Calling on Defra to include a more representative selection of mammals in its new terrestrial biodiversity indicator

Mammal Society is calling on Defra to include a more representative selection of mammals, including the harvest mouse, field vole, otter and hedgehog in its new terrestrial biodiversity indicator

You can see the evidence that Defra has provided in developing their proposals here.

Mammal Society Response

 You can download a PDF copy of the response written below here.

Terrestrial Biodiversity Indicators

We welcome the introduction of a long-term extinction risk target, and agree with the use of Red Lists as a metric to assess progress (proposed Target D5). We also support the attempt to create a biodiversity index that more closely matches the relative abundance of different taxonomic groups than has previously been the case: the inclusion of many more invertebrates, for example, is clearly a step forward.  However, we have serious reservations about the proposals for the species abundance index (proposed Target D4, 2030 and Long-term Abundance). 

There are considerable, and unacceptable biases, in the groups chosen to represent vertebrates. More than 15% of the species in the indicator are birds, despite these representing only 0.4% of all UK species, whilst fish and amphibians are clearly under-represented.  Mammals are also slightly over-represented, though not nearly to such an extreme extent as birds. More importantly, the proposal is very strongly biased towards bats, which make up ten of the fifteen suggested species. The evidence package does not state which species form the remainder, but given that the dataset is listed as the BTO Breeding Bird Survey, we would anticipate that they are red fox, roe deer, red deer, rabbit and brown hare (or possibly fallow deer). The selection of these species is problematic for several reasons:

1. Rabbit and brown hare are naturalised rather than native species, whereas the indicator is meant to represent the forty-nine native species.
2. The over-representation of deer and bats skews the indicator, making it unrepresentative of mammals as a taxonomic group. Specifically:
   1. There is considerable overlap in the ecological niches occupied by bats. They share many similarities in terms of habitat, dietary requirement, speed of response to environmental change (because they are all long-lived and breed slowly), and so on. Similar concerns apply to deer. This introduces redundancy within the indicator and neglects the very different circumstances of many other species.
   2. Deer and bats, alongside badgers, are the species that are increasing most rapidly in the UK, and contrast with most other species which are declining or stable. (For further information please see Mathews et al. 2018; Coomber et al. 2021.)
   3. Three of the bat species are found only in restricted southerly distributions in England and Wales, with the greater horseshoe bat being particularly geographically restricted. The trends of increasing population size for greater and lesser horseshoe bats are particularly marked (Boughey and Langton 2021), meaning that the indicators for each country and for the UK will be influenced by species only found in southern Britain.  
   4. Data on mammals from the Breeding Bird survey are not subject to any formal verification process, and are collected as a secondary objective.

The proposal given in the evidence pack that other indicators, such as hedgerow or broadleaved woodland availability, could act as a proxy for measurement of abundance (p. 73) is weak: those habitats contribute only a small fraction of the total population for most of the species listed, and even for the remainder the relationship is highly variable owing to differences in the quality of these habitats. These issues are described in detail in The Mammal Society’s Review of the Population and Conservation Status of British Mammals (Mathews et al. 2018).

The solution to these difficulties is to include a wider range of species in the proposed indicator. It seems that consideration has only been given to data derived from formal monitoring schemes of the type typified by butterfly transects and breeding bird surveys. This is surprising given the strong evidence that data from less formalised surveys also provide robust indicators of change, and the approach ties Defra into retaining the status quo in the absence of new support for additional systematic monitoring.  There are several surveys of mammals based on transect surveys, and which have thorough record verification procedures, that are not included because they do not necessarily involve follow-up of the same location (e.g. the Mammal Society’s Harvest Mouse Nest Survey; Volunteer Mountain Hare Survey of Scotland Nature Scot/Mammal Society/BTO; Mammals on Roads PTES). These more flexible survey designs encourage the participation of large numbers of citizen scientists, and permit the calculation of robust abundance indices. 

