Later in 2013, Michael Cramer and Nichole Barger suggested that the circles were the consequence of vegetation patterns that arose naturally from competition between grasses. They examined the conditions under which fairy circles arise and found that fairy circles are negatively correlated with precipitation and soil nutrition. This observation is consistent with resource competition being a cause of the crop circles. Grassy landscapes with a mixture of grasses can result in barren spots as a consequence of under-ground competition between different types of grasses. The patches are maintained because they form a reservoir of nutrients for the taller grasses at the periphery and possibly because of the activity of termites, as in the theory above. Using rainfall, biomass and temperature seasonality, they can predict with high accuracy the presence or absence of fairy circles in a region. According to Walter Tschinkel, this theory accounts for all the characteristics of fairy circles, including the presence of tall grass species. Other recent work has considered interacting combinations of both animal- and vegetation-induced patterning effects as a potential unifying theoretical explanation for the fairy circle phenomenon.