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A critical challenge in managing invasive alien plants is the prioritization in the allocation of resources to control measures (see Roura-Pascual et al. 2009). Managers need to decide which species to focus on and where and they have to select the best possible approach to take from the range of management options available (e.g. mechanical clearing, the application of herbicides, burning, and biological control).

Hakeas on the decline, Wilderness, southern Cape.
Photo: Brian van Wilgen.

The outcomes of integrated control of the invasive shrub Hakea sericea over four decades in South Africa were assessed in a recent paper published in Biological Invasions. This aggressively invasive shrub native to south-eastern Australia is a widespread invader in the Cape fynbos, and occurs mainly in rugged, inaccessible and fire-prone mountain areas. The species, from the family Proteaceae, is pre-adapted to ecological conditions in the Cape. It is serotinous (i.e. has a canopy-stored seed bank), and produces copious amounts of seed that are wind dispersed after fires. After its naturalization in 1883, H. sericea spread to an area of 800 x 200 km. A survey in 1979 estimated the total area occupied by this species to be ~530 000 hectares. Twenty-two years later after an extensive survey by the Protea Atlas Group, the overall distribution of the species had declined by 64% to ~190 000 hectares. In the interim, control measures were introduced and included a combination of felling and burning, augmented by biological control. The initial programs of mechanical clearing initiated in the 1970s were largely responsible for reducing the density and extent of infestations, and biological control was largely responsible for the failure of the species to re-colonize cleared sites, or to spread to new areas following unplanned wildfires.

Hakea sericea holds its seed in fire-proof woody structures called follicles. These open when the plant is killed in fires, resulting in the simultaneous release of the entire accumulated seed load. The winged seeds can be dispersed over long distances by wind.
Photo: DM Richardson.

Esler and co-authors propose that a significant portion of the resources used for clearing Hakea in the past can now be reallocated to mechanical control efforts against other invasive species (such as alien pines) for which effective biological control options are not available. A proviso to this recommendation is that adequate resources are made available to ensure the widespread and effective implementation of all biological control agents so that the advances on the control of this species are maintained.

Further details: Prof. Karen Esler

Esler, K.J., van Wilgen, B.W., te Roller, K.S., Wood, A.R., van der Merwe, J.H. (2010) A landscape-scale assessment of the long-term integrated control of an invasive shrub in South Africa. Biological Invasions, 12:211-218.

Other papers on Hakea sericea: