One of the critical mechanisms by which environment controls the population of a species is :

Options

A. Biotic control
B. Mortality
C. Fecundity
D. Environmental resistance

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a critical mechanism by which the environment controls the population of a species. This involves understanding the relationship between population dynamics and environmental factors.

2. **Identifying Key Theories**: One of the foundational theories in population control is proposed by Thomas Malthus. He suggested that populations grow exponentially while resources grow linearly, leading to a point where the environment cannot support the population.

3. **Defining Environmental Resistance**: Malthus introduced the concept of "environmental resistance," which consists of various checks that the environment imposes to control population growth.

4. **Explaining the Two Types of Checks**:
- **Positive Checks**: These are factors that increase mortality rates, such as disease, famine, and predation. When the population exceeds the carrying capacity of the environment, these factors help reduce the population size.
- **Preventive Checks**: These are measures that reduce birth rates, such as delayed marriage, contraception, and other forms of birth control. They help to prevent the population from growing too large in the first place.

5. **Conclusion**: The critical mechanism by which the environment controls the population of a species, according to Malthus, is through environmental resistance, which includes both positive and preventive checks.

6. **Selecting the Correct Answer**: Based on the explanation, the correct answer to the question is the fourth option, which refers to environmental resistance as the mechanism of population control.