Living Planet: Connected Planet

RUNNING MIGRATION ON LAND

Changes in precipitation, temperature and vegetation, as well as predation and disease risk, are drivers of mass migrations in large herbivores. Their migrations in turn deter- mine the movements of a number of carnivores. Populations of many migrating un- gulates have dropped by 35–90 per cent in the last decades. Fences, roads and railways have delayed or stopped migrations, or have exposed migratory animals to poaching as they move in large numbers along these barriers in search of safe passage (Bolger et al. , 2008). Migratory herbivores concentrate seasonally, often during calving, migration or at water sources in the dry season. This behaviour and its predictability makes them vulner- able to overharvesting.

Wildebeest, elephants, buffalo, caribou, chiru and Saiga ante- lopes, and many other ungulates have to migrate at the onset of dry season, summer or winter as the available water resources or forage diminish and become concentrated in certain areas, making the animals highly vulnerable to poachers and preda- tors. This resource-driven migration is well known, but the com- plexity of the ecological network is underestimated. One should also take into account forage, predators, social dynamics, physi- ology and predator avoidance, which form part of the dynamics between the species, its surroundings and the landscape. Habitat destruction, fragmentation, and poaching are particu- larly important threats to migratory species. Critically dependent upon certain bottlenecks and corridors, as well as specific sites along their migration for wintering, summer ranges, reproduc- tion and refuelling of body reserves, they become highly vulner- able to habitat loss or barriers in these locations. For millennia, ancient human hunters built pitfall and pit trapping systems to harvest migrating ungulates, such as caribou and Saiga antelope.

any ungulate herds is that of North-American caribou ( Rangi- fer ssp.). Migratory ungulates may be entirely dependent upon narrow corridors, sometimes a few hundred metres to a few kilometres at the narrowest points, as has been shown in the case of pronghorn ( Antilocapra americana ). Some of these cor- ridors have been used for at least 5,800 years (Berger, 2004), many most likely for far, far longer.

Indeed, in spite of journeys of several hundred and for some of several thousand kilometres, the largest range covered by

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