Density-dependent shoot-borer herbivory increases the age of first reproduction and mortality of neotropical tree saplings

Abstract

Shoot herbivory by the sapling specialist shoot-borer Cromarcha stroudagnesia (Lepidoptera, Pyralidae, Chrysauginae) is shown to have large direct and indirect effects on the rates of height increment and mortality of saplings of its host tree, Tabebuia ochracea (Bignoniaceae), in the secondary successional tropical dry forests of the Area de Conservación Guanacaste, Costa Rica. Experiments and field observations over 3-4 years show a substantial reduction in sapling height increments due to C. stroudagnesia herbivory, of equivalent magnitude to the difference in height increments between undamaged saplings in canopy gaps and full understorey shade. Extrapolating this data at average amounts of C. stroudagnesia herbivory increases the duration of the pre-reproductive sapling life stage by about 40% relative to undamaged plants. This is an underestimate, as top shoot herbivory by C. stroudagnesia also increased the probability of canopy gap saplings being overtopped and shaded by surrounding vegetation. Sapling mortality was increased by C. stroudagnesia herbivory, with 11.8% of the most heavily damaged young saplings dying in 3 years while no undamaged saplings died. Cromarcha stroudagnesia herbivory strongly increases with the number of conspecific T. ochracea saplings and the number of conspecific shoots within 50 m of focal saplings. It is therefore likely to disproportionately decrease the number of saplings and rate of recruitment to reproductive age in areas of high conspecific sapling density. These results suggest that sapling herbivory, especially herbivory of terminal meristems, has an important but largely unexplored influence on the population dynamics of tropical tree species. They further demonstrate that sapling herbivory by insects, in addition to the well-studied insect predation and herbivory of seedlings, is likely to influence tree species coexistence in tropical forests.

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

“…While H. grandella is undoubtedly a pervasive pest in Neotropical plantations, to the best of our knowledge its distance-and/or density-dependent eVect on Swietenia growth and mortality in natural populations has not been reported [but see Sullivan (2003) for the tree Tabebuia ochracea (Bignoniaceae)]. The forestry literature regarding H. grandella is largely anecdotal and focused on plantation settings (e.g.…”

Beneficial effect of spider presence on seedling recruitment of the tropical rainforest tree Dipteryx oleifera (Fabaceae)

“…Foliar herbivory damage can negatively affect seedling (Coley 1983, Dirzo 1987, Coley 1990, and can be severe enough to determine seedling fate (Sullivan 2003). Foliar insect herbivory accounts for about 10% of the leaf damage in tropical tree species (Dirzo 1987) and the majority of herbivory is by Lepidopteran larvae (Dirzo 1987, Coley 1990, Coley & Barone 1996.…”

Ecología de dispersión y reclutamiento de bosques húmedos tropicales

“…La mayoría de este daño es causado por larvas de mariposas (Coley y Barone 1996;Coley 1990;Dirzo 1987). Este tipo de daño puede afectar negativamente la demografía de plántulas (Coley 1983;Coley 1990;Dirzo 1987) y puede ser lo suficientemente severa para influenciar la dinámica de demografía de plántulas (Sullivan 2003). Las hipótesis especificas que estamos probando son: la "hipótesis de reclutamiento de plántulas"; sí la distribución del reclutamiento de plántulas es mayor que la esperada al azar, entonces, la distribución del reclutamiento es agrupado.…”