Chapter 8 Understanding Population Change. Principles of Population Ecology Populations exhibit...

Chapter 8

Understanding Population Change

Principles of Population Ecology

Populations exhibit characteristics unique to the individual.

Populations are central to many environmental problems.

Population ecology- deals with the number of individuals of a particular species that are found in an area and how and why those numbers increase or decrease over time, and how they respond to their environment.

1. Characteristics of Populations

© Brooks/Cole Publishing Company / ITP

Changes in population size, density, dispersion, and age distribution are known as population dynamics.

• population size is the number of individuals in a population at a given time;

• population density is the number of individuals per unit area in terrestrial ecosystems or per unit volume in aquatic ecosystems;

• dispersion is the spatial patterning of individuals;

• age structure is the proportion of individuals in each age group (e.g., prereproductive, reproductive, and postreproductive) of a population.

Characteristics of Populations


In terms of dispersion, individuals of a population can be clumped, uniform, or randomly distributed.

Fig. 10–2

2. Population Dynamics and Carrying Capacity


Population size is governed by births, deaths, immigration, and emigration:

[Population Change] = [Births + Immigration] – [Deaths + Emigration]

• If the number of individuals added by births and immigration are balanced by those lost by deaths and emigration then there is zero population growth (ZPG)

• populations vary in their capacity for growth, also known as biotic potential;

• the intrinsic rate of growth (r) is the rate at which a population will grow if it had unlimited resources.

Population Dynamics


Factors that tend to increase or decrease population size:

Fig. 10–3

Carrying Capacity

There are always limits to population growth in nature.

• carrying capacity (K) is the number of individuals that can be sustained in a given space;

• the concept of carrying capacity is of central importance in environmental science;

• if the carrying capacity for an organism is exceeded, resources are depleted, environmental degradation results, and the population declines.


Exponential vs. Logistic Growth

Exponential growth occurs when resources are not limiting.

Logistic growth occurs when resources become more and more limiting as population size increases.

Fig. 10–4© Brooks/Cole Publishing Company / ITP

Exponential Population Growth


Exponential growth occurs when resources are not limiting.

• during exponential growth population size increases faster and faster with time;

• currently the human population is undergoing exponential growth;

• exponential growth can not occur forever because eventually some factor limits population growth.

Fig. 10–4a

Logistic Population Growth


Logistic population growth occurs when the population growth rate decreases as the population size increases.

• note that when the population is small the logistic population growth curve looks like exponential growth;

• over time, the population size approaches a carrying capacity (K).

Fig. 10–4b

Exceeding the Carrying Capacity


During the mid–1800s sheep populations exceeded the carrying capacity of the island of Tasmania. This "overshoot" was followed by a "population crash". Numbers then stabilized, with oscillation about the carrying capacity.

Fig. 10–5

Exceeding the Carrying Capacity


Reindeer introduced to a small island off of Alaska in the early 1900s exceeded the carrying capacity, with an "overshoot" followed by a "population crash" in which the population was totally decimated by the mid–1900s.

Fig. 10–5

Population Curves in Nature


Natural populations display a broad diversity of population curves. Stable populations are relatively constant over time. Cyclic curves are often associated with seasons or fluctuating resource availability. Irruptive curves are characteristic of species that only have high numbers for only brief periods of times (e.g., seven–year cicada).

Fig. 10–6

Population Curves in Nature


Population cycles for the snowshoe hare and Canadian lynx are believed to result because the hares periodically deplete their food, leading to first a crash of the hare population and then a crash of the lynx population.

Fig. 10–8

r–Strategist Species


Characteristics of r–strategists, including production of many small and unprotected young, enable these species to live in places where resources are temporarily abundant. These species are typically "weedy" or opportunistic.

Fig. 10–7a

K–Strategist Species


Characteristics of K–strategists, including production of few large and well cared for young, enable these species to live in places where resources are limited. These species are typically good competitors. Fig. 10–7b

Survivorship Curves


Three kinds of curves:

• late loss (usually K–strategists), in which high mortality is late in life;

• constant loss (such as songbirds), in which mortality is about the same for any age;

• early loss (usually r–strategists), in which high mortality is early in life.

