Why do parasites exhibit reverse latitudinal diversity gradients? Testing the roles of host diversity, habitat, and climate

Aim: The latitudinal diversity gradient (LDG) – in which species richness
decreases from the equator toward the poles – is among the most
fundamental distributional patterns in ecology. Despite the expectation
that the diversity of parasites tracks that of their hosts, available
evidence suggests that many parasites exhibit reverse latitudinal
gradients or no pattern, yet the rarity of large-scale datasets on
host-parasite interactions calls into question the robustness of such
trends. Here, we collected parasitological data from a host group of
conservation importance – lentic-breeding amphibians – to characterize the
form and direction of relationships among latitude, parasite richness, and
parasite load. Location: The contiguous USA. Time period: 2000 to 2014.
Major taxa studied: Lentic-breeding frogs and toads and their helminth
parasites. Methods: We collected information on parasite richness and
infection load for 846 amphibian populations representing 31 species. We
combined these data with environmental and biological data to test for
LDGs and potential mechanisms. Results: Both parasite richness and
abundance increased across 20 degrees of latitude – a reverse LDG. For
parasite richness, this pattern was partially explained by latitudinal
increases in wetland area, landcover diversity, and the richness of
waterbirds – which function as definitive hosts for many amphibian
parasites. Host body size also correlated positively with latitude and
helminth richness, potentially reflecting increased habitat availability,
greater host longevity, or a persistent phylogenetic signal. Parasite
abundance associated positively with wetland area and landcover diversity,
but negatively with amphibian taxonomic richness. Longitude exhibited
non-linear relationships with parasite abundance and richness, which we
suggest stem from large-scale variation in host availability (e.g.,
migratory bird flyways). Main conclusions: With growing interest in the
distribution of parasites and pathogens, these results highlight the
importance of inverse latitudinal gradients while emphasizing the
explanatory influence of host body size, habitat availability, and host
diversity.