Final answer:
A keystone species is vital for the structure and biodiversity of an ecological community, with examples including the intertidal sea star and the banded tetra.
Step-by-step explanation:
A keystone species is a type of organism that plays a critical role in maintaining the structure of an ecological community and affecting the diversity and abundance of other species within the ecosystem. The presence and actions of a keystone species have a disproportionately large effect on their environment relative to their abundance. Examples of keystone species include the intertidal sea star, Pisaster ochraceus, which controls the population of mussels in the northwestern United States, preventing the mussels from dominating the ecosystem and allowing for greater biodiversity. Another example is the banded tetra in tropical streams, which is responsible for providing the majority of the phosphorus, an essential nutrient, thereby sustaining the community's ecosystem dynamics.
The removal of a keystone species often leads to significant changes in the ecosystem and can result in reduced biodiversity and altered ecological relationships, demonstrating the importance of these species in ecosystem management and conservation efforts.