Phenological shifts in a community context perspective
Community context proved to be a major modifying factor on the immediate and long-term consequences of phenological shifts in our study. Recent work has suggested that employing community contexts that extend the duration resources are available could be a useful management tool to help safeguard against climate-driven phenological mismatches in species interactions, such as plant-pollinator (Timberlake et al. 2019; Olliff‐Yang et al. 2020). While increasing intraspecific competition extended the duration of Drosophila ’s larval stage, our study indicates that it is not likely to promote host-parasitoid interaction persistence. Limited resources not only slowed host development, but increased host mortality, reducing the recruitment potential for the next generation of parasitoids. Other studies have found that, in addition to declines in individual survival, host populations experiencing strong competition may also be of lower value for parasitoids, reducing population growth to rates that are unable to support long term parasitoid persistence (Jones et al. 2009; Cunyet al. 2019). Furthermore, if parasitized hosts are more inclined to density-dependent mortality, as has been observed in some (Ives & Settle 1996), but not all host-parasitoid systems (Spataro & Bernstein 2004; White et al. 2007), this effect is likely to be amplified. Overall, these results suggest that extending the temporal availability of hosts, via competition, is unlikely to dampen the effects of phenological shifts, and that shifts towards later emergence times of parasitoids may reduce host-parasitoid persistence if parasitoids struggle to establish on high-density (e.g., low quality) host populations.
Most phenological studies have examined consequences of phenological shifts between pairs of interacting species, rarely considering whether outcomes of phenological shifts differ when more than two species are involved. Recent theoretical (Nakazawa & Doi 2012; Revilla et al. 2014; Takimoto & Sato 2020) and experimental (Timberlake et al. 2019; Olliff‐Yang et al. 2020) studies have pointed out this gap and suggest that the presence of alternative resources should weaken the effects of phenological mismatches, especially if those resources have complementary phenologies. By combining experimental and theoretical approaches, our results helped fill these gaps and showed that multispecies host communities were more likely to support the persistence of parasitoid populations across a broader range of phenological shifts, temperatures, and competition levels than pairwise interactions. Moreover, we found that the presence of an alternative host diminishes differences in average host survival and parasitism rates across different biotic and abiotic factors. Like in other studies, temporal complementarity between species (e.g., variation in growth rates), and a diversity of responses to environmental changes were major stabilizing mechanisms in our system (Zhang et al.2013; Craven et al. 2018; Sasaki et al. 2019), though this is the first example using a host-parasitoid system. While most studies recognize that functional redundancy and trait diversity at any given time can positively affect community resilience to environmental changes, it is rarely considered that these community parameters continuously change during the development of individuals within the community. Future studies should explicitly consider that the degree of functional redundancy within a community will likely vary during the development of individuals within communities. Regardless, our results suggest that the presence of alternative host species serves as an important buffer to phenological shifts in ecological communities, as it can provide complementary phenologies, increases functional redundancy, and can dampen community responses to variation in biotic and abiotic environments (Yachi & Loreau 1999; Yang 2020).