Specialization between foliar fungal endophyte and plant communities decreases toward the tropics

Associations among species are hypothesized to become fewer and more specialized toward the tropics. However, evidence for the latitudinal specialization gradient (LSG) remains mixed and restricted to plant-animal and host-parasite models, with increasing recognition that our understanding of LSG is often complicated by differences in species richness and sampling effects. Here, we tested whether specialization between foliar fungal endophytes - fungi that live within plant leaves - and their plant hosts increased toward the tropics by surveying 1,539 plants that spanned a 55-degree latitudinal gradient in Central and North America. Contrary to expectations, endophytes and plants formed a greater number of associations that were more evenly distributed toward tropical latitudes and were thus less specialized, even after accounting for differences in endophyte and plant community structure. Endophyte-plant communities were most specialized where evaporative demand and plant richness were low, and where water availability and primary productivity were high. Our findings show a reversed LSG where the specialization between endophytes and plants decreased toward the tropics and provides evidence that many species of endophytes can coexist even when there is high overlap in their host niches.