Thinking Like a Hornbill: Predicting Scale and Species Occurrence

--KT Mertes, PhD Candidate in the Jetz Lab of Yale EEB

Why do species occur where they do? How do we accurately predict their occurrence? These questions have inspired naturalists and scientists for many years. Accurate species occurrence information can be used to select protected areas, identify movement corridors, and develop successful management practices. The research I conduct at Mpala Research Centre in Laikipia, Kenya investigates how environmental conditions and spatial scale influence species occurrence patterns.

Both humans and animals are known to select resources at different spatial scales. For example, when choosing a city in which to live, I might consider weather patterns, local amenities, and job availability. When purchasing a home, I would want to know a house’s square footage, yard size, and safety. Animals respond to environmental conditions at different scales in similar ways. When selecting a home range, a bird might prioritize food resources, water availability, and nearby competitors. When selecting a nest site within a home range, ease of entry and shelter from predators may be more important. These habitat selection processes, combined across many individuals, determine a species’ local distribution.

The first step in studying scale-dependent occurrence patterns is to measure the scales where habitat selection takes place. One method uses movements made by individuals over long time periods. Places where animals move short distances and turn in many directions likely contain important resources like food; places where animals move long distances over short time periods might be dangerous or contain less food or water. Recent technological developments make it possible for very small GPS tags (less than 20 grams) to collect a precise GPS location every 20 minutes, and run for months on solar-powered batteries. During August – October 2012, my team attached GPS tags (built by the University of Konstanz, Germany) to six adult Von der Decken’s hornbills at Mpala. One tagged hornbill often flies by the research station and dining area, showing off his GPS “backpack.”

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Movement data are collected by placing weatherproofed antennas, receivers, and memory units near where tagged birds roost at night (search www.movebank.org to see where tagged birds spend their time at Mpala Research Centre). After several months, we will collect enough movement data to identify important spatial scales for von Der Decken’s hornbills. Once these critical features are measured, we will build models that incorporate data on environmental conditions to understand how hornbills select habitat and resources across spatial scales. Measuring the scales that are important to different species will also help us understand how species coexist by splitting environmental resources. Together, a fuller understanding of scale dynamics and the detailed information collected by GPS tags will move us closer to understanding and predicting species occurrence patterns.