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Otters of the world

HOW ACCURATE IS THE RED LIST FOR MAMMALS? A CASE STUDY

By Grace M Yoxon

Published by Highlnd Biological Recording Group and available at the Ottershop

In 2018, the Mammal Society published a Review of the Population and Conservation of British Mammals, (Mathews et al 2018).  This was followed in 2020 by a Red List for Britain’s Mammals with assessments according to IUCN criteria (Mathews and Harrower 2020).

Naturally our main focus is on otters and so we checked the information for this species and found that it is highly flawed.  According to the new Red List the otter in England is classed as “Least Concern” whereas in Scotland and Wales the species is classed as “Vulnerable”!

We queried this with the Mammal Society who said that the data was taken from sightings, National Surveys and the National Biodiversity Network (NBN).  Of course, any assessment such as this is only as good as the data on which it is based.  In HBRG we are very careful that our data is verified as otherwise it can give a completely false picture.

So let’s question the data a bit more.

Sightings

Unless the record has actually been verified by someone with experience in otters some of the “records” could actually be mink.  Furthermore, one animal may be recorded several times, especially in the larger home ranges in riparian habitats.  This will clearly give a considerable over-estimate when multiplied by many areas. 

Spraint surveys

The National Surveys are all based on spraint and so clearly there is the same issue of multiple records of an animal as it can easily spraint across several hectads.  It has been proven that spraint gives no indication of population size (Yoxon & Yoxon 2014) and yet this is the method used.  All spraint tells you is that an otter has been at that location – it does not tell you if it is resident or breeding.

It is certainly true that otters are being recorded in new areas, particularly in England, and also in more urban habitats.  But can we be sure that actual numbers have gone up?  For one thing there are a lot more people in England so this may well generate more records.  Just look at the distribution of HBRG records throughout the Highlands and the gap for any species is not necessarily due to non-existence but is often due to the fact that there is no-one there to record it.  There is also a lot of evidence that eel populations have crashed in many parts of England (Walker 2019) and as eels are a major part of the diet of freshwater otters, this is crucial.  Is it not more likely that the size of home ranges has increased, because they are having to travel further to obtain their prey?  Are they also expanding into more urban areas and into people’s fish ponds in order to get food? (Yoxon 2018)

Population figures

Population figures for otters are very difficult to obtain.  The figures quoted by the Mammal Society are 2,800 for England, 8,000 for Scotland and 900 for Wales, making a total of 11,700, not 11,000 as quoted in their paper.  These figures are then compared with Harris et al 1995 who quoted3,567 in mainland Scotland, 391 in Wales and 350 in England.  They then rounded that up and added on an estimated 3,000 for the islands to come up with a figure of 7,350.  This earlier work was based on DNA work in two rivers in England, data on Shetland from Kruuk et al 1989, spraint surveys and various other “estimates” based on length of suitable habitat and the percentage of occupation of sites in the latest published otter surveys.  These figures were extrapolated throughout the country.  However, as accepted by Harris et al 1995, habitat has an obvious effect on otter usage and so different habitats will not have the same population densities and this has also been shown on the Isle of Skye (Yoxon 2013).  Therefore theMammal Society are comparing two sets of figures which are based on different methodologies and therefore there cannot be any real comment about any population change. The only truly reliable way to obtain real numbers is by DNA and this has been done in very few areas and it is impossible to use this data for the whole country. 

The Mammal Society based their occupancy levels on the National Otter Surveys, which are carried out using spraint.  In Scotland this appears to suggest an increase from 57% in 1977-79 to 92% in 2003-04 (Strachan 2007) but a decline to 80% in 2011-12 (Findlay et al. 2015). This has been extrapolated out so that if the decline of 12% continues for the next 8 years there would be a decline of 34% over three generations.  On this basis that would indeed make otters in Scotland “Vulnerable”.  But is it really possible to say that?

Distribution map for otters in Scotland

The distribution map for Scotland in the 2018 Review appears to show a substantial “hole” to the east of Fort William.  And yet, IOSF does have records of otters in that area and our data goes into the NBN via HBRG and there is no evidence of such a “hole” in the NBN distribution map.  So the data on otters is there.

Otter distribution in Scotland from 2018 Review    

Otter distribution in Scotland from NBN

                                                                         

Discussion

The objective of the review was “to produce the most accurate assessment possible of population size, geographical range, and conservation trends since 1995”.   However, there are several flaws in this assessment of the status of Scottish otters:

  1. The latest Scottish survey was carried out in poor weather which does reduce the accuracy of the survey – this is admitted in the Report.
  2. The number and distribution of sites surveyed was greatly reduced from roughly 4,000 in the 2003-04 survey to just 1,000 in the 2011-12 survey so it is impossible to compare.  If only roughly 25% of the sites were re-surveyed, we have no idea if populations in the remaining 75% of the sites have gone up, down or remained the same based on that data.  So you cannot conduct a proper statistical analysis if you are not comparing the same data.

