Which statement best describes a keystone species
a) it has little genetic variation
b) it has an extremely high population
c) it remains unaffected by all environmental changes
d) it has a strong influence on the ecosystem
e) it's the first species to populate a new ecosystem

Option D,  it has a strong influence on the ecosystem

Explanation:

Keystone species are an essential part of an ecosystem and their functions/mark cannot be replaced by any other organism. Once the keystone species is removed from an ecosystem, the ecosystem struggles to sustain  or it changes radically. A keystone species in majority of ecosystems/food web is a predator but this is not true in all ecosystems/or with all keystone species.  

For example – there was a trophic cascade when the Gray wolves in the greater Yellowstone ecosystem were removed.  

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answer; 60-100; beats per minute