Response to Figures

Based upon figure 1 the effect of Ulva on Gigartina is a positive effect following the facilitation model. A facilitation model means that early colonists modify the environment so that it is more suitable for late successors and less suitable for other early successors. The Gigartina is a late species and when the Ulva is present then the number of Gigartina increases over time. When Ulva is removed the Gigartina levels are consistently low throughout both years. The pattern of mortality for fir and aspen changes when the aspen plots are thinned. When the aspen plots are thinned the fir mortality increases but the aspen mortality stays the same. This could be due to the fact that the fir are more susceptible to fire in the absence of aspen. The likely mechanism controlling these interactions is a tolerance model. In a tolerance model the earlier successors modify the environment so that is has a small effect on the later successional species. The later successor then takes over the colony and can eliminate the earlier species. In figure 2 Aspen is the earlier successor but over time the fir can still increase in density despite the presence of the aspen. However, as the Fir continues to grow the density of the aspen starts to decrease.