Bat Series: Extinction Vortex

Why should we make a big deal about near-threatened species? Why is it just as important to put effort into conserving near-threatened species as it is those which are vulnerable, for example? Looking at The International Union for Conservation of Nature’s Red List of Threatened Species (also referred to as the IUCN Red List) the category is the second after least concern. Some people may then ask why it is so important to put a high amount of effort into protecting these species when they are seemingly a long way off extinction. In response, I will introduce the extinction vortex.

The extinction vortex is a term which was coined to describe the phenomenon which occurs when species populations decline. As the population numbers reduce, several other factors coincide, including inbreeding and stochasticity (random declines in populations due to chance factors), driving the species further and faster towards extinction1. It occurs due to the loss of individuals and genetic diversity within a species. This makes conserving as many individuals of a species as possible vital for the long term health and survival of the population. 

In the United Kingdom, near-threatened bat species include Bechstein’s Myotis, the Western Barbastelle, the Lesser Horseshoe bat, and the Greater Horseshoe bat2. The western barbastelle was recently recorded in Jones Hill Wood, an area which is marked for destruction to make way for the High Speed Two line. For all these species, the IUCN lists natural system modification (conversion or degradation of habitat3) as a threat to the populations.

For many, it may seem that small declines in populations of specific species are not necessarily something terrible for biodiversity. This is where the principle of the extinction vortex comes in and why in many cases we don’t realise we are going to lose a species to extinction until it’s too late. This is a concept which applies to all species, whether an animal, plant, fungi or any other species – that is why it is vitally important that we protect and maintain suitable habitats. Should a population decline to the level where the extinction vortex begins, it becomes incredibly difficult to protect that species and avoid extinction. It is necessary to protect species far before they enter into the state and before population declines start.

In the governmental policy paper ‘High speed two, phase one information paper E2: Ecological Impact’, it is stated that “The Proposed Scheme has the objective of seeking to ensure no net loss to Biodiversity” and the mitigation measures to be put in place are laid out. 

Mitigation measures which HS2 Ltd suggests includes strengthening hedgerows to provide edge-habitat for bats to forage and commute along. However, this is not an appropriate mitigation measure for bats, such as Bechstein’s Bat, which forage in narrow space environments4, i.e. woodland habitats. Another mitigation measure proposed is planting new woodland habitat. This falls short of what would be required for Bechstein’s bats as they are less likely to forage in areas with only saplings than in mature woodland. The new habitats with saplings only will not become mature woodland for several decades. In the last couple of weeks, another mitigation plan – translocating soil from ancient woodlands – was scrutinised for the flawed logic utilised in proposing it and the limited evidence supporting its use. 

In essence, ancient woodland cannot be replaced once destroyed, no matter what mitigation measures are put in place. And what is the impact this has on some near-threatened bat species? It could potentially send them on a downward, irreversible spiral to extinction. 

[1] W.F. Fagan and E.E. Holmes (2006) ‘Quantifying the extinction vortex’, Ecology Letters, 9, pp. 51–60
[2] https://www.iucnredlist.org/search?landRegions=GB&searchType=species
[3] https://wildlife.state.nh.us/wildlife/documents/wap/chapter4-naturalsystemmod.pdf
[4] https://www.frontiersin.org/articles/10.3389/fphys.2013.00164/full

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