Variable strength of top-down effects in Nothofagus forests: bird predation and insect herbivory during an ENSO event

MAZÍA, C.N.; CHANETON, E.J.; KITZBERGER, T.; GARIBALDI, L.A.

AUSTRAL ECOLOGY

BLACKWELL

Año: 2008

1442-9985

Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. Evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variability in the strength of top-down forces created by environmental heterogeneity. We examined the magnitude of predation effects on plant damage by leaf-chewing insects and mean leaf size, by excluding birds from saplings in ‘dry’ and ‘wet’ Nothofagus pumilio forests of the northern Patagonian Andes, Argentina. The experiment lasted two years encompassing a severe drought during La Niña phase of a strong ENSO event, which was followed by unusually high background folivory levels. Leaf-chewer damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased leaf damage in both forests but did not affect tree leaf size. In the ensuing season (2000), insect damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size. The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss in dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing herbivory than at facilitating leaf area growth. Our results indicate that bird-initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above-normal herbivory, and also support the view that large-scale climatic events influence the strength of trophic cascades.