Introduction: Welcome to the World of Populations!
The term "population" might sound a bit technical, but simply put, it refers to "a group of individuals of the same species living in a specific area." For example, all the medaka (Japanese rice fish) living in a single pond or all the sika deer inhabiting a single forest make up a population.
In this chapter, we will explore how groups of the same species increase in number, how they maintain stable numbers, and how they compete or cooperate (sociality) with one another. It might seem challenging at first, but much of it is surprisingly similar to human relationships and social rules! Let's master it step by step!
*Note: Interactions between different species (such as interspecific competition and mutualism) will be covered in another chapter, "Biological Communities and Interspecific Interactions." Here, let's focus on "interactions within the same species"!
1. Population Size and Density
The most important measure for understanding the state of a population is population density.
Population density refers to the number of individuals per unit area or unit volume of living space.
\(\text{個体群密度} = \frac{\text{個体数}}{\text{生息場所の面積（または体積）}}\)
Because it is difficult to count every single individual in wild populations, the following estimation methods are frequently used. These often appear in experimental reasoning questions on the Common Test!
① Quadrat Method
Mainly used for plants and sessile or slow-moving animals (such as shellfish). Researchers place several quadrats (sample plots) in the survey area, count the individuals inside them, and then multiply by the total area to estimate the entire population size.
② Mark-Recapture Method
Used for freely moving animals such as fish, birds, and insects.
1. Capture a number of individuals, mark them, and release them back into their habitat.
2. After waiting enough time for them to mix, capture individuals again and check how many marked individuals are included.
Based on the proportion of marked individuals in the second sample, we estimate the total population size, \(N\).
\(\frac{\text{最初に捕獲して標識をつけた個体数 } (M)}{\text{推定したい全体の個体数 } (N)} = \frac{\text{再捕獲された中の標識個体数 } (m)}{\text{再捕獲された全体の個体数 } (n)}\)
\(\implies N = \frac{M \times n}{m}\)
[Key Concept] Assumptions for the Mark-Recapture Method
The Common Test frequently asks about the assumptions required for this calculation to be valid:
- No births, deaths, or migrations (immigration or emigration) occur during the survey period.
- Marking does not affect the survival rate or probability of recapture of the individuals.
- Marked individuals mix completely and uniformly with the rest of the population.
[Common Mistake]
Students often mix up the numerator and denominator when calculating the mark-recapture formula. Always write down the core concept first: "Proportion of marked individuals in the entire population = Proportion of marked individuals in the sample"!
2. Population Growth and Carrying Capacity
Population Growth Curve
If food and space were unlimited, a population would explode exponentially (geometric growth). In real-world nature, however, resources are limited.
As the population size increases, food shortages, limited living space, accumulation of waste products, and outbreaks of infectious diseases occur, putting the brakes on the growth rate. The factors that act as this brake are collectively called environmental resistance.
Because of environmental resistance, actual population growth follows an S-shaped curve. This is known as a logistic growth curve (sigmoid curve).
[Key Term] Carrying Capacity
The maximum population size that a given environment can sustainably support over an extended period is called its carrying capacity. In a logistic growth curve, the population size eventually levels off and stabilizes near the carrying capacity.
Survivorship Curves (Life Tables)
A graph showing how individuals of a cohort die off as they age is called a survivorship curve. The vertical axis shows the number of survivors on a logarithmic scale, while the horizontal axis represents relative age. These curves generally fall into three major types depending on the organism:
Type I (Late Loss): Organisms with high parental care (e.g., humans, large mammals). Infant mortality is extremely low, and most individuals die rapidly near the end of their lifespan. The curve is relatively flat at the top and drops steeply on the right.
Type II (Constant Loss): Organisms with a fairly constant mortality rate throughout their lives, regardless of age (e.g., many birds, lizards). The graph forms a straight diagonal line sloping downward to the right.
Type III (Early Loss): Organisms that provide no parental care and lay huge numbers of eggs (e.g., many marine fish, oysters, and other invertebrates). The vast majority die early due to predation, but individuals that survive to adulthood have high survival rates. The curve drops sharply in the early life stages.
[Fun Fact]
Why do fish lay millions of eggs at once? Looking at the Type III survivorship curve explains it all! Because the odds of an egg surviving to adulthood are exceedingly slim, producing an overwhelming number of offspring ensures that the species survives.
