May 18, 2011
Scientists have traditionally labelled species of hoofed mammals as either “migratory,” meaning they travel long distances from one place to another and back again, or “non-migratory” and based conservation plans on those labels. But now researchers at the Smithsonian Conservation Biology Institute and elsewhere are adding a third category, “nomadic.” And in their new study, published in Global Ecology and Biogeography, the scientists show that patterns in vegetation across a species’ range determine whether and how it moves.
The researchers looked at tracking data from four hoofed mammal species: guanaco, a llama-like creature from Argentina; barren-ground caribou in the Alaskan and Canadian Arctic; moose in Massachusetts; and Mongolian gazelle. They then compared this data with a 25-year set of satellite data showing how the landscapes in these places changed from season to season and year to year.
Moose were sedentary and stayed mostly in a small home range (non-migratory), while the guanaco ventured a bit farther (semi-migratory). The caribou had a long migration, covering hundreds of kilometers and crossing the U.S.-Canada border (migratory). Though the Mongolian gazelle also traveled hundreds of kilometers, they didn’t fit the standard “migratory” label, the researchers found. “When we put radio collars on [the gazelle],” said Thomas Mueller of SCBI, “we were surprised to discover that they go off individually in different directions.” Mueller and his colleagues labelled this third category as “nomadic.”
They also found a correlation between the variety in a landscape and how a species moved. The guanaco and moose, which moved the least, lived in areas where the vegetation had little variability. The caribou moved long distances in a coordinated manner, following the patterns of vegetation productivity, going where they’d find the best meal. The vegetation is less predictable in the landscape where the Mongolian gazelle live, however, and so their movements are also less predictable.
The findings have implications for the conservation of migrating animals. Traditional strategies run on the assumption that the critters move from one place to another with seasonal regularity, but this study shows that that’s not always the case.
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