Wildebeest Explosion

If you’ve spent time on Snapshot Serengeti, then you’ll know that wildebeest are rather abundant in the Serengeti – especially during the rainy season. But it wasn’t always this way. In the 1950’s there were fewer than a quarter of the wildebeest there are today.

Back then, there was something suppressing the wildebeest population, keeping it much lower than the land’s capacity. It wasn’t predators, though there are now more lions and hyena in the Serengeti thanks to the increase in wildebeest. It wasn’t poaching, though we know that poachers take a substantial number of wildebeest. It was disease.

In the early 1930’s rinderpest was detected in Serengeti’s wildebeest. Rinderpest is closely related to measles. In fact, it is believed that measles evolved from rinderpest some 800 to 1,600 years ago. But rinderpest doesn’t affect people; instead, it affects ungulates and most likely evolved in Eurasia. For a long time, the Sahara Desert probably acted as a sort of barrier, preventing the disease from reaching sub-Saharan Africa. But in the late nineteenth century, people transported infected cattle into the region.

Rinderpest has high mortality in wildebeest, especially in young animals. What was once known as “yearling disease” killed so many young wildebeest that the Serengeti population was only about 300,000 animals in the 1950’s. Rinderpest also causes high mortality in cattle, and so inoculation attempts started in the 1940’s. These got better over time, and in the 1960’s there was a largely successful push to vaccinate 80 million cattle across twenty-two African countries, including Tanzania.

Wildebeest themselves were not vaccinated, but as the number of rinderpest-infected cattle decreased with vaccination, so did the number of wildebeest that had rinderpest. Following the initial vaccination push, regular vaccination campaigns kept the infection rate very low in cattle. Despite a handful of small localized rinderpest outbreaks in the ensuing decades, the disease was essentially eliminated from the Serengeti wildebeest population. This pattern of infection shows us that for rinderpest, wildebeest are what is termed a spillover species, which means that the wildebeest population cannot by itself sustain the disease; wildebeest must constantly contract the disease from cattle for it to survive in the wildebeest population.

The Serengeti wildebeest population has since exploded. No longer constrained by rinderpest, it has soared to 1.2 to 1.5 million animals.

As for rinderpest, the vaccination campaigns of the mid twentieth century were only a start. The international Food and Agriculture Organization (FAO) continued to pursue the disease by vaccinating cattle and by the 1990’s had reduced it to only local outbreaks worldwide. In 2010, the FAO declared that they were confident that they had eliminated the disease from everywhere it had been known. And less than two years ago rinderpest was declared officially eradicated. It is the second of only two diseases that humanity has successfully eradicated, the first being smallpox.

Wildebeest all the way to the horizon