Patch use under predation risk often results in a change of feeding behaviour in the prey animals. However, such changes only appear if the animals are able to assess under which predation pressure they live. We investigated patch use of Mastomys natalensis under different conditions of avian predation pressure. In replicated maize field plots in Morogoro, Tanzania, avian predators were allowed under natural conditions (control), attracted with perches and nest boxes or kept out with nets. During four one‐week periods in late 1999, we measured rodent feeding decisions with the giving‐up density (GUD) method. Trays with known amounts of millet seeds in sand were placed in pairs, one of them under a cover, the other one in the open. M. natalensis mice were expected to give up sooner in the open trays than in those with cover. We hypothesised that M. natalensis mice could assess the ambient predation pressure leading to larger difference in GUD between covered and non‐covered trays in the plots where predators were attracted. We also made video recordings of the rodent activity at a pair of trays in each treatment. The GUD‐values were significantly lower for the covered trays but predation pressure did not affect this difference. The video observations showed that in the control and netted plots the animals visited trays equally frequently regardless of the cover, while the visits in the predator‐attracted plots occurred significantly more often in the covered trays. We conclude that M. natalensis can assess the ambient predation pressure and adapt its behaviour at a feeding patch. However, the variation in predation pressure in our experiment was not obvious from the GUD. Moreover, we found a strong relation between rodent density and GUD, which may mask variations in perceived predation pressure. Similar GUD values may be reached in different ways and we present models to investigate whether animals’ decision to forage at a food patch is only affected by the seed density at that patch, not by that at a neighbour patch.
Summary1. We investigate the effects of different levels of predation pressure and rodent dispersal on the population dynamics of the African pest rodent Mastomys natalensis in maize fields in Tanzania. 2. Three levels of predation risk were used in an experimental set-up: natural level (control), excluding predators by nets and attracting avian predators by nest boxes and perch poles. Because dispersal of the rodents could mask the predation pressure treatment effects, control and predator exclusion treatments were repeated with enclosed rodent populations. 3. Population growth during the annual population rise period was faster in the absence of predators and peak population size was higher, but otherwise dynamics patterns were similar for populations where predators had access or were attracted, indicating that compensatory mechanisms operate when rodents are exposed to high levels of predation risk. Reducing dispersal of rodents removed the effect of predation on population growth and peak size, suggesting that local predators may play a role in driving rodent dispersal, but have otherwise little direct effect on population dynamics.
Mastomys natalensis is the major pest rodent in sub-Saharan Africa. In this study, population genetic techniques were used to gain new insights into its dispersal behaviour, a critical parameter in pest management. Using 11 microsatellites, 272 individuals from a 300 ha area in Tanzania were genotyped. Genetic diversity was high, with no isolation by distance and little differentiation between field plots far apart, indicating a large effective population size and high dispersal rates in agreement with ecological observations. On the other hand, genetic differentiation between nearby field plots, isolation by distance within a single field plot and kin clustering were also observed. This apparent contradiction may be explained by yearly founder effects of a small number of breeding individuals per square area, which is consistent with the presence of linkage disequilibrium. An alternative, not mutually exclusive explanation is that there are both dispersing and sedentary animals in the population. The low-density field plots were characterized by low relatedness and small genetic distances to other field plots, indicating a high turnover rate and negative density-dependent dispersal. In one field plot female-biased dispersal was observed, which may be related to inbreeding avoidance or female competition for resources. Most juveniles appeared to be local recruits, but they did not seem to stay in their native area for more than two months. Finally, possible implications for pest management are discussed.
Development, survival and fecundity for Scatella (Teichomyza) fusca Maquart (Diptera: Ephydridae) were studied at 20 ± 1 °C and 85 ± 10% r.h. Mean (± S.E.) developmental times of eggs, larvae and pupae were 4.0 ± 0.06, 14.9 ± 0.19 and 14.6 ± 0.11 days, respectively, the mean (± S.E.) survival of the original egg cohort to the start of larval, pupal and adult stage being 77.2 ± 3.2%, 54.5 ± 2.6% and 47.9 ± 3.5%, respectively. Females and males displayed approximately straight survivorship curves during adult life, implying constant mortality rates. Mean (± S.E.) adult longevity was 41.6 ± 2.98 days for females and 51.2 ± 3.91 days for males. Assuming a stable age distribution the population consisted of 56% eggs, 31% larvae, 6% pupae and 7% adults. Oviposition peaked when females were 25 days old, and the highest reproductive values (RVx) (mean ± S.E.) ranged from 129.1 ± 7.57 to 138.5 ± 6.83 for individuals 17–27 days old. A female deposited a mean (± S.E.) of 614.7 ± 35.9 female eggs over a maximum life span of 93 days. The basic reproductive rate (R0) (mean ± S.E.) was 173.0 ± 14.2 female offspring per female and the intrinsic rate of natural increase of female individuals (r) (mean ± S.E.) was 0.088 ± 0.001 day−1. The mean (± S.E.) generation time (T) was 57.8 ± 0.78 days. In cultures with equal numbers of first instar S. fusca larvae and predacious third instar larvae of Hydrotaea aenescens (synonymous Ophyra aenescens) Wiedemann, mean (± S.E.) survival to the adult stage of S. fusca (16.7 ± 8.8%) was significantly lower than in controls with S. fusca alone (58.3 ± 7.4%). The potential significance of predation by H. aenescens on S. fusca in pig farms is discussed.
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