Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

Eldridge, D. J., Bowker, M. A., Maestre, F. T., Roger, E., Reynolds, J. F., & Whitford, W. G. (2011). Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecology Letters, 14(7), 709-722. PDF.

Summary

Shrub encroachment into grasslands and savannas is one of the best examples of native plant 'invasion'.  Shrub encroachment has been documented in drylands globally, including the western U.S., Mediterranean, Africa, and Australasia. However, it is unclear what impact shrub encroachment has on native ecosystems and what implications shrub encroachment has for understanding the impacts of native expansion under climate change. Eldridge et al. (2011) analyzed 244 case studies measuring impacts of shrub encroachment to assess impacts on ecosystems.  Not surprisingly, shrub encroachment increased the density of shrub cover.  Increased shrub cover also increased soil carbon and nitrogen storage and decreased the density of grasses.   However, there were no significant effects on the richness of grasses (i.e. number of species), nor on the richness of vertebrates or invertebrates.  Shrub encroachment to date was mainly linked to grass suppression and disturbance from grazing.  However, increasing atmospheric CO2 also facilitates shrub encroachment.  These results suggest that rising CO2 could lead to more rapid shrub encroachment, but the ecological impacts of encroachment per se appear relatively minor.

Take home points

  • Shrub encroachment is the main documented form of native plant 'invasion', but is currently mainly caused by grazing disturbance in drylands

  • Shrub encroachment increases shrub cover and decreases grass cover, but had no significant effects on plant, invertebrate, or vertebrate species richness

  • Ecological impacts of this best example of native plant 'invasion' are relatively minor.

Management implications

  • Shrub encroachment is mainly a concern in dryland ecosystems

  • Disturbance allows native species to expand, but expansion could be further facilitated by rising CO2

Keywords

Range Expansion; Impact Study; Meta-analysis; Invasive Plant; Terrestrial Habitat