By Rachel Weingart
On a balmy summer day in New Mexico, a small pond shrinks under the sun’s intense rays. Schools of spadefoot tadpoles swim about, indistinguishable from the shades of brown that make up their watery nursery. At first glance there seem to be two distinct species of spadefoots in the pond. The smaller tadpoles appear more streamlined with oblong bodies that flow into a thin tail. The larger ones are almost triple in size with more spherical bodies. Make no mistake, though: These tadpoles are all the same species. Nearly invisible to the untrained eye, these tiny creatures belong in fact to a vast tangled network of species, with which humanity is intertwined. Stressed spadefoots can be indicative of larger ecological distress.
Spadefoots are a species of frog that inhabit a variety of regions in the United States. Although called “spadefoot toads,” these creatures are technically frogs, not Bufonidae but Scaphiopodidae.
While frogs and toads are not always charismatic, they play a vital role as indicator species. Because frogs are specialists that can thrive only under very specific environmental conditions, their ecological well-being is indicative of the health of the ecosystems they call home. Like a canary in a coal mine, they can warn us of ecological dangers.
Many species adopt seemingly bizarre behaviors to survive existential threats. For the spadefoots, a major survival mechanism is cannibalism. Although scientists originally believed that it was a rare behavior only exhibited by animals under extreme human-caused stress, cannibalism is actually a natural part of the life cycle for creatures like spadefoots. This misunderstood practice is an example of environmental adaptation that, while harmful to some individuals, benefits the species as a whole.
David Pfennig, a researcher at the University of North Carolina at Chapel Hill, studies the toads to learn more about their unique relationship with members of their own species during the tadpole stage of development. He explains that some spadefoot tadpoles primarily consume decaying plant matter and other small organisms like fairy shrimp. These omnivorous tadpoles are the smaller ones — their diet is low in calories and nutrients. In contrast, some natal ponds are so rich in spadefoots that some of the tadpoles develop into the “carnivore morph, which exhibits enlarged jaw muscles and mouthparts.”
While varying dramatically in size, these tadpoles are still all the same species. This phenomenon is called phenotypic plasticity, or a physical change exhibited by an organism as a response to environmental factors. According to Pfennig, this plasticity “allows them to thrive in environments (such as deserts) where rainfall is highly variable. Specifically, in response to changing water levels in their pond, spadefoot tadpoles can either speed up or slow down their development.” Since their natal ponds are transient, spadefoot tadpoles are in a race against both time and each other to develop into toadlets. The more they eat, the faster they’ll grow. But there’s only so much food, so cannibalizing their smaller brethren is sometimes the best option to ensure survival. Evolution favors traits that benefit the survival of species, not individuals, and in this case survival means growing and developing faster than the pond can dry up. Cannibalism offers a unique way for spadefoot tadpoles to grow into froglets with lungs faster due to the nutritional benefits provided by members of their own species.
Kin recognition, the ability of an organism to distinguish between genetic relatives and non-relatives, also plays an important role in the spadefoot lifecycle. In an article published in Scientific American, Pfennig and Paul Sherman explain that spadefoots choose to associate based on morph: “If the tadpole remains an omnivore, it tends to congregate in schools that consist primarily of siblings. Its cannibalistic brothers and sisters, however, most often associate with and eat nonsiblings.” There is a delicate balance struck between practicing cannibalism and promoting the survival of their genetic relatives, and kin recognition defines this boundary line. Carnivore morph spadefoots will nibble on other spadefoots and “either eat [them] if they are not related or release them unharmed if they are siblings.” However, the survival of the individual comes first, especially if a carnivore morph spadefoot hasn’t had a good meal in some time. “The tadpoles stop discriminating against kin when their own survival is threatened — after all, a carnivorous tadpole is always more closely related to itself than to its sibling.”
In a meeting with Discover Magazine’s Bill Schutt, Pfennig explains that spadefoot tadpoles use tactile cues to determine the relative population density of their natal, transient pond. When there are too many spadefoots and too few resources available, some of the spadefoot tadpoles develop overnight into the carnivore morph and begin cannibalizing the omnivore morph tadpoles. The high population density compounds the already existing selection pressure caused by the threat of their natal pond evaporating in the desert environment.
