THREATS TO AMPHIBIANS: Grazing

by Laura Monti (monti@apollo.umenfa.maine.edu)

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Despite the vast amounts of land in the western U.S. devoted to grazing, little work has been done to evaluate the effects of this form of land use on the ecosystem. Roughly 70% of the area of the westernmost eleven contiguous states is devoted to grazing, a practice that has the potential to alter species composition within communities, disrupt ecosystem function and alter ecosystem structure (Fleischner 1994). Because amphibians and reptiles are commonly recognized as good indicators of habitat conditions, studies to document the effects of grazing on local herps should help us to evaluate the actual impacts of grazing. To date, such studies have been relatively sparse and results have been contradictory. The majority of the available literature focuses on reptiles, with very little published regarding amphibians. The following is a summary of the available literature and E-mail response to a query about this subject.

In southern England, cattle grazing is currently being tested as a management tool in maintaining habitat for Britain's rarest amphibian, the natterjack toad. The grazing regime in this instance is designed to prevent heath land succeeding to scrub and woodland, maintaining open areas for toads to forage in (Griffiths E-mail comm.). In Israel, moderate levels of pollution of breeding ponds with cattle dung has proven beneficial to some amphibian species. Anuran tadpoles, especially those of the Syrian spadefoot toad, Pelobates syriacus, feed directly on cattle dung. Urodele tadpoles receive indirect benefits, since dung encourages development of invertebrates, their preferred food. However, the researchers responsible for this study point out that excessive amounts of cattle dung destroy the habitat (Mendelssohn E-mail comm.). It appears that well-managed grazing may benefit invasive amphibians species as well. For example, Bury and Whelan (1984) found that the construction of reservoirs, irrigation canals and watering holes for cattle created permanent waters utilized by bullfrogs in California.

Researchers in Scandinavia stated in an E-mail that the green toad, Bufo viridis, is very dependent upon grazing. This species prefers to breed in relatively shallow ponds (<1m) in heavily grazed meadows. Eutrophication of the ponds caused by the animals churning up the bottom and defecating in the water favor green toad tadpoles. The adult toads prefer the very open nature of the grazed fields. These scientists feel that grazing, at least in Europe, may be important to the survival of species adapted to early successional communities. Europeans are constantly attempting to prevent natural catastrophes such as seasonal flooding. By keeping an area very open, grazing animals may serve to mimic the effect of such flooding (Tramontano E-mail comm.). Finally, a study in southeastern Idaho did not find a significant effect on western toads (Bufo boreas). Careful management by the local rancher may have contributed to this finding (Bartelt E-mail comm.). Thus, it appears that some amphibian species may benefit from the habitat alteration caused by grazing, while ecologically sound grazing management may prevent detrimental effects.

However, many studies suggest that the effects of grazing on habitat structure are devastating to the local herpetofauna. Most of the published material deals with reptiles. For example, Bock et al. (1990)found over 10 times as many bunchgrass lizards, Sceloporus scalaris slevini, on plots protected from grazing as on unprotected plots. These lizards appear to require intact bunchgrass to escape predation, a habitat component not found on grazed land. Jones (1988) cites his own work, which uncovered large differences in the presence and abundance of certain lizards in areas subjected to either heavy or light grazing. He attributes this difference to changes in habitat structure. Busack and Bury (1974) claim that sheep grazing reduces ground cover necessary for some lizard species. They report that an ungrazed plot within their study site had two times the number of lizards as a grazed plot. Species diversity was also greater on the ungrazed plot. Riparian areas seem to be especially susceptible to the loss of habitat structure described in these papers. Szaro et al. (1985) describe a complex relationship between dead-and-down material, earthworm abundance, and population levels of a riparian garter snake. Grazing pressure alters this balance to the detriment of the snakes. Taken as a whole, these papers suggest habitat structure is impacted by grazing. Loss of certain elements of the natural structure can have negative consequences for some reptiles.

