Which of the following is the functional unit of nature?

1. A plant

2. An animal

3. Ecosystem

4. Environment

Vertical distribution of different species occupying different levels in a biotic community is

known as 

1. divergence

2. stratification

3. zonation

4. pyramid

Which one of the following is not a functional unit of an ecosystem?

1. Energy flow                 

2. Decomposition

3. Productivity                 

4. Stratification

Which one of the following is one of the characteristics of a biological community ?

1. Stratification 

2. Natality 

3. Mortality 

4. Sex-ratio

Which vertical strata is occupied by trees in a forest?

1. Top

2. Middle

3. Bottom

4. Any of the above.