Step 1: Read the Assertion.
The Assertion lists four levels of biological organization as organisms, populations, communities, and biomes.
Step 2: Check the Assertion.
Ecology is studied at these rising levels, where single organisms group into populations, populations form communities, and communities together form biomes. So the Assertion is correct.
Step 3: Read the Reason.
The Reason says that because plant ecology and animal ecology cannot be kept apart, both are treated together as one subject called Ecology.
Step 4: Check the Reason.
This is also correct. Plants and animals live together and depend on each other, so their study is combined into a single field of Ecology.
Step 5: See if Reason explains Assertion.
The Assertion is about the levels of organization, while the Reason is about uniting plant and animal study. These are two separate true facts, so the Reason does not explain the Assertion.
Step 6: Conclude.
Both are correct, but the Reason is not the correct explanation for the Assertion.
\[ \boxed{\text{Both (A) and (R) are correct but (R) is not correct explanation for (A)}} \]