Which of the following would necessarily decrease the density of a population in a given habitat ?

Options

A. Natality `gt` mortality
B. Immigration `gt` emigration
C. Mortality and emigration
D. Natality and immigration

Explanation

To solve the question, "Which of the following would necessarily decrease the density of a population in a given habitat?", we can follow these steps:

### Step 1: Understand Population Density
Population density refers to the number of individuals of a species per unit area or volume in a given habitat. It can change due to various factors including birth rates, death rates, immigration, and emigration.

### Step 2: Identify Factors Affecting Population Density
1. **Natality (Birth Rate)**: An increase in the birth rate contributes to an increase in population density.
2. **Mortality (Death Rate)**: An increase in the death rate leads to a decrease in population density.
3. **Immigration**: The arrival of new individuals from other populations increases population density.
4. **Emigration**: The departure of individuals from the population decreases population density.

### Step 3: Analyze the Options
The question asks for the factor that would **necessarily** decrease population density. We need to look for options that include:
- **Mortality**: This factor will decrease density as it increases the number of deaths in the population.
- **Emigration**: This factor will also decrease density as it involves individuals leaving the population.

### Step 4: Evaluate the Options
- If the option states that mortality is greater than natality, the population density will decrease.
- If the option states that emigration is greater than immigration, the population density will also decrease.
- If the option involves a situation where birth rates exceed death rates or immigration exceeds emigration, the population density will increase.

### Step 5: Conclusion
The factors that will necessarily decrease the density of a population are:
- **Mortality** (death rate)
- **Emigration** (individuals leaving the population)

Thus, the correct answer is **Option C: Mortality and Emigration**.