Vanishing of the common species: Empty habitats and the role of genetic diversity

Biodiversity is declining, with major causes identified as habitat loss and a reduction of habitat quality. Recent studies have shown that particularly species with specific habitat demands are suffering in this way. Accordingly, habitat specialists have been nominated as umbrella species, which because they represent a much larger number of species, are thought best to fulfil the requirements of nature conservation. However, species which are ecologically intermediate between habitat specialists and generalists, and typically form networks of populations on adjoining habitats, might suffer even more severely under rapid habitat fragmentation than those specialists which had for a long time already occurred as discrete populations. Today, most of these formerly more widely distributed intermediate species also exist only as small and isolated populations which, because of their increasing geographic isolation, cannot counterbalance local extinctions by recolonisation. Furthermore, these species are mostly equipped with relatively high genetic diversity that is maintained by continual exchange of individuals between local populations. However, this high level of genetic variability frequently decreases after the collapse of population networks with negative effects on the viability of these species. Thus, factors at the population and molecular levels may lead that formerly common species vanish in the near future.