Brown rat. Image courtesy of Anemone Projectors CC BY-SA 2.0 license Originally posted to Flickr.
The brown or Norway rat is an adaptable and versatile species. It prefers habitats with dense cover, readily available water and an abundance of food resources. Prevalent in rural farm buildings, brown rat populations also occur in other rural habitats, including hedgerows, ditches and riparian environments. Densities here vary before and after harvest. Substantial populations also exist in urban areas, where they are typically associated with refuse tips, urban waterways, warehouses, older sewers, and other areas where human food waste is available, such as the vicinity of markets and fast-food outlets (Channon et al., 2006). In urban environments they inhabit buildings, make use of refuges such as sewers, and also build burrows (e.g., into banks of rivers and canals). Populations independent of humans occur in many coastal habitats, particularly salt marshes, and in grasslands (see Harris and Yalden, 2008).
• IUCN Red List (GB: n/a; England: n/a; Scotland: n/a; Wales: n/a; Global: LC.). • This species has not been assessed for Article 17 of the EU Habitats Directive.
The British population size for brown rats was estimated as 6,790,000 by Harris et al. (1995), comprising 5,240,000 in England, 870,000 in Scotland and 680,000 in Wales. The current estimate is largely based on the same data included in Harris et al. (1995), except for numbers of, and occupancy rates for, dwellings. Differences in population size are, therefore, owing to changes in these values. The comparison of the two reviews suggests that the population is approximately stable (7% increase but no confidence limits are available). However, the data are poor, and no information is available for habitats other than dwelling houses.
Differences in the way that landscape composition was measured between the current review and Harris et al. (1995) are not relevant to brown rats because estimates were based on the number of occupied dwellings rather than habitat-specific densities.
There are very few studies on the brown rat, and no recent documented trends in population size.
Future trends.-population stable/increase. Range increasing habitat stable.
Gaps in the species’ distribution in England and Wales are likely to represent areas lacking survey effort, rather than true absences. It is unclear whether some of the larger gaps in Scotland reflect a lack of recorder effort or true absences, although the range is highly likely to have been underestimated (Tony MitchellJones, pers. comm.). Further survey effort is recommended in these areas to increase confidence in the current distribution.
The total number of farm holdings per country was taken from the Agriculture category in the UK 2015 key statistics dataset (Office for National Statistics). The number of dwellings per country was taken from the 2014-2015 dwelling stock reports from the English, Scottish and Welsh governments. Density and occupancy estimates were available only for two categories of building: ‘farm’ and ‘non-farm’. The availability of non-farm buildings was obtained by subtracting the number of farms from the total number of dwellings.
The percentage of occupied non-farm dwellings was taken from the most recent English House Condition Surveys (EHCS; Department for Communities and Local Department for Communities and Local Government, 2015) as the summed percentage of indoor and outdoor dwellings infested by rats, whilst the percentage of occupied farm buildings was taken from Harris et al. (1995). The population estimate was obtained by multiplying the mean estimate of rats per holding by the availability of buildings and the relevant percentage occupancy.
Four papers were identified by the literature search, none of which contained pre-breeding population density estimates. Two papers contained percentage occupancy values for urban dwellings, including rats present outside as well as inside; one paper contained measures of relative abundance; and one outlined the eradication of brown rats from Lundy
Most data, including population density estimates for both urban and farm dwellings and percentage occupancy for farm dwellings, were taken from Harris et al. (1995), which, in turn, was based on very few studies. Percentage occupancy for urban dwellings was taken from the EHCS (Department for Communities and Local Government, 2015). Occupancy is likely to vary between dwellings in different areas (e.g., cities and rural), but this could not be accounted for with current data.
Owing to the lack of data on the size or density of brown rat populations in habitats other than dwellings, the current estimate does not account for populations in other types of man made structures, or for human-independent populations. This could be a very significant source of underestimation, and one that is not captured by the reliability scores shown below. The use of data from Harris et al. (1995), and lack of density estimates across habitat types, mean that a sensitivity analysis is not possible.
There is likely to be some under-recording of the distribution of brown rat because relatively few records are submitted to local biological records centres. The apparent gaps in the distribution in Scotland and elsewhere may therefore be artefacts of recording effort.
The black rat is a commensal species with an omnivorous diet, although it is notably more vegetarian than the brown rat (Harris and Yalden, 2008). Dietary investigations on the Shiant Islands in the Hebrides provide evidence for the consumption of seabirds during the nesting season (McDonald et al., 1997; Stapp, 2002), but the extent to which these are scavenged rather than actively predated has not been investigated. Also known as the roof rat, the black rat is highly dependent on buildings, and in Great Britain it has tended to live in dockside warehouses and similar structures. However, in some locations, such as the Shiant Islands and Lundy, it has also occupied rocky habitats and cliffs.
Status Non native -naturalised.
• IUCN Red List (GB: n/a; England: n/a; Scotland: n/a; Wales: n/a; Global: LC). • This species has not been assessed for Article 17 of the EU Habitats Directive.
The population appears to have been reduced to zero, compared with an estimate of approximately 1300 in the previous report (Harris et al., 1995), comprised of 750 in England and 550 in Scotland. Similarly, the distribution across 80 hectads shown by Arnold (1993) has been reduced to zero.
Population,habitat and range unknown.
Image courtesy of H.Zell CC BY-SA 3.0 license.
The black rat was present in Britain by Roman times, with well-stratified remains being recorded in Roman sites in London, York and Wroxeter dated from the 3rd to 5th centuries AD (Rackham, 1979; Armitage et al., 1984). The species was common throughout Great Britain until the introduction of the brown rat — which displaced it — in the early 18th century. Its greater dependency on buildings compared with the brown rat meant that it was more susceptible to rodenticide control; in addition, the switch to containerised storage and the use of grain silos reduced food availability in ports and warehouses (Symes and Yalden, 2002). By 1956, it was restricted to major ports, a few inland towns and some islands (Bentley, 1959); it was eradicated from many of these locations by 1961 (Bentley, 1964), and by 1983 permanent colonies were thought to persist only on the Thames, in Lundy and the Shiant Islands, Hebrides. Since then, there are regular though infrequent records, usually from seaports, where the species is presumably reintroduced with shipping consignments.
The species was eradicated from Lundy, together with the brown rat, in 2006 as part of the Seabird Recovery Project (Lock, 2006). In the Shiant Islands, the population was estimated to be 230-400 individuals in 1996 (McDonald et al., 1997). These were eradicated in 2016 by the Shiant Isles Seabird Recovery Project led by the RSPB, largely in efforts to encourage small-bodied burrowing seabirds such as Manx shearwaters and storm petrels to begin nesting on the islands.
No estimate was made of population size because of the lack of records.
According to current international guidelines (IUCN, 2001), a species may only be declared extinct in the wild when exhaustive searches fail to find even a single individual. The species therefore cannot formally be considered extinct in Great Britain even though this is likely.
It is plausible that there are still small populations of this species or occasional individuals present: all commensal animals tend to be under-recorded, and there is also high likelihood of confusion with the brown rat. There has been no systematic exhaustive survey of areas likely to retain the species (such as Tilbury or Cardiff).
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