How to invade an intact tropical rainforest: A case study of Psidium cattleianum

David Tng, School for Field Studies, Yungaburra, Australia and Susan G. Laurance, Centre for Tropical Environmental and Sustainability Science (TESS) and College of Science and Engineering, James Cook University, Cairns, Australia

Background/Question/Methods
The invasion of native habitats by exotic plants is a global phenomenon that threatens the integrity of ecosystems both in and out of protected areas. Disturbance is a major mechanism in the establishment of exotic plants and is a reliable predictor of an ecosystems invasibility. Intact tropical rainforests are rarely invaded by exotic species for a number of reasons but mainly because the environmental conditions (ie low light & low wind) do not support the establishment of plant species with pioneer plant characteristics. So what does it take to successfully invade a tropical forest? We compared the functional traits of the rainforest treelet Psidium cattleianum (Family: Myrtaceae) with equivalent native species and examined the environmental conditions of their current distribution.
Results/Conclusions
Like most tropical woody plants P. cattleianum is vertebrate dispersed, yet unlike native understorey species Psidium reproduces from both seed and clonally. These reproduction strategies allow the species to establish in the full range of forest light environments. The dense thickets formed by the multi-stemmed individuals of this species is a feature rarely found in other understorey species. Structurally, there are similarities with Psidium to the low-statured rainforest species that occur on mountain tops; with multi-stemmed individuals of relative high wood density and thick long-lived leaves. Yet it’s the propagule production and its attractiveness to frugivores that ensures this species effective dispersal. Seed dispersal is facilitated by a number of fruiting traits which makes it highly attractive to native or non-native frugivores, including a relatively large dependable crop of small seeded edible fruit, of a size which is suitable to generalist frugivores. Finally, much has been written about invasive plants escaping their predators and parasites in novel environments, yet an interesting scenario now exists in the Australian Wet Tropics where the invasive plant P. cattleianum and its parasitic Myrtle rust (Austropuccinia psidii) now occur. Yet Myrtle rust appears to be having a greater impact on the growth and seedling production of the native rainforest shrubs Rhodamnia spp. and Rhodamyrtus spp.--which would be the competitors of P. cattleianum, than on the invasive plant.