The role of pathogens in determining plant recruitment and distribution patterns in a western Amazonian floodplain

The main objective of this dissertation was to investigate plant host-pathogen dynamics and evaluate the Janzen and Connell (J-C) hypothesis explaining tropical ecosystem mechanism of diversity maintenance. The first chapter of this dissertation explores the influence of distance from fruiting trees and plant density on fungal disease incidence, insect damage and subsequent mortality of conspecific plants (J-C distance effect). I present novel data about plant pathogens, disease mechanisms, herbivores and host-pathogen interactions for one of the most common plant species of western Amazonia, Iriartea deltoidea. I found that insect herbivores are located in the vicinity of fruiting trees causing high mortality of conspecific seedlings as predicted by the J-C hypothesis. Surprisingly, the J-C distance pattern is not observed for lethal fungal pathogens such as Diplodia mutila.;The second chapter evaluates the nature and infection mechanisms of one of the most lethal pathogens found in I. deltoidea seedlings: Diplodia mutila. This fungus is ubiquitous and a generalist pathogen, causing disease and mortality in several host plants from different families. This characteristic could partially explain why I. deltoidea seedlings did not follow a J-C distance pattern. The potential implications of ubiquitous and pathogenic-endophytic fungi effects in tropical ecosystems are discussed. Endophyte-pathogens, hosts, herbivores and environmental conditions interact with each other, determining disease expression or repression.;The third chapter evaluates how environmental conditions, such as light availability, triggers disease expression and potentially defines plant distribution in tropical ecosystems. The ubiquitous and endophytic nature of many fungal pathogens also influences plant recruitment of dispersed propagules.;The fourth chapter examines the fate of dispersed seeds and seedlings in tropical ecosystems. Endophytic fungal pathogens could limit germination of dispersed seeds. Seedling mortality is high when dispersion is spatially and temporally aggregated. However seedling mortality is low when seedlings are randomly dispersed in the forest floor. Seedling mortality of dispersed propagules is produced by the synergistic effect of insect herbivores, fungal pathogens and environmental conditions. I conclude that pathogens in tropical ecosystems are not just agents of mortality or disease, but also organisms that influence survival and recruitment patterns of plant species.