3. Interactions Within a Population ①: Intraspecific Competition
Individuals of the same species require the exact same resources: food, light, water, territory, and mates. Therefore, when population density increases, fierce competition over these limited resources breaks out among members of the same species. This is called intraspecific competition.
Density Effects
The effects of population density on the development, morphology, survival, and reproduction of individuals are called density effects.
For instance, when fruit flies are reared at high density in a sealed container, food shortages lead to a decrease in average body weight per fly as well as a drop in the number of eggs laid.
Density Effects in Plants: Self-Thinning and the Law of Constant Final Yield
When plant seeds are sown at high densities, leaves begin to overlap as they grow, triggering intense intraspecific competition for sunlight.
- Self-thinning: A phenomenon where weaker individuals that lose out in competition naturally wither and die, causing population density to decrease over time.
- Law of Constant Final Yield: Regardless of differences in initial sowing density, as long as sufficient time elapses, the total biomass (total yield = overall weight of all plants combined) per unit area converges to an almost constant value. When density is low, each individual plant grows large; when density is high, many slender plants grow alongside self-thinning, meaning the total yield ends up the same!
[Common Mistake]
What remains constant in the "Law of Constant Final Yield" is the "total weight (total yield)", NOT the "weight per individual (average weight)"! As population density increases, the weight per individual becomes smaller. Always double-check whether the vertical axis of a graph shows "individual weight" or "total biomass per unit area"!
4. Interactions Within a Population ②: Sociality
While intraspecific competition involves conflict among individuals, living in groups also offers substantial benefits. Let's look at the mechanisms animals use to form groups and maintain order among their members.
Group Formation (Pros and Cons of Aggregation)
When animals gather to form herds, flocks, or schools, there are both advantages and disadvantages:
- Advantages: Spotting predators earlier, easier defense against enemies, cooperative hunting of larger prey, and greater ease in finding mates.
- Disadvantages: Intensified intraspecific competition for food and space, higher risk of disease transmission, and being more conspicuous to predators.
Order Systems to Avoid Pointless Conflict
If intense fighting were constant within a group, the entire population would become exhausted. To prevent this, animals have developed various social structures:
① Territory
An area of living space—such as a feeding ground or breeding site—that an individual or a specific group defends against intruders of the same species to maintain exclusive use (e.g., sweetfish [ayu], many songbirds).
*The overall area an animal traverses during daily activities is its "home range," and the actively defended portion within it is its "territory." Maintaining a territory prevents pointless conflict from overcrowding and secures a steady supply of resources.
② Dominance Hierarchy (Rank System)
A system where a clear pecking order based on strength or dominance is established among individuals in a group. A classic example is the "peck order" seen in chickens.
Once ranks are established, subordinate individuals yield to dominant ones during feeding or resting, which prevents bloody, fatal combat.
③ Leader System
A structure where a specific individual (the leader) directs the behavior of the entire group, such as during movement or foraging (e.g., Japanese macaques, wolves). Beyond directing the group, leaders also watch out for predators and mediate disputes among members.
④ Division of Labor and Social Insects (Eusociality)
A system where group members divide tasks among themselves. In particular, ants, termites, and honeybees exhibit eusociality.
These groups have specialized castes with distinct morphologies and roles—such as the "queen" (responsible for reproduction) and "workers" (non-reproductive individuals responsible for brood care and foraging)—and work cooperatively to raise offspring.
[Fun Fact] The Sweetfish's Switch: Territory vs. Schooling
Sweetfish (ayu) are famous for living solitary lives, each defending a territory around an algae-covered rock. However, if their density becomes extremely high, defending a territory becomes impossible. Amazingly, they abandon their territories and start swimming together in schools! It's a fantastic example of behavioral plasticity in response to environment and density.
Summary: Common Test Success Checklist
Let's review the main takeaways from this chapter. Can you answer the following questions in your own words?
- Mark-Recapture Formula: Can you set up the proportion equation to calculate the total population size?
- Carrying Capacity: Can you explain why population growth slows down midway along a logistic growth curve?
- Three Types of Survivorship Curves: Can you explain the characteristics and differences in early mortality between humans (Type I) and fish (Type III)?
- Density Effects: What happens to individual size as density increases? What happens to the final total yield?
- Territories and Dominance Hierarchies: Can you explain what kind of "wasteful cost" these systems reduce for a population?
Population dynamics and sociality are prime targets for graph interpretation and experimental data questions on the Common Test. Don't just memorize formulas—study the underlying ecological reasons ("why does this happen?") as you review!