AP Biology 8.3-8.4: Population Ecology

Population

All individuals of the same species living in a particular area and having the potential to interact.

Demography

The statistical study of population changes over time, including birth rates, death rates, life expectancy, immigration, and emigration.

Population size ($N$)

The total number of individuals in a population.

Population density

The number of individuals per unit of area or volume. Density can affect mate-finding, competition, disease transmission, and resource availability.

Quadrat sampling

A method for estimating the size and density of stationary or slow-moving organisms by counting individuals within multiple sample plots placed at representative, often random, locations.

Why can population size affect a population's ability to adapt to environmental change?

Larger populations generally contain more genetic variation, increasing the likelihood that some individuals have traits suited to the changed environment.

What is the typical relationship between body size and population density?

Population density generally decreases as body size increases because larger organisms require more food and other resources, so fewer can be supported in a given area.

Why should quadrats be sampled at multiple random locations?

Multiple random samples reduce the influence of local variation and allow the sample to better represent the entire habitat.

How should quadrat size be selected for a population study?

The quadrat should be large enough to include sufficient individuals for a representative sample, but appropriate to the organism's size and density. Larger organisms generally require larger quadrats.

Mark-and-recapture method

A population-estimation technique in which a sample of mobile organisms is captured, marked, released, allowed to mix with the population, and then sampled again to determine the fraction of marked recaptures.

What equation estimates population size using mark-and-recapture data?

$N \approx \frac{MC}{R}$, where $M$ is the number marked during the first capture, $C$ is the total number captured during the second sampling, and $R$ is the number of marked individuals recaptured.

A biologist marks 80 animals, later captures 100 animals, and finds 20 marked individuals. What population size is estimated?

$N \approx \frac{(80)(100)}{20}=400$ individuals.

What assumptions support a reliable mark-and-recapture estimate?

Marked individuals should mix randomly with the population, retain their marks, and have similar survival and capture probabilities as unmarked individuals. The population should not change substantially between samples.

How can capture behavior bias a mark-and-recapture estimate?

If marked animals avoid recapture, $R$ is too small and $N$ is overestimated. If marked animals are more likely to be recaptured, $R$ is too large and $N$ is underestimated.

How can marking itself bias a population estimate?

If marking harms individuals and reduces their survival, fewer marked animals may be recaptured, causing the population size to be overestimated.

Uniform dispersion

A spatial pattern in which individuals are approximately equally spaced. Territorial behavior and chemical inhibition between plants can produce this pattern.

Random dispersion

A spatial pattern in which an individual's location is independent of other individuals' locations, producing no predictable spacing pattern under the given conditions.

Clumped dispersion

A spatial pattern in which individuals occur in groups or patches. Social behavior, localized resources, seed fall near parent plants, and heterogeneous habitats can produce clumping.

How does dispersion provide information beyond population density?

Density measures how many individuals occupy a space, whereas dispersion reveals how individuals are spatially arranged and can indicate territoriality, social behavior, resource distribution, or interactions among organisms.

How do birth and death rates determine whether a population grows?

The population increases when births exceed deaths, decreases when deaths exceed births, and remains stable when births equal deaths, assuming immigration and emigration are negligible.

Immigration

The number of individuals entering a population over a given time period.

Emigration

The number of individuals leaving a population over a given time period.

How do immigration and emigration affect population size?

Immigration adds individuals to a population, whereas emigration removes individuals. Both can change population size even when births and deaths are equal.

What equation describes the change in population size over a time interval?

$N_1=N_0+B-D+I-E$, where $B$ is births, $D$ is deaths, $I$ is immigration, and $E$ is emigration.

Intrinsic rate of increase ($r$)

The maximum per-capita rate at which a population can increase under ideal conditions. It assumes environmental conditions and resources permit the population's greatest potential growth.

Exponential growth

Growth in which the per-capita rate of increase remains constant and positive, producing a J-shaped curve when resources are effectively unlimited and predation or other density-dependent limits are absent.

What equation represents exponential population growth?

A common continuous model is $\frac{dN}{dt}=rN$, where $N$ is population size and $r$ is the intrinsic rate of increase. The solution is $N_t=N_0e^{rt}$.

Carrying capacity ($K$)

The maximum population size that an environment can sustain over time, given its available resources and limiting conditions. Because environmental conditions can change, $K$ is not necessarily constant.

Logistic growth

Growth in which the per-capita rate of increase declines as population size approaches carrying capacity, producing an S-shaped curve. Environmental resistance and social factors limit growth near $K$.

How does population size affect per-capita growth in logistic growth?

Per-capita growth is greatest when the population is small relative to $K$ and approaches zero as $N$ approaches $K$. A common model is $\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)$.

Density-dependent limiting factor

A factor whose effect becomes stronger as population density increases. Competition, predation, parasitism, and disease are common examples.

Density-independent limiting factor

A factor that affects population size regardless of population density. Severe weather, fires, volcanic eruptions, and other natural disasters are examples.

How do density-dependent factors contribute to population regulation?

As density rises, density-dependent factors increase mortality or decrease reproduction, slowing growth and tending to keep the population near its carrying capacity.