There has long been debate over the primacy of top-down (consumption) and bottom up effects (primary productivity) as forces controlling population fluctuations of mammals. In deserts, rainfall driven fluctuations in food availability are often considered to be more important drivers of the population dynamics of mammals than predators’ top-down effects. Here I report the effects that the presence/absence of an apex predator, the dingo, has on population dynamics of mammals in Australia’s Strzelecki Desert. Results show that primacy of top-down and bottom-up effects as drivers of mammals’ population fluctuations scales with their body size and the composition of predator assemblages. Populations of herbivorous kangaroos (> 20 kg) and introduced mesopredators (4-7 kg) the red fox and feral cat increased with the availability of food resources in the absence of dingoes but showed negligible responses to increased resource availability where dingoes were present. Conversely, populations of small mammals (< 1.5 kg) increased with the availability of food resources in the presence of dingoes but showed muted responses to resource pulses where dingoes were absent. My study shows that that predators can decouple bottom-up effects and that top-down and bottom-up effects can have primacy on different species within desert mammal assemblages. This can occur because the strength predators’ top-down effects scales with body sizes of both predators and prey.