Sharon Holt, Beryl Wilson, Daryl Codron and Liora Kolska Horwitz.
Abstract
Studies of leopard tortoise (Stigmochelys pardalis) deaths in South Africa’s central interior have revealed the devastating role played by electric fences. Data on tortoise mortality show that adult female tortoises are especially vulnerable to electrocution, which in turn negatively impacts on reproduction and growth of the population. Ecological models predict an elevated risk of negative population growth and eventual extinction if this mortality pattern is allowed to continue unchecked. To ensure the survival of leopard tortoise populations, the relevant conservation bodies need to act decisively to legislate and enforce responsible design of electrified fencing on farms and reserves.
Introduction
Multiple factors threaten the future of tortoises worldwide including climate change, fragmentation and destruction of natural habitats, wildfires, over-predation, poaching, over collection and wildlife trafficking. All these dangers are currently faced by South Africa’s tortoise populations, a region with the most numerous and diverse range of tortoise species worldwide. In addition, for over two decades, researchers in South Africa have pointed out the detrimental effects of electrified fences for tortoises, especially where the electric strands are strung low. When tortoises make contact with the fence, rather than moving away, which would break the contact, they remain stationary and withdraw into their shell – often urinating, and so become part of the circuit leading to electrocution (Figure 1).
Any benefits that electric fences bring to conservation, like reducing poaching risk, must be balanced against its costs. For tortoises, these costs may be too high to justify fencing. Indeed, the first quantitative study on tortoise mortalities along electric fences undertaken in the early 1990s in the Thomas Baines Nature Reserve (Eastern Cape), showed that leopard tortoises comprised 91% of all reptile mortalities. Another, more recent study, undertaken in the south-eastern Karoo, demonstrated that tortoise mortalities were significantly higher along electric fences (56%) than regular fences (43%). Moreover, female leopard tortoises comprised more than double the number of male fatalities.
In a recent study by Holt et al. (2021), we set out to quantify the impact of electric fences on leopard tortoises in the arid central region of South Africa. Over four years (2016-2019), we collected 152 adult leopard tortoise carcasses along the outer borders of three farms in central South Africa; two in the Jacobsdal district (western Free State Province), and one in the Strydenburg district (eastern Northern Cape Province). That these animals all died from electrocution was evidenced by their close proximity to electric fences, and the fact that most carcasses were complete, with limbs present and shells intact. This contrasts to what we find if tortoises are killed by carnivores – they usually gnaw off the head and legs, and in the case of the larger carnivores, also break open the upper part of the carapace (shell) while leaving the plastron (underside of the shell) intact.
Demographic Effect of Fences
The lowest strand of wire in an electric fence is typically 100 to 150 mm above the ground. Hence, larger tortoises encounter fence wires more frequently, and so are more likely to be electrocuted than smaller ones. Female leopard tortoises are significantly larger – and so more prone to electrocution – than males. We therefore expected to find a bias towards adult females in the electrocuted death sample. To test this prediction, we measured the maximum length, maximum breadth and maximum height of the collected farm tortoise shells (Figure 2).
The resultant size distribution was indeed biased towards larger individuals (Figure 3), particularly adult females; this bias was in fact so strong it exceeded that previously recorded for tortoise deaths in the Thomas Baines Nature Reserve by Burger & Branch (1994). It was also evident when compared to size-sex break downs for a sample of dead leopard tortoises collected in the Sengwa Wildlife Research Area, Zimbabwe.
A Threatened Population
The over-representation of large adult females in the sample implies that electric fences are removing breeding-age individuals from the population at an abnormal rate. Unlike many mammals and birds, which invest (to varying extents) in survival of their offspring, species like tortoises compensate for high juvenile mortalities (due to predation, illness) by having high survival rates in adults. For example, the largest risk to sustainability of sea turtle populations is the high mortality of adults caught in fishing and shark nets, whereas excess deaths amongst turtle hatchlings have less of an impact on overall population dynamics.
Using an approach called ‘matrix modelling’ we computer simulated and compared the population ecology of tortoises affected and unaffected by electrified fencing (Figure 4). In these simulations, the excess adult female mortalities included in the population affected by electric fences, resulted in substantially lower predicted population growth rates. More importantly, the risk of negative population growth (and eventual extinction) was more than doubled by the effects of electrified fencing.
These results demonstrate that the leopard tortoise is facing a potentially serious threat, given that:
(i) reproductive age females are the main victims of the electrified fences (low strung fence wires and the high shell height of the adult leopard tortoise);
(ii) electric fencing is gaining in popularity and is widely used across southern Africa for safeguarding livestock and game.
(iii) there is no formal National guideline pertaining to the design of electrified game fences in South Africa, although there are documents compiled by Provincial authorities and/or privately-owned electric fencing companies.
Preventing Tortoise Deaths
Several strategies have been recommended to reduce tortoise deaths from electrification. These include: regular patrols to check fences and remove tortoises; attaching mesh wire – not connected to the electricity – to the lower part of the fence; raising the height of the lowest electric strand; building a barrier or rock aprons parallel to the electric fences; connecting the fence to a timer allowing the electricity to be switched off for the hours when tortoises are most active – a thermostatic switch for the fence for when it is hottest, or reducing the use of electrified fencing during the tortoise mating season and distancing such fences from water sources used by tortoises. However, many of these solutions are expensive, labour intensive or ineffectual. Ideally, as proposed by Macray (2017), timers attached to the electric fence could offer the best solution by regulating its activity to correspond to that of the tortoises i.e. electricity turned off during the hottest days, and during the hours when tortoises are most active. Such actions can significantly reduce leopard tortoise mortalities.
Why Protect Tortoises
In some African game parks, tortoise biomass is greater than that of some large mammalian herbivore species, and they are an important component in maintaining and regulating ecosystem functioning. Tortoises usually exploit large areas comprising mosaic habitats. Since they consume large amounts of grasses, fruits and seeds, do not masticate their food, have a relatively simple digestive system and long gut retention time, they serve a critical role in vegetation regeneration through seed dispersal and nutrient cycling through faeces and urine (and also as prey). In addition, through bioturbation such as burrow excavation, trampling and grazing, tortoises physically modify, maintain and create habitats thereby contributing to environmental heterogeneity in the landscape. Finally, tortoises are long-lived creatures, such that they have developed shared ecological and evolutionary histories of long duration, with plant communities.
Genetic analyses have identified seven genetic lineages of leopard tortoise, of which five occur in South Africa, with some probably endemic to the region. Although this species is currently listed as a species of least concern on the IUCN’s Red List, the findings of our study point to the long-term harmful impact of electric fences on leopard tortoise survival, raising the distinct possibility that we may lose some of these lineages. This highlights the urgent need to regulate electric fence use due to its deleterious impact on leopard tortoise demography and population viability in the near future.
Acknowledgements
We would like to acknowledge the Palaeontological Scientific Trust (PAST) for their financial support towards SH’s PhD research, on which this paper is based.
References
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