How Many Elephants? Understanding Population Numbers

We often get asked how many elephants there are in Amboseli, and if the population is going up or down. We think these questions are usually directed to getting some kind of "good" or "bad" metric for how the elephants are doing. But as usual elephant truth is a bit more complex than "good" or "bad", and some wider perspective is useful.

 

Kenya has far fewer elephants than it used to-- in 1974 there were estimated to be 167,000 elephants in the country; by the end of 1989 there were only 16,000 remaining and today there are between 30,000 and 40,000. While other populations were declining sharply Amboseli's population grew, and continues to grow. That early trend may be a bit misleading because the study population was still recovering from earlier poaching and some elephant families "joined" us in the late 1970s after agriculture displaced them from wetlands outside the Park. Although recent increases are due to more elephants, they are also using the ecosystem more widely (see Cynthia's letter above).

 

Elephant population growth rate isn't simple to calculate, because it is determined by a combination of factors that vary between individuals and over time such as  age at first reproduction, inter-birth intervals, and survivorship of calves. That intricacy makes output figures hard for non-specialists to understand.  Looking at short-term information like annual birth rates only tells part of the story: The number of births in any given year depends on the ecological conditions and the number of available females (old enough to give birth or without a calf so young it prevents her getting pregnant again) 22 months before. After terrible droughts like 2009, where almost 400 elephants died in Amboseli, all females came back into reproductive condition around the same time and we ended up with the "baby boom" we have written about before.

One easier-to-grasp measure is how quickly females can produce calves (the inter-birth interval). It is measured for each birth, from the previous birth.  This interval between successive calves depends on the mother's age, sex of both this and the previous calf, if the previous calf was first-born and whether or not it survived, as well as the ecological conditions the mother experiences. In good times, females can shorten the period between births, getting pregnant sooner and weaning their older calves earlier, which might contribute to big increases in population growth. This has been shown for recovering elephant populations elsewhere. Although the Amboseli interval varies between females and between years (see graph) there has been no overall change in the interval between births since we started the study.

Amboseli birth intervals are stable over time: average birth intervals recorded in any year can vary from 40 to 80 months (orange line), but average per-decade intervals (gold bars) don't change (error bars show standard deviation; overlap in error bars shows no difference between bars). Pale grey lines are standard deviations from the mean in any given year.

We know the Amboseli population is still self-regulating because females don't raise all their calves successfully, and the patterns of pregnancies and births  continue to reflect changes in the environment. Long-term analyses like these birth intervals show us that the elephants are not in "runaway success", although they are increasing.

 

For a look at how our families have done over the course of the study, please  Click Here. Pay special attention to how some families have become big and successful, while others have stayed small and some have gone extinct.