Chronic declines and Red Listing: Are hazel dormice Endangered?

Ellie Scopes describe her team’s latest article re-assessing the extinction risk and conservation status of hazel dormice (Muscardinus avellanarius) using long-term monitoring data.

Threats assessments and conservation priority often become entangled, despite different aims. Whilst threat assessments focus on the proximity of the species to extinction, usually with objective guidelines, conservation priority is more subjective and will involve considerations of the economic and cultural value of species, as well as conservation feasibility.

Despite these different aims, threat assessments, such as the IUCN Red List, are often mistakenly used within conservation priority. This can mean that species are only prioritised for action when they are near extinction, when the effort required to save the species is greatest.

It might be more beneficial to consider prioritising species based on returning populations to the point where they are self-sustaining, as implemented in the Favourable Conservation Status framework from the European Union.

In our study, we explored the challenges associated with threat assessments and conservation status by analysing the population trend for hazel dormice (Muscardinus avellanarius) in the UK.

Hazel dormouse collage
Left: A juvenile hazel dormouse © Ellie Scopes, Right: An adult hazel dormouse © John Webley

They were a suitable subject for robust population trend assessments thanks to the National Dormouse Monitoring Programme where volunteers check dormice nest boxes to record age, sex and weight of individuals encountered.

A previous study found that dormice are in continuous decline and based on this, they are currently classified as Vulnerable on the UK Red List. We assessed whether the population trend had stabilised due to conservation efforts, or continued to decline, and asked whether dormice might now be considered Endangered.

In Decline, but Endangered?

We used two models to assess the population trend, one based on the previous study, and one updated to reflect the new data and techniques. Both models are valid, and we wanted to include both to see how uncertainty around which model to choose would affect the results.

Population trend graph
A graph showing the overall dormouse population trend from 1994 until 2020. Model 1 (black) and model 2 (red) are both shown with their 95% confidence intervals (dashed lines). Population indices have been scaled to a value of 1 in the first year

We found only slight differences between the models when we looked at the overall population decline. The first model, our updated version, estimated the decline as 78% (95% CI = 72–84%) from 1994–2020. Meanwhile, our second model which was identical to previous analysis, estimated a slightly greater decline of 83% decline (95% CI = 75–86%) in the same period.

However, the differences become important when we consider the IUCN Red List assessment.

For dormice, the IUCN regulations indicate we should consider the decline in the most recent 10 year period (2011-2020) against the thresholds of 30% decline for Vulnerable and 50% decline for Endangered.

The first model shows 47% decline (95% CI = 38–56%), which would suggest hazel dormice stay in the Vulnerable category, but the second model shows a decline of 53% decline (95% CI = 45–59%), which would suggest an Endangered category. These model are equally valid and these estimates do not significantly differ as the confidence intervals overlap. However, this 5% difference in the central estimates, which is the only part used for threat assessments, spans the Endangered threshold and makes the assessment uncertain.

Estimated decline graph
A graph showing the estimated decline in the most recent 10 year period (2011-2020) for model 1 (black) and model 2 (red). The central estimate and confidence intervals are show – the high level of overlap indicates the models are not significantly different. The thresholds for IUCN Red List criteria for changes population size over a 10-year period for the categories Vulnerable and Endangered are indicated, and its clear that the small difference between models spans the Endangered threshold of 50% decline.

Further, it is clear from the overall trend from 1994 to 2020 that dormice are in a chronic decline, but this decline does not contribute to threat assessment. If we were to use the first model, then there has been a mean annual decline of 5.7% (95% CI = 4.7–6.8%).

If this estimated decline were to continue, by 2034 counts of dormice would have declined by >90% since 1994.Yet the decline over any 10-year period would never have exceeded the >50% threshold required for dormice to be categorised in a Red List assessment as Endangered.

Management implications

Our case study shows the impact of model uncertainty on Red List assessments, and how conservation priority based on threat assessments may miss species in chronic decline.

For species with clear evidence of declines, we need to use other methods to assign priority so that we act early instead of waiting for species to approach extinction. This is in line with the IUCN’s own views but needs to be more widely understood.

Using the evidence of the worst decline, we could label dormice as Endangered in order to generate more conservation action and funding. Regardless of this step, it is clear that more conservation action is required, as the effort to date has not stabilised the decline. Returning the population to the size it was in 1993 would mean at least doubling the current population in the next 10 years, then doubling the population again in the subsequent 10 years.

Read the full article: “Shifting baselines for species in chronic decline and assessment of conservation status. Are hazel dormice Muscardinus avellanarius Endangered?” in Issue 4:1 of Ecological Solutions and Evidence.

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