Effects of Cropping and Tree Density

on Earthworm Species Diversity in Southern Cameroon

Lindsey Norgrove*, Csaba Csuzdi, Stefan Hauser

University of Hohenheim project, IITA Cameroon, c/o Lambourn, 26 Dingwall Rd., Croydon CR9 3EE, UK

Abstract:

Earthworms can have positive effects upon crop growth in the tropics. So if these soils are to be managed so that their biological capacity for nutrient cycling and maintenance of soil structure is retained then more attention should be paid to the effect of cultivation and cropping practices upon earthworms. When trees are removed from land-use systems, such as when land is cleared, slashed, burned and tilled in slash and burn agriculture, earthworm density, diversity and activity are reduced. Conversely retaining trees in systems may maintain earthworm populations during the cropping phase.

Here we assessed the impact on earthworm species diversity and densities of crop cultivation in the understorey of timber plantations thinned to two tree densities and compared this with uncropped, undisturbed timber plantation controls. The plots were reassessed after the end of the cropping phase to assess whether populations had recovered. The experiment was in southern Cameroon. Sampling was conducted, using the formol explusion method in a circular frame of 50cm diameter. The experiment was a randomised complete block design in four replicates.

Twenty-three earthworm species were recorded from Eudrilidae sub families Eudrilinae and Parendrilinae, Ocnerodrilidae and Octochaetidae, most of which were endemics. This included two new species from two new genera from the sub family Parendrilinae, one new species from one new genus of Ocnerodrilidae, two new species of Dichogaster and one new species of Legonodrilus. Twelve species were endogeic, ten epigeic and one anecic.

We found no significant differences in earthworm diversity and densities between the different tree density treatments. Gernerally earthworm densities were decreased by cropping compared to the undisturbed plantation control. The most abundant species was a Legonodrilus sp novum with average densities in the undisturbed control plantation of 68 individuals m-2, across sampling periods. Densities in cropped plots  were reduced to 15% of the control densities, however, populations later recovered as densities during the fallow phase were not significantly different from those in the undisturbed plantation. The densities of the epigeic Octochaetidae were significantly reduced from 42 m^-2 to 12 m^-2 to 20% of the densities of the undisturbed plots and did not recover during the fallow phase.  Two Dichogaster species, Dichogaster kunguluensis and Dichogaster sp. novum were eliminated by cropping and populations did not recover during the fallow phase. The effects of cropping were species-specific and more work is required to identify which of these endemics are the ecosystem engineers in the system.