Avian frugivory and seed dispersal in some of the Taita hills forests fragments.
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This work was chiefly carried out in seven forest fragments in the Dabida massif of the Taita Hills -Ngangao,Chawia, Fururu, Ndiwenyi, Macha, Mwachora and Yale, between September 1997 and June 1998. Kasigau was also included for some analyses. The principal aim was to examine various aspects of the fleshy fruit-avian frugivore interface. Transects were laid in all fragments and used for bird and fruit censuses, and for assessment of regeneration. Timed watches were used to describe frugivore assemblages at selected trees. Data on fleshy fruits and avian frugivores were collected from transects in Dabida and grouped into sections (each comprising of 2-5 transects) for the analyses. Three rounds of data were collected in two distinct time periods-wet season (September to November) and dry season (January to March) In total, I identified 110 plants species (trees, shrubs and climbers) of which about 76percent were fleshy fruit- producers. Overall tree diversity was lower in the smaller fragments, which were also the most distrurbed. Fruit density varied spatially and temporally, peaking in January in Ngangao and in October- November in Chawia and the smaller fragments. Sections with the highest fruit densities fluctuated most. I recorded 14 avian frugivores, of which five (Cabanis's and stripe-cheeked Greenbuls, Hartlaub's Turaco, Taita Thrush and Taita white-eye) were relatively important in terms of density, distribution and frugivory levels. In general, frugivore densities were not linked to total fruit densities. During the period of lowest fruit abundance, but not at other times, significant explanatory variables for frugivore densities in a General Linear Model (GLM) included fragment size and tree diversity (corrected for fragment size): the relationship was positive in both cases. Frugivore numbers did not appear to track fruit supplies; density fluctuations were more likely connected with seasonal breeding behaviour, as the recorded densities of both frugivores and non-frugivores changed in a similar way, and showed declines during peaks in breeding activity. Using Mantel statistics I examined the frugivore of 58 individual trees belonging to II species growing in seven-forest fragments__six in Dabida plus Kasigau. Overall, there was little evidence of specialized frugivorous interactions. Site and fruit size significantly affected similarity of frugivore assemblages among conspecific and heterospecific tree species, respectively. Effect of location of tree could be attributed to forest fragments of different sizes varying in densities and composition of focal fruiting trees, and in the fruiting phenologies of these trees. Consequently, my findings indicate that at least one important processes of seed dispersal-fruit selection-may have been affected by habitat fragmentation. Lastly, I looked at the spatial patterns of young and adult individuals of mainly bird-dispersed plants (five tree and one climber species). Regenerating individuals (both of the trees and climber) occurred more on species, which were also fleshy fruiting, than expected from their relative densities in the habitat. This could be attributed, at least partly, to the foraging patterns of the avian frugivores, which suggests some potential to 'shape' their habitats. Secondly, I found little congruence between the spatial patterns of young and regenerating individuals of the five tree species, which was consistent for all sections. This could imply that the avian frugivores were unlikely to be highly efficient seed dispersers for these trees and that they had fairly comparable dispersal efficiencies. Lastly, pioneer species regenerated better in disturbed areas, whereas the non-pioneer species, since they do not require large gaps germination and establishment, were found to regenerate better in larger, less disturbed fragments.
- MST-Zoological Sciences