Counting polar bears from space
The icy (and increasingly ice-free) Arctic is a forbidding place. It’s isolated, it’s remote. The weather is tumultuous. If the polar bears don’t get you, the cold will. That makes it tough to conduct surveys of wildlife populations. Walruses and ribbons seals are so hard to monitor that they’re classified by the IUCN as “data deficient.” Polar bears are well studied, but only in certain easily accessible areas. Throughout most of their range, the data on the maritime bears remains elusive.
Most Arctic wildlife monitoring is done using airplane or helicopter surveys, or occasionally surveys done by ship. For some species, mark-recapture methods can be used. But those methods are either prohibitively costly or inadequate to detect ongoing trends over several years. Seth Stapleton of the US Geological Survey and colleagues wondered if satellite imagery could be used as a more cost effective method for monitoring Arctic wildlife. “Remote sensing affords access to vast expanses of otherwise inaccessible sites, at potentially reduced costs, without concerns about human safety and disturbance to wildlife,” he writes.
There’s one other benefit to using satellites to monitor wildlife: we have archives going back at least a decade. If satellite imagery can indeed be used for this purpose, then we can use those archives to establish population baselines.
Some researchers have effectively used satellite technology, with resolutions as high as half a meter, to monitor Weddell seals and Emperior penguins. And others have suggested that polar bears and Arctic marine mammals were visible on satellite imagery. But without some on-the-ground information to calibrate the satellite data, their visibility is not scientifically useful. So Stapleton and his colleagues set out to compare data derived from satellites with sightings from an aerial survey in a helicopter.
The research was centered on an area called Foxe Basin, Nunavut, located in the eastern Canadian Arctic. The area is completely free of ice and snow during the late summer. As the ice melts, bears become stranded on small ice floes and eventually congregate on land while they wait for the winter to return, and with it, the ice. In particular, the team focused on Rowley Island. It’s known to harbor lots of bears during the summer, and the white bears contrast well against the dark landscape. Combined with its flat topology, the island provides an idealized environment to evaluate the use of satellite imagery.
Overall, the researchers discovered that satellite imagery provided similar data as the more traditional aerial surveys, reflecting their utility and accuracy. They spotted 90 bears by satellite, and 100 from the helicopter. But that finding comes with two important caveats.
First, it took a considerable amount of time to pore over the images. They call the imagery review “tedious”; it took a combined 100 hours. “This,” they say, “made is unrealistic to re-examine the images a second time.” For satellite surveys to become more efficient, it would be prudent to develop a reliable, automated process for computers to analyze the images. That data could then be spot-checked by humans.
Second the ability to detect the bears on the imagery varied substantially from person to person. Recall that the site was chosen so that the bears’ white fur would stand out in stark contrast to the darker Earth underneath them. The two observers that looked for bears in the satellite images had opposite skills. One had “extensive experience interpreting remote sensing imagery” but no particular experience with polar bears. The other had moderate field experience studying polar bears in the eastern Canadian Arctic, but no experience examining satellite images. Surprisingly, it was the person with bear-specific experience who did better at spotting bears, not the person who was used to inspecting satellite images. “These experiences suggest that explicit search protocols and a rigorous training program including individuals with relevant, on-the-ground experience with the target species will improve implementation of the technique and ensure appropriate search images,” the researchers conclude. Similar techniques could probably also be used for darker species, like musk oxen and caribou, which stand out against the ground during the snowy winter months. – Jason G. Goldman | 11 July 2014
Source: Stapleton S., LaRue M., Lecomte N., Atkinson S., Garshelis D., Porter C. & Atwood T. (2014). Polar Bears from Space: Assessing Satellite Imagery as a Tool to Track Arctic Wildlife, PLoS ONE, 9 (7) e101513. DOI: 10.1371/journal.pone.0101513
Header image: shutterstock.com
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