Red List of South African Species

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Vulnerable (VU)
Assessors: Jan Venter Armin Seydack Yvette Ehlers-Smith
Contributors: Amanda Jones Lizanne Roxburgh Samantha Page-Nicholson Jeanetta Selier Peter Novellie
Reviewers: Roger Uys Matthew Child

Rationale

This sub-Saharan African species has a disjunct distribution between the eastern coastal forests of South Africa and the rest of its range. Within the assessment region, the species is inferred to be declining due to forest habitat loss from ongoing development along the coastal belt, illegal sand mining (which may represent an emerging threat) and indigenous timber extraction. Increasing bushmeat poaching and hunting with domestic dogs are also suspected to be directly causing a decline in the number of mature individuals. Preliminary data indicate that around half the subpopulations on protected areas and private lands are declining or have unknown trends (see Population). The estimated area of occupancy (AOO) ranges from 1,415–2,858 km2, depending on whether we include only currently occupied forests or all potentially viable forests within the extent of occurrence (EOO). Population estimates range widely: using a density range of 5–35 individuals / km2 yields a total mature population estimate of 3,538–50,015 individuals (using a 50% mature population structure). Blue Duiker are estimated to be unable to disperse further than 0.88 km between forest patches. Using forest clusters that fall within this dispersal distance as proxies for subpopulations, the largest cluster is estimated to be 314–687 km2, which yields 785–12,023 mature individuals. Under a precautionary purview, we suspect the lower estimates are more realistic given the wide variation in density and occupancy between patches on fine spatial and temporal scale, combined with multiple ongoing threats that may be causing local subpopulation decline or extinction.


Thus, we list Blue Duiker as Vulnerable B2ab(ii,iii,v) and C2a(i) using the lower estimates of AOO and mature population size. Further surveys, density estimates and occupancy levels across its range are necessary to more accurately calculate key parameters. This species should be reassessed when such data are available. Key interventions include effective management of the inter-patch matrix by minimising poaching rates; enforcement of legislation prohibiting illegal sand mining, development and timber harvesting; and coastal forest conservation and restoration through biodiversity stewardship schemes. As such, this species remains conservation dependent.


Regional population effects: There are no confirmed records from Swaziland or southern Mozambique, which suggests a gap in distribution between South Africa and the rest of its range. Hence, there is no rescue effect possible. Unlike the central African scenario, Blue Duiker in the assessment region occur in relatively small patches of suitable habitat within a forest/non-forest mosaic, which makes recolonisation of locally depleted patches difficult.

Distribution

This species occurs in forested areas throughout western, central, eastern and southern Africa. Within southern Africa, it occurs in eastern Zimbabwe, parts of central Mozambique (IUCN SSC Antelope Specialist Group 2016), and along the eastern seaboard of South Africa. Although once recorded in Inhambane Province, Mozambique, in 1952, extensive deforestation and civil unrest have resulted in no recent records in the region (Skinner & Chimimba 2005).

Within the assessment region, it is confined to the evergreen coastal and scarp forests and thickets along the coast from the iMfolozi River in northern KZN southwards to the eastern parts of the Western Cape Province. Additionally, although few data exist, this species is suspected to have been introduced into captive-breeding systems across many areas of the country, with confirmed presence in North West Province at least (Power 2014). There are no confirmed records from Swaziland or Lesotho (Lynch 1994; Monadjem 1998), and none from southern Mozambique, which suggests a discontinuous distribution between South Africa, eastern Zimbabwe and central Mozambique. The South African population is thus isolated.

