Effect

The effect of Ulva on Gigartina is best described as facilitation. This is because in the presence of Ulva, Gigartina is able to survive and grow. Ulva is making it more suitable for later species with its presence. When Ulva is removed the number of Gigartina recruits is close to 0. Which further backs up the idea that it is Ulva that creates the suitable environment. In figure 3 the control groups shows that the mortality for aspen and fir trees are about the same time with aspen having a slight higher mortality in comparison. When the aspen trees are thinned in early aspen successional stages the mortality for aspen stays around the same. However the firs experience an increase in mortality percentage by more than doubling their mortality. This means that there is a link between having thinner aspen and having less fir.The mechanism controlling these interactions is facilitation. Looking at Figure 1 it can be seen that aspen is the first colonizer after aspen is able to thrive fir then starts taking it’s place almost eliminating the fir. In figure 3 it can be confirmed that fir needs aspen in order to thrive. Thinning out the aspen creates higher mortality in the fir trees. This is facilitation because one of the species benefits with the introduction of another species and without that species fir is unable to grow.