Chapter 53

In what population statistic do DEMOGRAPHERS have a particular interest? How is this data often presented?

Demographers are particularly interested in birth rates and how they vary among individuals and death rates. This data is often presented in a life table.

What is DEMOGRAPHIC TRANSITION? Use the figure below to explain the process in Sweden and Mexico.

Demographic transition is a shift from rapid population growth in which birth rate out-paces death rate to zero population growth characterized by low birth and death rates. The demographic transition in Sweden took about 150 years, from 1810 to 1960, when birth rates finally approached death rates; in Mexico, the changes are projected to continue sometime after 2050, almost the same length of time as they took in Sweden. Demographic transition in associated with an increase in the quality of health care and sanitation as well as improved access to education, especially for women.

What is the difference between density and DISPERSION?

Density is the number of individuals per unit are or volume, while dispersion is the pattern of spacing individuals within the boundaries of the population.

Give an example of COMPETITION FOR RESOURCES.

Hirta Island-soay sheep. Reproduction by juvenile soay sheep drops dramatically as population size increases.

Is your biology class a COHORT? Explain.

My biology class is a cohort because the members of it are a group of individuals of the same age.

Explain the difference between SEMELPARITY (BIG-BANG REPRODUCTION) and iteroparity (REPEATED REPRODUCTION) as life history strategies.

Semelparity is reproduction in which an organism produces all of its offspring in a single event, while interoparity is reproduction in which adults produce offspring over many years.

After looking at the age structure graphs, describe the key features of Italy.

Small base, indicating individuals younger than reproductive age are relatively underrepresented in the population.

Explain the negative feedback mechanism TERRITORIALITY.

Territory space becomes the resource for which individuals compete.

Explain how two critical factors influence whether a species will evolve toward semelparity or iteroparity.

Two critical factors that influence whether a species will evolve toward semelparity or iteroparity are: the survival rate of the offspring and the likelihood that the adult will survive and reproduce again. In variable or unpredictable environments, survival rate of offspring is low and semelparity is favored. Similarly, in these environments, adults are also less likely to survive, so producing a large number of offspring will increase the chance that some will survive. In more dependable environments, interoparity is favored because adults are more likely to survive and breed again. In these cases, few relatively large, well-provisioned offspring are more likely to survive until they are capable of reproducing.

What are two examples of conditions that might lead to EXPONENTIAL POPULATION GROWTH in natural populations?

Two examples of conditions that might lead to exponential population growth in natural populations are access to abundant food and freedom to reproduce at one's physiological capacity.

What is the advantage to using per capita birth and death rates rather than just the raw numbers of births and deaths?

Using per capita birth and death rates allows for a simpler equation in calculating the change in population than raw numbers would.

Give an example of INTRINSIC FACTORS.

White-footed mice in a small field enclosure will multiply, but eventually their reproductive rate will decline until the population ceases to grow. High population densities in mice can include a stress system in which hormonal changes delay sexual maturation, shrink reproductive organs, and depress the immune system.

What is your predicted future growth of Afghanistan?

Continued growth.

If the carrying capacity (or K) is 1,000 and N is 10, the term (K-N)/K is large. Explain why a large value for (K-N)/K predicts growth close to the maximum rate of increase for this population.

A large value for (K-N)/K predicts growth close to the maximum rate of increase for that population because it is the fraction of the carrying capacity that is still available for population growth. A large number leaves more room for population growth.

What does a REPRODUCTIVE TABLE show?

A reproductive table (fertility schedule) is an age-specific summary of the reproductive rates in a population. It shows the reproductive output of a cohort from birth to death.

Explain the negative feedback mechanism TOXIC WASTES.

Accumulation of toxic wastes can contribute to density-dependent regulation of population size by poisoning organisms within an environment.

Explain the negative feedback mechanism PREDATION.

An important cause of density-dependent mortality-if a predator encounters and captures more food as population of prey increases.

Uniform pattern.

Animals often exhibit uniform dispersion as a result of antagonistic social interactions, such as territoriality.

After looking at the age structure graphs, describe the key features of Afghanistan.

Bottom-heavy, skewed toward young individuals who will grow up and sustain explosive growth with their own reproduction.

Give an example of DISEASE.

Caddis flies-peaks in disease-related mortality followed years of high insect abundance leading to fluctuations in population density.

In the LOGISTIC POPULATION GROWTH model, the per capita of increase approaches zero as the _____________ is reached.

