Population Consequences of Predation‐Sensitive Foraging: The Serengeti Wildebeest

Abstract

The "predation—sensitive food" (PSF) hypothesis proposes that both food and predation necessarily limit populations, because as food becomes limiting animals take greater risks to obtain more food, and some of these are killed. Alternative hypotheses are "predator regulation" where predators hold the prey population well below starvation levels; and "surplus" predation where predators kill only those prey that are excluded from optimal habitat and are dying from starvation. The predictions from these hypotheses were tested by examining body condition of Serengeti Wildebeest (Connochaetes taurinus) over 24 yr (1968—1991). Two phases of population growth were examined: 1968—1973 when the population was increased with superabundant food; and 1977—1991 when the population was stationary and regulated by intraspecific competition for food. Three categories of data were compared: live animals, predation kills, and nonpredation deaths. Body condition was measured from bone marrow, the last reserves of fat in ungulates. The predator regulation hypothesis predicts that the marrow condition should be similar to the predation and live samples. The surplus hypothesis predicts the predation and nonpredation samples should be similar. The PSF hypothesis predicts that marrow condition of the predation sample should be (1) poorer than that of the live sample, (2) better than that of the nonpredation sample, and (3) better when food is limiting than when it is abundant. Analyses of the frequency distribution of marrow categories showed that both the predation and nonpredation samples were significantly poorer than that of the live population. In both increase and stationary phases of population growth, the predation sample was in better condition than the nonpredation sample. The predation sample was not quite significantly better (P = 0.052) when food was limiting. These results are consistent with the PSF hypothesis and inconsistent with both of the alternative hypotheses. Female and male condition was similar in the predation sample, but females were killed at a younger age. Lions and hyenas killed animals in similar condition, but lions took animals at a younger age. The results suggest that (1) body condition affects the vulnerability of individual wildebeest to predation, and (2) predation jointly limits the population with intraspecific competition by removing animals from the population that are in better condition than those that are starving.