Trophic cascades in fragmented forests: birds as determinant of plant performance in the Maulino forest

Investigador responsable: Javier A. Simonetti
Coinvestigadores: Audrey A. Grez, Lohengrin A. Cavieres & Carlos E. Valdivia
Período: Marzo 2005 – Marzo 2009
Financiamiento: Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT 1050745

Land use is a significant threat to biodiversity world-wide. Forest loss and fragmentation might have far reaching consequences upon compositional, structural and functional biodiversity. Changes in species richness, population abundance and microclimate brought about by forest fragmentation can alter food-webs, impinging upon the dynamics of forest remnants. Despite its paramount importance, the effects of forest fragmentation upon ecological interactions, and particularly on food-webs have scarcey been addressed. Herbivory for instance, is usually lower in forst fragments than in continuos forest. Depressed herbivore populations is assumed to be the cause. Here, we hypothesize that low level of herbivory in forest fragments is the outcome of altered trophic cascades through changes in the abundance of predators with consequences at the plant individual, population and ecosystem level.

At the Maulino forest, central Chile, insectivorous birds are more abundant, herbivory and litterfall decomposition are lower in forest fragments compared to the continuous forest. This evidence strongly suggest cascading effects triggered by birds. On this regard, we aim to experimentally unravel if forest fragmentation can alter species-level cascades triggered by insectivorous birds. We expect plants to be indirectly benefited by fragmentation as the abundance of insectivorous birds is higher in forest fragments.

Focusing upon Aristotelia chilensis as our focal species, using natural and manipulative experiments, we plan to test the hypothesis of altered trophic cascades in a fragmented Maulino forest. Excluding birds from trees at both forest fragments and continuous forest, we will analyze the response of insects, herbivory levels, plant´s growth and reproduction, seedling recruitment and litterfall decomposition in bird-excluded and control trees. We expect that a) if insectivorous birds determine insect abundance, they should increase in A. chilensis from which birds have been excluded, particularly in forest fragments where bird abundance and pressure upon insects, is higher; b) leaf area lost to herbivorous insects should increase in A. chilensis from which birds have been excluded, particularly in forest fragments; c) at the plant individual level, growth and reproduction, should be lower in A. chilensis from which birds have been excluded, particularly in forest fragments; d) alternatively, if compensation mechanisms occur in A. chilensis, photosynthetic rate should be higher in trees from which birds have been excluded; e) at the plant population level, the density of seedlings should be lower under A. chilensis from which birds have been excluded as those trees sustain higher levels of herbivory, particularly in forest fragments; finally, f) at the ecosystem level, as herbivory enhances decomposition, litterfall generated by A. chilensis from which birds have been excluded should decompose slower than controls, particularly in forest fragments than the continuous forest.
The experimental analysis of altered trophic cascades in fragmented forests will help to unravel the role of disturbance upon food-web dynamics and offer strong guidelines for managing biodiversity in increasingly fragmented forests.