Three Decades of Monitoring Reveal That Differing Population Trends of Large Herbivores in Gonarezhou National Park, Zimbabwe, Are Related to Species Body Mass

ABSTRACT

Trends in large herbivore abundance in Gonarezhou National Park, Zimbabwe, were examined using population estimates from aerial surveys designed for elephant Loxodonta africana . For ~20 years after the particularly severe 1992 drought, the numbers of elephant, buffalo Syncerus caffer , eland Tragelaphus oryx , giraffe Giraffa camelopardalis , greater kudu Tragelaphus strepsiceros , nyala Tragelaphus angasii , warthog Phacochoerus africanus , waterbuck Kobus ellipsiprymnus , wildebeest Connochaetes taurinus and zebra Equus quagga increased. After 2013/2014, survey estimates for buffalo, eland, elephant, giraffe and zebra increased more slowly, suggesting that a logistic curve was an appropriate model for these species. In contrast, estimates for impala, kudu, nyala, warthog and wildebeest decreased after 2013. Species that declined had a body mass < 140 kg and species that did not had a mass ≥ 200 kg, which suggests that predation likely caused the declines. It is less obvious why trends in the abundance of smaller herbivores changed from increasing before 2013 to decreasing afterwards. Survey estimates for elephants in female herds suggest that there was an abrupt reduction in their rate of increase between the period before 2014 and that afterwards. Most herbivore population declines occurred during that latter period, when the number of elephants in female herds essentially stabilised, likely as a result of food limitation. From studies elsewhere, one can hypothesise that in Gonarezhou: (1) there was direct competition for food between elephant and the browser and mixed feeder species; (2) the food supply for grazers was reduced by dry‐season rainfall often being below‐average; (3) food shortages lead to poorer body condition and increased vulnerability to predation; and (4) elephant impacts on woody vegetation structure further increased the vulnerability of smaller herbivores to predation. The numerous elephants in Gonarezhou might appear a conservation success, but their high density may have adversely impacted populations of some large herbivores and predators.