Emily H. Waddell1,2,3, Lindsay F. Banin1, Colin J. McClean4, Susannah Fleiss2, Jane K. Hill2, Mark Hughes3, Ahmad Jelling5, Yeong Kok Loong6, Bernadus Bala Ola5, Azlin Bin Sailim5, Joseph Tangah7 and Daniel S. Chapman1,8
1Centre for Ecology and Hydrology, Bush Estate, Penicuik, EH26 0QB, UK
2Department of Biology, University of York, Wentworth Way, York, YO10 5DD, UK
3Royal Botanic Garden Edinburgh, 20A Inverleith Row, Edinburgh, EH3 5LR, UK
4Department of Environment and Geography, University of York, York YO10 5DD, UK
5The South East Asia Rainforest Research Partnership, Danum Valley Field Centre, PO Box 60282, 91112, Lahad Datu, Sabah, Malaysia
6Leverhulme Centre for Climate Change Mitigation, University of Sheffield, Sheffield, S10 2TN, UK
7Forest Research Centre, Sabah Forestry Department, P.O. Box 1407, 90715 Sandakan, Sabah, Malaysia
8Faculty of Natural Sciences, University of Stirling, Stirling, FK9 4LA, UK
Fragmentation and degradation of tropical forests reduce the native plant diversity in remnant forest patches, but consequences for invasion by non-native species are less clear. Occurrence of exotic plants may be greater in forests with increased edge habitat and disturbance, but biotic interactions with native plant communities may also be influential. Here, we examine the relative importance of fragmentation, forest disturbance, soil characteristics and native plant diversity for invasion of rainforest patches within agricultural landscapes.
We surveyed native and exotic plant communities in 49 plots at 18 sites, spanning gradients in landscape-scale fragmentation (i.e. measures of the amount of forest and edge in the landscape) and local forest disturbance (i.e. number of large trees and mean wood density) in oil-palm dominated landscapes of Sabah, Malaysian Borneo. We used partial least squares path modelling to examine the influence of fragmentation, disturbance, soil characteristics (i.e. pH, available phosphorus) and native community diversity (i.e. genus richness and phylogenetic diversity) on exotic occurrence. We first ran models using the total native community and then with different native community subsets (i.e. adult trees, saplings, seedlings and understorey plants excluding seedlings).
We surveyed 7268 stems from 332 genera. These included eight exotic species (0-51% of stems/plot, mean=5%) representing shrubs, herbs, graminoids and climbers. The best model explaining exotic occurrence (R2=0.277) included a direct negative correlation with native tree sapling diversity and an indirect positive effect of local forest disturbance. We found no evidence for either landscape-scale fragmentation or soil characteristics significantly influencing exotic occurrence in any of the models.
We found that occurrence of exotic species is associated with forest disturbance, likely due to previous commercial selective logging and biotic interactions with native tree saplings. Evidence of a negative correlation between sapling diversity and exotics may indicate biotic resistance from saplings during regeneration after logging, or that dominant exotic species outcompete saplings and reduce their diversity. Our results show that the strength of interactions between native and exotic plants, and hence the potential impacts of plant invasions, varies according to the life stage of the native community. Thus, studies quantifying biotic interactions using the total native community may underestimate the role of biotic processes in mediating invasion.