Search

The survival of hymenopteran populations especially that of ecologically and economically important pollinators and parasitoids, has become a major topic for empirical and theoretical studies. Complementary sex determination (CSD) in Hymenoptera may impose a substantial genetic load through the production of inviable or sterile diploid males. Modelling and laboratory studies have indicated that this genetic load may trigger a ‘diploid male vortex’ leading to rapid extinction of populations. Here we take a broader theoretical approach to analyze why populations exhibiting CSD persist in nature, even if they contain large proportions of homozygous diploid individuals. Using an individual-based model of spatially structured hymenopteran populations, we show that (i) inviability or reduced fertility of homozygous individuals, (ii) female-biased sex ratio, and (iii) strong intra-specific competition can mitigate the negative influence of CSD on population persistence and that (iv) already extremely low dispersal rates will result in long term survival. These findings underline the importance of life history traits for population survival and demonstrate that rather specific conditions must be met to initiate the ‘diploid male vortex’.

In metapopulation models it is common practice to use species-specific dispersal distances to predict the exchange of individuals between habitat patches. The influence of patch distribution on the reachability of a habitat patch is usually ignored. In a patch-matrix simulation, we investigated the effect of patch number, movement pattern and dispersal mortality on realised animal dispersal distances. In our spatially-explicit, individual-based simulations we demonstrate that: (1) with increasing number of patches the number of immigrants into patches further away from the release point decreases in all scenarios (“shadow effect”), (2) this effect is strongest for a random movement, (3) for rather uncorrelated walks a proper adjustment of mean dispersal distance in the negative exponential model can account for the effect of patch density on realised dispersal distances, (4) with a more directed walk this is not possible as the distribution of moving animals in the matrix becomes highly heterogeneous. This is due to the narrow but far reaching shadow patches exert on other patches further away from the release point.