The Invasive Woody Weed Ligustrum robustum subsp. walkeri Threatens Native Forests on La Réunion

Abstract

One of the last primitive island ecosystems in the Indian Ocean has been invaded since 1969 by the Sri Lankan privet, Ligustrum robustum. L. robustum is still spreading in the forests of La Reunion Island, where only 30% of the original vegetation remains, but where 98% of the primary native vegetation of the Mascarene Islands still exists. On Mauritius, where L. robustum was introduced about 1895, it now forms dense, impenetrable thickets, and its presence has been correlated with the inability of native vegetation to re-establish. We assessed the potential impacts of L. robustum invasion on the native ecosystems of La Reunion and identified the factors of invasibility. We determined the degree of invasion in 12 plots of 156 m2 and followed native flora and privet recruitment for 3 years in 12 seedling plots of 39 m2. The data show that monocultural L. robustum stands now exist in human-disturbed primary forest patches (3.3 individuals/m2 and 80% of total individuals) and high seedling densities (0.3–0.6/m2) occur in the least disturbed patches. L. robustum's rapid growth, high shade tolerance and seed production, bird-assisted seed dispersal and high seedling recruitment contribute to its invasiveness in intact forests. The conservation of the original ecosystems of La Reunion depends on the setting up of a long-term and immediate global control strategy.