Many models assume that disease acts independently of other causes of death. However, the researchers point out that it is possible for disease to kill those that are already doing poorly and would have died of starvation or been killed by predators, for example. Second, disease models usually assume constant environments, free from changes in food availability or catastrophic disturbances, such as drought or wildfires.
The researchers tested the assumptions by gathering field data on a herd of African buffalo struck by tuberculosis and following them through different conditions, including a devastating drought. They found that tuberculosis does explain increased mortality and decreased fecundity in some prime-aged buffalo, but older buffalo reflect the competing risks, and the increased survival and greater fecundity of buffalo in the herd without tuberculosis may compensate for some disease-related losses to the herd.
"Pathogens and parasites can have drastic effects, reducing survival or reproduction in infected hosts," write the authors. But they warn against drawing potentially misleading conclusions that may incorrectly influence how we think about epidemics--particularly chronic infections of long-lived hosts.
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Anna E. Jolles, Rampal S. Etienne, and Han Olff. "Independent and competing disease risks: implications for host populations in variable environments," The American Naturalist 167:5.
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