The appropriate response for controlling an invasive non-native species depends on the extent to which its invasion has progressed, which its invasion has progressed, which can be revealed by information on its distribution and abundance. Reeves muntjac is a native deer to China and Taiwan, but has been introduced and become well-established in Great Britain. Moreover, in recent years, reports and verified records in the wild from other European countries have become more frequent. We reviewed the status of Reeves muntjac in Britain and evaluated its national range expansion from 2002 to 2016. While the British population appears to have tripled in size since 1995, the rate at which it has expanded its range seems to have peaked at approximately 12 % per year between 2002 and 2005 and has since declined. We also consolidated observations on its international distribution, including a conservative evaluation of its presence in zoological collections. We predict that this species could expand its range to include every European country, although the availability of suitable landcover and climate is likely to vary substantially between countries. To prevent the significant impacts to conservation interests that have been observed in Great Britain from extending across Europe, national administrations should consider eradicating Reeves muntjac while that is still feasible.
Introduction
The Convention on Biological Diversity (https://www.cbd.int/invasive/done.shtml accessed 13 January 2020) and EU Regulation 1143/2014 on Invasive Alien Species (IAS) (Moore 2020) demand action against IAS since they can impact populations, species and the ecosystems in which they live. The options for controlling IAS include prevention of arrival, rapid removal to prevent establishment, or eradication to reverse an invasion and ongoing control of well-established populations (Simberloff 2003). Which of these options is appropriate depends on the availability of resources for management, the type of invasion curve exhibited by the species in question (Shigesada and Kawasaki 2002) and the location of the species on the invasion curve (Carboneras et al. 2018). Understanding this latter criterion requires knowledge of an IAS spatial distribution and abundance and change in these over time.
Reeves muntjac (Muntiacus reevesi, henceforth referred to as muntjac) is native to China and Taiwan and an IAS to Europe, which is known to cause substantial impacts to woodlands and plants of conservation concern (Cooke 2020). It was placed on the original list of IAS of Union concern by the EU in February 2015. The core of its European range is central and southern England, and it expanded rapidly across England and Wales during the latter 30 years of the twentieth century (Ward 2005). Despite resulting from a very small founder population (Freeman et al. 2016), the species was estimated to number approximately 50,000 in Great Britain by 1995 (Harris et al. 1995). Cooke (2019) described attempts to introduce muntjac to several locations throughout Europe, few of which were successful. However, increasing reports in the scientific and popular press have indicated that this species has now become established in continental Europe (see below) and in Japan (Ohdachi et al. 2009). To decide whether to and how to respond to invasion by muntjac, administrations for affected countries could benefit from understanding the degree to which populations of muntjac can grow and spread, and their potential for spread across Europe in the future.
We sought to evaluate the continued spread of Reeves muntjac across Great Britain since 2002 and to consolidate recent accounts of their presence throughout the rest of Europe. We also conducted a brief review of changes to their abundance in Britain since 1995 and speculated on their potential for spread across Europe if left unchecked. This included an evaluation of its maintenance in European zoological collections.
Method
We accessed the global distribution of muntjac from the IUCN red list of threatened species (https://www.iucnredlist.org/species/42191/22166608, accessed 22 May 2020). We assessed if this distribution was missing the presence of muntjac in any country from published literature (search term: Muntjac * country name * in Google Scholar, searched 22 May 2020). Based on these results we supplemented the global distribution with data from national biodiversity databases (Table 1).
Table 1 Global records of invasive non-native Reeves muntjac occurrence in the wild
Full size table
The extent of muntjac kept lawfully in licensed collections was evaluated by searching the Zoological Information Management System (ZIMS: https://www.species360.org/products-services/zoo-aquarium-animal-management-software-2/ searched 17 October 2019), a global database of 1100 zoological collections across 96 countries, for records of Muntiacus reevesi.