Fig. 10–9

1. Factors Affecting Human Population Size1. Factors Affecting Human Population Size


Human population is currently growing exponentially.

• What will be the ultimate size of the human population?

• What is Earth's carrying capacity?

Fig.1–1

Factors Affecting Human Population SizeFactors Affecting Human Population Size


Population ChangeBirths

+Immigration

Deaths+

Emigration–=

Population change is calculated as the difference between individuals entering and leaving a population:

• birth rate is reported as the number of births per thousand people;

• death rate is reported as the number of deaths per thousand people;

• zero population growth (ZPG) occurs when factors that increase and decrease population size balance.

Crude Birth and Death RatesCrude Birth and Death Rates


Developed countries tend to have lower birth rates and death rates than developing countries (data from 1998).

Fig.11–2

Population ChangePopulation Change


The annual rate of population increase is generally expressed as a percentage. Note the distribution of growth rates in 1998.

Fig.11–3

Population SizePopulation Size


The world's ten most populous countries in 1998, with projections of population size in 2025.

Fig.11–4



Fig.11–5

Population size by region in 1998, with projections of population size in 2025.



Fig.11–7

United Nations projections of human population increase, based on fertilities ranging between 1.7 (low), 2.1 (medium), and 2.5 (high) children per woman.



Fig.11–8

Average total fertility in 1998 was 2.9 children per woman. The total size of the human population depends upon the year by which the replacement rate of 2.1 children per woman is reached.

Total Fertility in the United StatesTotal Fertility in the United States


Fig.11–9

Total fertility in the United States had a major increase during the "baby boom" (1946–64) and is now hovering just below replacement level.

Birth and Death Rates Over TimeBirth and Death Rates Over Time


The rapid growth in the world's population has not resulted from an increase in birth rates, but rather a major decrease in death rates.

• death rates have decreased markedly during the past 100 years;

• birth rates have also decreased, but not as fast as death rates;

• the increasing difference between birth and death rates is what has lead to exponential population growth;

• the patterns of change in birth and death rates over time are different for developed vs. developing countries.

Death RatesDeath Rates


Infant death rates are lower in developed countries than developing countries.

Fig.11–12



In developed countries decreases in death rates are being accompanied by decreases in birth rates over time.

Fig.11–11a



In developing countries decreases in death rates have not been accompanied by as large of decreases in birth rates over time, leading to major population increase.

Fig.11–11b

2. Population Age Structure2. Population Age Structure


Age structure refers to the proportion of the population in each age class:

• prereproductive (0–14 years)

• reproductive (15–44 years)

• postreproductive (45 and up)

Population Age StructurePopulation Age Structure


Age structure of a rapidly growing vs. a slower growing population.

Fig.11–13a

Rapidly growing populations have pyramid–shaped age structures, with large numbers of prereproductive individuals. Slower growing populations have a more even age distribution.



Age structure of populations with zero growth vs. negative growth.

Fig.11–13b

Populations with zero population growth have nearly equal proportions of prereproductive and reproductive individuals; whereas populations with negative growth have a greater proportion of reproductive than prereproductive individuals.



Developing countries are expected to continue to have a pyramid shape through the year 2025, although the age structure will become somewhat more evenly distributed.

Fig.11–14a



Populations of developed countries are expected to have an increasingly even age distribution through the year 2025.

Fig.11–14b



Population age structure of the United States continues to show a bulge as the baby boom generation ages. This has been compared to watching a boa constrictor swallow a pig.

Fig.11–15

Demographic Stages

Populations undergo four stages as a society becomes industralized: Preindusrial, Transitional, Industrial, and Postindustrial

Factors that Affect Population Size

Density-Dependent Factors Tend to slow population growth by causing an

increase in death rate and/or decrease in birth rate Keep population at carrying capacity Predation, disease, and competition

Density-Independent Factors Random events that are not a result of population,

but can have an effect on population size. Blizzards, Frosts, tornadoes, hurricanes, etc.

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