Observations on non-additive predation: birds and grasshoppers

Background: Classic predator–prey theory assumes that predatory mortality is additive with other sources of mortality, but predatory mortality can be non-additive. If predators preferentially kill individuals already ‘doomed’ from other mortality causes, then predation can be less than additive. If predators preferentially kill individuals more likely to survive, then predation can be greater than additive. In either case, to be non-additive, predation must modify other causes of mortality such as intraspecific competition, decreasing it in the former case and increasing it in the latter. Non-additive predation also can modify selective pressures of predation and is seldom considered except when prey are much larger than their predators. Question: Can non-additive predation be observed in the most common type of predator–prey system (prey much smaller than their predators) where this is considered very unlikely? Organisms: Two sympatric grasshoppers and their avian predators. Field site: National Bison Range, Montana, USA. Methods: Field experiments to determine: (1) whether better foraging individuals survive longer in the absence of predators or if they die from non-predatory causes in the presence of predators; (2) whether predators preferentially kill better or poorer foragers; and (3) how predators affect prey populations. Conclusions: Both grasshoppers exhibit intraspecific competition with better foragers surviving longer. Both experience high predation rates, but their populations respond differently to predation. One increased with predation, due to the non-additive effect of the predators preferentially killing poor foragers, and thereby diminishing intraspecific competition, which counters predatory mortality. The other population declined more than expected with predation due to the non-additive effect of the predators preferentially killing good foragers, and thereby intensifying intraspecific competition.