We received muntjac distribution data for the UK from the British Deer Society constituting observation records from national surveys completed during 2007, 2011 and 2016. During these surveys, volunteers recording presence-only for each deer species. We plotted records at a resolution of 10 km2 on the British National Grid projection, with earlier records layered on top of later records to illustrate range expansion over time. Range expansion between survey periods was calculated as a compound annual rate as described by Ward (2005).
We used landcover variables significantly associated with the presence of muntjac in Great Britain (identified from Acevedo et al. 2010) to map the minimum extent of Europe that might be suitable for muntjac occupation. These variables were matched as closely as possible to those in the CORINE Land Cover inventory for 2018 (https://land.copernicus.eu/pan-european/corine-land-cover, accessed 22 May 2020) and plotted at a resolution of 25 ha. The variables were woodland, grassland and agriculture with natural vegetation. Mountain ranges, which were identified as unsuitable for muntjac occupation by Acevedo et al. (2010), were identified from the EEA Geospatial Data Catalog (https://sdi.eea.europa.eu/catalogue/srv/eng/catalog.search#/home, accessed 22 May 2020) and excluded from the dataset.
Results
The natural range of Reeves muntjac is subtropical in eastern China and Taiwan (Fig. 1) and possibly extends to tropical southern China. However, records from this latter region might be misidentifications of M. muntjak (Timmins and Chan 2016). The species was first liberated into the wild in England in 1901 (Chapman et al. 1994; Chapman 2020) and with many later releases has become widespread and abundant in England and increasingly in Wales, but not in Scotland (see below). More recently, it has been introduced to the island of Ireland, France, Belgium, the Netherlands, Denmark, Germany and Japan (Fig. 1, Table 1), where it is known to be breeding. The species is kept in licensed zoological collections across Europe and throughout the world and is highly likely to be kept in unlicenced collections too. The ZIMS database listed 99 institutions holding 355 individuals throughout Europe, 5 institutions holding 67 individuals in Asia and 40 institutions holding 105 individuals in North America during October 2019 (Table 2).
Carden et al. (2011) explained that muntjac were first reported in the Republic of Ireland in 2007, and Dick et al. (2010) confirmed the first verified report in Northern Ireland in 2009. The pattern of records is consistent with multiple release sites in counties Wicklow and Kildare in the Republic of Ireland and in the counties Down, Armagh, Fermanagh and Londonderry in Northern Ireland. There may have been other introductions in counties Wexford, Longford, Leitrim, Sligo, Roscommon and Donegal (Carden et al. 2011; Dick et al. 2010). The National Biodiversity Center, Republic of Ireland (https://maps.biodiversityireland.ie/Species/119475 accessed 19 June 2020) listed 133 records of muntjac sightings within 35 10-km squares since 2008.
Muntjac were introduced to France in 1891 at a number of locations, probably including Chambord, the largest enclosed park in Europe, and Rambouillet (Liger and Richard 1983). However, these introductions failed to establish (Baiwy et al. 2013) and muntjac were considered extinct in France at the time of the IUCN red list of threatened species entry for this species (Timmins and Chan 2016). However, records from the Indre et Loire, Loire et Cher and Indre regions of western central France continue to be reported, and probably have their source in a privately held collection that has included muntjac since 2000 (Terlin 2017). Occasional records have been reported from other parts of France, but these have mostly been of single males only (Terlin 2017), so are unlikely to constitute established populations.
Baiwy et al. (2013) stated muntjac were not established in Belgium in 2013, but reported isolated sightings increasing from 2005. Since 2008, reports have become more frequent, particularly near the cities of Brugge, Mol-Neerpelt, Braschaat and Hasselt. The authors did not discount the possibility of immigration from populations in the Netherlands. The website waarnemingen.be (www.waarnemingen.be/species/7700 accessed 10 June 2020) listed 385 observations involving 448 individual muntjac sightings since 2010, principally in the regions of East Flanders and Antwerp. Since 2010, muntjac have been reported from 56 10-km squares out of a total of 375 squares, four of which were in Wallonia and the remainder in Flanders and Brussels. Currently, a population genomic study is seeking to establish the extent to which the different populations share common ancestries and whether they are related to seized individuals from illegal private collections.
