Fitness responses to environment can shape species distributions, though
opposing eco-evolutionary processes can obscure environmental effects.
For example, host specificity influences parasite dynamics, but is
unclear how specialization of individual parasites or populations scales
up to continental distributions. Here, we develop a macroecological
framework to determine how host community structure affects
continent-scale specialization in Striga hermonthica, an African
parasitic plant of cereal crops. We find regional abundance of hosts in
cultivated cereal communities is associated with parasite specialization
observed in experiments. Moreover, abiotic environment at location of
origin predicts parasite performance on pearl millet and sorghum but not
maize, possibly due to the shorter coevolutionary history for maize and
Striga. Our study demonstrates that patterns of parasite local
adaptation to host communities can emerge at continental scales and that
differential environmental tolerances of hosts indirectly shape the
distribution of specialist and generalist parasites.