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Changes in land use that limit or impeed migration can increase pathogens in migratory animals. Another good reason to promote biodiversity and wildlife corridors in local ecosystems.
From ScienceDaily, Jan 23, 2011
"...Some kinds of parasites have transmission stages that can build up in the environment where host animals live, and migration allows the hosts to periodically escape these parasite-laden habitats. While hosts are gone, parasite numbers become greatly reduced so that the migrating animals find a largely disease-free habitat when they return. Long migratory journeys can also weed infected animals from the population: imagine running a marathon with the flu. This not only prevents those individuals from spreading disease to others, it also helps to eliminate some of the most virulent strains of pathogens.
"By placing disease in an ecological context," said Odum School dean John Gittleman, "you not only see counterintuitive patterns but also understand advantages to disease transmission. This is a classic example of disease ecology at its best..."
"...But for monarchs, and many other species, migration is now considered an endangered phenomenon. Deforestation, urbanization and the spread of agriculture have eliminated many stopover sites, and artificial barriers such as dams and fences have blocked migration routes for other species. These changes can artificially elevate animal densities and facilitate contact between wildlife, livestock and humans, increasing the risk that pathogens will spread across species. As co-author Han noted, "A lot of migratory species are unfairly blamed for spreading infections to humans, but there are just as many examples suggesting the opposite -- that humans are responsible for creating conditions that increase disease in migratory species."
And as the climate warms, species like the monarch may no longer need to undertake the arduous migratory journey to their wintering grounds. With food resources available year-round, some species may shorten or give up their migrations altogether -- prolonging their exposure to parasites in the environment, raising the rates of infection and favoring the evolution of more virulent disease strains. "Migration is a strategy that has evolved over millions of years in response to selection pressures driven by resources, predators and lethal parasitic infections -- any changes to this strategy could translate to changes in disease dynamics," said Han..."