Termite diversity across an anthropogenic disturbance gradient in the humid forest zone of West Africa

Abstract

Data are presented for termite assemblages across an anthropogenic disturbance gradient in the humid forest zone of West and Central Africa. Sampling was by standardised $\text{100}\text{m}\text{×2}$ m transects in: primary forest, several ages of regenerating forest, agroforestry plots, short fallows, mixed food crop fields, and mechanically cleared plots. Most sites were in southern Cameroon—two additional transects were conducted in primary forest in Congo (Brazzaville). Species richness was negatively correlated with the disturbance gradient, although transects in areas with a complete or near-complete canopy were broadly similar in species richness. The strongest negative correlation was found for termite species feeding furthest down the humification gradient (group IV soil feeders) probably because these species are energetically constrained from living in non-forest habitats. In contrast species feeding and nesting in dead wood (group I wood feeders) termites, which live in the most protected micro-habitats, were significantly positively correlated with the disturbance gradient. Species composition was also strongly correlated with disturbance level in multivariate redundancy analyses. Pest species were an insignificant element of the assemblage, in part because of the distance of the sites from savanna source pools. Overall, it appears that termite-mediated wood and litter decomposition may be relatively unaffected by light to moderate disturbance intensity, but that soil-conditioning by termites might be greatly reduced in agricultural land cleared from tropical forest.

Introduction

The humid forest zone of Africa stretches from Guinea in the west to Uganda in the east. It is formed of two large contiguous areas of continuous forest (the Guinea and Congo forest blocks), at various stages of anthropogenic degradation. The original forest systems were drier and poorer in tree species than Southeast Asian and South American wet tropical forests, and are on characteristically poor acidic soils.

The present forest is a mosaic of different land uses: patches of secondary forest and fallow, crop fields, some tree plantations and agroforestry and significant remnants of primary vegetation. The dominant soils are acidic (oxisols and ultisols derived from low activity clays), commonly exhibiting Al toxicity, low cation-exchange capacity, low base saturation and low P availability. The traditional food-production systems are those of shifting cultivation (slash and burn), with plantain, cocoyam, maize, cassava and groundnut as the principal staples, the last two of those having relatively low soil fertility requirements. Management schemes and agricultural rotations are variable, but cleared forest, first year or exhausted crop fields can all be short-fallowed for 2–4 years after productive use by the woody perennial weed Chromolaena odorata before recropping. Oil palm and cocoa may be introduced at a later stage, or planted directly after forest clearance. There is usually a progressive decline in the fertility and yield of foodcrop fields to about 5–6 years post-clearance, after which a long fallow of 10 or more years is desirable, returning land to secondary forest. Long fallows of as little as 4 years are increasingly common, however, reflecting socioeconomic changes, including population growth and declines in urban employment opportunities.

The fauna is increasingly recognised as a major influence on tropical soil fertility. In African forest biomes these are pre-eminently termites (Eggleton, 2000, Bignell and Eggleton, 2000. In particular, soil-inhabiting (especially soil-feeding) termites are thought to be important for the distribution, protection and stabilisation of organic matter, the genesis of soil micro-aggregates and porosity, humification, the release of immobilised N and P, the improvement of drainage and aeration, and an increase in exchangeable cations (Lavelle et al., 1997). Therefore, changes in the species richness, composition and functional characteristics of termite assemblages are likely to have major influences on soil properties. Termite diversity is known to decline where forest canopy cover is reduced or lost completely, soil-feeding termites being affected most severely (Eggleton et al., 1996, Dibog et al., 1999). This loss of diversity is examined here across a wider range of habitat/treatment types than have been previously examined for the humid forest zone, with an emphasis on functional characteristics.

The work described here forms part of the ‘Alternatives to Slash and Burn’ (ASB) Programme. The ASB is a global, system-wide programme designed to identify optimal schemes for tropical forest-based subsistence agriculture which are consistent with alleviating poverty, providing increased food security, enhancing environmental resilience and conserving biodiversity (Kenyatta, 1997). The general ASB objective is to answer the question: what is the effect of land use change on biodiversity and, what are the implications for ecosystem services and resilience, and for agricultural productivity?