The Plains Spadefoot Toad: Singer, Burrower, and Essential Part of the Water Cycle
By Alyssa Roggow
Broadcast 5.11 & 5.14.2022
Soon you’ll hear them. As spring rains tame the dust of the Montana plains and rinse the grasses briefly to green, spadefoot toads (Spea bombifrons) will stir for the first time in a year and clamber from burrows beneath the soil’s frostline to the surface. There they will congregate in pools of snowmelt and rainwater, and they will sing.
To our ears, “singing” may seem an over-generous description of this mating chorus. A pond well-populated with Spea bombifrons sounds a bit like a classroom full of kindergartners with kazoos. For the toads, however, there are no accompanying giggles, because they are in a race for survival—a race against evaporation.
Water doesn’t come often to the prairie east of the Rocky Mountain Front, and what little there is doesn’t stick around long. This region has a semi-arid climate, which means the amount of precipitation that falls in a year is less than the rate at which moisture is returned to the atmosphere through plant action and evaporation, a combination of processes called evapotranspiration. For an amphibian like the spadefoot toad, this water scarcity defines every aspect of its life cycle.
Once the toads have mated, females will lay an unseemly number of eggs—up to nearly four thousand—in the temporary pools and ponds near their burrows. Surviving adults will feed on small insects and spiders, then use the keratinous “spades” on their hind legs to dig new burrows in which to wait out the coming heat. The babies, meanwhile, hatch into tadpoles within a single day and metamorphose into adults within a few weeks—one of the speediest toad childhoods on the North American continent. Many tadpoles are omnivorous, feeding on algae and small invertebrates. If there is an abundance of fairy shrimp, or if the water is drying up quickly, some tadpoles morph into carnivores (and sometimes cannibals) with large, serrated mouthparts. These tadpoles become adults faster than their omnivorous siblings, but will be indistinguishable from them once reaching adulthood. If their aqueous home lasts long enough, the newly grown spadefoots join their parents underground just in time to avoid a scorching.
Spea bombifrons burrow to depths of almost a meter, where traces of moisture remain in the soil—but little do they know their burrows act as a positive feedback switch, an action in which a community of organisms alters its environment to be more suitable to the community’s needs.
As spring wanes and summer progresses, the sandy soils above the toads dry out, grasses and plants fall dormant, microbial life slows, and the soil compacts. Ironically, the drier soil becomes, the drier it wants to remain: compact soil becomes hydrophobic, so that sudden surges of water, like melting snow and spring rains, run off quickly instead of soaking in.
These same surges of water call Spea bombifrons from their burrows at just the right moment. As they scramble to the surface, they leave behind deep channels of loose soil: pathways for water to penetrate the hard ground and initiate a chain of reactions. As moisture returns to the deeper soil, hosts of microbes resume eating, pooping, and reproducing. They band together to create teeny-tiny hydrophilic pockets, or pores, in the soil, which hold water and create space for further infiltration. All the while, Spea bombifrons sing and carry on with their reproductive blitz.
Spadefoot toad burrows may be harder to spot than the toads are to hear, but they play an important role in the water cycle—the same water cycle that you and I depend on. Positive feedback switches like this are everywhere, and when they are running smoothly, they create densely interconnected, collaborative ecosystems that sustain themselves over long periods of time. And that is great news! It’s not always dog-eat-dog out there, after all.
Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.
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