Answer :
Final answer:
A keystone species has a crucial role in an ecosystem, influencing various species' presence and maintaining ecological balance. The intertidal sea star, Pisaster ochraceus, and the banded tetra are examples of keystone species with profound effects on species composition and nutrient cycling, respectively.
Explanation:
One example of a keystone species is Pisaster ochraceus, an intertidal sea star, which upon removal has been shown to result in increased mussel populations, leading to a significant alteration in species composition and a reduction in biodiversity. Likewise, the banded tetra is a keystone species that contributes inorganically by recycling phosphorus within tropical stream ecosystems.
Keystone species play critical roles even in low abundances because they dictate the presence and populations of various other species by their predatory behaviors, resource distribution, or other ecological interactions. The impact of keystone species on communities can be substantial, as they can influence the types of species that live in the ecosystem and ultimately preserve its biodiversity.