Dispersal in Mastomys natalensis mice: use of fine‐scale genetic analyses for pest management

摘要

Population genetic methods offer information on the degree of genetic diversity in individuals, social groups, family groups, and populations, as well as genetic similarities and differences. Factors contributing to such diversity include population size and density, population fragmentation, dispersal (including reproductive success of migrants), and social and mating structure. Therefore, population genetic methods have become a valuable augmentation to field experiments. Capture–mark–recapture experiments (CMR) can be time consuming, difficult to perform, and are often restricted to small areas. Dispersal behaviour across larger areas is difficult to study as recapture probability decreases with geographic distance. Although the population genetic approach has been used successfully for decades to characterize wild populations, it has only recently become sufficiently sophisticated to be applied to pest management (Rollins et al. 2006).The African multimammate mouse, Mastomys natalensis, is the major rodent pest in sub-Saharan Africa. It regularly causes severe agricultural damage, with 50–100% harvest losses in some outbreak years (Mwanjabe et al. 2002). M. natalensis occurs in natural grasslands, bushy areas, cultivated areas and human habitations. It exhibits yearly population cycles and irregular outbreaks, which are strongly related to rainfall (Leirs et al. 1994). Within one year, population densities may vary by a factor of 20 (Leirs 1995; Mwanjabe et al. 2002).Earlier studies in Morogoro, Tanzania (Leirs et al. 1993, 1994; Leirs 1995; Julliard et al. 1999), have resulted in a thorough basic knowledge of the local population ecology and the development of population dynamic models (Leirs et al. 1997a; Stenseth et al. 2001). These studies have contributed greatly to the improvement of pest management strategies. In Morogoro, the main breeding season lasts from April–May until September, and females produce on average five to six litters of 11–12 young (Leirs et al. 1993). There is multiple mating by both males and females. Multiple paternity occurs frequently and there is a large variation in male reproductive success (Kennis et al. 2008). In the breeding season, relatively many males die and the adult female:male sex ratio becomes 2.5–9 (Leirs 1995). Generally the lowest population densities (both sexes) are observed in June, sometimes less than two individuals per ha. There is generally little overlap between consecutive generations. In years with abundant rainfall, however, two generations can occur within one year, resulting in outbreaks. (This was not the case for the year sampled in this study).Dispersal rates and distances over the course of one generation are high. Between September and December, the monthly immigration rate for a 1 ha area is about 24% (Leirs 1995). As a result, the fraction of locally-born (1 ha) breeding individuals is only 0.4–12% (Leirs et al. 1993). About 1% (85/7650) of the animals captured in various field plots (7.9 ha in total) from a 125 ha area in Morogoro consisted of known dispersers with no clear sex bias (unpubl. data). Fifteen percent of these crossed a distance of more than 400 m (62% of the plot pairs separated by this distance). Females are sedentary during the middle of the breeding season, but show home range displacement at the start and at the end of the season (Leirs 1995). Sexually active males migrate slightly farther than non-breeding males (Leirs 1995).In this study, fine-scale genetic analyses were conducted on a spatial scale smaller than individual M. natalensis dispersal, using samples from a CMR study conducted in Morogoro between March 1998 and March 2002. The primary goal was to clarify to what extent population genetics can improve insight into the dispersal behaviour of M. natalensis. A detailed understanding of dispersal is vital for the improvement of pest management strategies (Rollins et al. 2006), especially when eradicated animals are quickly replace