The estimated EOO is 269,584 km2. Although previous assessments have asserted that the species exists inland in montane forest (Friedmann & Daly 2004; IUCN SSC Antelope Specialist Group 2008), there are no confirmed records for Afromontane forests along the foothills of the Drakensberg Mountains. Thus, this area is excluded from the AOO, which is estimated to be a maximum of 2,858km2 (which includes all remaining forest patches, as of 2013, within the EOO). Furthermore, they are patchily distributed within remaining forest patches, where Lawes et al. (2000) found that only 18% of the forest patches of Balgowan and Karkloof forests in KZN were occupied. Blue Duiker were not present in small forest patches further than 0.88 km from mainland populations, and smaller than 0.045 km2 in area. The probability of patch occupancy was > 50% if the patch was > 0.05 km2 and the distance between patches and a mainland forest was < 0.25 km (Lawes et al. 2000). Correspondingly, we fitted a 0.44 km buffer around patches larger than 0.045 km2 and merged patches that intersected into clusters. From 1,528 patches, we estimate there to be 637 viable clusters across the entire EOO. The largest cluster is 687 km2 with the largest single patch being 314 km2. However, not all available patches are occupied. For example, there are no current records of the species in the Kosi Bay area (S. Kyle pers. comm. 2015). Similarly, while the species was once described as “numerous” in Hluhluwe-iMfolozi and Mkhuze areas (Province of Natal 1935; Bourquin et al. 1971), they currently appear to be absent from these areas (Ezemvelo KZN Wildlife unpubl. data). Using only recent distribution records (post-2000) the estimated area of currently occupied forest clusters is 1,415 km2 (comprised of 350 forest patches within 51 clusters), assuming that any forest patch within 0.88 km2 of another forest patch is occupied.

Population trend

Trend

Blue Duikers are rare and secretive, making population counts and trend estimates difficult. The subpopulation in the Knysna forests declined to low levels between 1970 and 1980 and after 1992 in the Tsitsikamma Forest (Seydack et al. 1998). Before the decline (in the Knysna Forest) relatively high densities were encountered in moister forests, whereas after the decline population persistence was associated with drier forests (Seydack 1984; Seydack et al. 1998). This is not suspected to be caused by habitat loss or fragmentation. Instead, nocturnal warming negatively affecting digestible non-structural carbon contents of forage items is being implicated and currently investigated (A. Seydack et al. unpubl. data). It is not known whether the subpopulations have or will stabilise at current low densities. They occur in at least 27 formally protected areas in the Western and Eastern Cape provinces. The largest subpopulation is estimated to be between 400–800 individuals (based on   1–2 individuals / km2 over 400 km2 of forest cover) in the Garden Route National Park (Seydack et al. 1998); and, if we use forest clusters (defined above) as proxies for subpopulations, the largest forest cluster (687 km2) yields 344–687 mature individuals. Blue Duiker have a mean group size of four individuals (and are not limited to seasonal reproduction, but breeding occurs throughout the year). Considering the average family group in a home range, on average, consists of an adult male and female, one subadult (dispersal at about 18 months) and one infant/juvenile (Y. Ehlers-Smith unpubl. data), we infer a 50% mature population structure. There is a large overlap in offspring due to relatively short lambing interval (265 days) and they have a gestation period of 207 days (range=–216) (Bowland 1990; references within Skinner & Chimimba 2005). This, however, may not be applicable for moderate to lower quality habitats with lower fertility rates. Using 1–2 individuals / km2 to extrapolate across the AOO (1,415–2858 km2), yields a total population size range of 1,415–5,716 individuals (708–2,858 mature individuals).

However, other recorded densities are higher: throughout its continental range, densities encompass 5–35 individuals / km2 (references within Hart & Kingdon 2013). Using these densities reveals a much larger potential population size of 3,538–50,015 mature individuals within the assessment region and a largest subpopulation of 785–12,023 mature individuals. These densities are similar to recorded densities of 9–55 individuals / km2 in southern Cape forests and 13–19 individuals / km2 in the Tsitsikamma Forest specifically (Hanekom & Wilson 1991). Highest densities recorded are from Bowland (1990) and Bowland and Perrin (1995) in KZN coastal forests, ranging between 90–320 individuals / km2. The lower estimation is based on four survey methods, which had multiple repeats over eight years, whereas the upper limit is based on two survey methods that were not repeated (Bowland 1990), which equates to 2.38 individuals / ha on average. These upper values are clear outliers and cannot simply be extrapolated across the AOO as there is large amounts of variability in density and occupancy between forest patches across the range (Y. Ehlers-Smith pers. obs. 2014), and within patches between years (Bowland 1990). Recent (post-2000) density estimates suggest a density range between 11 and 24 individuals / km2 (Nakashima et al. 2013).

