- The diminutive Karoo dwarf tortoise may play a key role in seed dispersal of plant species in its semidesert habitat in South Africa, a new study finds.
- A germination trial showed the tortoises transport seeds to microsites suitable for germination, a potentially vital means for plants to survive drought in the arid Karoo region.
- The dwarf tortoise is highly endangered due to degradation of its habitat and increased predation by ravens and crows accompanying expanded human presence in the Karoo.
- The findings underline the broader ecological roles that small, understudied species play in landscapes.
The diminutive Karoo dwarf tortoise (Chersobius boulengeri) may play an outsized role as gardener in its arid South African habitat. Researchers gathered droppings of the miniature species — this endangered tortoise is one of the world’s smallest, maxing out at 11 centimeters (4.3 inches) in length — and analyzed them to identify its preferred diet. The newly published research suggests that the tortoises’ dietary preferences may play an important role in seed dispersal of at least four plant species.
C. boulengeri is a cryptic species endemic to South Africa’s Karoo, a vast semidesert area covering around 500,000 square kilometers (193,000 square miles), a region larger than California. Studying the species is challenging because these small tortoises spend most of the day hidden motionless in rock crevices, necessitating long hours peering under rock after rock to spot their tiny camouflaged shells, according to Victor Loehr, first author of the paper, published in the Journal of Arid Environments.
When they do venture out, they spend a lot of that time walking around and scanning their surroundings, behavior that doesn’t make sense for a small species that’s vulnerable to predation, Loehr told Mongabay.
The Karoo dwarf tortoise is highly endangered. In research published last year, Loehr and his team from the NGO Dwarf Tortoise Conservation found that the only known population is crashing due to a combined threat of habitat degradation due to livestock grazing and predation by ravens and crows, which spread across the Karoo landscape following human activity. These two threats have likely led to local extirpations, Loehr said.
As females only lay one egg at a time, and juveniles may take up to a decade to reach maturity, the tortoise populations grow very slowly, which can exacerbate such pressures.
That makes the diet findings particularly important. “If you don’t know anything about what a species is feeding on, then it’s impossible to assess an area for suitability for the species,” Loehr said. Until now, there was limited information on what these dwarf tortoises ate or their role in the surrounding ecosystem.
The research team analyzed tortoise feces using a range of methods and found that while the species ate a host of plants, it had a preference for 10, including doll’s roses (Hermannia spp.), which are relatively scarce in the arid Karoo landscape.
Tiny ecosystem engineers
These preferred meals explained the tortoise’s finicky scanning for food options on its daily forays, but also led the researchers to believe it may have a mutualistic relationship with some of its favored flora. A small germination trial indicated the tortoise does indeed aid some plants’ dispersal and germination.
While the tortoise gets a meal, the plants could benefit as their seeds are transported to microsites suitable for germination, Loehr said. “I do think that in areas with frequent droughts like the Karoo, tortoises may be especially important vectors for seed dispersal during and after drought,” he said.
“I think if we would expand another germination trial based on a larger number of tortoises and maybe more populations, then I would not be surprised if many more [plant] species would be dispersed by this species of tortoise,” Loehr added.
Conservation action to address predation and find other tortoise populations is underway, led by the South Africa-based Endangered Wildlife Trust. Loehr said his organization has collaborated with a range of institutions to breed Karoo dwarf tortoises in captivity with the aim of possibly carrying out wild reintroductions in the future.
“So far, we have discovered that it is possible to successfully keep, breed and raise Karoo dwarf tortoises in captivity,” he said. “That they are capable of dispersing plant species that are not abundant, [which] might make them a useful ‘tool’ in habitat restoration.”
Tomas Diagne, director of the African Chelonian Institute in Senegal, said the study provides a valuable methodology that could be applied to other species such as the much larger but similarly endangered sulcata tortoise (Centrochelys sulcata). “We suspect that this species is playing a key role in the Sahelian ecosystem. Now this paper is going to be something referential to conduct a similar study,” said Diagne, who wasn’t involved in the Karoo study.
“One of the key questions we need to answer is what is the role of these species in the ecosystem,” he added. “If we lose one species of tortoise it is going to impact the ability of the ecosystem to regenerate.”
For Craig Stanford, a researcher at the University of Southern California and chair of the Tortoise and Freshwater Turtle Specialist Group at the IUCN, the global wildlife conservation authority, the study’s results aren’t surprising as other tortoises play a similar role, but they do underline its importance, despite its tiny size: “It’s not like a giant tortoise, but nevertheless, they have the capacity to be small-scale ecosystem engineers,” he told Mongabay.
“The paper is kind of a small example of the ways in which these animals are underappreciated in their ecological roles,” said Stanford, who wasn’t involved in the study. “They’re not just kind of interesting little moving rocks on the landscape, they actually have ecological roles to play.”
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Citations:
Loehr, V. J. T., Keswick, T., & Barten, N. (2023). Karoo dwarf tortoises (Chersobius boulengeri) prefer and disperse doll’s roses (Hermannia spp.). Journal of Arid Environments, 219, 105094. doi:10.1016/j.jaridenv.2023.105094
Loehr, V. J. T., & Keswick, T. (2022). Structure and projected decline of a Karoo dwarf tortoise population. The Journal of Wildlife Management, 86(2). doi:10.1002/jwmg.22159
Banner image: A captive-bred Karoo dwarf tortoise hatchling. Image courtesy Victor Loehr.
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