Résumé :
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Tropical rain forests are regarded to be the most species-rich and one of the most complex terrestrial ecosystems on the earth. Many species show a high degree of specialization and dependence to other organisms in the same ecosystem. Species which live in tropical rain forests are particularly vulnerable to and threatened by deforestation and intensive agricultural activity. The bark and ambrosia beetle fauna (Coleoptera: Curculionidae, Scolytinae and Platypodinae) in wet tropical areas differs to some extent from the overall pattern because the most freequently encountered species are host generalists and because many species are more or less dependent upon disturbance to provide breeding material - dead wood. The focus of this thesis was to study the bark and ambrosia beetle fauna at La Selva Biological Station, a wet tropical lowland forest in Costa Rica. The following questions were central to study (1) Is the faunal composition the same in different forest types within the continuous forest? (2) Do the species colonize in unequal magnitude the different host trees? (3) Is there any relationship between host tree abundanceand occurrence of beetles? (4) If (3) is the case, can local adaptation be inferred to cause the pattern? and (5) Are the species, or populations of species, patchily distributed, and can patch size be deduced? In order to investigate these issues, log sections ('baits') from eight tree species were put with replicated sets of baits in five different forest types; two primary forests, one secondary forest, one abandoned, now regrowing cacao plantation and one polyculture plantation. The baits were colonized by a large number of wood-boring beetles, and their offspring were studied and analyzed. Even though most species were found in all forest types as well as in almost all host trees, substantial differences related to host choice, colonization and spatial distribution were detected. The regenerating logged forests (secondary forest and abandoned plantation) had the highest number of species. One primary forest site and the polyculture plantation) had the fewest species. These results must be seen in relation to how the collections were made. The baits do not cover a particularly wide array of habitats, as many species are preadapted to breed in large-sized material, others reproduce in smaller diameter material such as vines, twigs, leaf petioles, seeds and bamboo. Nevertheless, the baits collected almost 25% of the species we now know from La Selva. The results show that baits of the most common tree species in the areas also were colonized with highest numbers. Moreover, the results suggest that many bark and ambrosia beetles really are host generalists, but suggest also that they have the potential to adapt locally. The detected patterns indicate further that populations are patchily distributed. In some species, a significant relationship between position of baits and beetle offspring abundance was found, reflecting that occurrence in some baits influenced occurrence in neighboring baits. The dissertation enlightens thus some aspects of the existing documentation on host selection patterns and istribution in tropical bark and ambrosia beetles, but builds further on known information to include studies on a smaller scale. The results uncover patterns that provide material for a better understanding of the ecology of tropical bark and ambrosia beetles, and may form the fundament for further studies.
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