Carrying capacity

What is CARRYING CAPACITY?

Carrying capacity is the maximum population size that a particular environment can sustain.

Give an example of TERRITORIALITY.

Cheetahs use chemical communication to warn other cheetahs of territorial boundaries. Maintaining a territory increases the likelihood a cheetah will obtain food and reproducing.

Compare and contrast DENSITY INDEPENDENT REGULATION and DENSITY DEPENDENT REGULATION.

Density-independent and density-dependent regulation are both types of population regulation that deal with the variation of birth and death rate. However, density-independent regulation deals with birth and death rates that are not affected by population density, while density-dependent regulation deals with those birth and death rates that vary according to an increase in population density.

What is a Type III Curve?

Drops sharply at the start, reflecting very high death rates for the young, but flattens out as death rates decline for those few individuals that survive the early period die-off.

What are six examples of limiting resources that can influence carrying capacity?

Energy, shelter, refuge from predators, nutrient availability, water, and suitable nesting sites are six limiting resources that can influence carrying capacity.

Give an example of TOXIC WASTES.

Ethanol accumulates as a by-product of yeast fermentation. Alcohol content of wine >13% because that is the max. Concentration of ethanol most wine-producing yeast cells can tolerate.

What is a Type I Curve?

Flat at start, reflecting low death rates during early and middle life, and then drops steeply as death rates increase among older age-groups.

What is your predicted future growth of the United States?

Grow slowly through 2050 as a result of immigration.

Summarize human population growth since 1650. (Of all the reported statistics, which one surprised you the most?)

Human population skyrocketed after 1650 (500 million to 1 billion in 200 years). The annual rate of increase peaked in 1962 at 2.2% and declined to 1.15% by 2005. Current models project a continued decline in annual growth rate to just over 0.4% in 2050, a rate that would still add 36 million people per year. To me, the most shocking statistic is that the population grows by about 200,000 people each day, the equivalent of adding a city the size of Amarillo, Texas.

What does it mean for a population to be in EXPONENTIAL POPULATION GROWTH?

If a population is in exponential population growth, then the population is increasing under ideal conditions (abundant food and freedom to reproduce at physiological capacity).

Explain the importance of immigration and emigration in METAPOPULATIONS.

Immigration and emigration are particularly important when a number of local populations are linked, forming a metapopulation. Local populations in a metapopulation can be thought of as occupying discrete patches of suitable habitat in a sea of otherwise unsuitable habitat. Patches with many individuals can supply more emigrants to other patches.

Explain the impact of IMMIGRATION and EMIGRATION on population density. (To avoid confusion between these two terms, it might help to use this memory trick: Immigration is the movement INTO a population, while Emigration is the EXITING of individuals from a population.)

Immigration is the influx of new individuals from other areas, while emigration is the movement of individuals out of a population. The rates of immigration and emigration, along with birth and death rates alter population density. Although the percentages may seem small, they represent biologically significant exchanges between populations over time.

Explain the negative feedback mechanism COMPETITION FOR RESOURCES.

In crowded populations, increasing population density intensifies competition for declining nutrients and other resources, resulting in a lower birth rate.

Why do INFANT MORTALITY and LIFE EXPECTANCY vary so greatly between certain countries?

Infant mortality and life expectancy vary so greatly between certain countries due to the quality of life faced by children at birth and these differences influence reproductive choices the parents make. If infant mortality is high, then parents are likely to have more children to ensure that some reach adulthood.

What is a Type II Curve?

Intermediate, with a constant death rate over an organism's lifespan.

Explain the negative feedback mechanism INTRINSIC FACTORS.

Intrinsic (physiological) factors may regulate population size.

The end of this concept attempts to bring together the ideas of life histories and growth models. This is done with the introduction of two new terms: K-selection and r-selection. Explain the ideas behind the creation of these two terms.

K-selection (density-dependent selection) is the selection for life history traits that are sensitive to population density. R-selection (density independent selection) is the selection for life history traits that maximize reproductive success in uncrowded environments. Ecologists created these terms in an attempt to connect the differences in favored traits at different population densities with the logistic growth model.

Explain the effect of offspring care on parental survival in kestrels.

Kestrels reproduce in reduced (3-4), normal (5-6), and enlarged broods. Both males and females care for their offspring. It was found that kestrels with larger broods had lower survival rates the winter following reproduction, indicating that caring for more offspring negatively affects parent survival in kestrels.

