Contrasting effects of host or local specialization: Widespread haemosporidians are host generalist, whereas local specialists are locally abundant

Aim: Despite the wide distribution of many parasites around the globe, the range of individual species varies significantly, even among phylogenetically related taxa. Given that parasites need suitable hosts to complete their development, parasite geographical and environmental ranges should be limited to communities where their hosts are found. Parasites might also suffer from a trade-off between being locally abundant or widely dispersed. We hypothesize that the geographical and environmental ranges of parasites are negatively associated with their host specificity and their local abundance. Location: World-wide. Time period: 2009–2021. Major taxa studied: Avian haemosporidian parasites. Methods: We tested these hypotheses using a global database that comprises data on avian haemosporidian parasites from across the world. For each parasite lineage, we computed five metrics, namely phylogenetic host range, environmental range, geographical range and the mean local and total number of observations in the database. Phylogenetic generalized least squares models were run to evaluate the influence of phylogenetic host range and the total and local abundances on geographical and environmental ranges. In addition, we analysed separately the two regions with the largest amount of available data: Europe and South America. Results: We evaluated 401 lineages from 757 localities and observed that generalism (i.e., phylogenetic host range) was associated positively with both the geographical and environmental ranges of the parasites at global and European scales. For South America, generalism was associated only with geographical range. Finally, mean local abundance (mean local number of parasite occurrences) was negatively related to geographical and environmental ranges. This pattern was detected world-wide and in South America, but not in Europe. Main conclusions: We demonstrate that parasite specificity is linked to both their geographical and environmental ranges. The fact that locally abundant parasites present restricted ranges indicates a trade-off between these two traits. This trade-off, however, becomes evident only when sufficient heterogeneous host communities are considered.