Abstract
Pathogens are hypothesized to play an important role in the maintenance of tropical forest plant species richness. Notably, species richness may be promoted by incomplete filling of niche space due interactions of host populations with their pathogens. A potentially important group of pathogens are endophytic fungi, which asymptomatically colonize plants and are diverse and abundant in tropical ecosystems. Endophytes may alter competitive abilities of host individuals and improve host fitness under stress, but may also become pathogenic. Little is known of the impacts of endophytes on niche-space filling of their hosts.Here we evaluate how a widespread fungal endophyte infecting a common tropical palm influences its recruitment and survival in natural ecosystems, and whether this impact is modulated by the abiotic environment, potentially constraining host niche-space filling. Iriartea deltoidea dominates many wet lowland Neotropical forests. Diplodia mutila is a common asymptomatic endophyte in mature plants; however, it causes disease in some seedlings. We investigated the effects of light availability on D. mutila disease expression.We found I. deltoidea seedlings to preferentially occur under shady conditions. Correspondingly, we also found that high light triggers endophyte pathogenicity, while low light favors endosymbiotic development, constraining recruitment of endophyte-infested seedlings to shaded understory by reducing seedling survival in direct light. Pathogenicity of D. mutila under high light is proposed to result from light-induced production of H2O2 by the fungus, triggering hypersensitivity, cell death, and tissue necrosis in the palm. This is the first study to demonstrate that endophytes respond to abiotic factors to influence plant distributions in natural ecosystems; and the first to identify light as a factor influencing where an endophyte is placed on the endosymbiont–pathogen continuum. Our findings show that pathogens can indeed constrain niche-space filling of otherwise successful tropical plant species, providing unoccupied niche space for other species.
Highlights
Pathogens and other natural enemies have long been hypothesized to play an important role in the local maintenance of species diversity, especially in species-rich tropical forests [1,2,3]
We investigate if the widespread endophytic fungal pathogen D. mutila influences survival and recruitment patterns of its host plant the common tropical palm, I. deltoidea in natural ecosystems and whether this impact is modulated by the abiotic environment, potentially affecting host niche-space filling
We found that D. mutila is beneficial to I. deltoidea under understory conditions, but strongly reduces I. deltoidea’s capacity for recruitment in high-light forest gaps
Summary
Pathogens and other natural enemies have long been hypothesized to play an important role in the local maintenance of species diversity, especially in species-rich tropical forests [1,2,3]. A potentially important group of pathogens are endophytic fungi, which asymptomatically colonize plants [8] and are diverse and abundant in tropical ecosystems [9]. The variable virulence of the endophyte, the host defense response, and environmental conditions constitute the disease triangle [11], ‘or disease/mutualism triangle’, and could be influencing the where an endophyte is placed on the endosymbiont-pathogen continuum. None have looked for or identified environmental factors that alter the behavior of endophytes in natural ecosystems, their relationships to hosts, and the ecological implications for the niche-space filling (spatial and environmental patterns of distribution and abundance) of host plant species
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