Question

In many communities, one or few species appear to be very important in maintaining the community structure. these are called--

1)keystone species.
2)rare species.
3)endangered species.
4)vulnerable species.

Answer 1

Here is ur answer:

1) KEYSTONE SPECIES.

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Answer 2

Keystone species are important to maintain community structure in many communities.  

Explanation:

A keystone species are the entire organisms of the ecosystem. These species are important part of any ecosystem, without these species no ecosystem can survive.

The example of the keystone species is a predator prey relationship. Small predators eat herbivorous animal and helps to protect plants which can be eaten by herbivorous species. These small predators are considered as a keystone species of an ecosystem.