Hello there! Commensalism is where one organism benefits, but the other one remains unaffected. For example, spiders make webs on trees to protect themselves. The spiders benefit, while the tree remains unaffected. Commensalism is important, because organisms need to benefit somehow, and it's nice for an organism to be the host of another organism, while remaining unharmed. It helps some organisms out. It's necessary in an ecosystem, because without it, some organisms may not survive or it can be harder for them to find someplace else.
