A legacy of competitive exclusion: Host demography and amplified disease.

Dilution effects arise when increases in species diversity reduce disease risk, and amplification effects arise when the opposite occurs. Despite ample evidence for both phenomena, the mechanisms driving dilution and amplification effects and how they are mediated by environmental factors remain poorly understood. Mechanisms involving demographic rates or stage structure of hosts are particularly lacking in the diversity-disease literature. In Midwestern lakes, Metschnikowia bicuspidata parasites infect Daphnia dentifera focal hosts in autumn, with epidemics beginning when water is warm (∼25°C) and peaking when lakes have cooled (∼15°C). Epidemics are smaller in lakes with more Ceriodaphnia dubia alternative hosts, which serve as key diluters of disease. However, it is unclear whether seasonal changes in temperature affect their ability to alter host population dynamics and reduce disease. We conducted a mesocosm experiment to test how temperature (15, 20, or 25°C) mediated the effects of these key alternative hosts on density, stage structure, and disease dynamics in focal host populations. The experiment yielded several surprising results. First, focal hosts rapidly outcompeted alternative hosts at all temperatures. By the time parasites were added, alternative hosts had been almost completely excluded. Second, despite diluting disease in the field, initial presence of these alternative hosts amplified infection prevalence in the experiment. Third, this amplification arose as a legacy effect, lasting generations after alternative hosts were gone. Our explanation for this legacy amplification effect centers on focal host stage structure and demography. Competition with alternative hosts resulted in focal host populations that were more adult-biased when parasites were added, at all three temperatures. Additionally, host densities in these treatments increased more rapidly in the subsequent ten days, consistent with reduced background death rates. Since adults consume more parasites, and since exposed hosts must survive ten days before producing infectious spores, these initial conditions seem to have set disease dynamics along amplified trajectories. These results highlight the need for a broader understanding of the mechanisms that can amplify or dilute disease, including altered host stage structure and mortality of exposed hosts.