20 Populations



20 Populations

1. Explain how birth, death, immigration and emigration influence population size

Population Growth Rate

1) Demographers, scientists who study population dynamics, define the growth rate of a population as the amount by which a population’s size changes in a given time

2) The statistical study of all populations is called demography

a. Whether a population grows, shrinks, or remains the same size depends on four processes: birth, death, emigration, and immigration

1) Immigration is the movement of individuals into a population

2) Emigration is the movement of individuals out of the population

3) Two of these processes—birth and immigration—add individuals to a population

a) Every population tends to grow because individuals reproduce over their lifetime - Eventually, limited resources in an environment limit the growth of a population

4) While the other two processes—death and emigration—subtract individuals from the population

3) A simple population model that demonstrates the rate of population growth as the difference between the birthrate and the death rate (scientists assume that immigration and emigration are zero)

a. This model is called a stage I model

1) Birth rate – death rate = growth rate

2) In humans, birth and death rates are usually expressed as the number of births and deaths per thousand people per year

a) Note: when calculating population, to see how many new individuals will be added, take the growth rate and multiply it by the number of individuals in the population

i. If the growth rate is positive – the population is growing

ii. If the growth rate is negative – the population if shrinking

2. Growth rate affects population size

A. When population size is plotted against time on a graph, the graph generally shows a J-shaped growth curve

1) An exponential growth curve is a curve in which the rate of population growth stays the same; as a result the population size increases steadily (stage II model)

a. To calculate the number of individuals that will be added to the population as it grows, multiply the size of the current population (N) by the rate of growth (r)

1) Populations generally don’t grow unchecked

i. Their growth is usually checked by predators, disease, and the availability of resources (limiting factors) Eventually, growth slows, and the populations may stabilize - The population size that an environment can sustain is called the carrying capacity

3. Resources affect population size

A. As a population grows, resources eventually become limited, and eventually, depleted

1) The population model can be adjusted to account for the effect of limited resources, such as food and water

a. These resources are called density-dependant factors, because the rate at which they become depleted depends on the size of the population that uses them

1) The population model that takes into account the declining resources available to populations is called the logistic model of population growth (stage III) model

i. Logistic model is limited by a density-dependant factor

▪ It takes into account carrying capacity, the number of individuals the environment can support over time

▪ When a population reaches its carrying capacity, the birth rate equals the death rate and growth stops - This type of growth is known as logistic growth

1 Population Regulation

A. Real populations exhibit a range of growth patterns

1) Many species of plants and insects are generally not limited by density-dependent factors, but rather by density-independent factors

a. Examples: weather and climate

B. Rapidly growing populations

1) Many species, including bacteria, some plants and insects, are found in rapidly changing environments

2) Such species, called r-strategists, grow exponentially when environmental conditions allow them to reproduce

a. Sometimes the population is very large, other times, very small

b. Generally, these are the populations where the offspring mature quickly and with little or no parental care

C. Slowly growing populations

1) Populations that grow slowly generally have small population sizes

2) These species are called K-strategists, because their population density is usually near the carrying capacity (K) for the environment

a. They are generally categorized by a long lifespan, few young, and a slow maturing process, with reproduction late in life - They also provide extensive care for their young

5. Explain the differences between population size, density, and dispersion

Properties of Populations

1) A population is a group of organisms that belong to the same species and live in a particular place at the same time

a. The properties of populations differ from those of individuals

1) An individual may be born, it may reproduce, or it may die

2) A population study focuses on a population as a whole—how many individuals are born, how many die, and so on

2) Population Size

a. A population’s size is the number of individuals it contains

1) Size is a fundamental and important population property, but it can be difficult to measure directly

a) If a population is small and composed of immobile organisms, such as plants, its size can be determined simply by counting individuals

b) More often, though, individuals are too abundant, too widespread, or too mobile to be easily counted, and scientists must estimate the number of individuals in the population

i. Estimates of population size are based on certain key assumptions, so they all have the potential for error

(2) The number of individuals in a population, or population size,

can affect the population’s ability to survive

a) Small populations are more likely to become extinct

b) Random events, like fires or floods, endanger small populations

c) There is generally more interbreeding in small populations

3) Population Density

a. Population density measures how crowded a population is

b. Population density is always expressed as the number of individuals per unit of area or volume

(1) The number of individuals that live in a given area

c. If the individuals of a population are few and spaced far apart, they may never encounter each other

1) Big problem for reproduction

4) Dispersion

a. Dispersion is the spatial distribution of individuals within the population The way individuals are arranged in a space

1) In a clumped distribution, individuals are clustered together

a) Often occur when resources such as food or living space are clumped

b) Also occur because of a species’ social behavior, such as when zebras gather into herds or birds form flocks

2) In an even distribution, individuals are separated by a fairly consistent distance

a) Usually result from social interactions, but the interactions result in individuals getting as far away from each other as possible

b) For example, each gannet stakes out a small area on the coast and defends it from other gannets

c) Each gannet tries to maximize its distance from all of its neighbors, resulting in an even distribution of individuals

3) In a random distribution, each individual’s location is independent of the locations of other individuals in the population

a) Usually results from seed dispersal by the wind or by birds

b) Forests or a field of wildflowers result from random seed dispersal

Chapter 20 Populations

□ A population is a group of individuals of the same species living in the same place in the same time period.

□ A population’s size is the number of individuals it contains. Population density is a measure of how crowded the population is. The dispersion pattern—random, even, or clumped—indicates the distribution of individuals within the population.

□ A population’s age structure indicates the percentage of individuals at each age.

□ Populations show three patterns of mortality: Type I (low mortality until late in life), Type II (constant mortality throughout life), and Type III (devastating mortality early in life followed by low mortality for the remainder of the life span).

□ Four processes determine whether a population will shrink or grow: birth, death, immigration, and emigration.

□ The exponential model describes perpetual growth at an increasing rate in a population. The model assumes constant birth and death rates and no immigration or emigration.

□ In the logistic model, birth rates fall and death rates climb as the population grows.

□ The carrying capacity is the number of individuals the environment can support for an indefinite period of time. At the carrying capacity, birth and death rates are equal, and the population size is stable.

□ Density-independent factors kill the same percentage of a population regardless of its size. Density-dependent factors kill more individuals in large populations than in small ones.

□ Populations fluctuate over time due to environmental changes.

□ Small populations are less able to rebound from environmental changes, they are more likely to experience inbreeding, and their genetic diversity is often low.

□ About 10,000–12,000 years ago, human population growth was slow.

□ The development of agriculture increased the growth rate of the population. Improvements in hygiene, diet, and economic conditions around 1650 further accelerated population growth.

□ The population grew at its fastest rate in the decades immediately after World War II, largely because of better sanitation and medical care in the poorer countries.

□ Today, population growth is faster in the less-developed countries and slower in the more-developed countries.

Vocabulary List

Age structure

Agricultural revolution

Birth rate

Carrying capacity

Death/mortality rate

Density-dependent factor

Density-independent factor

Developed countries

Developing countries

Dispersion

Emigration

Exponential growth

Exponential model

Growth rate

Hunter-gatherer lifestyle

Immigration

Inbreeding

Life expectancy

Limiting factor

Logistic growth

Logistic model

Population density

Survivorship curve

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