Eye in the sky

AS THE Earth turns and seasons change, millions of animals migrate from one part of the globe to another. Some travel thousands of miles every year in search of food, water and amenable habitat. Human understanding of animal migratory behaviour has, up until now, been based more on conjectures than facts. But this might change soon. A study on zebra migration shows freely available satellite data can be used to gain insights into animal migration based on environmental cues.

Earlier research has proven that migratory behaviour increases extinction risk and that blocking a migratory route or loss of a preferred seasonal area for migration is often followed by a sharp decline in animal population. But it is the first time that remote sensing data has been used to monitor migration and travel speed in terrestrial mammals. The study, to be published in Journal of Geophysical Research: Biogeosciences, used NASA satellite data to document daily animal movements and map environmental conditions. It shows how zebras adjust their movement to changing environmental conditions during migration and are able to reverse migration to avoid adverse conditions or exploit renewed resource availability.

The research focused on the migration of zebras from the southeastern Okavango Delta to the Makgadikgadi grasslands in Botswana. The route witnesses the second-longest zebra migration on earth. Between 1968 and 2004, animal migration on this route was blocked by a man-made fence erected to separate wildlife from domestic livestock. But within three years after the fence was removed, zebras again started taking this route to migrate at the start of the rainy season and return at the end of the rains. “The study shows how zebras can rediscover old migratory routes that have been blocked off by man-made barriers in a matter of years, once those barriers are lifted,” says one of the authors, Pieter Beck, a research associate with Woods Hole Research Center in Massachusetts, US. For the study, researchers used satellite data to monitor environmental factors, like rainfall, for investigating environmental influence on departure date and movement speed of migrating zebras in the delta.

Researchers suggest models that predict migratory movements can act as key conservation tools by evaluating vulnerability of migrating animals to population or environmental changes. They now plan to design models that can help game and conservation managers, farmers and tour operators predict animal migration. “In the future, this wealth of information has to be made more available, and presented in more meaningful ways to decision makers,” says Beck.

According to Stephen Harris, a professor at the School of Biological Sciences of University of Bristol, in the UK, this study “is a crucial step forward in helping us protect migration corridors as hitherto migration studies had little understanding of stimuli animals use to decide when to start migrating, how fast to migrate, and how to assess the chances of success.”

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