An alternative method of estimating population size is to differentiate between coastal and inland forests and incorporate forest patch occupancy (based on habitat quality), which reveals a total mature population of 607 mature individuals in seven protected forests. Using these density splits (238 individuals / km2 for all coastal and lowland forests and 20 individuals / km2 for all inland forests) across all currently occupied coastal and inland forest patches (1,415 km2 in total) yields an estimated mature population size 27,621 individuals. However, further density estimates from various forest types are needed to refine this estimate.

Blue Duikers have very small territories (0.6 ha in coastal forests, Bowland & Perrin 1995) and live in small family groups, so large numbers can persist if there are no threats (for example, on private land, with active anti-poaching measures; Y. Ehlers-Smith unpubl. data). Clearly, there is a wide range in possible overall population estimates as the density estimates vary depending on the habitat type and habitat quality. While population estimates and densities may be higher than previously thought, this may reflect previous underestimates rather than an increasing population. Further field surveys are urgently needed to validate subpopulation sizes in various regions, forest types and land-uses across the range of the species. Thus, while there is no robust evidence to validate a mature population of fewer than 2,500 mature individuals, it is probable that the population is fewer than 10,000 mature individuals. For example, Rowe-Rowe’s (1994) total population estimate for KZN was 2,200 in 60 subpopulations.

Subpopulations also exist in suitable habitat outside of protected areas. A study currently taking place within southeastern KZN is showing that Blue Duikers occur in forest patches within residential areas and local conservancies, and many privately-owned patches of Indian Ocean Coastal Belt Forest (Y. Ehlers-Smith unpubl. data). Similarly, in a study of mesocarnivores and duiker species by Jones (2015), of 63 conservancies and private land owners surveyed across KZN, 47% confirmed Blue Duiker presence on their property, and 53% suspected the subpopulation trend to be either unknown or decreasing over the past five years (Jones 2015). Similarly, of 92 protected areas in the species’ range, only 36% confirmed Blue Duiker presence and 42% suspect an unknown or decreasing subpopulation trend (Jones 2015). Thus, it is possible that there is a continuing decline in mature individuals. The overall population is suspected to be declining due to ongoing habitat loss and habitat deterioration, especially within coastal forests. This is corroborated by long-term game count data in KZN protected areas, which generally show declining subpopulations from the 1980/90s to present (Ezemvelo KZN Wildlife unpubl. data). Generation length has been estimated at 4.9 years (Pacifici et al. 2013), which makes the three generation window c. 15 years. Subpopulation trends on a national scale should be calculated over this time period when such data are available. Similarly, throughout the continent, this species is thought to be declining (and may have already reached the threshold for Near Threatened) due to bushmeat hunting on both a subsistence and commercial scale (IUCN SSC Antelope Specialist Group 2016).

Blue Duikers are forest specialists and are sensitive to land-use change and anthropogenically transformed landscapes, although they appear to be capable of moving through plantations (Lawes et al. 2000) and well-wooded residential estates (Y. Ehlers-Smith pers. obs. 2014). However, plantations are considered to be low quality dispersal routes, with consequences such as increased mortality rates, and may act as a demographic sink for vulnerable populations, which may have impacts on regional abundance (references within Lawes et al. 2000). Blue Duikers are also sensitive to isolation (Lawes et al. 2000) and therefore limited connectivity between forest patches or large isolation distances results in small, fragmented subpopulations, which are suspected to not be genetically viable in the long-term (Y. Ehlers-Smith unpubl. data). Effectively, many of the remnant subpopulations will not be able to contribute to the continuation of the regional genetic variety of this species.

Threats

Within the assessment region, the main threat to the species is habitat loss. This occurs through farming, particularly sugar cane, and plantation forestry, as well as expanding human development and urban sprawl, particularly along the KZN coast. Indigenous timber harvesting is mainly a threat if it results in habitat destruction or is associated with heavy hunting pressure. In some rural regions of the Eastern Cape and KZN, habitat degradation due to illegal sand mining (sensu Masalu 2002), alien invasive plant invasions, and indigenous timber harvesting are causing significant habitat loss and habitat degradation.

