Mountain biking is a popular activity that has increased significantly over the last couple decades in the United States and throughout the world. This recreational activity offers frequent opportunities for interactions between humans and other species, especially plants. Of particular interest is human-mediated seed dispersal, due to its potential effects on native and invasive plant population dynamics. Seeds may become lodged in bike tires and dispersed relatively long distances. We sampled bikes from low desert trails in Tucson, Arizona to explore the role of mountain biking in seed dispersal. Data were collected throughout the year in both dry and wet conditions. Soil and debris were dislodged from mountain bike tires post-ride and collected. In the lab soil samples were combined with water to separate seeds from soil, as biotic material floats as compared to abiotic material which sinks. Additionally, surveys were administered to the riders to collect data on their personal statistics (e.g., weight and bike type) and their ride statistics (e.g., distance and duration).
Results/Conclusions
Over the course of eight months, we sampled soil from 54 bikes. Only very small amount of soil and debris were caught on tires, ranging from 0.0002g to 2.3g. We found only one seed among all the samples. It was from Pectocarya recurvata (Boraginaceae), a very common winter annual species. We also found that wider tires tend to pick up more soil and debris (r2=0.11, p=0.02). In addition, tires sampled in relatively wetter conditions acquired higher soil and debris loads. We expected the frequency of seed acquisition to be a function of the amount of soil and debris picked up. However, we did not detect such a pattern in our overall low-weight samples. We expect smaller soil and debris loads, as well as the reduced seed dispersal by mountain bikes, to characterize low desert areas. Such areas have fine soils that drain or dry quickly as well as low rainfall, both of which result in reduced pickup of material that might include seeds. For these reasons, we expect more mesic areas to exhibit increased seed dispersal by mountain bikers. We intend to address this hypothesis in future work.