CH 52 PPT Population Ecology Compatibility Mode

Overview: Earth’s Fluctuating Populations
• To understand human population growth, we must consider general principles of population ecology
• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size
• The fur seal population of St. Paul Island, off the coast of Alaska, has experienced dramatic fluctuations in size

Dynamic biological processes influence population density, dispersion, and demography
• A population is a group of individuals of a single species living in the same general area
• Density is the number of individuals per unit area or volume
• Dispersion is the pattern of spacing among individuals within the boundaries of the population

Density
• Determining the density of natural populations is difficult • In most cases, it is impractical or impossible to count all individuals in a population
• Density is the result of an interplay between processes that add individuals to a population and those that remove individuals

Births

Immigration

Population size Emigration
Deaths

Patterns of Dispersion
• Environmental and social factors influence spacing of individuals in a population
• In a clumped dispersion, individuals aggregate in patches • A clumped dispersion may be influenced by resource availability and behavior

Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South
Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

Random. Dandelions grow from windblown seeds that land at random and later germinate.

Demography
• Demography is the study of the vital statistics of a population and how they change over time
• Death rates and birth rates are of particular interest to demographers
• A life table is an age-specific summary of the survival pattern of a population
• It is best made by following the fate of a cohort
• The life table of Belding’s ground squirrels reveals many things about this population

Survivorship Curves

• A survivorship curve is a graphic way of representing the data in a life table
• The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate • Survivorship curves can be classified into three general types: Type I, Type II, and
Type III

Number of survivors (log scale)

1,000

100

Females
10

Males

1
0

2

4

6

Age (years)

8

10

Number of survivors (log scale)

1,000

I

100

II
10

III
1
0

50

Percentage of maximum life span

100

•

A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population. It describes reproductive patterns of a population

Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.

Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

The exponential model describes population growth in an idealized, unlimited environment

• It is useful to study population growth in an idealized situation
• Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth
• In animals, parental care of smaller broods may facilitate survival of offspring

Exponential Growth

• Exponential population growth is population increase under idealized conditions
• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase

The logistic growth model includes the concept of carrying capacity
• Exponential growth cannot be sustained for long in any population • A more realistic population model limits growth by incorporating carrying capacity
• Carrying capacity (K) is the maximum population size the environment can support
• In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached
• We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N increases

Per capita rate of increase (r)

Maximum

Positive
N=K

0
Negative
Population size (N)

Populations are regulated by a complex interaction of biotic and abiotic influences
• There are two general questions about regulation of population growth:
– What environmental factors stop a population from growing?
– Why do some populations show radical fluctuations in size over time, while others remain stable?

Population Change and Population Density

• In density-independent populations, birth rate and death rate do not change with population density
• In density-dependent populations, birth rates fall and death rates rise with population density

Density-Dependent Population Regulation
• Density-dependent birth and death rates are an example of negative feedback that regulates population growth
• They are affected by many factors, such as competition for resources, territoriality, health, predation, toxic wastes, and intrinsic factors
• In crowded populations, increasing population density intensifies intraspecific competition for resources •

Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries

• Oceanic birds exhibit territoriality in nesting behavior Health, Predation and Toxic Wastes
• Population density can influence the health and survival of organisms
• In dense populations, pathogens can spread more rapidly • As a prey population builds up, predators may feed preferentially on that species
• Accumulation of toxic wastes can contribute to density-dependent regulation of population size

Population Dynamics
• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

Stability and Fluctuation
• Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time
• Extreme fluctuations in population size are typically more common in invertebrates than in large mammals

Metapopulations and Immigration
• Metapopulations are groups of populations linked by immigration and emigration
• High levels of immigration combined with higher survival can result in greater stability in populations • Many populations undergo boom-and-bust cycles
• Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors

Regional Patterns of Population Change
• No population can grow indefinitely, and humans are no exception • To maintain population stability, a regional human population can exist in one of two configurations:
– Zero population growth =
High birth rate – High death rate
– Zero population growth =
Low birth rate – Low death rate
• The demographic transition is the move from the first state toward the second state
• The demographic transition is associated with various factors in developed and developing countries

Age Structure
• One important demographic factor in present and future growth trends is a country’s age structure
• Age structure is the relative number of individuals at each age
• It is commonly represented in pyramids
• Age structure diagrams can predict a population’s growth trends
• They can illuminate social conditions and help us plan for the future

Infant Mortality and Life Expectancy
• Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition

Ecological Footprint
• How many humans can the biosphere support?
• The carrying capacity of Earth for humans is uncertain • The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation
• It is one measure of how close we are to the carrying capacity of Earth
• Countries vary greatly in footprint size and available ecological capacity