Ron Verhagen

Stochastic seasonality and nonlinear density-dependent factors regulate population size in an African rodent

Ecology has long been troubled by the controversy over how populations are regulated,. Some ecologists focus on the role of environmental effects, whereas others argue that density-dependent feedback mechanisms are central. The relative importance of both processes is still hotly debated, but clear examples of both processes acting in the same population are rare,. Key-factor analysis (regression of population changes on possible causal factors) and time-series analysis are often used to investigate the presence of density dependence, but such approaches may be biased and provide no information on actual demographic rates,. Here we report on both density-dependent and density-independent effects in a murid rodent pest species, the multimammate rat Mastomys natalensis (Smith, 1834), using statistical capture–recapture models. Both effects occur simultaneously, but we also demonstrate that they do not affect all demographic rates in the same way. We have incorporated the obtained estimates of demographic rates in a population dynamics model and show that the observed dynamics are affected by stabilizing nonlinear density-dependent components coupled with strong deterministic and stochastic seasonal components.

Optimization of adult performance determines host choice in a grass miner

Models and empirical studies on host selection in plant–insect, alga–amphipod, host–parasite and prey–predator systems assume that oviposition preference is determined by the quality of the oviposition site for offspring development. According to the oviposition–preference–offspring–performance hypothesis, oviposition–preference hierarchy should correspond to host suitability for offspring development because females maximize their fitness by optimizing offspring performance. We show, we believe for the first time, that adult feeding site and related adult performance may explain most of the variation in adult feeding and oviposition site selection of an oligophagous grass miner, Chromatomyia nigra (Diptera). This study advances our understanding of the complex interactions between plants and herbivores because it shows that host–preference patterns are not only shaped by the optimization of offspring performance, as previously assumed, but also by the optimization of adult performance.

Forecasting rodent outbreaks in Africa : An ecological basis for Mastomys control in Tanzania

1. Rainfall data were collated for years preceding historical outbreaks of Mastomys rats in East Africa in order to test the hypothesis that such outbreaks occur after long dry periods. 2. Rodent outbreaks were generally not preceded by long dry periods. 3. Population dynamics of Mastomys natalensis rats in Tanzania are significantly affected by the distribution of rainfall during the rainy season. 4. All previous rodent outbreaks in Tanzania were preceded by abundant rainfall early in the rainy season, i.e. towards the end of the year. 5. A flow chart is constructed to assess the likelihood of rodent outbreaks up to 10 months beforehand, utilizing rainfall data.

Nutrient stress, host plant quality and herbivore performance of a leaf-mining fly on grass

Environmental stresses affect plant growth and performance in nature. Host plant quality in turn affects herbivore performance and population dynamics. In view of these interactions, two major hypotheses were formulated. The plant stress hypothesis proposes that physiologically stressed plants become more susceptible to herbivores. The plant vigour hypothesis proposes that plants that grow vigorously are favourable to herbivores. Here we test the plant stress/plant vigour hypotheses for a leaf miner, Agromyza nigripes (Diptera; Agromyzidae), on the grass Holcus lanatus. We assessed larval performance (survival, developmental time, pupal mass) on grasses growing under different levels of nutrients (Hoagland solution) and drought stress, under controlled field and greenhouse conditions. Plant vigour and nutrient content were high on soils with an intermediate nutrient concentration and lower under drought stress and soil nutrient shortage and overdose. Larval performance was also highest on wet soils with intermediate nutrient supply. The results of the mining flies support the plant vigour hypothesis (density, survival and development better on vigorous plants). Herbivore performance is higher on leaves with a higher protein content.

Random amplified polymorphic DNA (RAPD) and parsimony methods

Abstract Random amplified polymorphic DNA (RAPD) data possess a number of undesirable features for parsimony analysis. These features include their non-codominant inheritance, their anonymous nature, their different (a)symmetrical transformation probabilities, and their possible GC priming bias. As a consequence, no single parsimony method seems appropriate for RAPD data. Moreover, the presence/absence coding of RAPDs is equivalent to the invalid independent allele model for allozymes. These issues are discussed and the way in which parsimony analysis of RAPDs can be misleading is illustrated.

Population, environmental, and community effects on local bank vole (Myodes glareolus) Puumala virus infection in an area with low human incidence.

