As the sun descends behind sand dunes and wind-carved escarpments, central Qatar’s Al Rayyan region gets a reprieve from the intense heat of day. In the calm of twilight, a once-foreign sound emerges: the rasping croaking of toads drifts in over miles of parched rock and sand. Qatar’s harsh landscape is home to a growing population of African common toads, bounding into the broiling desert, and likely using humans’ artificial waterways and irrigation infrastructure to gain a toehold in a surprising new place.
Qatar’s climate is about as unfriendly toward amphibians as it gets. The Persian Gulf nation regularly has summer heat that exceeds 46 degrees Celsius, and only receives five to eight centimetres of rain a year. There are no permanent, natural sources of freshwater in the country, and no native amphibians to speak of.
So when Nobuyuki Yamaguchi — a zoologist now at University Malaysia Terengganu in Kuala Nerus — moved to Qatar in 2007 and saw toads all over the Qatar University campus, he was intrigued. Yamaguchi figured they had to have been introduced from somewhere.
“Later, when I started working on desert hedgehogs in northern Qatar, I found toads in some irrigated farms,” says Yamaguchi. This spurred Yamaguchi to work with a colleague, Abdulla Abdulkarim, to investigate how these toads were colonising such a hot and dry spot on the globe.
The researchers bounced all over the country, visiting farms, parks, and wastewater reservoirs in search of the toads. They would start by asking land-owners or site staff if they’d seen any toads on the premises, unfurling a large photo of a toad like a wanted poster to show them exactly what creature they were looking for. The team would then search the area, listening for the toads’ calls as well. After visiting 45 sites, they found toads at roughly half of them, mostly in the central part of the country surrounding the capital city of Doha, and in the northeast as well. Toads were less common in the south and west.
Their findings — published recently in the Journal of Arid Environments — suggest that the toads were likely introduced to Qatar near Doha likely by way of Egypt. They’ve since been infiltrating the desert through water supply networks used for irrigation. This is because the toads were more likely to be found in places where water is piped or brought in, rather than supplied on-site from a well.
“Toads are very unlikely to disperse long-distance over the hyper-arid environment in Qatar,” Yamaguchi said, adding that the reasonable alternative is using veins of moisture created by humans.
It’s possible the toads are flourishing in these networks in central Qatar — where the water transport system is well-developed — and spreading to more outlying areas by hitching rides in transport vehicles, which is how water and sewage tends to be shuttled around in these regions.
Giovanni Vimercati, a zoologist at the University of Fribourg in Switzerland not involved with this study, said he isn’t too surprised that the toads are able to pull this off. Unlike many other amphibians, toads can tolerate fairly dry conditions. Their parotid glands — conspicuous swellings on the back of their head — can retain water. The amphibians are also flexible, doing well in human-modified and urban environments the world over.
Vimercati agreed with Yamaguchi that the toads are probably exploiting “microhabitats” brought about by human terraforming rather than venturing out into the dunes like a bunch of miniature Paul Atreides.
Invasive toads have a grim reputation. Poisonous cane toads have wreaked havoc on Australian ecosystems for decades (and now South Florida) by eating native prey and poisoning native predators. Asian common toads are in the process of doing the same in Madagascar. It’s unclear what kind of impacts the African common toads will have on Qatar’s native wildlife, but Vimercati said it’s hard to compare this situation to Australia or Madagascar, which are far more biodiverse than Qatar’s desert habitats.
“However, this does not mean that the invasion will not have any impact,” he said. “They could be reservoirs of parasites and diseases or they can distress human communities with their breeding calls.”
The toads’ presence is also a potential threat to wild and domesticated predators (like dogs), since they have secrete poison from their parotid glands. Vimercati also noted that an established population in Qatar could act like a stepping stone for accidental introduction into Asian countries that do have diverse, already threatened ecosystems.
“The consequences may not necessarily be extremely bad in Qatar at the moment, but they can become devastating if the species will be moved from Qatar to other countries by trade routes, etc.,” he explained.
Going forward, Yamaguchi plans on testing the water network hypothesis directly, by comparing the water supply routes with the relative genetic relatedness (“phylogenetics”) of toads found in these systems.
“Two toad populations in two irrigated farms far apart may be phylogenetically very close to each other if those two farms are supplied water from the same source,” explained Yamaguchi.
For Vimercati, Qatar’s intrepid hoppers fit into an ongoing story of passive introductions and invasions in an increasingly connected world.
“With globalisation, the accidental translocations of species will increase in the future,” said Vimercati, “especially if adequate countermeasures and controls at the borders to avoid ‘contamination’ will not be taken.”
And if a toad can thrive in the desert, who knows what other potential invaders we’re underestimating?