dissertationThe host, the parasite, and the vector each shape disease dynamics. Vector-borne parasites spread by (1) getting into the next vertebrate host from an infected vector, and (2) getting into the next vector from an infected vertebrate host. I use an experimental approach to investigate pairwise interactions between organisms in a system composed of a vertebrate host, the Rock Pigeon (Columba livia), a hippoboscid fly vector (Pseudolynchia canariensis), and a malaria parasite of the pigeon (Haemoproteus columbae). Ultimately, such studies may reveal how ecological interactions shape evolutionary processes. Transmission requires an infected vector bite a vertebrate host. Fewer parasites would be transmitted if hosts could defend themselves against vectors. I tested the effectiveness of anti-vector defense by manipulating two pigeon defenses against flies: preening and antibody responses. Each independently decreased fly longevity and the defenses work additively. However, they were ineffective in decreasing malaria parasite transmission. This ineffectiveness may have little immediate consequence for the pigeon. In a field experiment H. columbae had no effect on nestling pigeon growth, survival, or fledging success. This was surprising since H. columbae is correlated with lower survival in older pigeons; however, nestling pigeons are provided a particularly rich diet by both parents and may be tolerant to infection. To complete transmission, the vector must bite an infected vertebrate host, but the effect of the infected blood on the arthropod host is unknown. I found malaria parasites decrease fly survival and fecundity, but only for female flies. Both sexes feed on blood and transmit parasites, but the comparatively high female reproductive costs may decrease infection tolerance through energetic constraints. Females also take larger meals to fuel reproduction, which may increase their exposure to parasites. In my work I found malaria harms arthropod hosts more than vertebrate hosts, counter to the conventional wisdom that a parasite should not harm its vector. However, if a "vector" is defined by host mobility, pigeons may be the actual vectors in this system compared to the more sedentary flies. Disease dynamics here may also differ because both fly sexes transmit the parasite. These two points warrant further investigation
