What makes a vintage year for the Serengeti lions?
### Today’s blog is an excerpt from Craig Packer’s forthcoming book, “Lions in the Balance: Man-eaters, manes and men with guns”, which will be published by University of Chicago Press in the fall of 2014. ###
When the Italians attempted to conquer Abyssinia in 1887, they provisioned their troops with livestock brought from India, but some of the cattle were infected with the rinderpest virus. By 1897, the disease had spread south from the Ethiopian plateau to South Africa’s Cape of Good Hope and across to West Africa, killing ninety percent of domestic livestock across the entire continent. Control programs were initiated throughout Africa; by the 1960s rinderpest was restricted to only a few areas, and Serengeti was the last major reservoir in Tanzania. A cattle-vaccination program around the Serengeti finally eliminated the disease from the wildlife inside the park in 1963. Liberated from rinderpest, the wildebeest, buffalo, warthog and other ruminant populations grew exponentially until they reached their current plateaus in 1979.
The lion population grew, too, but in a very different pattern.
The lions in the wooded habitat of our study area remained stable from 1966 until 1973 when the population suddenly leapt to a new equilibrium, then remained stable for another ten years before leaping again in 1983. The ruminant population had nearly tripled between 1966 and 1973; what held back the lions for so long? And what happened in 1973 and 1983?
Our lions endure an annual pattern of feast and famine; the migration brings the wildebeest and zebra within easy reach during the rainier months but sends the herds north to Kenya each dry season. In normal years, our study lions struggle to persist on warthog and buffalo, but these only sustain the adults – few cubs manage to survive.
The dry season of 1973 was the rainiest in decades, and the unseasonably green grasses attracted the wildebeest and zebra to our woodlands study area more or less continuously until the normal rains returned in November. Without the usual dry season famine, virtually every cub born in 1973 survived.
These surviving cohorts were large enough to form entirely new prides that could compete successfully against the prevailing social order and redraw the map of lion pride territories. Tough new gangs squeezed their way into the neighborhood, allowing the lion population to finally rise to a higher post-rinderpest plateau.
The recovering herds not only provided more meat on the hoof, but the wildebeest’s insatiable appetite for grass subsequently modified the habitat in the lions’ favor. An awful lot of grass was left uneaten when the wildebeest population was held low by rinderpest: grass fires roared through the park each year, burning the young acacia trees to stumps. But the expanding wildebeest population became the world’s largest lawn service, mowing the grass down to the nubs over thousands of square kilometers – creating fire breaks through much of the park. By the mid-1970s, less than a quarter of the Serengeti burned each year, and saplings were able to grow unhindered. Tree recruitment reached a peak in 1980 and persisted for ten years.
Lions need cover to hunt more successfully, and 1983 was the first year with favorable dry-season rainfall in this new improved world for hunting lions. Once again, the woodland prides recruited large numbers of young – large enough to spawn an expansion of new prides and redraw the map, with more groups packed more tightly than ever before.
The woodlands population crashed during a major disease outbreak in 1994 – lion numbers fell back to levels unseen since the late 1960s. But in 1999 – the first post-outbreak year with favorable dry season rainfall – the woodland population bounced all the way back up to the same level as in 1983-1993.
On the plains, the population’s initial post-rinderpest spurt occurred sometime after George Schaller’s departure in 1969, reaching a new plateau by 1974 when monitoring of the plains prides resumed. The plains population remained unchanged until November 1997, when El Niño brought the heaviest rains in forty years. The grasses on the plains had started growing taller during the early 1990s, and the El Niño floods kept the migration out on the plains for the longest period in decades. A single year with a more consistent food supply was enough to allow the plains lions to spawn whole new prides in the taller grasses.
As Lenin once said, “Sometimes decades pass and nothing happens; and then sometimes weeks pass and decades happen.”
Thanks to the Snapshot Serengeti camera trap grid, we can now watch the migration respond to year-to-year variations in rainfall. The next time the lions have a banner year, we will all be able to witness how food on the hoof translates into a baby boom of lucky cubs.