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 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.

Soil-borne pathogens restrict the recruitment of a subtropical tree: a distance-dependent effect

“…The inclusion of a large number of naturally occurring seedlings provides the opportunity to examine both distanceand density-dependent sources of seedling mortality caused by soil-borne pathogens. Two previous experiments that explicitly manipulated naturally occurring seedling density and pathogen abundance in tropical forests reported mixed results (Bell et al 2006;Norghauer et al 2010a). Bell et al (2006) demonstrated that pathogens were a key determinant of density-dependent survival in emerging Sebastiana longicuspis seedlings, whereas Norghauer et al (2010a) did not detect pathogen-induced mortality of post-establishment seedlings of two tree species.…”

“…Two previous experiments that explicitly manipulated naturally occurring seedling density and pathogen abundance in tropical forests reported mixed results (Bell et al 2006;Norghauer et al 2010a). Bell et al (2006) demonstrated that pathogens were a key determinant of density-dependent survival in emerging Sebastiana longicuspis seedlings, whereas Norghauer et al (2010a) did not detect pathogen-induced mortality of post-establishment seedlings of two tree species. Our natural field experiments suggested that fungal pathogens did accelerate the mortality of seedlings in a subtropical forest but that the effect of pathogens on mortality is independent of seedling density.…”

“…However, the emergence of a distance dependence induced by pathogens negates these possibilities, as pathogens did decrease seedling survival. A second possibility is that soil-borne pathogens caused pre-emergence mortality, whereby the low-density plots might have already suffered from high disease mortality (Norghauer et al 2010a;Reinhart and Clay 2009). Through this process, post-emergence density-dependent mortality might not be detected.…”

“…Norghauer et al (2010a) provided several explanations for the absence of a seedling density dependence, such as an insufficiently high host density for virulence and habitat characteristics selecting for strong plant resistance to pathogens. However, the emergence of a distance dependence induced by pathogens negates these possibilities, as pathogens did decrease seedling survival.…”

“…A few studies have utilized naturally geminated seedlings to test density or distance dependence in the field (Augspurger 1983a;Bell et al 2006;Packer and Clay 2000;Pigot and Leather 2008;Reinhart and Clay 2009). However, apart from a small number of studies (Norghauer et al 2010a;Packer and Clay 2000;Pigot and Leather 2008;Reinhart and Clay 2009), the overwhelming majority of the previous investigations on the J-C hypothesis have either not intended to or have failed to explicitly distinguish between density and distance dependence.…”