title: Shrub encroachment is linked to extirpation of an apex predator.
abstract: The abundance of shrubs has increased throughout Earth's arid lands. This 'shrub encroachment' has been linked to livestock grazing, fire-suppression and elevated atmospheric CO2concentrations facilitating shrub recruitment. Apex predators initiate trophic cascades which can influence the abundance of many species across multiple trophic levels within ecosystems. Extirpation of apex predators is linked inextricably to pastoralism, but has not been considered as a factor contributing to shrub encroachment. Here, we ask if trophic cascades triggered by the extirpation of Australia's largest terrestrial predator, the dingo (Canis dingo), could be a driver of shrub encroachment in the Strzelecki Desert, Australia. We use aerial photographs spanning a 51-year period to compare shrub cover between areas where dingoes are historically rare and common. We then quantify contemporary patterns of shrub, shrub seedling and mammal abundances, and use structural equation modelling to compare competing trophic cascade hypotheses to explain how dingoes could influence shrub recruitment. Finally, we track the fate of seedlings of an encroaching shrub, hopbush (Dodonaea viscosa angustissima), during a period optimal for seedling recruitment, and quantify removal rates of hopbush seeds by rodents from enriched seed patches. Shrub cover was 26-48% greater in areas where dingoes were rare than common. Our structural equation modelling supported the hypothesis that dingo removal facilitates shrub encroachment by triggering a four level trophic cascade. According to this model, increased mesopredator abundance in the absence of dingoes results in suppressed abundance of consumers of shrub seeds and seedlings, rodents and rabbits respectively. In turn, suppressed abundances of rodents and rabbits in the absence of dingoes relaxed a recruitment bottleneck for shrubs. The results of our SEM were supported by results showing that rates of hopbush seedling survival and seed removal were 1·7 times greater and 2·1 times lower in areas where dingoes were rare than common. Our study provides evidence linking the suppression of an apex predator to the historic encroachment of shrubs. We contend that trophic cascades induced by apex predator extirpation may be an overlooked driver of shrub encroachment.
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, 2751, Australia.|Centre for Ecosystem Science, University of New South Wales, Sydney, NSW, 2052, Australia.|Centre for Environmental Risk Management of Bushfires, University of Wollongong, Wollongong, NSW, 2522, Australia.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Gordon
authors.affiliation: School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, 2052, Australia.
authors.forename: David J
authors.initials: DJ
authors.lastname: Eldridge
authors.affiliation: Global Trophic Cascades Program, Forest Ecosystems and Society, Oregon State University, Corvallis, OR, 97331, USA.
authors.forename: William J
authors.initials: WJ
authors.lastname: Ripple
authors.affiliation: School of Life and Environmental Sciences, University of Sydney, Sydney, NSW, 2006, Australia.
authors.forename: Mathew S
authors.identifier: 0000-0001-8968-1161
authors.initials: MS
authors.lastname: Crowther
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, 2751, Australia.
authors.forename: Ben D
authors.initials: BD
authors.lastname: Moore
authors.affiliation: Centre for Ecosystem Science, University of New South Wales, Sydney, NSW, 2052, Australia.|School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, 2052, Australia.
authors.forename: Mike
authors.initials: M
authors.lastname: Letnic
mesh_terms: D000818:Animals; D004285:Dogs; D017753:Ecosystem; D020387:Food Chain; D019684:Magnoliopsida / Q000502:physiology*; D008322:Mammals / Q000502:physiology; D063148:Plant Dispersal; D011235:Predatory Behavior; D029631:Sapindaceae / Q000254:growth & development; D036226:Seedlings / Q000254:growth & development; D013021:South Australia
keywords: arid lands; dingo; fire; herbivore; mesopredator release hypothesis; shrub encroachment; top predator; trophic cascade