Invasive Cane Toads’ Predatory Impact on Dung Beetles is Mediated by Reservoir Type at Artificial Water Points

Abstract

Disruption to ecosystem functioning associated with biological invasions can have dramatic effects on the production and biodiversity values of ecosystems. In semi-arid rangelands of Australia, invasive cane toads (Rhinella marina) prey on dung beetles that were themselves introduced to promote nutrient cycling and reduce parasite burdens of livestock. Cane toads’ colonization of rangelands has been facilitated by artificial water points (AWP) which provide cattle with drinking water. Most AWP in northern Australia comprise bores that pump water into earthen reservoirs (dams). Dams typically support large toad populations because they allow toads’ access to water without which they could not survive. Here, we ask if restricting toads’ access to water at AWP can reduce toad populations, toads’ predatory impact on dung beetles, and the rate of dung decomposition by dung beetles. We contrasted toad and dung beetle populations, toad diets, and dung removal rates at AWP fitted with dams or tanks as reservoirs. In comparison to dams, tanks provide toads with little access to water. Population densities of toads were five times higher at dams than tanks. Conversely, population densities of dung beetles were 12 times lower at dams than tanks. Mass loss of experimental dung pats after 2 days was 13% greater at tanks than dams. Our study provides evidence that consumption of detritivores by an introduced predator can retard dung decomposition in a rangeland ecosystem. Restricting toads’ access to water at AWP should benefit livestock production by reducing both toad populations and toads’ predatory impact on dung beetles.