York University Tree Inventory: Commensalism

Disturbances are human-mediated or natural events that can lead to a change in the ecosystem by affecting the physical environment, leading to a change in the biotic relationships (between organisms) and the abiotic arrangements, such as water and soil. There are different hypotheses to predict the disturbance such as the intermediate hypothesis (IDH) and the enemy release hypothesis (ERH). The IDH hypothesis suggests that intermediate levels of disturbance will result in species diversity, while the ERH predicts that the number of invasive tree species will be increased due to the reduced number of predators in the new area. The purpose of this project is to examine the effect of subway disturbances on symbiotic relationships, such as presence of gypsy moths and lichens on trees, and overall trees’ health at the York university campus. 854 trees were assessed at various distances from the subway line disturbances. There were more gypsy moth egg masses within the subway line than outside. In addition, there was a relatively high richness in lichens on trees located outside of the subway station compared to within. Disturbances affect pathogens as it can lead to outbreaks in tree parasites, such as gypsy moths, in an area. These gypsy moths can then become a natural disturbance, affecting the trees’ health, and leading to a change in the symbiotic relationships. Overall, it is difficult to predict the effects of disturbances as it is measured using different factors: the intensity, timing, extent, duration, and the frequency of disturbance.