... If these two species meet they interbreed, and the abundance of the ruddy duck suggests that the outcome would be the eventual extinction of the native white-headed duck by genetic mixing1. The increasing utilisation of land for human activities such as agriculture, mining and forestry, has meant that habitats are becoming increasingly fragmented. Fragmentation of species' habitats results in reduced population sizes, which, for some species more than others, can mean that they become quite rare. Hybridisation between rare and common species has two potentially harmful consequences for the conservation of biological diversity. If the F1- or later-generation hybrids are partly sterile or have reduced vigour, then the rare species may be endangered by outbreeding depression. That is, rare populations may have reduced fitness due to gamete wastage in the formation of unfit individuals (this is particularly true of organisms that produce vast numbers of gametes, like plants and fish). ...

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