Population dynamics of Arvicanthis niloticus (Rodentia: Muridae) in Kenya

Abstract

The population dynamics of the Grass rat Arvicanthis niloticus were studied in relation to their diet and the climate in the Nakuru district of Kenya between September 1980 and May 1982. Numbers in the study area were low at the end of 1980 with groups of animals isolated in patches of grassy habitat. Local populations declined and some became extinct during the first half of 1981. Breeding began in April, one month after the start of the rains, but with a small initial population and high juvenile mortality, numbers did not increase significantly until September, when animals born earlier in the season were old enough to reproduce. This second generation reproduction was probably very important to the survival of the population over the following dry season when adult mortality was estimated at 15% per month. Reproduction was facilitated by a nutritious diet, consisting mainly of young grasses early in the season and seeds later on. Seed production was itself related to rainfall and as that declined towards the end of 1981, breeding ceased. Arvicanthis were considerably more numerous then than one year previously, with the difference being ascribed to the longer rainy season experienced in 1981 than 1980. The density of animals in favourable habitats increased with immigration from less suitable areas between January and March 1982, and then declined until the end of the study. Breeding resumed in April, one month after the start of the rains. Peak numbers of Arvicanthis were recorded in the middle of the dry season, when few crops were in the ground. They would only become a serious pest to agriculture if they were still abundant at the start of the breeding season some months later. Expansion then could result in very high numbers by harvest time, in August or September.

A quantitative study of diet in three species of rodents in natural and irrigated savanna fields

“…Although that subject lies outside the scope of this paper, suffice it to say that reproduction in Arvicanthis and Mastomys was continuous in the irrigated fields of northern Nigeria but was discontinuous (seasonal) in the natural fields, a fact likely related to the quality and availability of food at the two sites. Vegetation is an important source of cover and food to rodents (Delany and Monro 1986). Indeed, we observed drastic declines in population levels when the grasses in our demographic study grids were slashed for routine management purposes.…”

“…The significance of diet, especially the role of plant products that stimulate reproduction in rodents, has been extensively discussed in the literature (Gautun 1975, Delany andMonro 1986). Although that subject lies outside the scope of this paper, suffice it to say that reproduction in Arvicanthis and Mastomys was continuous in the irrigated fields of northern Nigeria but was discontinuous (seasonal) in the natural fields, a fact likely related to the quality and availability of food at the two sites.…”

“…Rabiu and Pisher (1989) observe that the importance of seeds and monocots in the diet of Ariicanthis in northern Nigeria was less than that in east Africa, findings also suppcrted in the present study. In Kenya, the diet of Arvicanthis is mainly of mono;ots during the late dry seasons (rather than dicots as in the natural fields of northern Nigeria), and of seeds in the later part of the rains up to the early part cf the dry season (Taylor andGreen 1976, Delany andMonro 1986).…”

“…Experimental tests and discussion on the validity of this assumption are given in Hansson (1970). For a determination of the mean percent food composition of the series of stomachs (eg for each species in a given season), the diet estimation formula described in Delany and Monro (1986) was used: a = X (la) / T x 100 where a is the mean contribution of food A to the diet in a particular period, I is the percentage of a sample that was identifiable, a is the percentage contribution of food A to the identifiable fraction of that sample, and T = I [Z (la)] for all food types in the period considered. Data thus obtained were pooled on monthly bases and transformed using arcsin square root prior to statistical evaluations.…”

“…In addition, much of the research involving seasonal changes of diets of small mammals in Africa has been on the influence and regulatory role of diet in reproduction (Sheppe 1972, Neal 1977, Ghobrial and Hodieb 1982, Neal 1982, Delany 1986, Delany and Monro 1986, Leirs et al 1989.…”