American Bison and their unexpected effects on the water snails of the Wild West

Author: Julián Monge Nájera, Ecologist and Photographer 

Thanks to the fact that a few survived the great slaughter of the 19th century, today we can see herds of bison majestically crossing a few streams in North America. Ecologist Dwayne W. Meadows wondered what effect the trampling of thousands of hooves can have on snails and other small animals that live in these streams. And he not only asked, but went to find out on an island in the Great Salt Lake, Utah.

Fuente: Wikimedia.

Antelope Island, in Utah’s Great Salt Lake, is a nature reserve with all the wildlife we associate with the Old West, such as coyotes, badgers, porcupines, and bison. But there are also inhabitants who, although they have great ecological importance, do not appear in cowboy movies, such as the snails from streams, rivers and swamps that, as part of the cycle of livestock parasites, played a forgotten role in the history of the Old West.

To understand the effect of bison on snails and other freshwater organisms, ecologist Dwayne W. Meadows compared parts of streams where bison cross the water, with calmer ones. He found that, although, in general, both parts have the same invertebrate species, if you look closely there are important differences. Bison hooves grind sand and pebbles, kill aquatic plants and animals, and increase the amount of sediment dissolved in the water. Some species suffer, but others benefit from these changes in the microhabitat. 

In my sketchbook: the three species of this story.

Physella gyrina. Fuente: Roy Anderson.

An example of the species that benefit from the passage of bison are snails of the genus Physella or Physa(perhaps Physella gyrina, common in the area, although Dr. Meadows did not identify the species): this snail lives better in shallow, muddy waters, which are richer in algae and debris that are their food.

On the other hand, the snails of the genus Stagnicola (perhaps Stagnicola montanensis, typical of the region) are scarce where bison cross; they are more common in the quiet parts of streams, with coarse sand, clean water, and denser aquatic vegetation, characteristics that, thanks to the work of another American malacologist, Dwight W. Taylor, are known to favor them².

Stagnicola montanensis. Fuente: G y Ph. Poppe.

When preparing this article, I was pleased to learn that Dr. Taylor, whom we knew in Costa Rica as “Don Guillermo” ³, had done this study of snails from the American West just when I was a little boy speaking my first, clumsy words. Who would have imagined that decades later we both would meet and that I would help edit his magnificent monograph on the physical snails of the world?⁴. 

Originally published  in Blog Biología Tropical: 13 august 2020

*Edited by Zaidett Barrientos, Katherine Bonilla y Carolina Seas.

REFERENCES

¹ Meadows, D. W. (2001). Effects of Bison trampling on stream macroinvertebrate community structure on Antelope Island, Utah. Journal of Freshwater Ecology, 16(1), 83-92.

² Taylor, D. W., et al. (1963). Freshwater snails of the subgenus Hinkleyia (Lymnaeidae: Stagnicola) from the western United States. Malacologia, 1(2), 237-281.

³ Barrientos, Z., & Springer, M. (2007). Dwight Willard Taylor, «Don Guillermo». Revista de Biología Tropical, 55(1), ix-xii.

⁴ Taylor, D. W. (2003). Introduction to Physidae (Gastropoda: Hygrophila); biogeography, classification, morphology. Revista de Biología Tropical, 1-287.