However, Blue Duikers can exist on mixed land-use areas. For example, forest patches surrounded by plantations seem to have less of a negative effect on Blue Duikers than other forest-dwelling species, where probability of patch occupancy when the patch was surrounded by plantations was twice that of Tree Hyraxes (Dendrohyrax arboreus) (Lawes et al. 2000). Similarly, the likelihood of patch occupancy increased when the patch was closer to human habitation (Lawes et al. 2000). Subpopulations on private land are mostly wild and free-roaming and thus count towards this assessment. However, the numbers on private land are unknown. Conservancy chairs in KZN have confirmed that numbers are on the increase since there has been a private initiative to remove snares from conservancy managed land and undeveloped municipal stands (C. Hoskins, Crags View Rehabilitation Centre, pers. comm. 2015). However, snare removal is ongoing, which suggests continuous trapping effort within semi-urban areas. Snaring within the rural and farming communities is a big problem, with continual snare removal on a bi-weekly basis necessary to keep poaching at bay. Often animals are left in snares, dead or dying, suggesting that the practice of snaring is not out of desperation. Snaring may be the major cause of a continuing decline in mature individuals. They are easily caught in snares as they create well-marked paths between bed sites and feeding sites, over which snares can be laid (Skinner & Chimimba 2005). For example, locals claim disappearance of Blue Duiker from Ingele Forest (Weza), KZN, and researchers have yet to confirm their presence in the forest after 30 km of transect surveys (Y. Ehlers-Smith pers. obs. 2015). Similarly, illegal dog hunting for sport is suspected to be causing localised subpopulation declines or extinctions (see Use and Trade). 

In a recent survey of private landowners within Blue Duiker range (Jones 2015), respondents listed poaching and dogs as common threats. Natural predation (for example, Caracal, Caracal caracal) was rarely listed and only two respondents had heard of incidents of Caracal preying on Blue Duiker in KZN (Jones 2015), which corroborates a study from Tsitsikamma National Park that found resource availability, rather than predation, to be the limiting factor (Hanekom & Wilson 1991). 

Uses and trade

Off-take under controlled trophy hunting is not suspected to be high enough to have any effect on the population, especially if trophies are increasingly sourced from introduced captive-bred subpopulations. This supposition needs to be verified with empirical data. However, Blue Duikers are suspected to be adversely affected by bushmeat hunting. The species is subject to extensive hunting for bushmeat throughout its range, and is arguably the most important wild ungulate economically and ecologically in Africa (Wilson 2001). The bushmeat trade in Africa is increasing and could, in many cases, not be considered as subsistence hunting anymore but rather commercial (Robinson & Bennett 2004; Lindsey et al. 2013). Demand tends to exceed supply, which puts tremendous pressure on wild populations of forest animals (Robinson and Bennett 2004). For example, 47% of traders at the Faraday market in Johannesburg (N = 32) sold duiker products such as horn and skin (Whiting et al. 2011), however, the species were not identified.

Illegal taxi-hunting (organised hunting using dogs), sensu Grey-Ross et al. (2010), has been observed in two of the major reserves in southeastern KZN, which showed considerably fewer duiker present possibly as a result of these hunting pressures (Y. Ehlers-Smith unpubl. data). Conversely, in areas where dog hunting practices are absent due to anti-poaching measures, Blue Duikers displayed 100% occupancy (Y. Ehlers-Smith unpubl. data). In the absence of dog racing in the province, taxi-hunts may become a business in the region and is a source of income for many (Y. Ehlers-Smith pers. obs. 2014). When poachers are caught hunting with dogs in the Eastern Cape and parts of KZN (for example, the Umzimkhulu valley), they are often in possession of a Blue Duiker. Blue Duiker are also frequently caught in snares in areas where bushpig are the target species (D. de Villiers pers. comm. 2016).

Although conservancies and private wildlife areas could benefit this species in terms of habitat conservation and protection from poaching, landowners should be careful not to introduce or maintain high stocking rates of Bushbuck (Tragelaphus sylvaticus), or extra-limital species like Nyala (Tragelaphus angasii), as this potentially negatively affects Blue Duikers through increased interspecific competition and opening up of forest habitat, exposing them to predation (Coates & Downs 2005; Y. Ehlers-Smith unpubl. data).

