Population Dynamics and Habitat Use by Murid Rodents in Maize Cropping Systems in Rongai Division, Nakuru District, Kenya
Murid rodents are an important pest group in eastern African agriculture by causing crop depredation, and as reservoirs for epizoonotic infections. On a global scale, they also pose some of the most complex management problems leading to huge capital expenditures towards their control. Gaps in our knowledge of how to plan for effective control need ecological theory and field data to provide vital ingredients in the design of sustainable management strategies. Efforts to obtain such information have gathered momentum especially in southeast Asia and Tanzania. Relatively less work has been done in Kenya and information lacking includes long-term ecological studies of pest species in cropping systems. This study, therefore, aimed at providing information towards bridging this gap. In order to achieve the above-mentioned goal, I set to investigate the demographic patterns and habitat use by murid rodents in maize cropping systems between May 2000 and December 2001. Capture Mark Recapture (CMR) technique was used on four permanent one-ha square grids established along an altitudinal gradient. Rodents were captured using Sherman's live traps placed at 10m spacing between trapping stations. Traps were baited with fried coconut cubes mixed with peanut butter and Elianto cooking oil. A total of 1240 captures including recaptures of 837 individuals were made of rodents and shrews in 20,100 trap nights. Fifty-three animals were found dead in the traps during handling. The rodents included six murid rat species (Aethomys cf kaiseri Noack 1887, Arvicanthis cf neumanni 1822, Lemniscomys striatus Linnaeus 1758, Mastomys cf erythroleucus Smith 1834, Rattus rattus and Mus minutoides Smith 1834) and a cricetid (Tatera cf. robusta Cretzschmar 1830) while the shrews were all soricids (Crocidura species). Mastomys was the dominant species within and across grids, accounting for 71.0 % of all captures. Other common rodents were Arvicanthis (14.1 %) and Mus (8.0 %). The other species represented less than five percent of captures, with Aethomys being represented by three individuals and Rattus by one. Aethomys and Rattus were captured only at the Mugo grid located at highest altitude (1922m ASL) while Tatera was restricted to lowest altitude (1712m) at the Moto grid. Small mammal species diversity was highest at Mugos (D=2.411 and H'=1.639) and least at Kurt's (D=1.333 and H'=0.829). Rodent species equitability was highest at Moto grid (E lID = 0.586) and least at Beth's (E IID = 0.266). Beth was closer to Kurt in terms of species assemblages, while Mugo and Moto grids were least similar. Rodent population densities showed similar dynamics, with most captures being made towards the end of the cropping season. Density fluctuations were low but influenced by the distribution of annual rainfall. Mastomys cf. erythroleucus population structure changed over time. Age distribution was influenced by the amount of precipitation. Sex ratio was of the expected 1:1 ratio with exception of a few months. The overall body mass dynamics showed no sexual dimorphism between males and females. Seasonal variations had strong effect on body mass dynamics (F = 14.268, P = 0.0004, n = 48), and field growth rates (F = 8.791, P = 0.005, n = 58). Larger individuals and higher growth rates were recorded during the wetter seasons. Population density did not have any significant effect on body mass dynamics or field growth rates. Breeding in Mastomys was depressed by drought but when there was adequate distribution of rainfall throughout the year, reproductively active individuals were encountered in all the months. The first breeding commenced following 100mm of rainfall between May and June 2000. The second followed 332mm of rainfall In November 2000 and January 2001, leading changes in population structure and subsequent increase in population densities. Heterogeneity in recruitment and survival occurring between sexes and age classes across grids were perhaps influenced by rainfall distribution. Maturation rates were low (mean ± SEM: 0.113 ± 0.029 males and 0.156 ± 0.035 females), respectively Rodents exhibited very low population persistence with individuals staying in an area for an average of 37 ± 1.635 days. Male and female Mastomys cf erythroleucus showed similar range lengths. However, between any given trapping period, males exhibited wider range distances than females. There was significant seasonality in range lengths with animals covering longer distances during wet than dry conditions. Distribution of rodents was aggregated at the edge of the farms. Ground cover had stronger influence than habitat structure on capture frequency of rodents. This study has documented the presence of seven rodent species with potential pest statuses in maize cropping systems of Rongai Division, Kenya. Mastomys, a multimammate rat, was the most important species due to numerical dominance in all the maize crop fields. Multimammate rats are recognised across Africa as important pests of crops and as disease reserviors. The prolonged drought occurring in two consecutive years prior to the commencement of the study led to low rodent numbers in the area. Population recovery was noted towards the end of the first cropping season. This suggests higher growth rates may occur during unusually higher rainfall leading to population outbreaks. The strong association between rodent captures and patches of dense weed cover reiterate the importance of cover removal as a strategy in ecologically based rodent pest management. Rodents were also captured largely along farm edges, and crops at these points are likely to get damaged. Field edges may therefore be important target areas during chemical control operations. This study has contributed vital though not exhaustive information on rodent population ecology in maize crop fields, which is important in designing ecologically based rodent pest management for Kenya.
- MST-Zoological Sciences