Red List of South African Species

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Near Threatened (NT)
Assessors: Neville Pillay Peter Taylor Debbie Jewitt Genevieve Pence Rod Baxter
Contributors: Lizanne Roxburgh Nico Avenant Margaret Avery Duncan MacFadyen Ara Monadjem Guy Palmer Beryl Wilson
Reviewers: Matthew Child

Rationale

African Marsh Rats are dependent on intact rivers and wetland ecosystems, as they have not been found in artificial or degraded wetlands, and are thus patchily distributed within the assessment region. Furthermore, they are rare relative to Otomys spp., occurring at low densities with low reproductive rates within fragmented subpopulations. Although previously assessed as one species (D. incomtus), new data reveal three species within the assessment region: D. capensis endemic to the Cape region, D. incomtus probably endemic to the eastern areas of the assessment region, and D. robertsii occurring throughout northern South Africa and Zimbabwe. The extent of occurrence for both D. incomtus and D. robertsii is estimated to be far greater than 10,000 km2 while that of D. capensis is inferred to be 17,880 km2. The area of occupancy, calculated by summing the amount of remaining natural vegetation around wetlands within each species’ range, is estimated to be between 615–4,262 km2 for D. capensis, 2,382–13,823 km2 for D. incomtus, and 1,030–11,382 km2 for D. robertsii. These values, however, do not account for degraded habitats and include potentially unoccupied patches (resulting from the poor dispersal ability of the species). Thus, we take a precautionary purview to this assessment by using the lower bound of the occupancy values.

Wetlands are continuing to be lost with agricultural and human settlement expansion, which in turn increases wetland degradation from overgrazing, water abstraction, pollution and invasive alien plant sprawl. For example, between 2005 and 2011 in KwaZulu-Natal Province, 7.6% (7,217 km2) of natural habitat was lost (1.3% per annum), which equates to a 13% loss of habitat over ten years (projecting to 2015). Given the restricted range, habitat fragmentation and ongoing loss of suitable wetland habitat we list D. capensis as Vulnerable B1,2ab(ii,iii,iv), D. incomtus as Near Threatened B2ab(ii,iii,iv), and D. robertsii as Vulnerable B2ab(ii,iii,iv). We note that D. incomtus would qualify for Vulnerable C2a(i) if density estimates suggested a population size of < 10,000 mature individuals. We consider these genuine changes, as 32.8% of wetlands nationally have been lost between 1990 and 2013/14 (although this includes many wetlands that are simply temporarily dry due to the wetter conditions in 1990). This species should be reassessed once density estimates in suitable habitat are available. Key interventions for these species include conserving and restoring strips of natural vegetation around wetlands and riverside, and extending protected wetland habitat area and connectivity through biodiversity stewardship schemes.

Regional population effects: Not applicable for D. capensis. Although we assume that D. incomtus is endemic to the eastern region of the assessment region, future molecular work is needed to affirm this. For D. robertsii, dispersal may be possible through corridors or riverine vegetation within the Greater Limpopo Transfrontier Park and Mapungubwe Transfrontier Conservation Area. However, wetland habitats are fragmented and this species is a poor disperser and thus it is unknown whether significant rescue effects are possible.

Distribution

These species are associated with rivers and wetlands within the northern and southern African savannas from Senegal in the west to Ethiopia in the east and south to the Western Cape Province of South Africa (Monadjem et al. 2015). The type specimen for the genus was collected in Durban. Mullin et al. (2002, 2005) provided molecular and biogeographical evidence for the existence of D. incomtus in the KwaZulu-Natal Province region, D. robertsii in the lowveld and northern provinces of South Africa, and D. capensis in the Western Cape Province. No Dasymys species has been recorded from Lesotho (Lynch 1994).

