Casuarina Invasion Alters Primary Succession on Lava Flows on La Réunion Island

Invasive plants can alter community dynamics and the successional trajectories of ecosystems they colonize. We explore how interactions between disturbance and invasion govern successional trajectories in the case of Casuarina equisetifolia invading lava flows on La Réunion Island. Surveys from 1972 and 1990 were compared with results of a survey in 2012 to detail progression of the invasion over time. General additive models were used to estimate the influence of altitude, distance to putative source of introduction, and lava flow age on the abundance of C. equisetifolia. Based on the predictions, we estimated the likely rate and eventual extent of spread of the species in the area through time. We placed our findings in the context of a conceptual model of successional processes in the area to highlight how the invasion of C. equisetifolia and natural and human‐mediated disturbances are changing natural vegetation dynamics. The extent of invasion by C. equisetifolia has increased twentyfold over the past 40 yr from 110 ha in 1972 to 2373 ha in 2012. Lava flows have facilitated this spread, and in turn C. equisetifolia has started to radically change successional trajectories, increasing the rate of succession sevenfold. The continued spread of this species poses a major threat to the small area of remaining native lowland rain forests on La Réunion Island, which cover <2 percent of their original extent.