What are the impacts of interspecific hybrids on species conservation?

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What are the impacts of interspecific hybrids on species conservation?

Rates of hybridisation are increasing dramatically worldwide because of human activities such as the translocation of organisms, habitat fragmentation and modification.  As a result, many species have faced, or are facing, extinction.  However, hybridisation is also a natural process which has long been recognised as playing an important role in the evolution of plants, and recent studies have found that hybridisation has also played an important role in the evolution of animals.  Thus, conservationists need to distinguish between species that have arisen through recent, anthropogenic means and those that have arisen through ancient, natural hybridisation.  Conservation policies need to protect the latter species, and also reduce rates of anthropomorphic interspecific hybridisation.

Humans often translocate organisms and introduce them into new areas, often with catastrophic effects on the native species.  This problem is exemplified by the introduction of the ruddy duck Oxyura jamaicencis from North America into Europe.  This species is closely related to the white-headed duck O. leucocephala which is commonly found in Britain and north-western Europe.  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 mixing.  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).  In this way, the rare species becomes extinct through demographic means: small populations are more vulnerable to extinction.  On the other hand, if the hybrids are fertile and vigorous, hybridisation may lead to the genetic assimilation of the rare species by a numerically larger one.  In this way, the rare species becomes extinct through genetic means.

Early conservation policies generally did not allow the protection of hybrids, however, the recent recognition of the historical role of hybridisation as an evolutionary process, has caused a re-evaluation of these policies.  An early series of interpretations of the US Endangered Species Act (ESA) concluded that a species or subspecies that had undergone hybridisation with another named entity was prohibited from protection as a federally endangered species or subspecies. The view was taken that ‘impure’ species containing ‘foreign’ genetic material did not merit protection as they would not help recover the listed species, and may jeopardise its continued existence.  Strict and prolonged adherence to this policy would have discontinued conservation programs and reintroduction strategies for many plants and animals.  However, based on scientific information on hybridisation, this Hybrid Policy was withdrawn, and a new more flexible policy was proposed: The Intercross Policy (1996).  The overall goal of this revised policy is to maintain flexibility in interpretation and to allow policy to be guided by scientific understanding of hybridisation.  However, the fact that this policy has not yet been approved, illustrates the difficulty in writing a hybrid policy that is flexible enough to apply to all situations, but that would still provide helpful recommendations.

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In order to fully understand the impact of interspecific hybrids have on species conservation, it is important to distinguish between the different categories of interspecific hybridisation: those that are a natural part of the evolutionary legacy of taxa, and those that are due to human effects.  The former group of species should be eligible for protection, however, the latter can be further divided into categories which have different consequences from a conservation perspective.  Allendorf et al. divide ‘natural’ hybridisation into three categories.  Type 1 hybridisation results in natural hybrid taxa that have arisen by natural genetic admixture.  Species resulting from ...

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