Drewes’ Moss Frog Survey

There was great excitement when Sheraine van Wyk and colleague Anina Lee heard the call of the Drewes’ Moss Frog in Fernkloof Nature Reserve. But why get all worked up over a tiny frog that could only be heard and not seen at the same time?

Sheraine, Whale Coast Conservation’s Education Manager, has a special interest in frogs. Frogs are interesting little critters. Being amphibians, most of them hatch from eggs laid in water into aquatic tadpoles, which then metamorphose into terrestrial frogs. Their skins are permeable and very sensitive to pollutants, so they are very good bio-indicators of the state of health of an eco-system where they occur. This makes frogs very useful in assessing whether wetlands, for example, are polluted. If there is pollution, you won’t find any frogs there.

Sheraine and some of her colleagues have, over the years, become especially interested in the frogs of the Overstrand. It’s not always easy to spot them amongst vegetation, but they are a dead giveaway during the mating season when the males call out for attention from the females. Each species of frog has a distinct mating call, so they can be identified from their calls, as one can with birds.

Drewes’ Moss Frog, (Arthroleptella drewesii) is a small, extremely well-camouflaged frog and, as the name implies, it hides under moss and other dense vegetation. It even lays its eggs in the damp vegetation and fully formed froglets emerge from the eggs without having gone through the usual tadpole stage in water. The adult is only 18 mm in size and the colouring varies from blotchy brown to black. It’s a rare privilege to catch a glimpse of one.

In fact, so seldom are they seen that Louis du Preez and Vincent Carruthers in their book Frogs of Southern Africa (2009) describe the conservation status of the Drewes’ Moss Frog as “Data deficient” and the distribution range as “uncertain.”

So not surprisingly, frog enthusiasts were thrilled to hear these frogs calling in Fernkloof Nature Reserve in October 2015. The calls were surprisingly wide-spread in the reserve. But it was evident that not much was known about the exact locations and the numbers of this species in the Reserve. When Anina heard them again at the end of May 2016 it sparked a decision to survey the population to find out more about this Fernkloof favourite. Whale Coast Conservation undertook the survey with the financial support of the Hermanus Botanical Society.

Sheraine devised the survey method and supervised the two field workers. They were Michael Brits, a student at Stellenbosch who is experienced in GIS mapping, and Denfred Bruintjies, Sheraine’ s colleague at Whale Coast Conservation who is a whizz at recognising frog calls. In the first week of July 2016 they traversed more than 28 km of hiking trails in Fernkloof. They listened for the distinct calls, closed in on them and, using a hand-held GPS device, marked the location. These little frogs are so confident of being unseen that they don’t bother to stop calling when one approaches. So it was possible to estimate the number of frogs calling at each identified location. Michael and Denfred recorded the calls and, where possible, took a picture of the habitat. The GPS data was used to create maps to show where the Drewes’ Moss Frog occurs and roughly how many there were.
As one might expect, the little frog was heard after the first winter rains in all the damp places and rain seepages in the central Fernkloof water catchment.

Figure 1: Michael Brits (left) and Denfred Bruintjies took up the challenge of finding a specimen of the Drewes' Moss Frog

Although Michael and Denfred could hear the frogs, it proved to be nearly impossible to find one in the dense vegetation. It became a personal challenge for them to find at least one! For four days they tried in vain until, on the very last day, Denfred heard one calling from a little temporary rivulet high up in the mountain. And there, under a stone, was the prized little fellow. What a find! Denfred was very excited and his hand shook so much that most of the photographs he took of the frog were out of focus. But he managed one great shot of the frog sitting on his finger.

And how many frogs were there? Well, let’s say there is a very healthy population of several hundred at least. Although a concentration of one species in such a small area makes them vulnerable to any catastrophe, Fernkloof is a well-managed reserve. The frogs can even survive a fynbos fire (such as the one that swept through Fernkloof in December 2015). Our very special little Drewes’ Moss Frog is as well protected and looked after as one could wish.

Further research funding needed

1. We now have to do a follow-up survey of the frogs during 2017 to confirm data. We would also like to collect data from areas adjacent to the Fernkloof Reserve.
2. We would also like to do a survey of the plants in typical moss frog habitat.

Funding needed for further research is R10,000 p.a.