The grazing impacts of four barren ground caribou herds (Rangifer tarandus groenlandicus) on their summer ranges: An application of archived remotely sensed vegetation productivity data

Barren ground caribou (Rangifer tarandus groenlandicus) herds experience significant fluctuations in abundance through time. A proposed hypothesis for these fluctuations involves the overgrazing of vegetation on herd summer ranges at high herd densities resulting in a reduction in available forage. Semi-domesticated reindeer (Rangifer tarandus) herds in Scandinavia negatively affect vegetation productivity at high herd densities and exclosure experiments have indicated that overgrazing may also affect barren ground caribou herds. However, overgrazing of summer range vegetation has yet to be tested at the landscape level or related to herd densities for barren ground caribou herds. Accordingly, the question examined in this study was - does barren ground caribou herd density have an effect on summer range vegetation productivity? To answer this question, summer home ranges for four herds in the Northwest Territories and Nunavut, Canada, were delineated using caribou telemetry data. Yearly overall productivity and vegetation seasonality (the change between the maximum and minimum productivity throughout the year) metrics were calculated by year using Moderate Resolution Imaging Spectroradiometer and Advanced Very High Resolution Radiometer data and related to herd densities. Overall vegetation productivity was negatively related to herd density, indicating that some summer range degradation may have occurred at high densities. Conversely, and contrary to our expectations, vegetation seasonality was positively related to herd density. The Bathurst herd, which experienced densities more than three times those of the other herds examined, had the least negative association with overall productivity. Given the negative relationship detected between each herd's densities and overall vegetation productivity, it is likely that some summer range degradation occurred as herd densities increased. Considering the results of the Bathurst herd however, it is difficult to conclude that overgrazing has been a major factor governing herd abundance in the herds examined here. This study demonstrated the utility of archived remotely sensed productivity data and how productivity indices can be useful tools for providing information on large mammal grazing impacts.