Demography, Reproduction, Diet and Crop Depredation in Three Species of Rodents in Natural and Irrigated Fields in Nigeria

Rodent populations of Arvicanthis, Mastomys and Tatera were studied during July 1990 to August 1992 in both natural and irrigated savannah fields of northern Nigeria. Tatera was entirely absent from the irrigated fields. Irrigation enhanced higher rodent population densities in the magnitude of three to four times over densities in the natural field. Arvicanthis was the most abundant of the three species in both sites. Tatera was the least common in the natural site. Adult survival rates in Arvicanthis and Tatera were significantly higher than Mastomys. Survival rates of both adult and young differed between the wet and dry seasons, being lower during the peak rainy months of July-August. Monthly recruitment in the irrigated fields was nearly four-fold higher than that in the natural fields. In the natural fields all species showed strict seasonal breeding in tune with the rainy months (May-October), with Arvicanthis starting at the same time as the onset of the rains but a month or two ahead of Mastomys and Tatera. In contrast to reproduction in the natural fields, the breeding seasons in Arvicanthis and Mastomys in the irrigated fields were continuous. Seeds formed an important food group for all species in both sites during the rainy season. Mastomys (in both sites) and Tatera (in the natural site) maintained almost the same levels of seed intake throughout the year. The amount of seed intake by Arvicanthis was enhanced by irrigation. In both sites during the rainy periods, monocots and insects made up the second and third most important food groups by mass of the diet intake of the species. The damage and yield loss caused by rodents in cereal crops in irrigated savannah fields was investigated. The high level of yield loss in wheat, compared to low and moderate levels in rice (grown for the most part during the rains), was probably a result of the greater depredation rate on the wheat, grown by irrigation during the dry season when other rodent food sources were less abundant. By the absence of Tatera in the irrigated fields this study provides evidence that the responses of African rodents to irrigation differed among the species. This study also demonstrates the opportunistic tendency of the rodents to exploit new, highly productive habitats as seen in their continuous reproduction, higher population densities and greater depredation of seeds and cereals in the irrigated fields. Arvicanthis, Mastomys and Tatera stand indicted as crop pests in both rainfed and irrigated agriculture