Complex plant-pathogen interactions maintain tropical forest diversity
Pathogenic fungi account for significant mortality in tropical seedling communities and their host-specific interactions can influence plant community dynamics through negative feedbacks. The long-standing Janzen-Connell hypothesis suggests that specialized pests such as insects and pathogens maintain high plant diversity by preventing species dominance through their disproportionately negative effects on locally abundant plant species. While much is known of the identities and specificity of pathogens and pests in agriculture and forest management systems, the diversity and complexity of natural systems highlights the challenges involved with characterizing and identifying cryptic plant-fungal interactions. An experimental test of this hypothesis in the Chiquibul Maya Mountain Forest Reserve of Belize shows that fungal plant pathogens, but not insects, have a community-wide role in maintaining seedling diversity in this forest. Culturing microfungi from diseased leaf tissue and experimentally infecting healthy seedlings with fungal spores tested the virulence and specificity of strains of putatively pathogenic fungi within the seedling community.
Putative pathogens were identified from a diverse assemblage of seedling-infecting fungi. Inter-annual and spatial variation in seedling disease incidence and severity suggest important interactions between biotic (e.g., seeding density) and abiotic (e.g., soil moisture) variables. Genetic characterization (using nrITS gene region) of fungal taxa infecting a range of tropical seedling hosts highlights the predominance of low specificity of plant-associated fungi in species rich communities. Experimental infection of healthy seedlings of seven species with putative pathogens revealed broad host range or polyphagy of pathogens but species-specific virulence. Seedling hosts also varied in susceptibility, with certain species resistant to any infection and others experiencing 100% mortality when exposed to certain Colletotrichum sp. (Glomerellaceae) strains. The original assumption of specialized pests driving the negatively density dependent mortality thought to regulate plant populations does not seem to hold either for plant-pathogen interactions in tropical forests. Polyphagy in fungi is the more common strategy in species-rich communities with high numbers of locally rare species. Nevertheless, plant preferences of pests and the variation in plant responses to common pests appears to be sufficient to facilitate coexistence as described in the Janzen-Connell hypothesis. It is clear that native endemic diseases are a crucial component in maintaining biodiversity and ecosystem services, and further characterization of these complex interactions could shed light on potential emerging and invasive diseases.