Bats on the Red List

by Fiona Mathews

Many bats have a perilous conservation status. Globally, 40% of all species are currently listed as either Threatened with imminent extinction, Near Threatened or Data Deficient by the International Union for the Conservation of Nature (IUCN). In Great Britain, the first Red List for Mammals — published last week — lists 4 species as Threatened, 2 as Near Threatened and 3 as data deficient.  Some species listed on Annex II of the Habitats Directive on the other hand — greater horseshoe, lesser horseshoe, and Bechstein’s — are classified as Least Concern.  Here is why.

Derivation of the Red List

The Red Listing process uses an internationally-agreed set of criteria to assess the threat of extinction.  These criteria are based on the rate of decline, population size, area of geographic distribution, degree of population and distribution fragmentation, and the probability of recolonisation from neighbouring countries. A final criterion which is based on the results of quantitative population viability modelling is not relevant to bats in Britain as no such research has been conducted. Crucially, the Red List takes a rather short time window as its period of assessment. So under criterion A (Population Decline), which is the criterion that qualified most species for inclusion in a threatened category, there needs to be evidence of a decline of at least 50% over the past 3 generations for the species to be classified as Endangered, or at least 30% for it to be classified as Vulnerable. These are very major declines indeed. 

It is fair for us to give some thought to the shortcomings in the data available to make these assessments. For many species we have very little information on trends on population size. This is partly because bats, as many people will realise, are wretchedly difficult to study. The propensity of some species to switching roosts is just one factor that makes roost count data complex to interpret. And for species such as Natterer’s and Brown long-eared bats, we only have sensible information from the fraction of the population that lives in buildings, whilst we know virtually nothing about tree roosts (which could well account for most of the population). For many other mammal species, we can look to changes in distribution in order to make inferences about changes to population size. Unfortunately, in the case of bats, this is very difficult because trends over time are confounded with changes in survey effort. With the increasing availability of broadband acoustic recorders, there are increasing numbers of records from across the country; and where these are located within, or adjacent to, the known range, it is difficult to separate out true and erroneous records. Excellent work by the Bat Conservation Trust to increase volunteer numbers, as well as legislative changes that have led to increasing numbers of professional bat surveys, also means that there are many more people recording bats than there were 20 years ago.

This time frame is also significant. In drawing up the Red List we defer to the IUCN agreed list of ‘generation times’ which are based not on time to breeding, but on the average age of adults in a population. These standard values range from 10 for Brown long-eared bat and 9 for Greater horseshoe, to 4.2 for the noctule. So, when we look for a decline of at least 30% over 3 generation times for Brown long-eared, we are looking for a very much greater change than would be sufficient to qualify a noctule bat.  To my mind, this source of uncertainty is one of the areas that most urgently needs attention: unfortunately, very long-term ringing studies are rare.  I would particularly urge anyone ringing species such as noctule, common pipistrelle, soprano pipistrelle, and Daubenton’s to continue their work so we can get a better understanding of true generation length.

Which species are now officially classed as threatened, and why? 

(Full details can be found in the Red List document on our website here, which also gives further details of country-specific assessments).

Greater mouse-eared bat (Critically endangered)

A hibernating population of this species, discovered in Sussex in 1969 has declined  from a maximum of 30 individuals to a single male, first recorded as a juvenile in 2002. A small hibernating population of up to 10 individuals known in Dorset was lost by 1980. The last known male was not found this year in hibernation surveys. There still remains a possibility that there could be an unknown colony; though it is also possible that the lone male was a migrant from continental Europe.

Grey long-eared bat (Endangered)

This species is classified on the basis of the sharp decline in the number of known breeding roosts, and the substantial decline in the availability of species-rich meadows (particularly wet-meadows) which the species uses extensively for foraging. The population is severely fragmented, with >50% of the total occupancy found in patches smaller than would be required by sustainable populations and these are separated from the other colonies by a large distance. The population is very small, with the best estimate being approximately 1,000 individuals.

