The effects of forest fragmentation on ecological interactions and particularly on food webs have scarcely been analysed. There is usually less herbivory in forest fragments than in continuous forests. Here we hypothesize that forest fragmentation enhances top-down control of herbivory through an increase in insectivorous birds and a decrease in herbivorous insects, with a consequent decrease in plant reproductive success in small forest fragments. In the Maulino forest in central Chile, we experimentally excluded birds from Aristotelia chilensis (Elaeocarpaceae) trees in both forest fragments and continuous forest, and analysed herbivore insect abundance, herbivory and plant reproductive success during two consecutive growing seasons. We expected that insect abundance and herbivory would increase, and reproductive success would decrease in A. chilensis from which birds have been excluded, particularly in forest fragments where bird abundance and predation pressure on insects is higher. The abundance of herbivorous insects was lower in the forest fragments than in the continuous forest only in the first season, and herbivory was lower in forest fragments than in the continuous forest throughout the study. Moreover, during the second growing season herbivory was greater in the excluded trees than in the control trees, and as expected, there was a greater difference in the fragments than in the continuous forest, but this was not statistically significant. Exclusion of birds did not affect the reproductive success of A. chilensis. Our results, after 2 years of study, demonstrate that birds affect the levels of herbivory on A. chilensis in the Maulino forest, but do not support our hypothesis of enhanced top-down control in fragmented forests, as the strength of the effect of excluding birds did not vary with fragmentation.
SummaryFrom March to July 2000, four seabird colonies in coastal ecosystems of Sinaloa, northwest Mexico were visited: on islets, a sandy beach and a long-abandoned salt-panning flat. There were partial and total breeding failures, most of them due to human activity. Amongst colonies of nine species, the breeding success of three was severely affected. All 250 Royal Tern Sterna maxima eggs were taken for direct consumption; 50% (75) of Bluefooted Booby Sula nebouxii chicks were killed for crab (Portunidae) fishing; and in a Least Tern Sterna antillarum colony (97 pairs), 50% failed due to disturbance from a fishing championship. Some of these species have priority status within Mexican and international conservation regulations. Certain aspects of these problems are discussed, and actions are suggested to balance conservation and the development of economic activities.
Composición, riqueza de especies y abundancia de insectos defoliadores de actividad nocturna asociados a Aristotelia chilensis (maqui) en el bosque maulino fragmentado Composition, species richness and abundance of nocturnal folivorous insects associated with Aristotelia chilensis (maqui) in the fragmented Maulino forest
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