The African Marsh Rat, D. incomtus, was previously thought to range widely across Africa, including across South Africa (Friedmann and Daly 2004), but recent genetic and morphometric analyses show that it is endemic to eastern South Africa and Swaziland (Monadjem 1998; Mullin et al. 2002, 2004). Based on morphological similarity, the species may also occur in southeast Zimbabwe in the eastern Highlands (Mt. Selinda and Mazoe) (Mullin et al. 2005), but this has not been confirmed by chromosomal or molecular analysis. If it is the same species, there is a disjunct distribution between the populations of the assessment region and Zimbabwe. Its full distribution still needs to be confirmed by sampling more localities in Zimbabwe (Mullin 2003). It is associated with Tongaland-Pondaland coastal mosaic vegetation. The Drakensberg Mountain range largely separates D. incomtus and D. robertsii but the two species are connected via a possible dispersal route through Mpumalanga Province on the eastern side of the Drakensberg and thus it is not clear why there is such a strong distinction between D. incomtus and D. robertsii (Mullin et al. 2005). We suspect this species is range-limited within KwaZulu-Natal Province due to past and ongoing wetland modification, destruction and deterioration (Driver et al. 2012).

The Cape Marsh Rat, D. capensis, is endemic to the Western Cape Province where it is known from just a few localities, occurring from Wolsley to Knysna, and may occur in Tsitsikamma. It may represent an isolated relict population (Mullin et al. 2005). It is morphologically more similar to D. incomtus than D. robertsii, which indicates a coastal distribution between the two species and supports the hypothesis that there was a link between the lowlands of the Western Cape and Ethiopia through an east coast grassland corridor that was once inundated with flood-lands (Davis 1962).

The newly described D. robertsii is patchily distributed in the lowveld of northern South Africa and Zimbabwe. Although habitat may be contiguous between the two regions, as the species is a wetland specialist, we suspect that dispersal rates are hindered by the fragmented nature of wetland systems. Within the assessment region, it occurs predominantly in Limpopo, Mpumalanga and Gauteng provinces (Mullin et al. 2005), which corresponds to the Limpopo watershed area. Additionally, Power (2014) recently recorded the species in the North West Province for the first time in a wetland on a tributary of the Waterkloofspruit at Kgaswane. The species is also expected to occur in the Kgomo Kgomo floodplain wetlands (Power 2014).

The extent of occurrence (EOO) and area of occupancy (AOO) for each species is summarised in Table 2. While the EOO was calculated using all available records for each species, the AOO was systematically estimated by buffering wetlands within the EOO by both 500 m (strip width used to assess habitat condition around wetlands in the National Biodiversity Assessment, as it provides a good proxy for wetland condition; Driver et al. 2012) and 32 m (minimum buffer zone of no development around waterbodies, as set in the National Environmental Management Act, Activity 9 and 11 Listing 1 of Government Notice R544 and Activity 16 Listing 3 of Government Notice R546 of 2010), using the wetland layer created by the National Biodiversity Assessment (Nel et al. 2011). The amount of remaining natural vegetation was then calculated using a 2013–14 national land-cover dataset (GeoTerraImage 2015a). However, these data do not include degraded habitat and thus, in reality, AOO is likely to be smaller.

Population trend

Trend

The abundances and population sizes of these species is unknown. They are rare, and exist at low densities. For example, at Karkloof Forest in KwaZulu-Natal Province, where Otomys and Dasymys co-occur, sustained collecting in suitable habitats by K. Willan over a number of years resulted in 130 Otomys and only one Dasymys (P. J. Taylor, unpub. data based on records in the Durban Natural Science Museum). Similarly, D. capensis was relatively infrequently sampled through Barn Owl (Tyto alba) pellets in the Western Cape Province (Avery et al. 2005). As a wetland specialist that rarely emerges from the wetlands, it is trap-shy and thus difficult to monitor. Compared to other African rodents of similar size, D. incomtus has low reproductive output and delayed postnatal development, which may compromise its ability to cope with continued habitat loss (Pillay 2003).

