The Influence of Fragment Size on Biotic Interactions that structure Plant Communities in the Asian Tropics

Abstract

Diverse rainforests in the tropics are being rapidly modified and fragmented for human use. Plants that persist in remnant forest patches are threatened by many biotic and abiotic changes that are associated with fragmentation. Several of these fragments consequently contain fewer plant species than they once did but, in the absence of contiguous forest, remain the only strongholds of plant and animal diversity in many parts of the world. However, the future of diversity in forest fragments may be uncertain as fragmentation can influence plant-animal interactions that shape plant communities. Some of these changes may already be apparent in the youngest life stages of fragmented plant communities, which would then form the template for the future of those fragments. By examining the compositions of differently aged plant communities in forest fragments, we may be able to identify interactions that are especially influenced by fragmentation, as various plant-animal interactions play structuring roles during different stages of a plant’s life cycle. In this thesis, we primarily investigate relationships between fragment area and processes that structure plant communities in a tropical forest. In the first data chapter (the second chapter of this thesis), we examine the compositions of four life stages of woody plants across a gradient of fragment size. We show that plant species are primarily affected by fragment size during their transitions from seeds to saplings. We discuss several possible explanations for the observed patterns but suggest that altered interactions between plants and their natural enemies (insects and fungal pathogens) may be particularly important drivers. Such plant-enemy interactions have the potential to maintain plant diversity by causing the negative density-dependent mortality of locally abundant plant species, and by allowing the persistence of locally rare species (the Janzen-Connell Hypothesis). As the modification of diversity-maintaining interactions can have catastrophic long-term consequences for the plant diversity in small fragments, it was important to investigate whether plant-enemy interactions are indeed sensitive to fragment size. In the second data chapter (the third chapter of this thesis), we investigate the relationship between fragment area and (soil-borne) fungus-induced mortality of six woody plant species in a shadehouse experiment. We present evidence that the pathogenic effects of fungi on one plant species increased with increasing fragment size. Although we show that plant fungus-interactions can be influenced by fragment size, further experiments are required to investigate whether such effects (even when spread across the community) can influence the diversity maintained in a forest fragment. In the third data chapter (the fourth chapter of this thesis), we first examine the roles of insects and fungi in maintaining woody plant seedling diversity in an Indian rainforest. We then investigate whether the diversity-maintaining abilities of these natural enemies are influenced by fragment size. We present evidence that insects play important roles in maintaining gamma diversity primarily by suppressing common plant species independent of density. We show that they maintain more diversity in large fragments than in smaller fragments, as smaller fragments were dominated by insect-resistant species. We present evidence that fungi caused the density-dependent mortality of one plant species but infer the presence of more such interactions in the community. We show that fungi may be sensitive to fragment size, and that fungi maintain more beta diversity of woody plant seedlings in large fragments than in small fragments. In conclusion, I show in my thesis that insects and fungi play important roles in maintaining woody plant seedling diversity. I further show that these essential plant-enemy interactions are predictably influenced by the size of a forest fragment, and that the plant diversity in small fragments may consequently be at risk from the breakdown of such interactions.