Supplementary materials for "The importance of biological variables in SDMs of species with strict interactions: a case study with a hairworm and its hosts"

Species distribution model (SDM) has conventionally been based mainly on abiotic factors, i.e., temperature and precipitation variables. Although recent studies have shown that incorporating biotic factors may improve the model performance, only few empirical studies could effectively incorporate biotic factors in SDMs. Here we used a parasite and host system to investigate whether incorporating biotic information can improve SDM. Specifically, SDM of the parasite based on hosts’ distributions can be helpful because the distribution of the parasite may depend on the dispersal ability of the hosts. We evaluated the most important layer, including the SDMs of its three hosts, for predicting the distribution of the parasitic horsehair worm Chordodes formosanus.
We retrieved presence points for three host species and the parasite. We first built the three hosts’ SDMs. We subsequently built SDMs for the parasite with MaxEnt implemented in ENMeval by using bioclim variables (chosen according to VIF values) and the aforementioned host SDMs built at 30 seconds scale. In addition, we evaluated niche overlap among hosts and parasite according to five similarity indices (Schoener’s D, I statistic, relative rank, Pearson correlation coefficient and the rank correlation coefficient rho).
According to percentage and permutation importance, the presence of the Taiwanese giant mantis, Titanodula formosana, was the main variable that explained the distribution of C. formosanus. In addition, habitat suitability models showed high similarity between hosts and the parasite, according to niche overlap estimations.
Accounting for biological interaction is critical for SDM as shown here that the distribution of the main final host effectively predicts the distribution of the parasite. Additionally, SDMs can be helpful to prioritize host sampling and by consequence parasite sampling, too. SDMs are reliable tools for host and parasite conservation management and it is hoped their use will help to improve biological and ecological knowledge of parasites.