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Prediction of invasive species requires delineation of the fundamental niche and has included development of Habitat Suitability Models. Buffelgrass (Pennisetum ciliare, synonym Cenchrus ciliaris) is a non-native invasive perennial in the arid southwest. Perennial grasses such as buffelgrass can have their geographic distribution constrained by repeated grazing. Grazing can play an important part in integrated weed management programs as a biological control. Free-roaming horses overlap buffelgrass populations in specific areas. In these areas, what is the effect of horses in the control of buffelgrass? The objective of this study was to quantify the effect of horse grazing on buffelgrass. We asked the question: what is the fundamental niche of buffelgrass in the lower Sonoran Desert of Mesa, Arizona? And do horses constrain the potential habitat of buffelgrass, creating a realized niche of less geographic area? We performed an observational study to answer these two questions. Data collection was performed between November 20-30, 2016. Chi-square analyses were used to test the assumptions that buffelgrass has a fundamental niche along the study highway and that horse grazing creates a niche contraction that hinders further buffelgrass expansion. The study showed that buffelgrass is significantly less frequent in areas grazed by horses than in areas ungrazed by horses. The result suggests that horses may be an effective tool against the expansion of buffelgrass populations into areas susceptible to its establishment. Further study is recommended.