استفاده از پهپاد برای اندازه گیری نشانه های طیفی خرس های قطبی
Abstract
1- Introduction
2- Materials and methods
3- Results
4- Discussion
References
Abstract
The increasing spatial resolution of earth observation satellites is creating new opportunities to survey wildlife. Satellites could be particularly valuable for surveying polar bears (Ursus maritimus) because of their remote circumpolar distribution and status of concern in the face of Arctic warming. However, the white coloration of bears does not contrast well with sea ice or snow in panchromatic imagery. We took advantage of the close-range observation capabilities of a drone to determine the spectral signature of polar bears as they would appear in multispectral satellite imagery, capturing low-altitude (≤100 m) multispectral images of bears in natural landscapes in Churchill, Manitoba, Canada. The bears' spectral curves were similar to those previously measured from pelts, with reflectance increasing with wavelength through the visible spectrum, although live bears had higher reflectance than pelts in the red-edge and near-infrared region. Bears had sufficiently consistent reflectance across the overhead surface of their body that ≥50% of pixels comprising each subject could be confidently matched to its core spectral signature, boding well for detection in coarser satellite imagery. Bears were clearly distinguishable from snow by their much lower reflectance in the blue and green region, but could potentially be confounded with large bright boulders. Currently available multispectral satellite imagery may still be too coarse (1.2 m/pixel) to reliably detect polar bears on sea ice, but resolution will likely continue to increase in future systems. Drones are a useful tool to resolve the spectral signature of wildlife species that could potentially be detected in satellite imagery.
Introduction
Satellite-based remote sensing technologies are rapidly advancing and increasingly being used to monitor and model the abundance and distribution of wildlife species (He et al., 2015). The tremendous value of earth observation satellites for environmental and ecological monitoring over vast swaths of the Earth's surface has long been recognized (Kerr and Ostrovsky, 2003), but only recently has satellite imagery achieved sufficiently high spatial resolutions to enable direct detection of wild animals or their signs. One of the first such studies involved detection of colonies of emperor penguins (Aptenodytes forsteri) (BarberMeyer et al., 2007). Subsequently, an increasing number of papers have reported detection of a variety of wildlife species in several parts of the world using satellite imagery (LaRue et al., 2017), including other penguins species (Lynch et al., 2012), flamingos (Sasamal et al., 2008), masked boobies (Sula dactylatra) (Hughes et al., 2011), seals (LaRue et al., 2011; McMahon et al., 2014), large African mammals (Yang et al., 2014; Xue et al., 2017), polar bears (Ursus maritimus) (Stapleton et al., 2014a; LaRue et al., 2015), whales (Fretwell et al., 2014) and albatrosses (Fretwell et al., 2017). The polar bear is a species for which satellite-based monitoring could be particularly attractive for several reasons. Polar bears occupy large geographic areas at low densities in remote locations throughout the Arctic and, as a result, can be challenging and expensive to monitor using traditional techniques such as mark-recapture methods (Lunn et al., 2016; Regehr et al., 2018) or low-level aerial surveys (Stapleton et al., 2014b). Both approaches are expensive, labour-intensive, and dangerous—several researchers have been killed or injured studying polar bears in the Arctic (Stirling, 2011). Moreover, several conservation concerns related to polar bears necessitate an increase in monitoring frequency and geographic scope. Shifts in the distribution and timing of sea ice melt affect where and when bears can hunt and may affect population health (Laidre et al., 2018).