Importance of accounting for context: a re-analysis as proof of concept
To illustrate the importance of considering context when testing the ERH, we re-analysed a recent synthesis (Xu et al. 2021). Xuet al. used the dataset of Turcotte et al. (2014), which reports annual herbivory damage for a range of plant species. Xuet al. categorised each plant species in the dataset as ‘exotic’ or ‘native’ and compared their damage rates, accounting for plant growth form and latitude of each observation. We repeated their analysis, but also included information about enemy type, which was available in Turcotte et al. (2014) (context = type of enemy). See Supplementary Analysis for details.
We found that damage rates were generally higher on native than exotic species (F1,1660=9.12, p=0.003, Fig. 6a; N=137 [exotics], N=1527 [natives]), consistent with the conclusion of Xu et al. (2021). However, when we included the type of enemy as a random variable, we found no difference in damage between native and exotic species (χ21=0.13, p=0.719, Fig. 6b). We suggest that these distinct findings can be largely attributed to vertebrate impacts. Effects of vertebrates were only recorded on natives (Fig. 6b; N=0 [exotics], N=76 [natives]), and as vertebrates cause higher mean damage than other types of enemy in the database, average damage on natives was disproportionately higher than damage on exotics. In this case study, unless type of enemy is explicitly accounted for, it can erroneously appear that overall damage is lower on exotics, as found in Xu et al. (2021)’s study. In fact, if only damage from insects is examined (N=84 [exotics], N=1341 [natives]), damage appears higher on exotics than natives (Fig. 6b), contradicting the ERH and the conclusion of Xu et al.(2021). This brief example demonstrates how evidence for the ERH is contingent on context, and failure to account for context affects conclusions. It also highlights that examining context can reveal data gaps (here, vertebrate herbivory on exotic plants) that should be prioritised in future work.