were limited to a single delay window (2-day), while two windows of delay (2- and 4-day) indicated parasitoid persistence in 24°C, supporting predictions that accelerated development rates narrow the window of vulnerability in host-parasitoid interactions. Importantly, warming reduced the probability of host-parasitoid persistence, irrespective of phenological relationships (OR = 83.5, P < 0.001).
Our simulations indicated that high levels of resource competition among hosts did not increase the number of relative emergence times that supported parasitoid persistence in either ambient or elevated temperatures. Instead, the chances of parasitoids persisting for extended generations was significantly greater when competition levels were low, and this was consistent across all phenological delays, except 0-day (OR = 1.8, P = 0.19). Importantly, the presence of alternative host species (i.e., the sulfurigaster-birchii combination) significantly increased the probability of parasitoids persisting when averaged across all phenological shifts and temperatures (Fig. 3). This suggests, that while our single generation experiment identified intermediate levels of survival and parasitism rates when alternative host species were present, those differences in survival and parasitism rates had meaningful, positive effects on host-parasitoid coexistence across multiple generations.