avoiding predators has a positive impact on the organism & cactus wrens build their nests in cholla cacti to avoid predators ->  cactus wrens building their nests in cholla cacti has a positive impact on themselves  cactus wrens building their nests in cholla cacti has a positive impact on themselves & commensalism is a relationship between two organisms in which one benefits while not harming the other one & cactus wrens building nests in cholla cacti does not harm the cholla cacti -> the relationship between cactus wrens and cholla cacti is commensalism