Finally, we urge Defra to consider basing the biodiversity species indicator on occupancy trends.  This would permit a much broader array of species to be used as bioindicators. We assume that the proposed indicator was intended to provide evidence of changes in population size, but the bat data already include a mixture of relative abundance data (roost counts) and occupancy (bat detector surveys recording presence/absence in transect sections). In any case, the relationships between changes in relative abundance indices and changes in underlying population size are uncertain for most, if not all, UK species. The exclusion of taxa for which only occupancy data exist therefore seems curious, given the benefits it would generate in terms of species coverage. Recent advances in analytical approaches to occupancy modelling mean that it is now possible to correct for biases even in completely unstructured data (van Strein et al. 2013), and we have recently developed these methods for application to mammals, accounting for differences in detection probabilities across species assemblages (Coomber et al. 2021). This approach has shown clear declines across most small mammals in the UK. In addition, an occupancy-based indicator could be applied across multiple species and make maximum use of data submitted by the public. A strict verification process is already in place, through the Mammal Society and its network of county recorders, to assure data quality. In addition, mammal distribution and occupancy approaches have already been developed within APHA (e.g. Croft et al. 2017) and it is appropriate that Defra should be making use of its own previous research.

We therefore propose that, as a minimum, the following species should be added to the indicator:

Species	Rationale	Datasets
Harvest mouse	Indicator of mammalian fauna in arable and marginal habitats. Herbivore. A highly responsive indicator of short-term change.	Mammal Society Harvest Mouse Survey (relative abundance) Ad hoc records (occupancy)
Field vole	Indicator of mammalian fauna in grassland ecosystems (which form approximately 50% of UK land cover, and are very poorly represented in the other indicators). Britain's most abundant mammal and critical to many predatory species. Herbivore. A highly responsive indicator of short-term change.	Ad hoc records (occupancy) If resources were available, Mammal Society could set up an annual monitoring programme based on trapping (abundance)
Otter	Indicator of health of riparian systems, particularly chemical pollutants. Carnivore. Responsive to long-term changes, so a useful indicator of ecosystem status over longer time-periods.	Periodic national surveys (occupancy) Ad hoc records (occupancy)
Hedgehog	Indicator of urban/peri-urban and rural habitats. Insectivore. An indicator of medium-term changes.	PTES mammals on road survey and Living with Mammal Survey (relative abundance) Ad hoc records (Mammal Society) (Occupancy)
Water vole	Indictor of habitat quality in riparian systems (especially bankside vegetation) and success/failure of invasive species control. A responsive indicator of short-term changes.	Water vole survey (PTES) (Occupancy) Water vole records from periodic National Otter survey (Occupancy Ad hoc records (Occupancy)
Dormouse (England and Wales)	Indicator of broadleaved woodland (notably coppice and ancient semi-natural woodland). An indicator of short-medium term changes.	Dormouse Monitoring Programme (PTES) (relative abundance) Ad hoc records from tunnels/nest boxes (occupancy)
Wood mouse	One of the most ubiquitous mammals in UK. Not restricted to woodlands but also commonly found in peri-urban habitats, marginal habitats and agricultural environments. Responsive indicator of short-term changes.	Ad hoc records (occupancy) If resources were available, Mammal Society could set up an annual monitoring programme based on trapping (abundance) Ad hoc records (occupancy)

References

Boughey K, Langton S. 2021. UK Biodiversity Indicators: Technical Background Document. https://data.jncc.gov.uk/data/5299ab4d-c442-4e20-880e-b651297cb579/ukbi2021-techbg-c8-a.pdf

Coomber, F.G., Smith, B.R., August, T.A., Harrower, C.A., Powney, G.D. and Mathews, F., 2021. Using biological records to infer long-term occupancy trends of mammals in the UK. Biological Conservation, 264, p.109362.

Croft, S., Chauvenet, A.L.M. and Smith, G.C., 2017. A systematic approach to estimate the distribution and total abundance of British mammals. PLoS One 12, e0176339. https://doi.org/10.1371/journal.pone.0176339.

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.  A report by the Mammal Society under contract to Natural England, Natural Resources Wales and Scottish Natural Heritage. Natural England Joint Publication JP025.