More than a burning issue: Does grass invasion disrupt plant-animal interactions and threaten persistence of semi-arid woodland communities?

A project undertaken at the School of Environment, Charles Darwin University and supervised by Christine Schlesinnger

Introduced pastoral grasses have become major global drivers of ecosystem change, and transformers of native plant assemblages. One such species, Cenchrus ciliaris (buffel grass), is becoming ubiquitous in extensive areas of semi-arid Australia, North and South America. Buffel grass invasion is known to be associated with declines in the diversity of the ground-layer flora but the effects of invasion on the structurally important and more persistent components of the plant community - trees and shrubs - remain unknown. In central Australia buffel grass is most prevalent on alluvial plains associated with inland watercourses. The native woodlands and shrublands of these floodplains provide rich resources compared to the surrounding landscape and shifts in their structure and function are likely to have serious adverse consequences for numerous fauna. A novel and poorly explored indirect mechanism by which invasive species may be changing native plant populations is by disrupting their interactions with mutualist or predatory fauna, such as pollinators, seed dispersers and granivores. Recent research has confirmed that the diversity and abundance of ants is reduced in areas with buffel grass and that birds spend less time overall, and proportionally less time foraging, at sites with buffel grass. The implications of such shifts in abundance or behaviour of seed dispersers and predators for plant communities that are already under stress are unknown, and this represents a substantial gap in our ability to predict the longer term impacts of buffel grass on plant and animal communities and to effectively manage invaded ecosystems.

This collaborative project between Charles Darwin University and University of Wollongong aims to investigate how buffel grass affects population and recruitment dynamics of keystone woody perennial plants that are characteristic of riparian woodlands in central Australia, and identify whether disrupted recruitment is linked to invasion-induced shifts in plant-animal interactions.

Figure 1. Buffel grass invaded area with sparse Acacia shrubs

Figure 2. Acacia tetragonaphylla seed

Figure 3.The galah can be a voracious predator of a variety of Acacia seeds, and is here seen foraging on a buffel grass seed head