Symbiosis Between Mistletoe & A Tree Symbiosis In this case the mistletoe is the parasite and the tree is the host. Mutualism If the relationship is competitive, it means that only one organism gains something.f the relationship is competitive the parasite eventually kills the host. Here is an example of how the mistletoe lives: Even though mistletoe is often thought of as a cheery plant, it is actually quite deadly to other plants.