The availability of food and the chances of becoming prey themselves determine the choice of location for grazers in the Serengeti. This is the conclusion of zoologist Grant Hopcraft. Knowledge about the spatial distribution of grazers is not only important for the Serengeti, currently under threat from a proposed motorway, but also, for example, for the Oostvaardersplassen in the Netherlands. Hopcraft will be awarded a PhD on 17 December 2010 by the University of Groningen for research into the dispersal and migratory behaviour of large and small grazers in the Serengeti.
Hopcraft investigated how the choice of location of herbivores in the Serengeti is associated with their body size in relation to food and predation. He analysed data from the past 25 years concerning the specific locations where large and small grazers gather in the savannah. He also studied the migratory behaviour of a group of zebras and wildebeest with the help of GPS transmitters. The grazers investigated by Hopcroft made various choices of location, both on a daily and a seasonal basis. The balance between predation risk and food supply turned out to be crucial in these choices.
Small grazers like gazelles have very limited dispersal areas, Hopcraft concluded. They are easy prey and therefore seek out locations where there are few natural enemies. In addition, small grazers have to rely on the scarce patches of grass with high protein content. ‘Small grazers make high demands on their food because they have a limited digestive system’, explains Hopcraft. ‘They have to graze the most nutritious pieces of land in order to get enough energy inside them.’
For large grazers like buffalo, for example, natural enemies and food quality do not play such a major role; rather, the amount of food available determines their dispersal. ‘Large grazers have hardly any enemies, they are only eaten by lions. Because they are large ruminants they can also digest food with a low nutritional value. Food quantity is the most important factor for these animals,’ says Hopcraft.
The most successful grazer in the Serengeti, the wildebeest or gnu, grazes on nutritious grass just like the gazelles but appears to follow a different strategy. Instead of grazing in a limited area, they migrate the whole year round from one nutritious spot to another. It’s a successful strategy because at 1.3 million animals, the wildebeest is the most common grazer in the Serengeti. ‘Wildebeest don’t have to be frightened of predation either,’ states Hopcraft. ‘They are so numerous that the chances of being eaten are very small indeed. In addition, their natural enemies, particularly lions, cannot follow the herds over such huge distances.’
However, the successful migration of the wildebeest through the Serengeti is under serious threat from a 450 kilometre-long motorway the Tanzanian government is planning through the Serengeti. Hopcraft predicts that such a road will have a drastic negative effect on the whole ecosystem. He sees a lot of drawbacks associated with such a road.
The proposed motorway will divide the nature reserve in two, straight through the most important wildebeest migration path. But is a strip of asphalt really such a huge barrier? Of course it is, states Hopcraft: ‘Because we are dealing here with such enormous numbers of animals, fences will have to be placed alongside the road to protect road users. That means that it really will be a barrier. The expectation is that the wildebeest population will crash from 1.3 million to 300,000 animals. Such huge numbers alone will mean a massive disruption of the ecosystem.’
Hopcraft sees another danger in the spread of infectious diseases such as rabies, bovine tuberculosis and rinderpest due to cattle transport through the nature reserve. ‘A rinderpest epidemic would be catastrophic for the ecosystem’, states Hopcraft. ‘We know from past experience that such a disease can wipe out 90% of the animals.’
Knowledge about the way grazers move within a nature reserve is not only applicable to the Serengeti but also to other nature reserves such as the Dutch Oostvaardersplassen. ‘In the Oostvaardersplassen you can see a similar mix of herbivores (konik horses, deer and cattle) as in the Serengeti. It’s therefore very interesting to compare the two regions to each other’, says Hopcraft. ‘The Oostvaardersplassen is also a globally unique ecosystem. There are many more animals to the square metre than in the Serengeti.’
J. Grant C. Hopcraft (Kenya, 1971) studied zoology at the University of British Columbia in Vancouver, after which he conducted various research projects in the field in the Serengeti. Hopcraft will be awarded a PhD by the Faculty of Mathematics and Natural Sciences. His supervisors were Prof. H. Olff and Prof. A.R.E. Sinclair. His thesis is entitled Ecological Implications of Food and Predation Risk for Herbivores in the Serengeti.
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