Rationale
(Changed due to Not applicable)
Disturbance and destruction of riverine vegetation, and degradation of river systems, has occurred throughout the White-backed Night Heron's range outside of protected areas, and the species may have undergone a decline in area of occupancy of 30% in the last three generations, qualifying it as Vulnerable. The regional population is estimated to be less than 1 000 mature individuals, occupying a highly fragmented range of less than 400 km2. Because the species is inconspicuous and nocturnal, declines could easily go unnoticed. Acute water shortages facing South Africa suggest that water abstraction, detrimental changes in hydrology, and degradation of riverine habitats are set to continue, as is a decline in water quality due to increasing turbidity, siltation, erosion, and pollution.
Distribution
A secretive and easily overlooked species that is widespread but generally sparse throughout its extensive Afrotropical range. Occurs widely in West Africa from Senegal eastwards to Nigeria, and southwards to Cameroon, Gabon, Congo and northern Angola. In Central and East Africa there are scattered populations from Democratic Republic of the Congo to eastern Sudan and western Ethiopia, and southwards from Tanzania to Malawi, Zambia and southern Angola (del Hoyo et al. 1992).
In southern Africa most records are east of 20°E, with concentrations in the Okavango Delta and Chobe regions of northern Botswana where it is particularly regular (Randall 1994, Martin 1997), as well as the Caprivi zone in northern Namibia. Isolated records along the lower Kunene possibly represent a localised but largely overlooked population (Braine 1988). In Zimbabwe it occurs primarily along the Zambezi and Save rivers, with scattered records elsewhere (Martin 1997). In Mozambique the species is surprisingly scarce but almost certainly widely overlooked (Clancey 1996), with only one record at Cahora Bassa mentioned by Parker (2005), and no recent records south of the Save River (Parker 1999).
Within the region, the species occurs very sparsely in low-lying, high-rainfall areas of northern and eastern South Africa and Swaziland, extending westwards along the south coast to about Knysna, Western Cape (Martin 1997). Due to its nocturnal habits and secretive nature it is probably widely overlooked, even in well-monitored areas, and local populations have recently been discovered in several locations. Nevertheless, its distribution is fragmented and highly localised, and the only areas where clustering of atlas records is discernable are Kruger National Park and the escarpment region of Mpumalanga and Limpopo provinces.
Martin (1997) proposed that a comparison of SABAP1 atlas data and the map presented by Brooke (1984) may suggest local extinction events have occurred in southern KwaZulu-Natal and the littoral belt of Eastern Cape. Brooke (1984) was concerned that the population on the lower Vaal River and its tributaries may have been lost, and noted that the species was previously recorded down to the Kimberley area in Northern Cape; however, the population on the Vaal persists, and it was recently recorded on the Vaal near Sasolburg and Bothaville (SABAP2 data). Similar concerns about an apparent contraction in the former Transvaal are hopefully unfounded as the species still occurs at low densities in North West, Gauteng and Limpopo provinces (Tarboton et al. 1987, Martin 1997, SABAP2 data).
A comparison of SABAP1 and SABAP2 datasets reveals a similarly scattered occurrence pattern but relatively little overlap of records and localities between the two atlas periods; this lack of congruence is probably a reflection of the secretiveness of the species, and possibly nomadic movements, rather than actual changes in distribution. Nevertheless, the most recent atlas data did not re-confirm its occurrence along the upper Limpopo, the KwaZulu-Natal/Free State border region, or the KwaZulu-Natal coastal belt, and the species has not recently been recorded west of Uitenhage in Eastern Cape. There are a small number of records from the lower Orange and Fish rivers close to Vioolsdrif on the border of South Africa and Namibia (Shaughnessy and Shaughnessy 1980, Simmons and Allan 2002), which may represent vagrants or more likely a small overlooked population (Parker and Barnes 2000).
