Conservation biologists and natural resource managers are increasingly concerned with the invasion of exotic or non-native species in natural ecosystems (Mooney and Drake 1986, Soule 1990, Williamson 1996, Vitousek et al. 1997). The primary concern is that invasive species will alter species composition, which in turn, makes the ecosystem “unhealthy” or susceptible to degradation (Mooney and Drake 1986, Williamson 1996). Disturbances, particularly unnatural ones, appear to make ecosystems more susceptible to biological invasions (Braithewaite et al. 1989, Binggeli 1996). No ecosystem is free from disturbance, and since European settlement, the forest landscape has experienced increased logging, wildfire, road building, and the introduction or accidental release of non-native animals that transport invasive plants and/or their seeds. Recently some environmentalists have suggested that trail horses facilitate the dispersal and spread of invasive or non-native plant species(Land 1994); however, there are no data to support or refute this assertion.

The objectives of this research project are to: (i) assess the importance of different mechanisms by which horses may introduce non-native plant species, (ii) determine if invasive species introduced by horses germinate and colonize horse trails, (iii) determine if horse-borne invasive species spread from trail edges into natural ecosystems, and (iv) compare the presence and abundance of invasive plant species along horse trails to other recreation activities that may also be responsible for the spread of non-native species in natural ecosystems. The systematic study will provide valuable data to help land managers mitigate the introduction of invasive plant species by horses, if horses are found to be an important source for invasive species. Alternatively, the comprehensive study may provide convincing data that horses are not an important source of invasive plant species in natural ecosystems.

The study sites span a broad south - north environmental gradient and include Biltmore Estates (Asheville, NC), Land Between The Lakes (Golden Pond, KY), Kickapoo State Park (Oakwood,IL), Kettle Morraine State Forest Southern Unit (Eagle, WI), and Northern Highland American Legion State Forest (Boulder Junction, WI).