Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar

Abstract

Herbivores and pathogens with acute host specificity may promote high tree diversity in tropical forests by causing distance- and density-dependent mortality of seedlings, but evidence is scarce. Although Lepidoptera larvae are the most abundant and host-specific guild of herbivores in these forests, their impact upon seedling distributions remains largely unknown. A Wrm test of the mechanism underpinning the Janzen–Connell hypothesis is dificult, even for a single tree species, because it requires more than just manipulating seeds and seedlings and recording their fates. Experimental tests require: (1) an insect herbivore that is identiWed and highly specialised, (2) linkage to an in situ measure (or prevention) of herbivory, and (3) evaluation and conWrmation among many conspecific adult trees across years. Here we present experimental evidence for a spatially explicit interaction between newly germinating seedlings of a Neotropical emergent tree, bigleaf mahogany (Swietenia macrophylla, Meliaceae), and caterpillars of a noctuid moth (Steniscadia poliophaea). In the understory of a southeastern Amazon forest, the proportion of attacks, leaf area lost, and seedling mortality due this specialised herbivore peaked near Swietenia trees, but declined signiWcantly with increasing distance from mature fruiting trees, as predicted by the Janzen–Connell hypothesis. We conclude that long-distance dispersal events (>50 m) provided an early survival advantage for Swietenia seedlings, and propose that the role of larval Lepidoptera as Janzen– Connell vectors may be underappreciated in tropical forests.

Citation

Norghauer, Julian M.; Grogan, James; Malcolm, Jay R.; Felfili, Jeanine M. 2010. Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar. Oecologia. 162: 405-412.