Using GPS technology and GIS cluster analyses to estimate kill rates in wolf—ungulate ecosystems

Abstract

Predatory behavior of wolves (Canis lupus) was studied in 2 wolf territories in Scandinavia. We used hourly data from Global Positioning System (GPS)-collared adult wolves in combination with Geographic Information System (GIS) for detailed analyses of movement patterns. We tested the hypothesis that wolves spend 1–2 days close to larger prey such as moose (Alces alces) and reasoned that 1–2 locations per day would be enough to find all larger prey killed by the wolves. In total, the study period comprised 287 days and yielded 6,140 hourly GPS positions, with an average of 21.4±2.4 (SD) daily positions. Depending on the radius used to define clusters, 4,045–5,023 (65.9–81.8%) positions were included in 622–741 GPS-clusters. We investigated all positions within clusters in the field, and 244 (22%) single positions. In total, we found 68 moose and 4 roe deer (Capreolus capreolus) and classified them as wolf-killed within the study period. Another 10–15 moose may have been killed but not found. The GIS analyses indicated the proportion of wolf-killed ungulates included in GPS clusters to be strongly dependent on both number of positions per day and the radius used for defining a set of spatially aggregated GPS positions as a cluster. A higher proportion (78%) of killed prey in clusters based on nighttime (2000–0700) than those based on daytime (0800–1900) positions (41%). Simulation of aerial search during daylight hours for killed moose resulted in a serious underestimation (>60%) as compared to the number of wolf-killed moose found during the study. The average kill rate, corrected for 14% nondetected moose, in the territories was 3.6–4.0 days per killed moose. We concluded that the feeding behavior of wolves in Scandinavia was either different from wolves preying on moose and living at the same latitude in North America, or that estimates of wolf kill rates on moose may have been seriously underestimated in previous North American studies.