Relationship of nematode communities to human demographics and environment in agricultural fields and fallow lands in Senegal

One hundred and nine soil samples were collected in 69 different localities along two transects, one North– South, about 900 km in length, and the other East–West (750 km in length), across Senegal and Gambia. The first transect followed a rainfall gradient and the second a human density gradient. The relationship between carbon content, C/N ratio and soil type on the abundance and species distribution of the nematodes along the transects was studied. Results showed that short-term fallows did not influence the specific structure of the communities, when compared with the nematode communities of fields located in the immediate vicinity, where Scutellonema cavenessi and Tylenchorhynchus gladiolatus were the dominant species. The expected negative influence of human disturbance on nematode occurrence seemed to be compensated by greater crop diversity, mainly near the towns. Less-disturbed areas also maintained a high diversity, but were characterized by the presence of particular species such as Xiphinema spp. Soil type was the most important factor affecting the species composition of the nematode community. As a result, nematode communities followed a distribution in areas, corresponding to the successive soil types, but did not change in relation to the human or climatic gradients. At a large scale, the study of plant-parasitic nematodes can give both different and complementary information on the ecological trends of an area to that of free-living nematodes.