Distribution and movement patterns of Mongolian gazelle: Does unpredictability of resources lead to nomadism?

Background/Question/Methods Variability and predictability of resources has important conservation and ecological implications. In grassland ecosystems, changes in resource availability can occur at broad spatial scales translating into large area requirements for ungulate populations.  Such large area requirements may increase the vulnerability of such species to habitat loss and fragmentation. Understanding and predicting such large scale dynamics, although critical, has received little attention so far.
Mongolia’s Eastern Steppe is one of the world’s largest remaining temperate grassland ecosystems. Mongolian gazelles (Procapra gutturosa Pallas) are the dominant wild large herbivore in this steppe habitat. While these gregarious gazelles still roam Mongolian grasslands in large numbers, the species has experienced a major reduction in range during the past century, and is further threatened by continued habitat loss, fragmentation and excessive hunting.
We investigated habitat use and movement patterns of gazelles. We performed extensive, seasonally repeated field surveys to determine gazelle occurrence and relate gazelle presence to remotely sensed data of vegetation productivity (normalized difference vegetation index, NDVI). We also tracked individual movements of gazelles via collars equipped with Argos and GPS.

Results/Conclusions Gazelles preferred an intermediate range of vegetation productivity, which may illustrate a trade-off between forage quality and quantity: areas with low NDVI may be limited by low ingestion rates whereas areas with high NDVI may be limited by the low digestibility of mature forage. Spatiotemporal variation of gazelle habitat areas was high. Only 15% of the study area was consistently gazelle habitat throughout all four survey periods, indicating that gazelles must range over vast areas in search of food.
The tracking data revealed that gazelle movements are wide ranging and show high inter-individual variation. Individuals marked at the same location moved in different directions with dissimilar patterns within weeks and did not repeat the previous year’s movement patterns. Such irregular, non-repeating movement behavior is likely tied to the unpredictability of the foraging resources of gazelles. We define these irregular long distance movements as nomadism, and draw a conceptual contrast between nomadism and migration, in which other wide ranging ungulates exhibit regular movements to and from defined and disjunct seasonal ranges.