Work through Figure 53.2, doing the math to make sure you get the same answer as the text. Note and understand what the letters of the formula mean. Next, try the following problem. A population ecologist wished to determine the size of a population of white-footed deer mice, PEROMYSCUS LEUCOPUS, in a 1-hectare field. Her first trapping yielded 80 mice, all of which were marked with a dab of purple hair dye on the back of the neck. Two weeks later, the trapping was repeated. This time 75 mice were trapped, out of which 48 of the mice were marked. Using the formula N=mn/x, what is the population of mice in the field? (Answer is at the end of this reading guide.)

N=mn/x N=(80)(75)/48 N=125 individuals

What is your predicted future growth of Italy?

Population decrease.

Explain the negative feedback mechanism DISEASE.

Population density can influence health and survival of organisms. If transmission rate of a disease depends on a certain level of crowding in a population, its impact may be density-dependent.

Give both biotic and abiotic reasons for population fluctuations over the last 50 years in the moose population on Isle Royale, based on POPULATION DYNAMICS.

Population fluctuations over the last 50 years in the moose population on Isle Royale are do to predation by wolves (biotic) and harsh winter weather (abiotic) which increased energy needs of the animals and made food harder to find.

Explain how negative feedback plays an essential role in the unifying theme of regulation of populations. Does negative feedback play a role in both density-independent and density-dependent regulation?

Population would never stop growing without negative feedback. Density-dependent regulation provides that feedback, operating through mechanisms that help reduce birth rates and increase death rates, halting population growth.

After looking at the age structure graphs, describe the key features of the United States.

Relatively even until older, post reproductive ages except for a bulge that corresponds to the "baby bloom" post-World War II. Nation's current total reproductive rate-2.1 kids per woman.

Clumped pattern.

Sea stars group together in ride pools, where food is abundant and they can breed more successfully. Animals may also form groups to increase the effectiveness of predation.

Can the world's population sustain an ECOLOGICAL FOOTPRINT that is currently the average American footprint? Explain.

The ecological footprint concept summarizes the aggregate land and water area required by each person, city, or nation to produce all the resources it consumes and to absorb all the waste it generates. One way to estimate the ecological footprint of the entire human population is to add up all the ecologically productive land on the planet and divide by the population. This calculation yields approximately 2 hectares (ha) per person (1ha=2.47 acres). Reserving some land for parks and conservation means reducing this allotment to 1.7 ha per person-the benchmark for comparing actual ecological footprints. Anyone who consumes resources that require more than 1.7 ha to produce is said to be an unsustainable share of the Earth's resources. A typical ecological footprint for a person in the US is 10 ha.

On what is the LIFE HISTORY of an organism based?

The life history of an organism is based on the traits that affect an organism's schedule of reproduction and survival.

In the graph below, explain why the line with the value of 1.0 shows a steeper slope that reaches exponential growth more quickly than does the line with the value of 0.5. On this graph, add a third line that approximates a population with an exponential value of 1.25.

The line with the value of 1.0 shows a steeper slope that reaches exponential growth more quickly than the line with the value of 0.5 because its species has a higher maximum rate of increase.

In the graph below, explain why the logistic model predicts a sigmoid (S-shaped) growth curve when the population density is plotted over time. Hint: The critical part of this answer is why growth slows as N approaches K.

The logistic model predicts a sigmoid (S-shaped) growth curve when the population density is plotted over time because new individuals are added to the population most rapidly at intermediate population sizes, when there is not only a breeding population of substantial size, but also lots of available space and other resources in the environment. The population growth rate slows dramatically as N approaches K. For a population's growth rate to decrease, either the birth rate must decrease, or the death rate must increase, or both.

What two pieces of data are needed to mathematically determine DENSITY?

The number of individuals (mass) and unit area (volume) are needed to mathematically determine density.

What will the per capita birth and death rates be if a population is demonstrating ZERO POPULATION GROWTH?

The per capita birth and death rates will be equal if a population is demonstrating zero population growth.

What three variables form the life history of a species?

The three variables that form the life history of a species are: when reproduction (the age at first reproduction or at maturity), how often the organism reproduces, and how many offspring are produced during each reproductive episode.

Random pattern.

This pattern occurs in the absence of strong attractions or repulsions among individuals of a population or where key physical or chemical factors are relatively homogeneous across the study area.

Give an example of PREDATION.

Trout concentrate on one insect emerging from its aquatic larval stage for a few days then switch to another species as it becomes more abundant.