Final answer:
Foundation species have a crucial influence on their ecosystems due to their abundance and their role as primary producers; e.g., kelp in kelp forests.
Step-by-step explanation:
The true statement about foundation species is: Foundation species play a crucial role in shaping and maintaining their ecosystems. Unlike the claim in option A that they are rare with minimal impact, foundation species typically have substantial influence due to their abundance and role as primary producers. For instance, kelp is a foundation species in the kelp forests off the coast of California, contributing significantly to the ecosystem's structure and function. Although top predators are important, not all foundation species fulfil this role (as stated in option C), and foundation species are not exclusively found in aquatic ecosystems (as stated in option D).