Effects of Habitat Degradation and Fragmentation on the Genetic Population Structure of Phytophagous Beetles in an African Rainforest

Abstract

In the presented study the genetic population structure of the phytophagous beetles Amphitmetus transversus (Coleoptera, Curculionidae) and Monolepta vincta (Coleoptera, Chrysomelidae) was analysed in a highly disturbed and fragmented rainforest remnant in Kenya – the Kakamega Forest. It is assumed that the patchy distribution of forest habitats prevents geneflow between populations and leads to an impoverishment of their genetic resources. Previous consequences are a loss of genetic variability within and an increase of genetic differentiation between populations. The main questions were, whether recent degradation and fragmentation of the Kakamega Forest influences the genetic variability and differentiation of the beetle populations. <br /> Newly established microsatellite systems for A. transverus with six polymorphic markers and for M. vincta with nine polymorphic markers were used in order to analyse the genetic population structure of the insects. Both, the marker system of the weevil and the leaf beetle showed deficiencies of Hardy-Weinberg-Equilibrium (HWE) at several markers. The heterozygote deficits were explained by the existence of null alleles at the concerning markers, which was also confirmed by the existence of null homozygotes. <br /> The results of the present study partly indicate that anthropogenic introduced habitat fragmentation affects the genetic structure of the weevil A. transversus as predicted by theory. It has been found that populations of the small fragments in the Kakamega Forest show significant lower genetic diversity than populations of the moderate fragment as well as of the continuous forest. A. transversus was genetically differentiated across the observed range (FST = 0.12), which was partly explained by an ‘isolation by distance’ pattern. Furthermore, a separation in three population cluster was revealed. It was not found that fragmentation of habitat generally leads to an increase of genetic differentiation due to the restriction of migration and hence gene flow between isolated sites.<br /> The highly variable microsatellite system of M. vincta exhibited a weak genetic differentiation (FST = 0.01) across the geographical scale that could not be attributed to the geographical distance between populations. If the result is caused by the high mobility of the leaf beetle or by the high variability of the marker system in combination with the limited sampling remains unclear. <br /> In conclusion it was found that the genetic population structure of A. transversus in the Kakamega Forest appears to be influenced by both, contemporary and historical habitat changes. The study is evidence that anthropogenic fragmentation has always to be interpreted in the context of long-term population history.