The Table Mountain massif is a sandstone ridge with granite outcrops, constituting the Northern Section of South Africa’s Table Mountain National Park. Twelve streams flowing off the mountain were chosen for this study (Figure 1). Afromontane Forests are the natural riverine vegetation with adjacent Sandstone and Granite Fynbos (Mucina & Rutherford 2006). The fynbos is fire-driven (Van Wilgen et al. 2010). Forests exclude small fires while anthropogenically altered landscapes alter fire return intervals (Slingsby et al. 2020). CapeNature, the nature conservation authority for the Western Cape Province, monitors H. rosei tadpole populations within the Table Mountain National Park and Kirstenbosch National Botanical Garden estate at the end of the southern summer (ed. Measey 2011; Measey et al. 2019), before the wet season. This allows for the greatest tadpole visibility during the lowest flow. This monitoring is through an annual timed (30 min) count on five streams at altitudes of about 300 m – 400 m, and two streams at about 800 m. The former, lower-altitudinal monitoring provides the presence/absence response variable for this study. SANParks, the conservation authority for national parks in South Africa, measures (in situ, seasonally) the water chemistry of selected streams within Table Mountain National Park in order to compare conditions where tadpoles are present/absent, controlled for altitudes of 300 m and 400 m above sea level.

Habitat suitability for aquatic larvae is entirely dependent on water conditions. The variables measured in this study are a global standard for monitoring, and prescribed by SANParks’ Biodiversity Monitoring System: the Freshwater and Estuarine Monitoring Programme (Russell, Roux & Randall 2012). A hand-held, multi-probe, water chemistry instrument (models pH100, DO200, EC300; YSI, Yellow Springs Instrument, Yellow Spring, United States) was used. Monitoring began in the summer of 2014 (inclusive of the El Niño event), ending in autumn 2016. Measuring these variables in situ is cost-effective and convenient. Measurement was taken in the middle of a stream, mid-profile, after at least 10 min of stabilisation. The explanatory variables assessed in this study are: aspect, electro-conductivity (salinity), dissolved oxygen, pH, water temperature, and permanence of water flow. Although this is not a comprehensive water analysis, these integrated metrics provide an overall indication of habitat health.

A stepwise linear regression was used to remove variables that do not explain observed presence/absence. Linear models are constructed using the retained explanatory variables. The binomial response variable (presence or absence) was tested for, while abundances are displayed in relation to the ecological boundary for the H. rosei tadpoles (Table 1). The log-likelihood of 15 models was derived, and Akaike’s information criterion calculated (Akaike 1973). All analyses were conducted in R (R Core Team 2019).

Water yield and water temperature are known to be influenced by land-use practices (Bosch & Hewlett 1982) that affect adjacent riparian vegetation cover and fluvial connectivity, and they are also affected by water abstraction and water storage (Baxter 1977; Cooper et al. 2013; Davies & Day 1998). Both no-flow of water and water of too high a temperature are indicative of habitat-disconnect.

Nursery Ravine, Cecelia Ravine and Fernwood Gully each have only one of two altitudes with favourable conditions for tadpoles. Of these, Nursery Ravine is the only stream that supports tadpoles in the 21st century. Much like Cecelia Ravine, adverse conditions are most likely due to lack of water flow rather than higher temperatures.

Re-colonisation of H. rosei tadpoles in the streams of Cecelia Ravine would represent an indication for successful rehabilitation of that system, but this has not yet occurred. The average summer temperature for the Cecelia/Spilhaus stream is 17.0 °C; however, the summer of 2015 had an average of 17.4 °C. Presumably this stream was cooler in the past if greater volumes of water flowed through it then.

Factors influencing water temperature are flow rate and the presence of dams. Both tributaries of the Disa River, Disa Stream (in Disa Gorge) and Original Disa Stream, flow from dams. The precautionary approach should apply in this regard: release of cooler (hypolimnion) waters is preferred, rather than the over-flow of warmer surface waters (from the epilimnion). The mean water temperature of mountain streams is expected to increase, largely due to increased air temperature attributed to climate change (Isaak et al. 2012).

Riparian vegetation has a cooling effect on the water in the adjoining stream (Davies & Day 1998). The Table Mountain Ghost Frog is associated with Afromontane forests. These forests have shrunk from their pre-colonial extents (Sim 1907), but have rebounded on the southern slopes of Table Mountain (Poulsen & Hoffman 2015). The latter authors suggest that in the past the indigenous forest canopy was significantly greater than its current extent, and the rebound is limited to the southern slopes of Table Mountain. Campbell and Moll (1977) show that the Platteklip and Kasteelspoort (Valley of the Red Gods) streams flowed under the cover of indigenous trees (circa 1600s) for much of their length. Mitigation of high temperatures could include the arrest of energy (heat) inputs through the restoration of indigenous forest canopies for cooling and shade.

In addition to existing measures (e.g. alien clearing), two actions to mitigate temperature extremes and droughts could be considered: (1) Streams can be shaded by rehabilitating natural vegetation (through a properly managed burn cycle, not manual planting), and (2) the environmental water reserve should be determined, met and/or mitigated, by supplementing water inputs (to the upper catchment) where possible, and by limiting water abstractions to an altitude below that used by H. rosei tadpoles. There is no forum currently to debate solutions. The City of Cape Town, CapeNature, Kirstenbosch Botanical Gardens, and SANParks (and perhaps the national and provincial authorities that issue water-use licenses) need to be open to options and innovations regarding water resilience, research and monitoring.

A Cape Peninsula freshwater conservation forum would be helpful to co-ordinate conservation and freshwater management, not only for human needs but for freshwater biodiversity and ecosystem functionality as well. CapeNature, South African National Biodiversity Institute and Table Mountain National Park should co-ordinate non-invasive tadpole surveys and water chemistry monitoring more than once a year; perhaps on consecutive days, and weeks apart. The mimicry of natural fire-return cycles in an appropriate, controlled manor is needed to rehabilitate the upper catchment area of the Table Mountain plateau. Supplementation of water as a mitigation measure is conceivable for Cecelia Ravine if some waters of the Victoria and Alexandra dams are not diverted to the De Villiers Dam, but rather left on their original eastward course towards Cecelia and Rooikat ravines. The Cecelia/Rooikat streams, Nursery stream, and the Front and Back Table (north and south plateau) are the sources of four of Cape Town’s rivers. These four streams represent four approaches to water course rehabilitation (in addition to alien clearing): reservoir-overflow diversion for environmental water requirements (Cecelia/Rooikat is the source of the Zand River catchment), mitigation of water-abstraction from small catchments (Nursery is the source of the Liesbeek River catchment), indigenous shading-plant rehabilitation (Platteklip is the source of the pre-colonial – now subterranean – Camissa River), and ecological burns (the Back Table is the source of the Disa River).