Threats

There are several major threats to this species, which revolve around habitat loss and degradation. Wetlands are the country’s most threatened ecosystem, with 65% of wetland ecosystem types threatened (48% of all wetland types Critically Endangered, 12% Endangered and 5% Vulnerable) because they are highly productive and hence become transformed for agriculture (Driver et al. 2012). The 1990–2013/14 South African National Land-Cover change report found a 32.78% decline in wetlands, nationally, during the study period (GeoTerraImage 2015a). However, this is partially confounded by 1990 being generally wetter than 2013/14 and so many wetlands in the drier western regions may not be lost, but just temporarily dry. Habitat loss due to land transformation in the surrounding matrix further isolates wetlands from one another and exacerbates the degradation of individual wetlands. For example, sugarcane plantations are often planted right up to wetlands edges, not respecting the appropriate buffer (D. Jewitt pers. obs. 2015; Photo 1). Water abstraction or filling in of wetlands from human settlement and industrial expansion also leads to habitat loss. Compounding this is wetland degradation from overgrazing rank grasses surrounding wetlands, which leads to the loss of ground cover and decreases small mammal diversity and abundance (Bowland and Perrin 1989, 1993). Similarly, suppression of natural ecosystem processes, such as fire, can also lead to habitat degradation through bush encroachment or loss of plant diversity through alien invasive species, and is suspected to be increasing with human settlement expansion. Overall, 45% of our remaining wetland area exists in a heavily modified condition, due primarily to onsite modification from crop cultivation, coal mining, urban development, dam construction, and overgrazing (and thus erosion) and off-site modifications from disruptions to flow regime and deterioration of water quality (Driver et al. 2012). For example, the number of dams in KwaZulu-Natal Province has increased from approximately 14,455 in 2005 to over 20,980 in 2011 (Jewitt et al. 2015), which represents a 45% increase in the number of dams and a 26% increase in the extent of dams. This has obvious impacts on flow levels causing drying out of hydromorphic grasslands during dry periods.

Uses and trade

This species is not known to be traded or utilised in any form.

Conservation

D. capensis presumably occurs in several protected areas in Western Cape. Further surveys are needed to compile a protected area checklist for the species. In KwaZulu-Natal, D. incomtus occurs in Ndumo Game Reserve and the Maloti Drakensberg Park at least. In Limpopo and Mpumalanga, D. robertsii occurs in Kruger National Park and presumably other formally and privately protected areas. Similarly to D. capensis, protected area managers should compile an inventory of protected areas in which Dasymys species currently exist. There are two important types of intervention that are a priority for these species:
  1. Conserve and create wetland clusters and corridors. Biodiversity stewardship schemes should be promoted if landowners possess wetlands close to core protected areas or remaining habitat patches, and the effects on small mammal subpopulations should be monitored. Protecting such habitats may create dispersal corridors between patches that will enable adaptation to climate change.
  2. Conserve or restore riparian vegetation around wetlands. Retaining ground cover and rank vegetation is the most important management tool to increase small mammal diversity and abundance around wetlands. This can be achieved through lowering grazing pressure (Bowland and Perrin 1989), or by maintaining a buffer strip of natural vegetation around wetlands (Driver et al. 2012). Small mammal diversity and abundance is also higher in more complex or heterogeneous landscapes, where periodic burning is an important tool to achieve these (Bowland and Perrin 1993). Removing alien vegetation from watersheds, watercourses and wetlands is also an important intervention to improve flow and water quality, and thus habitat quality. Education and awareness campaigns should be employed to teach landowners and local communities about the importance of conserving wetlands.
Recommendations for land managers and practitioners:
  • Working for Water managers should continue to work with private landowners in key wetland areas to remove alien vegetation.
  • Landowners and communities should be incentivised to stock livestock or wildlife at ecological carrying capacity and to maintain a buffer of natural vegetation around wetlands.
  • Enforce regulations on developments that potentially impact on the habitat integrity of grasslands and wetlands.
  • Publicise these species for conservancies as symbols of wetland conservation and thus biodiversity stewardship agreements.
Research priorities:
  • Field surveys to more accurately delineate geographic distribution, especially in the Eastern Cape Province, and investigating particularly whether these species occur within artificial waterbodies, agricultural landscapes and urban/rural gardens.
  • Similarly, density estimates need to be produced. Dasmys incomtus should be re-assessed once such data become available.
  • Estimating current and future rates of wetlands and/or natural habitat loss within the species’ ranges.
Encouraged citizen actions:
  • Private landowners should continue to work with Working for Water to conserve wetlands and improve ecosystem functioning.
  • Similarly, citizen scientists can collect owl pellets for deposit at natural history museums and help experts to identify small mammal species.

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