Is woody shrub encroachment a legacy of mammal declines?

A Project undertaken at University of New South Wales  supervised by Mike Letnic and David Keith

Population declines of once common species often precedes understanding of their roles within ecosystems. Consequently, important drivers of environmental change may remain undiagnosed because we do not know how rare or extinct species shaped ecosystems in the past.
 
This project has provided evidence for links between shrub encroachment and declines of native mammals in arid Australia. Using field studies we have shown that the abundances of shrubs, shrub seedlings and shrub seeds were higher in landscapes where native mammals, including hopping mice and burrowing bettongs were rare or absent. Our field experiments also show that now rare native mammals are significant consumers of shrub seeds and that where they persist small and medium sized mammals are more significant consumers of shrub seeds than ants. This finding challenges the paradigm that ants are the dominant consumers of seeds in arid Australia, and suggests instead that the dominance of ants may be an artefact of mammal declines.
 
Shrub encroachment has adversely affected pastoralists and the integrity of conservation reserves by suppressing grass cover and restricting access to land. Our findings providing evidence that the consumptive effects of native mammals impose a recruitment bottleneck on woody shrubs lend support for the idea that restoration of native mammal populations could be used as a tool to limit recruitment of woody shrubs.

Picture- View of the dingo fence in along the NSW/SA border in the Strzelecki Desert. Shrub encroachment has occurred on the NSW side of the fence where native rodents are rare.

Picture- Burrowing bettong feeding on leaves of Acacia ligulate at the Arid Recovery reserve in South Australia. Reintroductions of bettongs have revealed that they are significant consumers of shrubs and shrub seedlings.

Figure 1. View of the dingo fence in along the NSW/SA border in the Strzelecki Desert. Shrub encroachment has occurred on the NSW side of the fence where native rodents are rare.

Figure 2. Burrowing bettong feeding on leaves of Acacia ligulate at the Arid Recovery reserve in South Australia. Reintroductions of bettongs have revealed that they are significant consumers of shrubs and shrub seedlings.