Negative impacts of grazing on amphibians have been documented primarily anecdotally (though see also Worthylake and Hovingh 1989, Jennings 1988 and Jennings and Hayes 1993). For example, the aforementioned research in Scandinavia also found that the fire belly toad, Bombina bombina does not breed successfully in ponds subject to use by grazing cattle and sheep (Tramontano E-mail comm.). In British Columbia, Dr. S. Orchard (E-mail comm.) has described the effects of cattle congregating at ephemeral pools that provide spawning habitat for Great Basin spadefoot toads. The shallow depressions are quickly reduced to a labyrinth of hoof-created pits no longer able to support tadpole populations. He also states that small mammals appear to be excluded from grazed areas. Blotched tiger salamander, found in these grazed areas, use rodent burrows as retreats, and thus may be negatively impacted by this reduction in rodent numbers. In the Sierra Nevada region, Dr. Roland A. Knapp (E-mail comm.) claims that mountain yellow-legged frogs have disappeared from a meadow since that area was incised by the South Fork Kern River. The incision, he believes, was due at least in part to heavy grazing. In Zion National Park, Dr. Breck Bartholomew (E-mail comm.) feels that their may be a link between the presence of livestock (in this case, horses) and an increased incidence of fungal infections in egg masses.

It is obvious that these observations suggest that grazing has a negative impact on amphibian populations. It is also obvious, however, that considerable research is necessary before any conclusions can be drawn. Overall, there is a great need for a well-organized effort to examine the effects of grazing on the ecosystem in general and the local herpetofauna in particular. it is important to note that results appear site and species specific. Before we either condemn all grazing as the ultimate evil, or praise the benefits of cattle as habitat management tools, we need to have our facts straight. Without a doubt, grazing alters the ecosystem. To what extent and to what end are questions that still beg answers.

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References

Berry, K. H. 1978. Livestock grazing and the desert tortoise. North American Wildlife and Natural Resources Conference Transactions 43: 745-750.

Bock et al., 1990. The effect of livestock grazing upon the abundance of the lizard Sceloporus scalaris in southeastern Arizona. J. Herp. 24:445-446.

Bock, C. E., H. M. Smith, and J. H. Bock. 1990. The effect of livestock grazing upon abundance of the lizard, Sceloporus scalaris, in southeastern Arizona. J. Herpetol. 24(4): 445- 446.

Bury, R. B., and J. A. Whelan. 1984. Ecology and management of the bullfrog. U.S. Fish and Wildlife Service Resource Publication 155, 23 pp.

Busack, S. D. and R. B. Bury. 1974. Some effects of off-road vehicles and sheep grazing on lizard populations in the Mojave Desert. Biol. Conserv. 6(3): 179-183.

Fleischner, T. L. 1994. Ecological costs of livestock grazing in western North America. Conserv. Biol. 8(3): 629-644.

Jennings, M. R. 1988. Natural history and decline of native ranids in California. Pp. 61-72 In: H. F. DeLisle, P. R. Brown, B. Kaufman, and B. M. McGurty, eds. Proceedings: Conference on California Herpetology. Southwestern Herpetologists Society, Special Publication No. 4. 143 pp.

Jennings, M. R. and M. P. Hayes. 1993. Decline of native ranids in the desert Southwest. 1994. Pp. 183-211 In: P. R. Brown and J. W. Wright, eds. Proceedings: Herpetology of the North American Deserts. Southwestern Herpetologists Society, Special Publication No. 5. 311 pp.

Jones, K. B. 1988. Distribution and habitat associations of herpetofauna in Arizona: comparisons by habitat type. Pp. 109-128 in: R. C. Szaro, K. E. Severson, and D. R. Patton, technical coordinators. Management of amphibians, reptiles, and small mammals in North America: Proceedings of the symposium. USDA Forest Service, General Technical Report RM- 166.

Oldemeyer, J. L. 1994. Livestock grazing and the desert tortoise in the Mojave Desert. Pp. 94-103 In: R. B. Bury and D. J. Germano, eds. Biology of North American tortoises. National Biological Survey, Fish and Wildlife Research 13.

Szaro, R. C., S. C. Belfit, J. K. Aitkin, and J. N. Rinne. 1985. Impact of grazing on a riparian garter snake. Pp. 359-363 in: R. R. Johnson, C. D. Ziebell, D. R. Patton, P. F. Folliott, and F. H. Hamre, techincal coordinators. Riparian ecosystems and their management: Reconciling conflicting uses. USDA Forest Service, General Technical Report RM-120.

Worthylake, K. M., and P. Hovingh. 1989. Mass mortality of salamanders (Ambystoma tigrinum) by bacteria (Acinetobacter) in an oligotrophic seepage mountain lake. Great Basin Nat. 49(3): 364-372.

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