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.
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
Mills, C.H., Gordon, C.E, and Letnic, M. (2017). Rewilded mammal assemblages reveal the missing ecological functions of granivores. Functional Ecology doi: 10.1111/1365-2435.12950
Mills, CH and Letnic, M. (2018). Reversing functional extinction of mammals promps a rethink of paradigms about seed fate in arid Australia. R. Soc. open sci. 5:171977. http://dx.doi.org/10.1098/rsos.171977