An analysis of numerical trends in African elephant populations

Abstract

The elephant debate deals largely with population size, how elephant numbers change over time, how they may affect vegetation, and how their populations should be managed. Trends in elephant numbers frequently motivate management decisions, and past efforts to alleviate elephant impact aimed at controlling population size. However, methodological and statistical constraints may influence interpretation of trends and lead to incorrect management decisions. Furthermore, inferences about the response of elephant populations to specific management actions are seldom based on scientific evidence. In this thesis I assess the consequences of survey design and monitoring features on the interpretation and statistical reliability of population trends as well as the effect of population management on elephant densities and population growth rates. To do this, I collated information on elephant population estimates and past management actions across Africa. I used information from the northern Botswana elephant population to clarify temporal trends in elephant densities and numbers. Elephant numbers in northern Botswana increased from 1973 to 1993 while densities remained relatively stable. This difference in trends is due to an associated increase in survey area during the same time. In contrast, from 1996 to 2004 surveyed areas remained constant in size and neither elephant numbers, nor densities changed significantly during this time. This apparent stabilisation in numbers may have resulted from density-related elephant dispersal. This case study suggests that in open populations movements may complicate the interpretation of trends, and that differences in the rates of change in numbers and densities may have different management implications. The precision of population estimates, sample size, population size, and the magnitude of the annual rate of population change to be detected, affect power to identify trends. Two-thirds of the 156 time series that I assembled apparently were stable, and only 30 % of these had sufficient statistical power to detect population changes. These apparent stable trends without sufficient statistical power are inconclusive and should not be used to inform management decisions. Past elephant population management practices may have increased densities and growth rates in African elephant populations. Case studies of populations that were exposed to different management actions indicated that fencing of populations and water supplementation may have enhanced growth rates probably by influencing dispersal patterns. Thus, past management practices may have contributed to the ‘elephant problem’ by enhancing local elephant densities and population growth rates. In this thesis, I showed that trends based on elephant numbers may be misleading when the area over which elephants were counted, increased in size. Second, despite much effort and resources devoted to the monitoring of elephant populations for more than 50 years, population estimates and time series including such estimates had low quality, thereby reducing statistical power to detect trends in population change. Third, population growth rates were associated with management, where elephant population densities grew at faster rates when managed. Future conservation efforts should take into account the methodological and statistical constraints that may influence trend analyses of elephant populations and take cognizance of the fact that management decisions need to be evaluated against expected outcomes. Copyright