Plant invasion of tropical rainforest remnants

Intact tropical rainforests are considered to be robust to exotic plant invasions. However, land-use change across the tropics is altering the structure and native species composition of rainforests, potentially making these landscapes vulnerable to invasion. I examined local and landscape-scale factors that affect exotic plant invasion in remnant forest sites surrounded by oil-palm plantations across Sabah, Malaysian Borneo, spanning gradients in landscape-scale fragmentation and local forest disturbance. Invasion (exotic richness and abundance) was higher when there was greater local forest disturbance and propagule pressure, the latter due to increased landscape fragmentation. Invasion and young native tree diversity were negatively related, indicating the potential for impacts on regeneration processes. At 21 forest sites within oil-palm plantations, exotic species richness declined from oil palm (mean=9.2 species/transect) to forest edge (7.8 species) to inside rainforest remnants (3.1 species in disturbed forest), and only one species, Clidemia hirta, invaded intact forest. Exotic communities inside forest remnants were functionally distinct from those found in oil- palm, (generally taller, woody and dispersed by vertebrates), providing evidence that trait-filtering is occurring. Herbivory in the most common invader, C. hirta, reduced the reproductive output of C. hirta plants and was found to be higher when co-occurring with a native confamiliars, suggesting host-sharing is occurring between C. hirta and related natives. Nonetheless, C. hirta plants experienced overall lower herbivory and more individuals were reproductively active, possibly placing C. hirta at a competitive advantage relative to native confamiliars. This research shows that rainforest remnants within highly fragmented landscapes experience low invasion if local forest disturbance is low, but that C. hirta is almost always present and so other exotic plant species with similar traits to C. hirta may readily invade these forests in future. Forest fragmentation is predicted to increase, but improving canopy cover may provide protection from exotic plant invasion.