Muntjac were first recorded in the Netherlands during 1997–1998 in the provinces of Gelderland and Overijssel (Hollander 2015). Hollander (2015) reported that the population in Gelderland still persisted around the Veluwe, but that other observations across the Netherlands were infrequent. Since 2000, the trade and possession of any species of muntjac has been forbidden in the Netherlands and zoos must be licensed to keep them (Hollander 2013). Nevertheless, some road deaths have been reported each year and illegal ownership is suspected (M. La Haye, pers. Com). In the province of Noord-Brabant, initial reports were recorded in 2005, and a mother and fawn were observed during 2013, but breeding in the wild could not be confirmed (Hollander 2015). The south of Noord-Brabant province borders Belgium, and migration from there into the Netherlands was not ruled out by Hollander (2015). The waarneming.nl website (https://waarneming.nl/species/7700/ accessed 10 June 2020) listed 42 observations and 36 photographs of muntjac since June 2019, covering the provinces of Gelderland, Limburg and Noord-Brabant.
Confirmed records of muntjac in Denmark were not discovered in the scientific literature during our searches, but two online newspaper articles (Table 1) recorded the species as breeding in central Jutland during 2015 and at least one individual, which probably escaped from a farm known to breed muntjac on Læsø island during 2019.
After the first free-living sightings of muntjac in Germany in 2004, Nehring and Skowronek (2017) reported records from eight widely scattered areas within five states, mostly of single deer, and some of which were subsequently eradicated. Later reports have been widely scattered within Rheinland-Pfalz, including Bad Kreuznach, Kusel, Birkenfeld, Mayen and near the towns of Trier and Koblenz (Hofmann 2018). A muntjac skull was found in a forest near Hildesheim, Lower Saxony a few years ago (U. Kierdorf, pers. Comm.).
In Austria some muntjac were taken in the latter part of last century to a private deer park at Altenfelden in the Oberosterreich (F. Marshall, pers. Com.). Some 90 km to the south-east of Altenfelden around 2018, there were unconfirmed reports close to Mondsee, not far from where muntjac had been held privately.
The range of muntjac in England has continued to expand since 2002 (Ward 2005). The annual compound rate of range expansion accelerated from its 1972–2002 rate (8.57%) to nearly 12% per year by 2007 but has since declined to 2–3% (Table 3). The invaded area of England and Wales grew substantially between 1972 and 2002, with a pattern consistent with movement by human agency and expansion of their core range (Chapman et al. 1994; Ward 2005; Chapman 2020). During that period, muntjac expanded their range into East Anglia, Lincolnshire, all four counties of Yorkshire and into South Wales, and isolated reports were received throughout western England and the Midlands. Records from isolated parts of Scotland were not verified (Ward 2005). Since 2002, new records of muntjac observations have in-filled the core range in central England and have expanded to cover the central south coast (Fig. 2). Expansion in the south west of England and Wales seems to have been much slower, with greater patterns of expansion into Wales across the border with England, and markedly into Cheshire and Lancashire in the north west of England. Records continue to be reported in the north east of England, although an extensive search for muntjac in county Durham revealed no tangible evidence of their persistence (I. Smales, pers. Comm.). At the time of the 2016 survey, muntjac were present in 39.8% of the 2653 10-km squares throughout mainland Great Britain (excluding the Outer Hebrides, Orkney and Shetland islands), including 61.3% of the 1715 10-km squares of England and Wales.
The pattern of range expansion across Britain has been consistent with those identified previously: gradual in-filling of the range within the core area of central England, gradual expansion at range edges and frequent new reports from locations at considerable distance from core populations (Fig. 2).
The pattern of suitable landcover for muntjac is highly variable across Europe (Fig. 3). Every Western European country has some landcover likely suitable for muntjac occupancy, and most countries have abundant landcover that may be suitable. Moreover, there appear to be many potentially suitable corridors that could facilitate expansion from the populations in France, Belgium, the Netherlands and Denmark to the Russian border and the eastern Mediterranean.
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