Conservation

The Blue Duiker occurs in several protected areas and state forests within the assessment region: Mkambati Nature Reserve (NR), Silaka NR, Hluleka NR, Dwesa-Cwebe NR (Hayward et al. 2005), East London Coast NR, Ongoye Forest NR, Dlinza Forest NR, Entumeni NR, Nkandla Forest NR, Harold Johnson NR, Enseleni NR, Addo Elephant National Park (NP), Garden Route NP, Mpofu NR, Fort Fordice NR, Thomas Baines NR, Groendal NR, Durban Bluff NR, Mpenjati NR, Kenneth Stainbank NR, Oribi Gorge NR, Mount Currie NR, Krantzkloof NR, Vernon Crookes NR, Umdoni Park Forest, Umtamvuna NR and Mbumbazi NR. As such, protected area expansion will benefit the species but is not the key intervention.

They are generally resilient to moderate anthropogenic disturbance, and can live in small forest patches, if the matrix is managed correctly (Lawes et al. 2000). Reducing disturbance in the matrix through enforcement of trespassing and illegal hunting regulations is a key intervention, and should improve connectivity between forest patches. The land-use of the matrix needs to be considered too. Although commercial plantations are used by the species as corridors (Lawes et al. 2000), they are unable to live there and plantations cannot be used as corridors for most species. Thus, the use of plantations as corridors is not recommended and instead conservancies are suggested as a land-use type conducive to connectivity. As such, biodiversity stewardship agreements may be crucial in conserving high-quality corridors for Blue Duikers and other species.

Increasing education and awareness in local communities adjacent to key remaining forest patches should be employed to highlight the plight of this species and curb sport-hunting and snaring. Alternative livelihoods can be trialled in such communities to stem poaching rates.

From a policy perspective, increased enforcement by government agencies on illegal development, sand mining and indigenous timber harvesting needs to be effected. Researching and setting quotes for the sustainable trophy hunting of this species should be undertaken.

Recommendations for land managers and practitioners:

  • Secure all suitable remaining habitat under some form of formal or semi-formal conservation legislation to prevent further habitat loss.
  • Develop and implement a metapopulation strategy to ensure the continued genetic integrity of the regional variety of this species. A precautionary approach to reintroductions and translocations should be employed due to unknown subpopulation structure.
  • Develop trophy hunting quotas that reflect local subpopulation size and recruitment rates.
  • Systematic long-term monitoring at key sites across its range to quantify population trends over three generations.
  • Registration and control of external breeding programs.
  • Increased efforts to monitor population status and effects of threats.
  • Maintain other browsing ungulates (for example, Bushbuck) at moderate densities in small fenced estates or properties.

Research priorities: The main research priority is generating taxonomic information. Little is known about subpopulations or ecotypes. Another data deficiency is accurately estimating the rate of perceived population decline. Specifically:

  • Taxonomic investigation into the species to clarify whether the species within South Africa is a regional endemic or similar to those recorded elsewhere in Africa.
  • Surveys to refine population estimates, utilisation and distribution. Once this information has been acquired then future Population and Habitat Viability Analyses (PHVAs) can be considered. Baseline monitoring with camera trap surveys are currently being conducted by Eastern Cape Parks and Tourism Agency in protected areas within the Eastern Cape. Ezemvelo KZN Wildlife conducts drive counts in Kenneth Stainbank and North Park nature reserves and has attempted camera trapping and home range mapping, but none have proved reliable methods given the resources required to implement them.
  • Quantifying the effects of habitat degradation and bushmeat hunting on the South African population.
  • Research on the impacts of changing land-use on biodiversity, particularly for mammals such as Blue Duiker, using camera trapping. Metapopulation dynamics of forest mammals in the fragmented subtropical coastal forests of southern KZN is being conducted by the University of KZN in collaboration with Ezemvelo KZN Wildlife (June 2014–June 2016).
  • Research investigating Blue Duiker as prey for Caracal is currently being conducted in KZN through the University of KZN in collaboration with Ezemvelo KZN Wildlife.

Encouraged citizen actions:

  • Report sightings on virtual museum platforms (for example, iSpot and MammalMAP) and the KZN Wildlife Watch application, especially outside protected areas.
  • Create conservancies that provide better quality habitat for this species.
  • Create biodiversity stewardship sites through the National Biodiversity Stewardship Programme that gives key sites legal recognition.

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,]
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