Ecological interaction without co-occurrence is not evidence of spillover risk: Predicting landscape conversion impact on small mammal occurrence and the transmission of parasites in the Atlantic Forest

Changes in landscape configuration significantly impact ecosystems and the services they provide, including disease regulation for both humans and wildlife. Land use conversion usually favors disturbed-adapted species, often known reservoirs of zoonotic parasites, thereby potentially escalating spillover events (i.e., the transmission of parasites to new hosts, including humans). While there is growing evidence indicating a higher risk of spillover in anthropogenic landscapes, the effect of landscape conversion on small mammal distribution and the intricate relationship between parasite sharing, spillover events, and co-occurrence remain open questions. Here we aim to investigate how alterations in landscape configuration influence the distribution and co-occurrence of small mammals, potential hosts of zoonotic and epizootic diseases in the Brazilian Atlantic Forest. To address this, we integrated ecological network metrics and joint distribution models while accounting for phylogenetic relationships and functional traits to answer two main questions: (1) Do small mammal species considered central hosts in the transmission of parasites exhibit a higher probability of occurrence in landscapes with reduced native vegetation areas? (2) Do small mammal hosts that share a higher number of parasites have higher co-occurrence probabilities? Our results demonstrated that species identified as significant hosts in our centrality analysis displayed an increased probability of occurrence in landscapes that are both more heterogeneous and have greater agricultural activity, hence fewer natural areas. Regarding the relationship between species co-occurrence and parasite sharing, our findings indicated that although most strong co-occurrences were prevalent within groups with higher parasite fauna similarity, not all species sharing parasites had a higher probability of co-occurring. Consequently, species that do not share parasites but are likely to co-occur might have a higher potential for spillover events between them.