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Mutualism management enables forest tree recovery

The Pisonia trees of the Seychelles Islands are of great importance to the island ecosystem. These trees provide nesting habitat and nest materials for Lesser noddies, White terns, and Black noddies, and endemic insectivorous birds such as the Seychelles warbler and the Seychelles fody often forage on invertebrates living in Pisonia canopies. Pisonia trees also produce a peat-like humic soil, which is rich in phosphate and uncommon on oceanic islands. However, these forests are threatened by the highly destructive West-Indian scale insect Pulvinaria urbicola, which forms a mutualism with invasive ants.

On Cousine Island in the Seychelles, the African big-headed ant Pheidole megacephala protects infestations of scale insects, which cause dieback and death of Pisonia trees, and in some cases near-complete loss of forested areas. As Pisonia trees occur mostly in conservation areas, the development of effective low-impact methods for managing the scale insect infestations is a high priority. Yet few management attempts have been made, and it is unclear whether the impacts of management can be mediated.

In a recent publication in Biotropica, C·I·B PhD student René Gaigher and C·I·B core team member Michael Samways evaluated the outcomes of a management program designed to reduce pressure on Pisonia trees on Cousine Island. They used a highly selective and targeted ant baiting method to suppress the invasive ants throughout the infested area, with the aim of decoupling the mutualism and reducing the density of scale insects. Eleven months after baiting there was a 93% reduction in ant foraging activity, followed by a 100% reduction in scale insect density on host trees. Native herbivorous insect abundance, as well as herbivory on Pisonia trees, increased after baiting, but there was still a significant overall improvement in the trees’ shoot condition and foliage density, and a noticeable decrease in sooty mold, a fungus that grows on scale insect secretions on tree trunks and leaf surfaces (Fig. 1).

Pisonia grandis before baiting and eleven months after baiting

Figure 1 Pisonia grandis condition a) before baiting in May 2010 and b) the same location eleven months after baiting in May 2011 in an area of high Pheidole megacephala and Pulvinaria urbicola densities, clearly indicating a significant overall improvement in Pisonia shoot condition and foliage density.

Considering the poor condition of Pisonia trees in the ant-infested area before baiting, it is likely that management intervention succeeded in preventing significant damage to this important component of the forest ecosystem. These results demonstrate the conservation benefits of managing the invasive mutualists, both to the native forest trees and native canopy herbivores.




Read the paper: Gaigher, R and Samways, M.J. 2012. Strategic management of an invasive ant-scale mutualism enables recovery of a threatened tropical tree species. Biotropica. DOI: 10.1111/j.1744-7429.2012.00898.x

For more information contact Rene Gaigher at reneg@sun.ac.za or Michael Samways at samways@sun.ac.za