The species was recorded in 57 grid cells during SABAP1, suggesting a maximum range of 40 000 km2, of which less than 1% (i.e. less than400 km2) comprises its specialised riverine habitat (Parker and Barnes 2000). Conversely, it was recorded from only 32 15' x 15' grid cells in SABAP2, corresponding to an actual range of only c. 224 km2 and a decrease of 44%. However, the scarcity of data complicates interpretation and this may be an underestimate.
Population
The total regional population was estimated at 500-1 000 individuals by Parker and Barnes (2000), and this estimate is probably still accurate. These authors considered Kruger National Park (40-60 pairs) and the Middle Vaal River (50-60 pairs) to be the two most important sites for the species, supporting 10-25% of the regional population between them (Barnes 1998). Gauteng Province is estimated to support approximately 3.7% of the region's population, i.e. 10-20 pairs (Whittington-Jones 2011). The population in Swaziland is estimated at five breeding pairs (Parker 1994). Population densities are not documented.
Population trend
The global population is suspected to be stable, in the absence of evidence for any declines or substantial threats (BirdLife International 2014). While the species is naturally rare and localised, having highly specialised habitat requirements and thus being confined to limited riverine habitats, the regional population is thought to be declining, based on apparent reductions in range and continued threats to its habitats. Similar patterns occur in syntopic riverine specialists such as African Finfoot Podica senegalensis, Pel's Fishing Owl Scotopelia peli and Half-collared Kingfisher Alcedo semitorquata.
Threats
The White-backed Night Heron is one of eleven local waterbird species primarily restricted to rivers; seven of these are considered Threatened or Near Threatened. Degradation and clearance of sensitive riverbank habitats is the primary threat to this species, especially for agriculture and through overgrazing in densely populated rural areas (Parker and Barnes 2000). It is unclear whether clearing of alien vegetation along rivers (such as through the Working for Water programme) has a positive or negative effect on the species.
Because of the White-backed Night Heron's reliance on rivers, consequences of detrimental factors are not limited to the point of impact but also occur downstream. Likewise, dams and other impoundments may have major ecological impacts downstream, with attendant impacts likely to accrue to species such as the White-backed Night Heron. Impoundments cause reduced river flow and attenuated flood peaks, as well as altered seasonality and temperature of flow, sediment loads, channel morphology and water chemistry (Parker and Barnes 2000). Dramatic rises in water levels and aseasonal floods in regulated waterways are a threat during the breeding season, as nests are usually built close to the water surface. Reduction of water flow, either through changes in river catchments, e.g. from invasive alien plants, or through increased water abstraction is a further threat.
Aquatic systems are also threatened by siltation, soil erosion, pollutants and eutrophication. The species is very sensitive to diurnal disturbance and the presence of secluded roosts is an important factor in habitat selection (HN Chittenden pers. comm.). Ribbon development along the coastline and particularly coastal rivers is of concern (Parker and Barnes 2000).
Conservation
Underway
No species-specific conservation measures are currently underway. The species is represented reasonably well in IBAs.
Proposed
While local protection and restoration efforts are commendable, the long-term conservation of the White-backed Night Heron and other aquatic and riverine species will ultimately depend on effective management of rivers at the ecosystem-level through national, provincial and municipal initiatives. In particular, riverside vegetation cannot be protected until alternative economic strategies are available to rural farmers, who modify it for cultivation (Parker and Barnes 2000). In the short-term, the species is thus dependent on existing protected areas. As it has bred successfully at small artificial impoundments, simulating its ecological requirements at such sites may be an effective conservation measure (Parker and Barnes 2000). A full census of suitable river systems should be conducted to modify current population estimates, determine linear densities and investigate exact habitat requirements; such efforts could simultaneously contribute to research on other co-occurring threatened riverine specialists.
Research
* Conduct surveys of river systems to obtain accurate estimates of population size and occurrence densities.
* Investigate ecological requirements, with the aim of simulating suitable breeding, roosting and foraging conditions at artificial impoundments.
* Identify, protect and manage new conservation sites.
* Conduct research into the nature and severity of threats to the species and its habitat, and investigate effects of e.g. clearance of alien trees along rivers, infestation of waterways by alien fish and other foreign species, widespread amphibian declines and other potential threats.
