Anthropogenic Hybridisation: A Biodiversity Threat Under the Radar

Anthropogenic, or human-induced, hybridisation refers to the interbreeding between otherwise distinct populations facilitated by deliberate or accidental human activity. While in some circumstances (e.g., the florida panther case1) this type of hybridisation may be considered a management tool to introduce genetic variance and thus increase genetic diversity in a small population, it is more generally deemed a serious conservation threat2. This issue is what has led species like the Scottish wildcat to functional extinction3, and what currently threatens the conservation of wolf populations in Europe4.

Anthropogenic hybridisation is grouped into two broader categories:

  1. human-induced interbreeding between two distinct wild populations; or
  2. interbreeding between a wild species and its domestic counterpart.

In the former case, issues such as habitat loss or, increasingly, climate change, may bring species to shift their natural ranges and thus come into contact and breed with other evolutionary distinct populations. In situations where the two populations can interbreed with fertile offspring, this may lead to the genetic extinction of one or more species. If the hybridising populations cannot produce fertile offspring, their interbreeding may result in wasted reproductive effort which in the long term may jeopardise the conservation of species.

The latter case is documented when animals such as cats and dogs, cows or even farmed salmon, come into contact with their wild counterparts (e.g., wildcats, wolves, bisons, wild salmon, respectively) and interbreed, usually producing viable offspring. Since domestic animals resulted from the process of artificial selection, their traits may be maladaptive in the wild and deleterious to wild populations. Moreover, domestic and farmed animals are usually present in much higher densities than their wild counterparts, and extensive interbreeding between the two may cause the genetic extinction of the latter.

Another issue applicable to both categories is that, since current conservation policies at a global scale rarely extend their legal protection to hybrid individuals, the presence of hybrids in a wild population may allow a loophole for the killing of protected taxa. Species subject to high levels of human-wildlife conflict, such as the wolf, may be particularly concerned by this issue.

Alongside the lack of policies, effective management is also largely missing worldwide, which is critical since effective mitigation of this issue relies on rapid intervention to prevent the spread of “unwanted” genes in wild populations.

There are two main issues possibly hindering effective management to date. First, the technical tools to detect hybridisation are continuously evolving, and our ability to detect the unwanted genes (or introgressed genes in scientific jargon) is accordingly improving. This begs the question of how we should define “hybrid”, as we are increasingly capable of detecting hybridisation events that occurred many generations in the past.

Second, lethal removal is a tool available to management to address and mitigate the hybridisation issue, for instance by removing the hybrid individuals from wild populations. However, views on the use of this tool are highly polarised both amongst the public and amidst the scientific community, which may delay the intervention choice and ultimately hinder action.

While the issue of anthropogenic hybridisation remains largely unbeknownst to the public, its rate of occurrence may be on the increase due to growing anthropogenic pressures. This may ultimately mean that this issue will pose an increasing threat to biodiversity conservation in the forthcoming decades.  

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References

  1. Pimm, S.L., Dollar, L. and Bass Jr, O.L., 2006. The genetic rescue of the Florida panther. Animal Conservation9(2), pp.115-122
  2. Allendorf, F.W., Leary, R.F., Spruell, P. and Wenburg, J.K., 2001. The problems with hybrids: setting conservation guidelines. Trends in ecology & evolution16(11), pp.613-622
  3. https://www.bbc.co.uk/news/uk-scotland-46617965
  4. http://www.exeter.ac.uk/news/featurednews/title_717931_en.html

Cover image credits: https://bit.ly/32TvEo6

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