Assessing water conditions for Heleophryne rosei tadpoles and the conservation relevance

Abstract

The Table Mountain Ghost Frog (Heleophryne rosei) is endemic to the Table Mountain massif and is Critically Endangered. Other than clear, clean perennial stream flow, the optimal aquatic conditions required by their larvae are unknown. Dissolved oxygen, temperature, pH, electro-conductivity, aspect and permanence of flow are the independent variables measured seasonally at two sampling altitudes at 12 rivers of the massif. Using a logistic regression model we found that a permanence of water flow and lower water temperature were significant predictors of tadpole presence. Streams with mean summer temperature above 17.2 °C, at 300 m – 400 m above sea level, do not have tadpoles. Summer and autumn abstraction should be avoided, while a summer water temperature above an average of 17.2 °C is a threshold of potential concern for management authorities responsible for biodiversity conservation, threat mitigation efforts, and bulk-water supply and abstraction.Conservation implications: The Environmental Water Reserve has not been determined for streams of Table Mountain. The requirements of the Critically Endangered Table Mountain Ghost Frog (Heleophryne rosei) can be adopted as the minimum conditions to support this species and associated communities. Perennial flow, an average January water temperature of 17.2 °C or lower.

Impacts of invasive fishes on ghost frog tadpoles

“…The positive effect of water temperature is unexpected, as previous literature reports 17.2°C to be the upper limit for tadpoles of a congeneric species, such as H. rosei (Ebrahim, de Villiers & Measey, 2020). It is, however, important to consider the range of temperatures sampled in this study is 10.1°C – 23.9°C (Figure S1).…”

“…Additionally, we provide valuable information on what constitutes suitable habitat for heleophrynid tadpoles. Acidic streams are associated with higher tadpole abundance, as previously suggested (Ebrahim, de Villiers & Measey, 2020). The pH of a stream is affected by terrestrial biotic mechanisms, including acidification of water by riparian vegetation such as in the fynbos biome, with indigenous forest and pine plantations also having the capacity to acidify water (Midgley & Schafer, 1992).…”

“…The significant effect of riparian vegetation type on tadpole abundance shows that alien pine plantations are associated with a decrease in tadpole abundance, either because of a direct effect on the aquatic habitat or because they alter the terrestrial habitat for adults. Ebrahim et al (2020) hypothesise that pine plantations slow the flow of water and provide less shading to the stream, which negatively affects tadpole occurrence of H. rosei . This effect specifically on heleophrynid tadpoles remains untested, but pine and eucalyptus plantations have shown to significantly reduce water flow in South African streams during the first 6 to 20 years of tree growth (Scott & Prinsloo, 2008).…”

“…Tadpoles spend between 18 and 24 months in the larval stage, requiring permanent flow to survive. This permanence of flow, along with a mean annual water temperature that does not exceed 17.2°C were identified as key habitat descriptors for tadpoles of the Table Mountain ghost frog, Heleophryne rosei (Hewitt 1925) (Ebrahim, de Villiers & Measey, 2020). Given their specialist life history and limited suitable habitat, ghost frogs are susceptible to population declines hrough either human mediated threats (invasive species, habitat loss) or natural phenomena (floods, fires) (SAFRoG & IUCN-ASG, 2016a,b).…”

Fine-scale drivers of extinction risk: tadpole occupancy dynamics of the Table Mountain Ghost Frog ( Heleophryne rosei )

“…The lack of conservation effort for Heleophryne rosei may be in part influenced by a paucity of data that confirms these threats as tangible rather than anecdotal. Only two studies, focussing on distribution and conservation (Boycott and de Villiers 1986), and broad patterns of tadpole ecology (Ebrahim et al 2020) have been published on H. rosei since its description in 1925 (Hewitt 1925). The last survey of the species took place 25 years ago and included documenting the first localised extinction event from an entire stream (de Villiers 1997).…”