Which of the following can be inferred from the figure?
(1) Age class 2 has maximum fecundity
(2) Age class 2 has maximum survival
(3) Age class distribution is at equilibrium
(4) Age class distribution is not at equilibrium

What Can You Infer from an Age Class Distribution Figure? Understanding Population Equilibrium

Population ecologists use age class distributions to gain insights into the structure and dynamics of a population. A figure showing the proportion or number of individuals in each age class can reveal important information about survival, reproduction, and whether a population is stable, growing, or declining. This article explains how to interpret such a figure and what conclusions can be drawn about fecundity, survival, and equilibrium.

Understanding Age Class Distribution

An age class distribution (also called an age structure or age pyramid) visually represents the number or proportion of individuals in each age group at a given time. The shape of the distribution can indicate whether a population is:

• Growing: More individuals in younger age classes.

• Stable: Similar numbers across age classes.

• Declining: More individuals in older age classes.

Age class distributions are fundamental for understanding population dynamics and predicting future changes.

Key Concepts in Age Class Distribution

Fecundity

• Definition: The average number of offspring produced by an individual in a given age class.

• Inference from Figure: A figure alone does not provide direct information about fecundity unless it is explicitly labeled or described. You cannot infer which age class has maximum fecundity just by looking at the number of individuals in each age class.

Survival

• Definition: The proportion of individuals that survive from one age class to the next.

• Inference from Figure: Survival rates between age classes can sometimes be inferred by the relative sizes of adjacent age classes. For example, if age class 2 is much larger than age class 3, it might suggest lower survival from age class 2 to 3. However, survival is not directly visible unless the figure shows mortality data or survivorship curves.

Equilibrium

• Definition: A population is at equilibrium (stable age distribution) when the proportion of individuals in each age class remains constant over time.

• Inference from Figure: If the figure shows a stable or balanced distribution (e.g., similar numbers in each age class), it might suggest equilibrium. However, a single snapshot does not confirm equilibrium unless you know the population has been stable for several generations or the figure is explicitly labeled as such.

Interpreting the Age Class Distribution Figure

Suppose you are given a figure showing the age class distribution of a population at a given time. The figure may show the number or proportion of individuals in each age class (e.g., age class 1, age class 2, age class 3). Here’s how to interpret the possible options:

(1) Age class 2 has maximum fecundity

• Interpretation: Fecundity is not directly visible from an age class distribution figure unless it is explicitly stated or shown. The number of individuals in an age class does not indicate how many offspring they produce. Therefore, this option cannot be inferred from the figure alone.

(2) Age class 2 has maximum survival

• Interpretation: Survival is about the proportion of individuals moving from one age class to the next. If age class 2 is larger than age class 3, it might suggest lower survival from age class 2 to 3, not maximum survival in age class 2. Maximum survival would mean almost all individuals in age class 2 survive to age class 3, which would make the numbers similar or age class 3 larger. This inference is not supported by the figure unless specific survivorship data are provided.

(3) Age class distribution is at equilibrium

• Interpretation: Equilibrium is achieved when the proportion of individuals in each age class remains constant over time. A single figure does not provide information about changes over time or whether the distribution is stable. Unless the figure is labeled as an equilibrium distribution or you have additional information, this cannot be inferred.

(4) Age class distribution is not at equilibrium

• Interpretation: This is the most cautious and generally correct inference from a single snapshot. Without evidence of stability over time or explicit labeling, you cannot assume the population is at equilibrium. Populations often fluctuate and only reach a stable age distribution after several generations under constant vital rates.

Stable Age Distribution and Equilibrium

A stable age distribution occurs when the proportions of individuals in each age class remain constant over time, typically after the population has been growing or declining at a constant rate for several generations. At equilibrium, the population size may remain constant, and the age structure is balanced.

In practice, most natural populations are not at perfect equilibrium due to environmental variability, changes in birth and death rates, and other factors. Therefore, unless the figure or accompanying text specifically states that the population is at equilibrium, it is safer to assume that the age class distribution is not at equilibrium.

Real-World Implications

Understanding age class distribution is crucial for:

• Wildlife management: Predicting future population trends and setting harvest quotas.

• Conservation: Identifying vulnerable age classes and targeting conservation efforts.

• Ecological research: Studying the effects of environmental changes on population structure.

Summary Table

Conclusion

From a figure showing the age class distribution of a population at a given time, you cannot infer which age class has maximum fecundity or survival unless additional data are provided. You also cannot assume the population is at equilibrium from a single snapshot. The most accurate inference is that the age class distribution is not at equilibrium unless explicitly stated otherwise.

Correct answer:
(4) Age class distribution is not at equilibrium