Water, Migration and the Serengeti Ecosystem
Understanding the mechanisms that control the timing of wildlife migrations may prove vital to successful management
A big weather wind from the southeast came up in the late morning, and by noon it was shifting to the east, turning the dark clouds on the Crater Highlands. Once again, the herds were moving.
The Tree Where Man Was Born
Each year, some time near the end of the wet season in Serengeti National Park, Tanzania, one of nature's most spectacular phenomena begins. Approximately 1 million wildebeests (Connochaetes taurinus) and a smaller number of zebras set off on a journey that will lead them from their wet-season range in the south of the park to the northwestern dry-season range, then to the southwest and finally back to the south. Over the course of the year, the animals travel an average of 10 kilometers per day.
Not only does the migration extract a large cost in energy, but calving also coincides with the arrival of the wildebeests in the southern range, and many of the newborn fail to survive the trek to the northwest. Indeed, it has been estimated that the migration increases wildebeest mortality by 3 percent per year compared with populations that remain in the northwest. Clearly, powerful environmental factors must be forcing this migration. But what might those factors be?
Many mechanisms have been proposed as triggers for the onset of the migration. Biomass models based on forage and nutrients and the effects of carnivores have all been invoked, but none has explicitly included rainfall, and the models' predictive power has proved to be low. They all fail to consider unused forage, do not explain why animals migrate and cannot predict the timing of the migration, which may vary by as much as three months. Curiously, although hydrological data have been collected for the park for many years—for example, rainfall for 38 years and river discharge for four years in the 1970s—this information seems to have been ignored when searching for causes for the migrations. Likewise, vegetation changes over decadal time scales have been reported only casually.
To look deeper into the driving forces of the Serengeti ecosystem, we chose to collect water-quality data and merge them with the available hydrologic data. Using both numerical and graphical techniques, we then analyzed the data to reveal any correlations between water quantity and quality and both the timing of migration and vegetation types and availability. Our studies have led us to propose that water quality, as well as quantity, makes up the dominant force driving the Serengeti ecosystem. Not only do these factors explain the timing of the wildebeest migration, but they also suggest why vegetation occurs in the patterns that it does.