I think it is keystone. If they were not there the population of urchins would go up and the kelp would go down.
Convergent evolution in my understanding is the independent evolution of similar features in species of different lineages.
Hello!
So, day and night are caused because the earth makes a full rotation on its axis once every 24 hours.
That way, have of Earth is facing the sun (it is morning on that side) and the other half isn't (which is why it is nighttime over there).
For example, if it's nighttime in Canada and America, it's morning in Pakistan and India. This is because Pakistan and India are facing the sun whilst Canada and America are not.
A N S W E R:
The balls should be moved so that half the Earth is facing the sun and the other half is not. This would be 24 hours. As for the sun, it is a gas, not a solid, so it doesn't rotate like one. However, that does not mean that it is not rotating to some extent.
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