Current ecological research
Because of palaeo-climatological peculiarities of the different continents, Danielsen (1997) postulates that the African forest birds might be much less sensitive to logging operations and/or forest fragmentation than the forest avifauna of South East Asia or South Central America. Danielsen’s (1997) conclusion has only been supported by very few African studies, which were difficult to compare due to different logging techniques and different census methods. However, quantitative studies in Ghana’s forest reserves (Holbech 1992, 1996) did at least not falsify Danielsen’s hypothesis. Species richness and diversity of forest avifauna seemed unaffected by logging per se, but more important in terms of species loss was fragmentation (Holbech 1996). Forest reserves in Ghana of less than 50 sq. km size did loose many forest species irrespective of logging. Holbech’s (1996) study was carried out on a large scale and, as many reserves were studied, it was based on relative abundance data (encounter and capture rates) and did not deal with particular management techniques. Relevant studies in Africa concerned with logging and bird faunas underline the necessity to focus on population data, bird behaviour, and population dynamics rather than to rely on species richness, diversity and relative abundance data only (e.g. Dranzoa 1998). Gartshore et al. (1995) similarly emphasises the importance of further ecological studies in this respect. Many forest bird species may be relatively common in secondary habitats such as tree plantations, but the role of these habitats for population dynamics remains largely unknown. Another reason, for impact assessments in forest reserves in Côte d’Ivoire is the overall condition of its Forêts Classées:
Whereas in Ghana’s forest reserves logging was moderate (1-2 trees/ha) and felling cycles were adapted to the regeneration potential of stands, there was no management attempting to obtain sustainable yields of trees in Côte d’Ivoire in the past (Sayer et al. 1992). Consequently, many Forêts Classées experienced heavy overexploitation.
As mentioned above, there is only a minor understanding of the consequences of the drastic recent ecological changes in the Upper Guinean region, and more information about the effects on biological diversity in this region is highly needed. Only in recent times, projects started to document the ecological status of forest reserves and to monitor long-term effects (e.g.
Mühlenberg et al. 1995, Hoppe-Dominik 1999, Mühlenberg et al. 1999, Waltert 1999). In order to involve the local communities directly in conservation issues and to guarantee a long-term supervision of the areas, the organisation of monitoring programs is increasingly community-based. In such programs the numbers of indicator species are limited and adequate methods are still to be developed (Waitkuwait 1992, Mühlenberg et al. 1995, Mühlenberg et al. 1999). Researchers are faced by many methodological problems concerning density estimation (e.g. in duiker censuses, see Struhsaker 1997, Nett 1999). Therefore, supplementary short-term studies are highly valuable in providing cross-reference data for comparison.
Objectives
This study has the following objectives:
x To document the structure of the vegetation mosaic in a logged-over forest and to assess the impact of forest management on its composition;
x To document the diversity, composition and structure of the forest bird community in a (heavily) logged-over forest after major deforestation in Côte d’Ivoire;
x To assess the impact of the forest management, especially the establishment of tree plantations and liberation thinning compartments on the rainforest birds;
x To provide baseline information, especially density estimates of forest birds for an evaluation of their conservation status and for cross-referencing with data from regular monitoring.
Hypotheses:
The principal hypotheses of this study were:
1. Bird species composition, diversity, and abundance structure in the heavily degraded Bossematié Forest in south-east Côte d’Ivoire should differ significantly from primary forests and from large, moderately disturbed forest reserves. It is, however, possible that some forest species can persist in the patchy mosaic of small, closed forest habitats (Johns 1996), but large species such as hornbills might need larger forest areas when disturbance has been high (Johns 1987, Kavanagh & Bamkin 1995, O’Brien & Kinnaird 1996). Due to high habitat heterogeneity created by heavy logging, overall species richness might still be high in degraded forest (Yahner 1988), but due to the extreme ecological changes, some forest specialists, especially understorey insectivores (Thiollay 1992) and species with restricted geographical ranges (Kattan 1992, Marsden 1998) might be extinct.
2. It has been shown that tropical lowland forest bird communities can be highly diverse and equitability in abundance between species can be extremely high (Terborgh et al. 1990, Thiollay 1994b). Abundance distribution in degraded forest, however, should follow log-series more than log-normal or broken-stick models (relationships between disturbance and abundance distribution e.g. in Johns 1992, Herremans 1995).
3. In an ecologically complex primary forest, spot-diversity (on a few hectares) is very high due to the year-round availability of major resources (Terborgh et al. 1990). In a degraded forest, this high alpha-diversity should be much reduced.
4. The bird community in the heavily logged forest should be dominated by geographically widespread species with low habitat specificity (Kattan 1992). Due to the predominance of productive vegetation layers near the ground, species with highest densities are found among understorey species. In general, less specialised species, e.g. with mixed diets, are expected to reach high densities.
5. In the Neotropics, obligate members of mixed foraging flocks are uncommon in logged forest (Thiollay 1992, 1994a). This should be the case also in obligately flocking African species in the Bossematié Forest. There, the most frequent members should be non-obligately flocking species. In contrast to undisturbed communities, associations between birds and other vertebrates (primates or larger mammals) should be a rare phenomenon.
6. Liberation thinning produces a shift in the vegetation mosaic of the heavily logged forest, creating more gaps and reducing mature forest patches by increased treefall rates. Forest management such as liberation thinning and plantation management affects species composition and abundance in the degraded forest. Species with restricted ranges, and specialised species such as large understorey insectvores should have higher abundances in less disturbed forest compartments.
7. In view of source-sink population theories (Pulliam et al. 1992, Dias 1996), low recapture rates of bird species can indicate differences in habitat quality between logged and unlogged forests, even when no apparent differences are found in relative abundances (Uganda, see Dranzoa 1998). Lower recapture rates in more disturbed areas of the study plot can can also be expected from capture-recapture data (Winker et al. 1995.).