But there’s more to this survival story. Riparian ecosystems are home to more forms of life than just spadefoot toads, and the compounding effects of climate change will continue to weigh heavily on all riparian species. The extinction of one species can create a cascading effect, rippling throughout the food web and causing the extinction of other species that depend on each other for survival. This phenomenon is known as a trophic cascade. These major ecological upheavals result in the loss of biodiversity, which is our last, best barrier against the climate crisis. With the rate of extinction skyrocketing, each passing season leaves us less and less time to understand the complex inner workings of the natural world and take action to preserve vulnerable species.
Losing frogs would mean losing vital indicator species, as well as diminishing the rich biodiversity in riparian areas that are already continuously threatened by anthropocentric factors. What can be done? University of Illinois researcher Valerie Buxton and her colleagues are exploring means of encouraging spadefoots to reproduce. Frogs and many other animals tend to live among members of their own species, a phenomenon known as conspecific attraction. This poses a challenge to repopulation efforts, however. If a habitat is devoid of frogs, reproductive frogs from nearby habitats will typically refrain from using the area as a breeding ground. Buxton’s team, however, has shown that the New Mexico spadefoot can successfully spawn in unoccupied, artificial breeding ponds amidst a chorus of recorded spadefoot calls. Overall, “spadefoots colonized playback ponds faster and more often than control ponds,” meaning that there is a potentially viable method of human-guided conservation, should the need arise to artificially increase spadefoot populations to maintain biodiversity in riparian areas.
The complex relationships between species make up the backbone of the biotic world. In his book, Flight Ways: Life and Loss at the Edge of Extinction, Thom Van Dooren explores the idea of multispecies entanglements. All forms of life on this planet are connected, and therefore humans are entangled in the ways of life practiced by other species, both thriving and disappearing. Escaping from an anthropocentric framework and looking back at it with a critical eye allows us to face hard truths. In Van Dooren’s mind, the most important step we as human beings can take is to acknowledge that the animals we share our planet with “inhabit richly storied worlds” just as we do. Van Dooren blurs the line between that which is human and that which is alive, creating a sense of oneness with all that lives on our planet. But he also acknowledges the inherent brokenness of our connections to other species.
Nature is not gentle or benevolent, and the pressure of anthropogenic extinction is immense. While humans try to care for species whose members are few in number, this human care for near-extinct animals can end up becoming a form of violence in itself. In the words of Van Dooren, “the care that is practiced at the dull edge of extinction is often intimately and inextricably entangled with various forms of violence.” There is an experience of violent care that occurs when human beings make judgments about the value of cannibalism in enabling spadefoot populations to survive and ensure that the species as a whole is not pushed into extinction. In a world where the days grow hotter and the ponds dry up faster, the phenotypic plasticity and subsequent cannibalism among spadefoot tadpoles will only grow more common.
Unlike spadefoots, humans cannot simply develop overnight into a new, cannibalistic morph to survive modern anthropogenic pressures. From climate change to environmental degradation and habitat destruction, humanity doesn’t have a great track record. The behavioral and physiological adaptation of spadefoots should thus be taken as a cautionary tale. Survival demands adaptation in one way or another, through either a physiological or societal change. But as spadefoots have shown us, adaptation may not always align with our social norms.
Today, people seem more willing than ever to grapple with the challenge of making our world a better place for all species. Climate change is not going away, and neither will our entangled relationships with the natural world. We can work together to create a world in which these impacts are mitigated, and biodiversity is preserved. Maintaining riparian habitats, removing invasive species, eliminating toxic pollutants and runoff, and conducting more ecological research should be top priorities.
Perhaps the most powerful tool at the average person’s disposal is the ability to raise awareness for other species who cannot speak for themselves. We must teach others where the riparian places in our world are and work to encourage appreciation of the incredible creatures that call them home. Love the wild places in our world — and teach others how to love them, too. It is hard to know where to start when faced with so much brokenness, but why not begin with the humble frog?
About the Author …
Rachel Weingart is Q Magazine‘s Volume 5 Student Editor. She is a senior majoring in Earth, Society, and Environmental Sustainability, with minors in Spanish and Political Science. She is also pursuing the Certificate in Environmental Writing (CEW).
This piece was written for ESE 360, the CEW introductory course, in Spring 2022.