In this study, the distribution of Puumala hantavirus (PUUV) infection in local bank vole Myodes glareolus populations in an area with low human PUUV infection (nephropathia epidemica [NE]) incidence in northern Belgium was monitored for 2 consecutive years. Bank voles were trapped in preferred habitat and tested for anti-PUUV IgG. Infection data were related to individual bank vole features, population demography, and environmental variables. Rare occurrence of PUUV infection was found and PUUV prevalence was low compared with data from the high NE incidence area in southern Belgium. Small-scale climatic differences seemed to play a role in PUUV occurrence, vegetation index and deciduous forest patch size both influenced PUUV prevalence and number of infected voles in a positive way. The data suggested a density threshold in vole populations below which PUUV infection does not occur. This threshold may vary between years, but the abundance of bank voles does not seem to affect the degree of PUUV seroprevalence further. We found indications for a dilution effect on PUUV prevalence, dependent on the relative proportion of nonhost wood mice Apodemus sylvaticus in a study site. In conclusion, we regard the combination of a dilution effect, a possible threshold density that depends on local conditions, and a higher fragmentation of suitable bank vole habitat in our study area as plausible explanations for the sparse occurrence of PUUV infection and low prevalence detected. Thus, beside human activity patterns, local environmental conditions and rodent community structure are also likely to play a role in determining PUUV infection risk for humans.

The basis of reproductive seasonally in Mastomys rats (Rodentia: Muridae) in Tanzania

Reproduction of the African murid genus Mastomys, multimammate rats, is seasonal, starting after the rains and extending well into the dry season. During a two-year study in Tanzania, we tested three hypotheses to investigate the proximal causes of this seasonality. Food availability was no limiting factor since food supply could not induce continuous breeding. Temperature was always high and thus not a restrictive climatic factor. Diet was probably always sufficiently varied and protein-rich to allow reproduction. This contradicts earlier hypotheses that consider Mastomys as an opportunistic breeder in which reproduction is seasonally limited by poor conditions; environmental predictors were believed not to be involved since the animals live in an unpredictably unstable environment (...)

Ecological and epidemiological data on Hantavirus in bank vole populations in Belgium

SummaryEpidemiological and ecological data of a Hantavirus infected bank vole population were collected using live-trapping methods. A close association between the distribution of HV-infected bank voles and wet habitat types was found. Several seroconversions were observed during the life of individual bank voles. All seroconversions occured within the wet habitat types and only after a certain age was reached. Survival of HV-infected bank voles did not differ from not-infected animals. In the laboratory passive antibody transfer was observed from antibody-positive females to their off-spring.

Effects of environmental pollution on microsatellite DNA diversity in wood mouse (Apodemus sylvaticus) populations.

Ten microsatellite DNA loci were surveyed to investigate the effects of heavy metal pollution on the genetic diversity and population genetic structure of seven wood mouse (Apodemus sylvaticus) populations along a heavy metal pollution gradient away from a nonferrous smelter in the south of Antwerp (Flanders, Belgium). Analysis of soil heavy metal concentrations showed that soil Ag, As, Cd, Cu, and Pb decreased with increasing distance from the smelter. Genetic analyses revealed high levels of genetic variation in all populations, but populations from the most polluted sites in the gradient did not differ from those of less-polluted sites in terms of mean observed and expected heterozygosity level and mean allelic richness. No correlation was found between measures of genetic diversity and the degree of heavy metal pollution. However, an analysis of molecular variance and a neighbor-joining tree suggested a contamination-related pattern of genetic structuring between the most polluted and less polluted sites. Pairwise F(ST) values indicated that populations were significantly genetically differentiated, and assignment tests and direct estimates of recent migration rates suggested restricted gene flow among populations. Additionally, genetic differentiation increased significantly with geographical distance, which is consistent with an isolation-by-distance model. We conclude that, at least for our microsatellite DNA markers, genetic diversity in the studied wood mouse populations is not affected greatly by the heavy metal pollution.

Host Nutritive Quality and Host Plant Choice in Two Grass Miners: Primary Roles for Primary Compounds?

The relationship between host plant choice and plant nutritional quality was investigated in two oligophagous grass miners Chromatomyia milii and C. nigra (Diptera, Agromyzidae). We tested whether host choice is determined by chemically mediated host suitability for offspring performance and/or adult performance. A second goal was to relate the observed variation among the different fitness parameters to quantitative and qualitative variation in foliar food quality. Choice experiments illustrated that both miners discriminated among grass species, and that C. milii has a smaller host range than C. nigra, as observed under natural conditions. Oviposition preference was correlated with adult feeding preference and related adult performance (longevity and fecundity) for both miners. Offspring performance measures (survival and pupal size) of at least C. nigra were more weakly related to host preference. Nearly all variation in adult performance of both miners was explained by foliar protein content, which had a positive effect on adult longevity and fecundity. Pupal size of both miners was positively related to foliar water and amino acid content and negatively related to lignin content. No clear relationship between host chemistry and offspring survival was observed. These observations show that fitness parameters are differentially related to host chemistry. Secondly, they suggest that chemically mediated host suitability for adult performance is an important determinant of host choice in this species. Finally, the results suggest a primary role for foliar protein content in host choice of the study species in general and in shaping the host range of C. milii in particular.