Top-down effects have primacy over bottom-up effects on the population dynamics of a flightless desert bird

Abstract

In desert ecosystems, some argue that primary productivity controls vertebrate populations while others contend that predators' top-down effects are under-appreciated. While seldom used to explain population dynamics in desert environments, the exploitation ecosystems hypothesis (EEH) unites bottom-up and top-down processes by conceptualizing how the biomass of plants and consumers vary along spatial productivity gradients in the presence and absence of predators. Here, we test predictions from the EEH along temporal primary productivity gradients, by comparing abundances of an apex predator (dingo) and a large omnivorous flightless bird (emu) on either side of Australia's dingo-proof fence over a nine year period. Where dingo populations were not suppressed by humans, their abundance increased with rainfall in the previous 12 months but did not respond to rainfall where their populations were suppressed. Conversely, emu abundance increased in response to antecedent rainfall where dingoes were rare but showed a negligible response to rainfall where dingoes were common. Our results accord with the EEH by suggesting that dingoes' top-down effects can decouple the linkage between emu populations and primary productivity. More generally, our findings show that top-down effects can have primacy over bottom-up effects for both an apex predator and an omnivorous prey species.