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 will determine if there is a link between the decline of Australia's mammal fauna and increased abundance of woody shrubs in arid ecosystems. This shrub encroachment is symptomatic of land degradation (1) and is thought to impair the livestock production and biodiversity values of landscapes. However, encroachment is not restricted to pastoral land and its causes are not well understood (2). Existing hypotheses propose that encroachment results from interactions among grazing, burning, and elevated CO2 favouring recruitment of shrubs over grasses (2).

Data we have collected showing that endangered native mammals predate on shrub seeds and seedlings and that shrub seedling abundance is lower in their presence lends support to the hypothesis, that encroachment is a legacy of mammal decline. If this hypothesis is correct, restoration of endangered mammal populations could be conducted to prevent encroachment.

Shrub encroachment has adversely affected pastoralists and the integrity of conservation reserves by suppressing grass cover and restricting access to land. If endangered mammals do limit shrub recruitment and thus potentially provide ecosystem services, restoration of native mammal populations could be become a greater priority for pastoralists and government agencies concerned with both biodiversity conservation and management of pastoral lands.

Publications

Gordon, C.E. and Letnic, M. (2015). Functional extinction of a desert rodent: implications for seed fate and vegetation dynamics. Ethography 38: 001-010. doi: 10.1111/ecog.01648

Gordon, C.E., Eldridge, D.J., Ripple, W.J., Crowther, M.S., Moore, B.D. and Letnic,. M. (2017). Shrub encroachment is linked to the extirpation of an apex predator. Journal of Animal Ecology doi: 10.1111/1365-2656.12607

Figure 1. Burrowing bettong (Mike Letnic)

Figure 2. Dusky hopping mouse (Ben Moore)

Figure 3. Sifting for shrub seeds in the Strzelecki Desert. Note the shrub encroached landscape in the background (Mike Letnic)