Serotine bat (Vulnerable)

A negative trend in population size is estimated from roost counts (-1.6% per annum) from the National Bat Monitoring Programme (NBMP), and whilst this is not statistically significant, the wide confidence intervals reflect the small sample size: it is therefore reasonable to assume that the trend would be significant were further data available, and to adopt a precautionary approach, particularly given the higher rate of decline noted for the whole of GB using a larger sample size. There is suspicion of a recent decline in range and population size in the east of England, but there also appears to be a corresponding increase in populations in the north and west of the range. It is emphasised that this assessment is based on very poor data, with the estimates of population size and availability of suitable habitat being very unreliable. The species should be re-assessed as soon as further information becomes available.

Barbastelle bat (Vulnerable)

This species is listed on the basis of small and fragmented populations. It is plausible that the total population size is <10,000 individuals; and that the number of mature individuals in each subpopulation may be ≤1,000. This is coupled with a decline in the availability of veteran trees and ancient woodland, to give a classification of Vulnerable.

Near Threatened Species

Near Threatened species are not necessarily those undergoing a slower decline – indeed a species could decline 10% every 30 years without showing up on a Red List as it would fail to meet the threshold criteria!  Rather, they are species where there is an identified risk that means that they could rapidly change to swifter rates of decline in future if aversive action is not taken. Leisler’s bats and Nathusius’ pipistrelle both fall into this category. Their populations appear to be patchy (i.e. fragmented), there are very low numbers of known roosts, and they are at risk from wind turbines. In the case of Nathusius’ pipistrelle, this risk extends to offshore wind farms and installations in northern Europe, because the species is migratory.  For both of these species, further information about roost size and locations are urgently needed.

Data deficient species

For whiskered, Brandt’s and Alcathoe bats, we simply have insufficient information to make any assessment. It is notable that the IUCN system is set up to avoid species ending up in the data deficient category as far as possible, not least because of the risk that these species are simply forgotten about. Therefore, if ANY information is available to suggest no change in range, then — unless there were other evidence available to point out a problem — a species will automatically fall into the Least Concern category.  For whiskered, Brandt’s and Alcathoe, we are not able to interpret historical distribution maps because of the strong likelihood of species misidentification. It is also unclear whether the current apparent rarity of Alcathoe is a true reflection of its status, or whether it is simply not being recognised at present. The only realistic way to sort out this problem is through extensive DNA confirmation of species ID (both of animals in the hand and from droppings collected in building and bat-box surveys).

What about everything else? Can we assume other species are OK?

Greater and lesser horseshoe bats are consistently showing population increases, and are also our best surveyed species. We can therefore be confident that in Great Britain they are doing pretty well. However, this does not mean that we should be complacent.  Both are on Annexe II of the Habitats Directive for a very good reason; namely that historically they suffered catastrophic declines. The success seen with their conservation in Britain is also not mirrored throughout Europe, meaning that their conservation here is important on a continent-wide scale.  Bechstein’s bat is another species listed on Annexe II which does not feature on the Red List. This is because there is no evidence of recent declines: indeed, there has been a consistent increase in the number of records. This, of course, could be an artefact of increased survey effort, and the growing use of tools such as acoustic lures. Therefore, it is important that going forward we also look at other indicators of potential threat, such as changes in the availability of suitable habitat. We should also use modern tools such as genetic markers of population size provide more robust estimates of change in population size, as for this (and other woodland bats), direct counts of population size are not feasible.

And what about everything else? We can be reasonably confident that common and soprano pipistrelle populations are gradually increasing.  For other species, I recommend careful monitoring. We also need to remember our shifting perceptions of baseline populations. The Red List provides only a snapshot of very recent change, and picks up only the worst affected species. We must not forget that for many species, current populations are likely to be a fraction of those seen historically, and our efforts to restore and protect them must continue.