Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar |
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Authors: | Julian M Norghauer James Grogan Jay R Malcolm Jeanine M Felfili |
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Institution: | 1. Faculty of Forestry, University of Toronto, Earth Sciences Building, 33 Willcocks Street, Toronto, ON, M5S 3B3, Canada 4. Institute of Plant Sciences, University of Bern, 21 Altenbergrain, 3013, Bern, Switzerland 2. Yale University School of Forestry and Environmental Studies, 360 Prospect Street, New Haven, CT, 06511, USA 3. Departamento de Engenharia Florestal, Universidade de Brasília, CP 04357, Brasília, DF, 70919-970, Brazil
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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 firm
test of the mechanism underpinning the Janzen–Connell hypothesis is difficult, 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 identified and highly specialised, (2) linkage to an in situ measure (or prevention) of herbivory, and (3)
evaluation and confirmation 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, big-leaf 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
to this specialised herbivore peaked near Swietenia trees, but declined significantly 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. |
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