High School

While the most abundant and dominant species within a particular ecosystem is often crucial in perpetuating the ecosystem, a "keystone" species, defined as one whose effects are much larger than would be predicted from its abundance, can also play a vital role. However, identifying a keystone species by removing it and observing changes in the ecosystem is problematic due to complex species interactions.

For example, Pisaster ochraceus is often a keystone predator because it consumes and suppresses mussel populations, which, in the absence of this starfish, can become a dominant species. However, such predation on a dominant or potentially dominant species occurs in systems with and without species playing keystone roles.

Moreover, while P. ochraceus occupies an unambiguous keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats, the impact of P. ochraceus predation is weak or nonexistent, and sand burial is sometimes responsible for eliminating mussels. Keystone status appears to depend on context, whether it be geography, community diversity (such as a reduction in species diversity thrusting more remaining species into keystone roles), or the length of species interaction (since newly arrived species may dramatically affect ecosystems).

Which of the following factors affects whether P. ochraceus has a keystone role in a habitat?

A. The degree to which the habitat is sheltered from waves
B. The degree to which other animals within a habitat prey on mussels
C. The fact that mussel populations are often not dominant within some habitats occupied by P. ochraceus
D. The size of the P. ochraceus population within the habitat
E. The fact that there is great species diversity within some habitats occupied by P. ochraceus

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.