There is therefore no evidence whatsoever of the true status of Scottish otters – is the population declining, increasing or stable? 

It is accepted that more data is necessary but for a truly valid Red List assessment of Scottish otters any statistical analysis must be based on sound comparative data.

The report states “The future prospects for each species were assessed, in terms of the likely changes in population size, range size and habitat quality, based on a combination of empirical evidence and expert opinion. The assessment considered historical changes in population size and range over the preceding 20 years, and evaluated direct and indirect drivers of change (for example, hunting pressure, habitat loss, and climate change).”  In many areas, particularly in England, habitat loss and fragmentation is rampant due to increased construction of housing, roads and even HS2 so clearly things have and are changing.  So how can the otter assessment for England be “Least Concern”.

The Red List is an international process, and not one that the Mammal Society or any other organisation, can change.  The argument is that it does create awareness of issues being faced by different species and although it is not perfect “a blunt tool is better than no tool at all”.   However, this can be very dangerous. It may indeed increase awareness, but once this sort of information is in the public domain and is picked up by the media it can lead to false impressions.  There are already a lot of problems with fisheries following reports that otters are “everywhere”.  By now classifying English otters as “Least Concern” it just adds to the impression that there is no need for concern.

The main problem is that Red List assessments are used by governments worldwide to create conservation policy.  If they see that a species is “Least Concern” then they will turn their conservation attention elsewhere when the situation is in fact dire. 

It is essential that any assessment is based on sound scientific data which can be validated and tested statistically.  At HBRG, we all submit our records which are then transferred to the NBN for use in assessments such as this.  If someone requests records from our Records Secretary they receive a full list of designations including Scottish Biodiversity List, Habitats Directive, etc and also the context.  It is therefore vitally important that we DO submit our records so that we can ensure that the data is there and if it is not used then we can question it.  We have examined the case for otters and found it wanting, but how do we know that the assessments for other species are equally flawed. We would encourage other members of the Group to check their own specialist species to see how accurate the assessment has been there. 

References

Findlay, M., Alexander, L. and Macleod, C., 2015. Site condition monitoring for otters (Lutra lutra) in 2011-12. Scottish Natural Heritage Commissioned Report No. 521.

Harris, S., Morris, P., Wray, S., and Yalden, D., 1995. A review of British mammals: population estimates and conservation status of British mammals other than cetaceans. Peterborough: The Joint Nature Conservation Committee.

Kruuk, H., Moorhouse, A., Conroy, J.W.H., Durbin, L., and Frears, S., 1989. An estimate of numbers and habitat preferences of otters (Lutra lutra) in Shetland, UK. Biological Conservation, 49: 241-254

Mathews F., Kubasiewicz L.M., Gurnell J., Harrower C.A., McDonald R.A. and Shore R.F., 2018.  A review of the population and conservation status of British mammals: technical summary. A report by the Mammal Society under contract to Natural England, Natural Resources Wales and Scottish Natural Heritage. Natural England, Peterborough https://www.mammal.org.uk/science-research/mammal-review-2018-technical-summary/

Mathews F., and Harrower C., 2020. IUCN – compliant Red List for Britain’s terrestrial mammals. assessment by the Mammal Society under contract to Natural England, Natural Resources Wales and Scottish Natural Heritage. Natural England, Peterborough  https://www.mammal.org.uk/science-research/red-list/

Strachan, R., 2007. National survey of otter Lutra lutra distribution in Scotland 2003-04. Scottish Natural Heritage Commissioned Report No. 211

Walker, P., 2019. Why saving the European eel matters. The Ecologist https://theecologist.org/2019/jul/16/why-saving-european-eel-matters

Yoxon, P., 2013. A Model of the Effect of Environmental Variables on the Presence of Otters along the Coastline of the Isle of Skye.  International Journal of Biodiversity, vol. 2013, Article ID 386723, 7 pages, 2013. https://doi.org/10.1155/2013/386723

Yoxon, P. and Yoxon, K., 2014. Estimating otter numbers using spraints: is it possible?  Journal of Marine Biology, Volume 2014, Article ID 430683, 6 pages http://dx.doi.org/10.1155/2014/430683

Yoxon, P., 2018. Are Eurasian otters “flooding back”?  OTTER, Journal of the International Otter Survival Fund, Volume 4 pp 29-31

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