Long-term trends in parasite diversity and infection levels: approaches and patterns

Parasites exist in every ecosystem, affecting nearly all organisms and playing a complex role in human societies. On the one hand, they contribute substantially to biodiversity and support ecosystem stability by performing essential ecological functions. On the other, they can impose health burdens on their hosts, causing diseases in both animals and humans. Despite their significance, our understanding of how parasitic organisms are affected by human-driven environmental change remains poor. In other well-studied groups such as free-living birds, mammals and insects, long-term ecological data sets have been instrumental in elucidating temporal trends in abundance or diversity and linking them to anthropogenic drivers. For parasites however, overarching long-term trends in infection levels or diversity have yet to be identified. Here we provide an overview of the research approaches developed to study long-term changes in parasite systems and the trends highlighted by these studies. Our aims were to help researchers make informed methodological decisions when designing their research, and to provide recommendations for future long-term research on parasite ecology. To this end, we performed a systematic literature search on long-term analyses of eukaryotic parasites of wild animals and identified four types of approaches deployed to gather long-term data: (i) long-term monitoring; (ii) snapshot resampling; (iii) literature-based research; and (iv) natural history collection-based studies. Our results revealed striking differences in the temporal scope, geographical scale of sampling, sample sizes and taxonomic resolution of parasite identification among these approaches. However, no overarching trends in parasite infection levels or diversity were identified. When detected, significant temporal changes were often linked to anthropogenic disturbances, but these claims were rarely supported by inferential analyses. Overall, our results show that our understanding of long-term trends in parasite systems remains hampered by data scarcity and research biases. To address these issues, we advocate for the establishment of large-scale parasite monitoring programmes combined with existing ecological monitoring projects, as well as the development of new scalable biomonitoring tools. We also highlight the importance of valorising historical data and preserved biological material in museum collections to obtain baseline information on parasite systems.