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The Impact of Trail Horses on Plant Communities

November 14, 2016
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Horseback riders on a recreational trail. Photo Credit: http://www.morguefile.com

Exotic plant species, also known as non-native plant species, are a potential threat to the native plants in an environment. A “healthy” plant community is desirably dominated by native vegetation, so the presence of non-native species often suggests competition of the available resources. This may cause native species to die off and the biodiversity of plants and plant-dependent organisms in the community to decline, deeming a community as “unhealthy”. It is important to find the source of non-native plant species in different environments in order to control or eliminate the introduction of these potentially invasive plants. In many forests and parks, there are recreational horseback riding trails available to the public. These trails could very well be the sites where exotic plant species are introduced or transported to a plant community. Horses have the ability to disperse seeds via manure (endozoochory) and also by carrying the seeds on their coat (epizoochory). Depending on the home location of the horses, the seeds may be of native or non-native species. Since non-native species may be introduced to plant communities via horses, we can assume that trail horses pose as a threat to these communities.

Can Horses Introduce Non-Native Plant Species Along Trails?

Horses have a tendency to defecate indiscriminately—or poo where they please. Since non-native plants are often found in pastures, trail horses with pasture access are able to act as shuttles for non-native seeds by consuming them and  transporting them to a new community where they may germinate, or start to grow. Seeds from the pasture may also stick to the horses and simply fall or get knocked off while on a trail. Hay-fed horses can also obtain seeds from hay. Horses that consistently consume hay transport more seeds through their manure than by their coat or hooves (Gower 2008). It has been established that seeds may remain viable in the horses’ digestive tract for up to two months, although a seed generally passes through the digestive tract within 48 hours of being eaten (Campbell and Gibson 2000; Törn et al. 2009). With this knowledge, it is possible that a seed found in manure came from a distant community, which makes it a little more likely to be exotic.

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Horses grazing in a pasture are susceptible to the discrete seed dispersal of non-native species. Photo Credit: http://www.photobucket.com

How Do Horses Impact the Germination of Seeds?

There are many ways in which a horse affects seed germination along trails. They trample the ground, defoliate vegetation, and cause changes in the soil nutrient status by urination and defecation (Törn et al. 2009). Disturbance of the ground has been shown to assist in the spreading of seeds along trails. It has also been known to enhance the germination of seeds by creating open gaps and pockets in the ground. The amount of different species, known as richness, has been found to be higher right along trails, although this includes both native and non-native species (Campbell and Gibson 2000). This suggests that the soil disturbance along trails assists in germination. Many invasive plant species are easily germinable and therefore, if present, may benefit from ground disturbance by horses.

The effects of trampling, however, don’t only include disturbance of the soil. Often times trampling compacts the soil rather than stirring it up. Soil compaction has been associated with a decrease in seed germination. It increases the soil strength and reduces the ability for water to soak in. It also affects plant root growth and raises soil temperature (Ostermann-Kelm et al. 2009). In areas with high soil strength and compaction, there is usually low plant diversity along the trail. So, depending on the dryness or wetness of the environment, trampling may assist in or stop the germination of species.

Despite the species of the seeds, manure serves as a favorable growing site and seems to be an ideal substrate for germination in a controlled environment (Törn et al. 2009). Areas near horse manure have shown significant plant diversity compared to areas further from manure, yet the diversity consisted mostly of native plants to the area of study (Ostermann-Kelm et al. 2009). Nonetheless, if a horse defecates in a plant environment and there happens to be an exotic seed nearby (whether or not it was present due to consumption by the horse or by some others means of dispersal), the seed may germinate and grow to be an invasive exotic plant that competes and takes over native plants in the area.

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The sprouting of a seed in nutrient-rich soil. Photo Credit: http://www.photobucket.com

So, Are Horses a Threat to Natural Plant Communities?

It is true that horses may carry a variety of non-native plant species and contribute to seed germination, but the environment itself plays a major role on whether or not an exotic seed will grow along a trail. Without enough nutrients and moisture available to the plant as it matures, the species has a low survival rate (this also depends on the plant species). The growth of the plant will also depend on factors such as sunlight and wind. Ultimately, horses are a limited threat to natural plant communities due to the exotic species that may be present in manure (Campbell and Gibson 2000) and the resources that may be available. This form of non-native seed dispersal is only found where horses travel and therefore is not as threatening as some other modes of seed dispersal, such as wind or pollinators. Although exotic seeds are not found all the time in horse poo, it is still important to take precautions toward preventing the introduction of non-native plant species.

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Different hay sources contain a variety of seed species. Photo Credit: http://www.morguefile.com

How Can We Help Prevent Invasion of Non-Native Species?

Overall, research has shown that horses have the ability to introduce non-native plant species to a plant community along a trail. Although many other factors contribute to the germination and growth of an exotic seed, the horse is the first step that makes the seed available to the environment. In order to help with the prevention of non-native species growth along horse trails in protected areas, we can collect manure along trails and limit the amount of horses allowed on the trail (Törn et al. 2009). Proper disposal of any unused or spoiled hay would also decrease chances of exotic plant growth or invasion (Gower 2008). While recreational horseback riding may be enjoyable to humans, it is not always as fun for plant communities, so it is important that we be helpful and clean up after ourselves and the horses. If the communities could thank us, they would!

References


1. Gower S.T. (2008) Are horses responsible for introducing non-native plants along forest trails in the eastern United States?. Forest Ecology and Manag 256:997-1003. doi: 10.1016/j.foreco.2008.06.012


2. Törn A, Siikamäki P, Tolvanen A (2009) Can horse riding induce the introduction and establishment of alien plant species through endozoochory and gap creation?. Plant Ecology 208:235-244. doi: 10.1007/s11258-009-9701-5


3. Campbell J.E., Gibson D.J. (2000) The effect of seeds of exotic species transported via horse dung on vegetation along trail corridors. Plant Ecology 157: 23-35. doi: 10.1023/A:1013751615636


4. Ostermann-Kelm S.D., Atwill E.A., Rubin E.S., Hendrickson L.E., Boyce W.M. (2009) Impacts of feral horses on a desert environment. BMC Ecology 9:22. doi: 10.1186/1472-6785-9-22

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