don't need a weatherman to know which way the wind blows

Conservative republicans and the oil companies that they invest in want you to believe that driving your car doesn't contribute to global warming. They say there aren't enough studies to validate it. 99% of atmospheric scientists claim otherwise, but GW Bush says we need more "sound science" before anything is done about it. User groups, from the Blue Ribbon Coalition to BATCO, use the same argument when it comes to recreational trails. They say that there is not enough science to support the claims that making more trails or increasing the number and type of users on trails is bad for wildlife and the habitat they call home.

Increased human use of trails and the wild places that house those trails is bad for not only soil erosion but for wildlife. To me this is common sense. From direct experience to reading journal articles during my Master's education as a Conservation Biologist. The more you use something the more it gets degraded. With wildlife, this results in animals not using areas highly used by humans. Ecologists call areas where wildlife don’t reproduce or thrive: habitat sinks. Habitat sources on the other hand are productive areas where wildlife increase their numbers and migrate from once the area’s carrying capacity is reached. The IMBA rule of “Never spook animals” is pretty laughable considering that it’s inevitable if you’re riding into an animal’s habitat. What happens when you ride by a group of deer? They run away. Birds get spooked and fly away. This increases their stress response because they don’t know that we’re not trying to kill them (are we just riding by or are we their predator?). Constant increases in the release of stress hormones in animals, just as in humans, reduces the animal’s “fitness” or their likelihood of reproducing and passing on their genes to the next generation because they are expending too much energy flying or running away from bikers when they should be eating and finding mates. Disturbing wildlife in winter is especially dangerous when they are already stressed from reduced food resources and with the cold temperatures burning away at their fat reserves.

There are habituated species, like some colonies of prairie dogs with trails that run through them. They become so used to people that they don’t even run and hide when humans enter their habitat. Prairie dogs can become very habituated and end up endangering their lives when they don’t think twice about a dometic dog that chases it and kills it, or a biker that runs them over because they didn’t move out of the way in time (I know this to have happened in Boulder). Birds or mammals that are habituated are mostly non-native species that shouldn’t be there anyway and which habitat away from native species. Trails are also a source for the colonization and movement of non-native plant species (weeds) that also exclude native species and result in less food for the native animal species that rely on them.

The results of human presence are just starting to be studied by scientists. It is definitely an emerging field of research that holds promise for upcoming biologists and ecologists. There is more known about the effects of roads (“road ecology”) from direct mortality of individual animals to the disturbance effect of roads (noise, pollution, habitat fragmentation, etc.).

I did a 5 minute Google Scholar search and found a few citations that I thought I’d pass on just in case you want to read more. There aren’t many studies but there are enough to realize recreationists are having a significant effect on wildlife. There aren’t that many studies because (1) it’s a new field of study, and (2) people don’t want to have to think about their negative effects when they are trying to have fun and recreating…so few studies are done.
One thing I believe is that the burden of proof shouldn’t fall on those trying to preserve wildlife and their habitat, but on the people trying to USE that habitat. Unfortunately this is opposite of how most people think. They think “innocent until proven guilty.” I would like the burden of proof to fall on the recreationalists (like it is on the developers making a housing development). Once a given user group can prove that there is no significant effect of that type of recreation on a given habitat or on a given species, then I’m all for opening up that area to recreation. Since that will never happen, I suggest we all exercise a bit of restraint and caution when doing whatever we want recreationally and going where ever we want simply because we have fun doing it. Obviously, since I ride and hike myself quite a bit, I'm fine with opening up certain areas, even if there is a significant effect, as long as it's regulated so to minimize the effect.

Here are some articles from scientific journals if you're interested:

From: “Effects of Urbanization and Habitat Fragmentation on Bobcats and Coyotes in Southern California,” in The Journal of the Society for Conservation BiologyVolume 17 Issue 2 Page 566 - April 2003.
“Human-dominated areas were less suitable than natural areas in some important ways. Animals more associated with non-natural areas had higher levels of night activity, and both bobcats and coyotes were more likely to be in developed areas at night than during the day.”

Nocturnal and Diurnal birds show reduced use of areas being used by humans for recreation and lower reproductive success with some birds. Owls are very sensitive to the presence of humans and are strictly nocturnal:
Title: Effects of recreation on Rocky Mountain Wildlife: A review for Montana. http://www.montanatws.org/PDF%20Files/3bird1.pdf

EFFECTS OF WINTER RECREATION ON WILDLIFE OF THE GREATER YELLOWSTONE AREA:
A LITERATURE REVIEW AND ASSESSMENT
http://www.nps.gov/yell/publications/pdfs/wildlifewint.pdf

Recreational trails, human activity, and nest predation in lowland riparian areas:
http://www.public.iastate.edu/~jrmiller/trails.pdf

We do need people to experience the outdoors so that they want to preserve it. That is true. But with as many people as we have experiencing the outdoors we are loving many places to death. If we didn't have so many people that wanted to recreate as much as possible then I wouldn't be bringing this up because the study of the effects of recreation on wildlife wouldn't be a research field at all.
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as an aside:

nocturnal = active at night
crepuscular = active at dawn and dusk
diurnal = active during the day

Most mammals are crepuscular or diurnal, except cats like mountain lions which are crepuscular and nocturnal so to better hunt under the cloak of darkness. They have physiological differences that enable them to see better at night. Most small rodents are the same (mice, rabbits, voles). Foxes, coyotes, bobcats, deer, elk and bears are some of the many that used to be dirunal and crepuscular until humans took over the world. Now they have changed their behaviour to adapt the the changing landscape and forage when there is less human disturbance. We still see these animals in the early morning and at dusk but rarely do we see them during the daylight hours. We especially don't see them near human development too often because we are their predators - car and human alike.

If you don't ride that day and decide to ride that night...don't you think that there are others that rode that day but didn't ride that night? That means that the diurnal creatures are still getting it from the diurnal recreationists and you are also disturbing the nocturnal creatures. So you have just extended the amount of time that wildlife is getting disturbed and harassed, not lessened it by riding at night. I believe we need to give the wildlife a rest sometimes so that they don't leave us for good.