Abstract
Published analysis of genetic material from field-collected tsetse (Glossina spp, primarily from the Palpalis group) has been used to predict that the distance (δ) dispersed per generation increases as effective population densities (De) decrease, displaying negative density-dependent dispersal (NDDD). Using the published data we show this result is an artefact arising primarily from errors in estimates of S, the area occupied by a subpopulation, and thereby in De. The errors arise from the assumption that S can be estimated as the area () regarded as being covered by traps. We use modelling to show that such errors result in anomalously high correlations between and and the appearance of NDDD, with a slope of -0.5 for the regressions of log() on log(), even in simulations where we specifically assume density-independent dispersal (DID). A complementary mathematical analysis confirms our findings. Modelling of field results shows, similarly, that the false signal of NDDD can be produced by varying trap deployment patterns. Errors in the estimates of δ in the published analysis were magnified because variation in estimates of S were greater than for all other variables measured, and accounted for the greatest proportion of variation in . Errors in census population estimates result from an erroneous understanding of the relationship between trap placement and expected tsetse catch, exacerbated through failure to adjust for variations in trapping intensity, trap performance, and in capture probabilities between geographical situations and between tsetse species. Claims of support in the literature for NDDD are spurious. There is no suggested explanation for how NDDD might have evolved. We reject the NDDD hypothesis and caution that the idea should not be allowed to influence policy on tsetse and trypanosomiasis control.
Highlights
Using critical assumptions about gene flow, a model developed by Rousset [1], and analyses of trap samples of tsetse flies (Glossina spp), de Meeus et al [2] claimed to have found strong support for the hypothesis that the dispersal distance per generation, in tsetse, increases as a power function of decreasing population density
We show that the methods they used to estimate parameters are subject to large errors, and that such errors create the false signal of negative densitydependent dispersal (NDDD), even in simulated populations where NDDD has been proscribed
We recognised that the main support for the NDDD hypothesis is built around Eq (1): pfiffi d pbDe pbNe where δ is the predicted dispersal distance per generation of a tsetse population, and Ne is the effective population size, roughly defined as the number of adults in a population that will leave a genetic signature to the generation
Summary
Using critical assumptions about gene flow, a model developed by Rousset [1], and analyses of trap samples of tsetse flies (Glossina spp), de Meeus et al [2] claimed to have found strong support for the hypothesis that the dispersal distance per generation, in tsetse, increases as a power function of decreasing population density. De Meeus et al concluded that negative densitydependent dispersal (NDDD) probably applied to all tsetse species [2] They predicted that mean dispersal distance (δ) would increase by 200-fold when the effective population density (De) of adult tsetse decreased from about 24,000 to 1 per square kilometre, the order of density decline commonly associated with tsetse control operations [12]. This led them to warn that such control measures could unleash enhanced invasion of areas cleared of tsetse, so prejudicing the long-term success of tsetse control campaigns. In turn, prompted them to suggest the necessity of using "area-wide and/or sequential treatments of neighboring sites" to counter the added invasion, and they implied that small control operations risk doing more harm than good
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