Here in Colorado, at least along the Front Range, we enjoy a variety of "wilderness" recreational opportunities from hiking to rock climbing. Sometimes differing modes of trail enjoyment come into conflict, and sometimes the wildlife suffers even worse consequences. Here I will relate why people traveling over trails at high speed (mountain bikers and trail runners specifically) need to understand their potential impacts on reptiles in particular.
I have been fortunate to encounter at least four species of snakes along popular trails in various parks and open spaces. Snakes make use of patches of earth unobstructed by vegetation in order to bask, warming their bodies for active hunting later, to help digest a meal, or otherwise regulate their metabolism. Trails are ideal for this purpose and it is not uncommon to find serpents stretched across the full width of a trail.
Obviously, venomous snakes such as the Prairie Rattlesnake also pose a threat to people using trails. Trail runners must be mindful of the potential to encounter rattlesnakes, especially in early morning or early evening hours, and on overcast days. I personally recall one afternoon when a trail runner passed me, and I almost tripped over a Prairie Rattlesnake shortly thereafter. I suspect the runner never saw it.
When traversing terrain at a high rate of speed, it is essentially impossible to notice a snake or other small organism on the trail. If it is noticed it may be mistaken for a branch or other object. Snakes are generally well-camouflaged and overlooked even by those seeking to find them.
Besides the fact that a snake cannot move rapidly enough to avoid an oncoming bicycle tire, or running shoe, it is usually not programmed to do so behaviorally. A snake's primary defense against a perceived threat is to rely on camouflage and stillness in hopes the predator or danger does not detect it. So, snakes are, figuratively if not literally, "sitting ducks" when it comes to oblivious mountain bikers and trail runners.
There is evidence that other types of small animals also suffer from collisions or other encounters with mountain bikers and trail runners. While my chief personal interest is in insects and spiders, and I find a great deal of carnage related to even regular bicyclists, most invertebrate species have robust, widespread populations that can withstand even heavy mortality. Not so with many reptiles, small mammals, ground-nesting birds, and other organisms. Remember that all animals are subject to many non-human mortality factors as well: Predators, parasites, disease, and infertility for example.
What can you do to avoid conflict and still enjoy yourself? Consider recreating at a time of day when reptiles are not normally active. Yes, that means late morning through afternoon during the warmer months. Winter riding means little or no conflict with wildlife. Slow down, especially in open habitats such as prairies, meadows, and glades, south- or east-facing hillsides, and along rock outcrops. Ride only on trails designated for mountain biking (but you do that already, I'm sure). If there is a park headquarters, visitors center, or nature center, consult with personnel there to find out which trails are best, and whether there is frequent wildlife activity along them.
Thank you for reconsidering your riding and running habits and becoming more "wildlife-friendly" to other creatures. Those of us who share the trails but move at a slower pace and enjoy our encounters with animals will be grateful for your thoughtfulness.
Sources: Burgin, Shelley and Nigel Hardiman. 2012. "Is the evolving sport of mountain biking compatible with fauna conservation in national parks?," Australian Zoologist 36(2): 201-208.
Craver, Monica. 2009. "The Impacts of Mountain Biking on Amphibians and Reptiles, 2008," e-mail to Council ATdnv.org
Davenport, John and T. Adam Switalski. 2006. "Environmental Impacts of Transport, Related to Tourism and Leisure Activities," in Davenport, John and Julia L. The Ecology of Transportation: Managing Mobility for the Environment. Dordrecht: Springer. pp. 333-360.
Goode, Matthew J., Jeffrey M. Howland, and Michael J. Sredl. 1995. Effects of Microhabitat Destruction on Reptile Abundance in Sonoran Desert Rock Outcrops. Nongame and Endangered Wildlife Program Heritage Report. Arizona Game and Fish Department, Phoenix, Arizona.
Vandeman, M.J. 2011. "The Impacts of Mountain Biking on Wildlife and People - A Review of the Literature," ARPN Journal of Science and Technology 4(7): 418-426.