Invertebrate diversity and the ecological role of decomposer assemblages in natural and plantation forests in Southern Benin

The aim of this thesis was to investigate the diversity of invertebrates in natural semi-deciduous forest and plantation forests of the Lama forest reserve in Benin. Litter decomposi-tion, being a key ecosystem process, was studied, and the importance of decomposer assem-blages was investigated. An inventory of termite assemblages in semi-deciduous forest and teak plantations was conducted, and the effects of different forest types and seasons on the activity of termites and soil- and litter-dwelling invertebrates investigated. Furthermore, a new diplopod species, Stemmiulus lama n. sp. (Diplopoda: Stemmiulidae), the first record of Stemmiulidae for Benin, was described. Finally the diversity of true bugs (Heteroptera) in different habitats of the Lama forest was investigated.
Leaf litter decomposition is influenced by litter quality, climatic factors and soil biota. In Chapter 2 (“Leaf litter breakdown in natural and plantation forests of the Lama forest reserve in Benin”), we used the litterbag technique to examine the breakdown of leaf litter from two indigenous (Afzelia africana and Ceiba pentandra) and two exotic tree species (Tectona grandis and Senna siamea), and investigated the relationship between litter breakdown and relative abundance of litter-dwelling invertebrates. The study focused on semi-deciduous for-est, teak plantations and firewood plantations. We showed that litter species and forest type had significant effects on litter breakdown. We found that decay rates were highest for Afzelia africana in natural forest (k = 4.7) and lowest for Tectona grandis in firewood plantations (k = 1.3). We also found a significant litter × forest interaction, indicating dissimilar changes in litter breakdown across forest types. We observed higher frequencies of occurrence of in-vertebrates in indigenous than in exotic litter, and litter bags in natural forest attracted more invertebrates than those in plantations.
Our results indicate that litter breakdown is strongly influenced by litter type, forest type and the activity of litter-dwelling invertebrates. Therefore, management practices should aim to enhance biological activity of decomposer communities to avoid soil degradation and main-tain productivity.
Conversion of natural semi-deciduous forest to teak plantations may influence termite species richness and composition. In Chapter 3 (“Termite assemblages in a West-African semi-deciduous forest and teak plantations”), we used a modified standardised transect method to establish the first termite inventory in the Lama forest reserve. Overall termite diversity turned out to be surprisingly low (19 species), irrespective of forest type. This was due to the soil conditions which were unfavourable for soil feeders, the most species-rich termite group in African forests. Nevertheless, termite species richness was significantly higher in natural forest than in teak plantations. Termite assemblages were characterised by Kalotermitidae in natural forest, whereas fungus-growers (Macrotermitinae) dominated in teak plantations. Termite relative abundance (= encounter density) was higher in teak plantations than in natu-ral forest. The difference in termite assemblages was due to differences in two environmental variables, litter biomass and soil water content.
Forest type and season can influence the activity of termites and soil invertebrates. In Chap-ter 4 (“Activity of termites and other epigeal and hypogeal invertebrates in natural semi-deciduous forest and plantation forests in Benin”), we used a cardboard baiting method to monitor the activity of termites and soil and litter-dwelling invertebrates in semi-deciduous and plantation forest. The overall frequency of occurrence of invertebrates was highest in semi-deciduous forest, followed by firewood plantations, young teak and old teak plantations. Collembola, Isopoda, Isoptera, Diplopoda, Araneae and Hymenoptera (ants) were the most common soil invertebrates. We found that the activity of the most abundant taxa varied among forest types (except for Diplopoda and Araneae), with a higher activity in natural for-est. We observed a significant effect of season on the frequency of occurrence of soil- and litter-dwelling invertebrates, the lowest value being recorded during the long dry season. The frequency of occurrence of termites was higher in old teak plantations than in the other for-ests, but only one species, Microtermes? pusillus?, showed a significant difference.
Many invertebrate species occurring in tropical ecosystems are unknown to science. In Chap-ter 5 (“Stemmiulus (Diopsiulus) lama n. sp., a new millipede from Benin (Myriapoda, Diplo-poda, Stemmiulidae)”), we describe a new species, Stemmiulus lama n. sp., from the Lama forest. This species is the first record of a stemmiulid millipede in Benin.
Forest use may influence the diversity and community structure of true bugs (Heteroptera).
In Chapter 6 (“Diversity of true bugs (Heteroptera) in various habitats of the Lama forest reserve in southern Benin”), we compare Heteroptera assemblages in relation to forest use in different habitats, including natural forests, degraded forest, plantations and isolated forest fragments. We sampled 893 Heteroptera over a 12-month period, representing 104 species in 16 families. We found no significant effect of habitat type on species richness and evenness. However significant differences in abundance, Shannon-Wiener diversity and Berger-Parker dominance were found. In isolated forest fragments, Shannon-Wiener diversity was signifi-cantly higher than in lowland forest, and the abundance of Heteroptera was higher in lowland forest than in young teak plantations. We also found that the Berger-Parker dominance index was lower in isolated forest fragments than in lowland forest. Species richness and Shannon-Wiener diversity were significantly higher, and Berger-Parker dominance lower in disturbed than in undisturbed forest patches within the Noyau central, a now fully protected part of the reserve. We identified two indicator species for two forest types, one for lowland forest (Stenocoris southwoodi Ahmad) and one for dry forest (Lygaeidae sp.11). We found that Het-eroptera community diversity was mainly a function of habitat age, while other habitat char-acteristics had no influence on the diversity of true bugs.
To sum up, the present thesis provides baseline data on the diversity of invertebrates in the Lama forest reserve in Benin and gives ample evidence of the ecological significance of de-composer assemblages in natural as well as plantation forests. It shows that management prac-tices should aim to enhance decomposer communities in order to safeguard the productivity and sustainable use of the Lama forest reserve.