Assertion : The community of an ecotone commonly contains the organisms of each of the overlapping communities and in addition the organisms which are restricted to the ecotone.
Reason : The tendency for increased variety and density at community junctions is known as the edge effect.

Explanation

To analyze the assertion and reason provided in the question, we will break down the concepts involved step by step.

### Step 1: Understanding the Assertion
The assertion states that the community of an ecotone contains organisms from each of the overlapping communities, as well as organisms that are unique to the ecotone itself.

- **Explanation**: An ecotone is a transition area between two different ecosystems. In this zone, we can find species from both adjacent ecosystems (overlapping communities) and also some species that are specifically adapted to live in the ecotone. This unique biodiversity is a characteristic feature of ecotones.

### Step 2: Understanding the Reason
The reason provided states that the tendency for increased variety and density at community junctions is known as the edge effect.

- **Explanation**: The edge effect refers to the phenomenon where the biodiversity (variety of species) and population density (number of individuals) are higher at the boundaries or edges of different ecosystems compared to the interior of those ecosystems. This is due to the mixing of species from both ecosystems and the presence of unique species that thrive in the ecotone.

### Step 3: Evaluating the Relationship
Now, we need to evaluate whether the reason explains the assertion.

- **Analysis**: Both the assertion and the reason are correct. The assertion accurately describes the composition of an ecotone, and the reason correctly identifies the edge effect as a phenomenon observed at community junctions. However, the reason does not directly explain why the ecotone has a unique biodiversity; it merely describes an observation related to community junctions.

### Conclusion
Based on the analysis:
- Both the assertion and reason are correct.
- The reason does not explain the assertion.

Thus, the correct answer is that both statements are correct, but the reason does not explain the assertion.

### Final Answer
**Option B**: Both statements are correct, but the reason statement is not explaining the assertion.

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