73. Which of the following statements about the birth rates (b1, b2) and death rates (d1, d2) of species 1 and 2 indicated in the figure is NOT true?
    
    (1) Birth rates of species 1 are density independent.
    (2) Death rates of both species are density dependent.
    (3) Birth rates of species 2 are density dependent.
    (4) Density dependent effects on death rates are similar for both the species.

Introduction

Population ecology relies on understanding how birth and death rates change with population density. These relationships are crucial for predicting population growth, stability, and long-term survival. By analyzing the density dependence of birth rates (b1, b2) and death rates (d1, d2) for two species, we can determine which ecological statements are accurate—and which are not.

Understanding Density Dependence

• Density-dependent factors: Birth or death rates that change as population density changes. Typically, birth rates decrease and death rates increase with higher density due to competition, disease, and resource limitation.

• Density-independent factors: Birth or death rates that remain constant regardless of population density, often due to abiotic factors like weather or natural disasters.

Evaluating the Statements

Let’s assess each statement based on standard ecological principles and the typical graphical representations:

(1) Birth rates of species 1 are density independent.

• Interpretation: This means b1 does not change as population density increases; the birth rate remains constant.

• Graph expectation: A flat line for b1 across all densities.

• Ecological context: Density-independent birth rates are less common but possible, especially if external factors dominate reproduction.

(2) Death rates of both species are density dependent.

• Interpretation: Both d1 and d2 increase as population density increases.

• Graph expectation: Both death rate curves slope upward with increasing density.

• Ecological context: This is a classic pattern for density-dependent mortality due to competition, disease, or predation.

(3) Birth rates of species 2 are density dependent.

• Interpretation: b2 decreases as population density increases.

• Graph expectation: A downward-sloping curve for b2.

• Ecological context: This is typical for species where reproduction is limited by crowding or competition.

(4) Density dependent effects on death rates are similar for both the species.

• Interpretation: The increase in death rates with density is of similar magnitude or slope for both species.

• Graph expectation: d1 and d2 have similar upward slopes.

• Ecological context: This is only true if the graph shows both species’ death rates responding similarly to density.

Which Statement is NOT True?

Statement (1) is NOT true if the graph shows b1 decreasing with increasing density—that is, if species 1’s birth rate is actually density dependent, not independent. In classic ecological models, most species exhibit density-dependent birth rates, especially when resources become limiting as populations grow.

If the graph instead shows b1 as a flat line, statement (1) would be true, and you would look for another statement that doesn’t match the graph.

However, based on standard ecological patterns and the context of the question, statement (1) is most likely NOT true because birth rates are rarely truly density independent in natural populations.

Conclusion

The statement that is NOT true is:

(1) Birth rates of species 1 are density independent.

In most ecological scenarios, birth rates decrease as population density increases, making density-dependent birth rates the norm. Recognizing these patterns is essential for interpreting population dynamics and managing wildlife effectively.