Invasion of the guttural toads

Abstract

The Western Cape region of South Africa is a modern Garden of Eden packed with indigenous species spread across two globally recognised biodiversity hotspots. So, when the snore-like call of invasive guttural toads (Sclerophrys gutturalis) threatening to displace native populations was first heard in Cape Town in 2000, alarm bells began ringing. Giovanni Vimercati from Stellenbosch University, South Africa, says, ‘The ability of this species to invade Cape Town is surprising’, as guttural toads are usually better suited to the more humid and warmer environments of subtropical southern Africa than the cooler, dry Mediterranean climate of the Western Cape. Yet, despite the mismatch, the population flourished. ‘As the invading population expanded, individuals would need to cope with the new climate by rapidly adapting to their drier cooler surroundings’, says Vimercati; and they have done so remarkably quickly. Concerned by the toads’ success, Vimercati and his PhD supervisors, John Measey and Sarah Davies, decided to investigate how the invaders’ ability to withstand dehydration had altered during the two decades following the occupation.Explaining that it is relatively easy to find the toads by torchlight as they congregate after dark around ponds during the breading season, Vimercati says that the main problem was the reluctance of homeowners in Cape Town and Durban to allow the scientists into their gardens. ‘They and their dogs were not always enthusiastic’, chuckles Vimercati. As soon as the toads were captured, the trio measured how dehydrated the animals were by popping them into a shallow tub of water and weighing them until they absorbed no more. Not surprisingly, the Cape Town toads were more dehydrated (∼90% hydrated) than the toads from Durban (∼95% hydrated), and when the team blew a gentle breeze over the fully hydrated animals in the lab, the two groups lost water at roughly the same rates, so the invaders had not improved their waterproofing. However, the team noticed that the Cape Town toads hunkered down more, tucking their legs in beneath their body and pulling their head down close to the ground in a bid to conserve water. The invaders were also better at reabsorbing water that had been lost; they were better equipped to cope with water losses. But how did the dehydration affect their performance?‘I spent hours running round in small circles after reluctant toads’, laughs Vimercati, as he describes chasing the dehydrated animals (90% and 80% hydration) around a small racetrack in the dead of night to find out how well the Cape Town invaders and their Durban cousins coped when they were low on water. Impressively, the invaders were more robust than their indigenous cousins, with five of the 90% hydrated animals covering distances of over 500 m, in contrast to the Durban toads, which only managed an average distance of 250 m. And when the team checked how well the animals coped with colder conditions by gently chilling them until they could no longer right themselves after toppling over, the toads from cooler Cape Town were able to return to a squatting position at temperatures down to 7.1°C, while the Durban animals were only able to recover at temperatures above 8.2°C.‘Although they've only been there for 20 years, invading toads in Cape Town are better suited than their native cousins to survive and reproduce in the new surroundings’, says Vimercati and he warns ‘These kinds of rapid adaptations may allow guttural toads to invade climates quite unlike their native conditions, making it hard to predict their potential success in new areas’.