Final answer:
The symbiotic relationship between spiders and trees is an example of commensalism, where spiders benefit from using trees to support their webs without affecting the trees.
Step-by-step explanation:
The type of symbiotic relationship where spiders depend on trees for their webs, but the tree is not affected, is commensalism. In a commensal relationship, one species gains a benefit while the other remains unaffected. This is seen in various examples throughout nature, such as birds that nest in trees, gaining shelter without harming the tree. Similarly, spiders use trees as an anchor point for their webs, which provides spiders with a platform to catch prey. The tree does not derive any benefit from this arrangement, nor does it suffer any consequence.
It is important to differentiate commensalism from other symbiotic relations such as mutualism, where both species benefit, and parasitism, where one species benefits at the expense of the other. Spiders and trees are a textbook examples of commensal relationships, illustrating the diverse ways in which species interact within their ecosystems.