What Is Life? A Guide To BiologySecond Edition

Jay Phelan

© 2012 W. H. Freeman and Company

CHAPTER 14Population Ecology

Ecologists have a challenging task managing natural populations when it's difficult to count the number of individuals present or determine how many the environment can support.

Ecology is the study of interactions between organisms and their environments. It can be studied at many levels, including:

INDIVIDUALSIndividual organisms

POPULATIONSGroups of individual organisms that interbreed with each other

COMMUNITIESPopulations of different species that interact with each other within a locale

ECOSYSTEMSAll living organisms, as well as non-living elements, that interact in a particular area

Population ecology examines features that cannot be studied on an individual organism, such as population size.

Populations cannot grow unchecked forever, so exponential growth never lasts long in nature.

Exponential growth occurs when each individual produces more than the single offspring necessary to replace itself. According to realistic (and moderate) estimates of birth and death rates, a population of just 500 elk would grow to more than a billion individuals within 80 years and eventually would cover the earth.

15

102550

100

5001,0372,580

39,7483,791,849

34,507,489,384!

Exponential growth

DENSITY-DEPENDENT FACTORS• Food supply• Habitat for living and breeding• Parasite and disease risk• Predation risk

Logistic growth describes population growth that is gradually reduced as the population nears the environment’s carrying capacity.

CARRYING CAPACITY (K)

Exponential growth

Logistic growth

Exponential growthPopulation decline

Density-independent forces include natural disasters such as floods, earthquakes, and fires.

Mortality events resulting from density-independent forces

In an environment in which a “bad luck” event repeatedly occurs, a population can be in a perpetual state of exponential growth with periodic massive mortality events.

Development of agricultural technologies is one example of how carrying capacity can be increased.

Snowshoe hares (prey)

Lynx (predator)

1 A growing hare population provides more food for the lynx, which then reproduce at higher rates.

2 Lynx eat too many hares, thereby reducing their food source and causing their own population to crash, which enables the hare population to grow.

1

2

Effective and sustainable management of natural resources requires the determination of a population’s maximum sustainable yield, the point at which the maximum number of individuals are being added to the population (and so can be harvested or utilized).

CARRYING CAPACITY

MAXIMUM SUSTAINABLE YIELD

When a population is halfway to the carrying capacity, K/2, it is growing at its fastest rate.

When a population is halfway to the carrying capacity, K/2, it is growing at its fastest rate.

Maximum sustainable yield is a useful concept, but it is difficult to put into practice because it is hard to accurately determine population size and carrying capacity in nature.

The concept of maximum sustainable yield generates insights into fighting biological pests such as cockroaches.

Antechinus

BIG-BANG REPRODUCTION• Reaches sexual maturity at one

year• Mates intensely over a three-

week period• Males die shortly after mating

period• Females usually die after

weaning their first litter

Greater bulldog bat

SLOW, GRADUAL REPRODUCTIVE INVESTMENT• Reaches sexual maturity at one

year• Produces about one offspring

per year

House mouse

FAST, INTENSIVE REPRODUCTIVE INVESTMENT• Reaches sexual maturity at

one month• Produces litters of six to ten

offspring every month

Humans are at the slow extreme of life histories, waiting two and sometimes several more decades before reproducing. (And they generally have just one offspring at a time.)

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In species with type II survivorship curves, individuals experience approximately the same risk of death at any age.

In species with type II survivorship curves, individuals experience approximately the same risk of death at any age.

Giant tortoise

Cedar waxwing

Mackerel

TYPE IHigh survivorship until old age, then rapidly decreasing survivorship

TYPE IISurvivorship decreases at a steady, regular pace

TYPE IIIHigh mortality early in life, but those that survive the early years live long lives

REPRODUCTION AND SURVIVALBig-bang reproducers such as salmon make a single, exceptionally high investment in reproduction, then die shortly afterward.

REPRODUCTION AND GROWTHBeech trees grow much more slowly in the years when they produce many seeds than they do in years when they produce few seeds.

NUMBER AND SIZE OF OFFSPRINGFemale lizards (Uta stansburiana) produce medium-size eggs as a compromise between many small eggs (with poor survival of offspring) and few large eggs (with better survival of offspring).

ALLELES THAT CAUSE SICKNESS AND DEATH EARLY IN LIFE

ALLELES THAT CAUSE SICKNESS AND DEATH LATER IN LIFE

Natural selection “weeds out” alleles that cause sickness and death early in life, because those alleles never get passed on.

Mutations that cause sickness and death after reproduction will be passed to future generations.

Infant born with mutant allele that causes death at age 10

Infant born with mutant allele that causes death at age 150 Infant inherits mutant

allele that causes death at age 150

REPRODUCTION

SEXUAL MATURITY

The cumulative effects of the build-up of late-acting harmful alleles are responsible for what we see as aging.

Time

Rodent

HIGH HAZARD FACTOR• Relatively high risk of death at each age• Individuals tend to reproduce earlier• Earlier aging• Shorter life spans

2 years

Tortoise

LOW HAZARD FACTOR• Relatively low risk of death at each age• Individuals tend to reproduce later• Later aging• Longer life spans

150 years

In environments characterized by low mortality risk, populations of slowly aging individuals with long life spans evolve. In environments characterized by high mortality risk, the opposite occurs.

New generation

Eggs

= 500 fruit flies

1 INITIAL SETUPStart with a cage that contains a large number of fruit flies and fresh food.

2 START NEW GENERATIONAfter 2 weeks, put a fresh dish of food into the cage and collect the eggs laid on it. Transfer the eggs to a new cage. Sample some of the flies hatched from these eggs and measure their longevity.

3 INCREASE GENERATION TIMERepeat the procedure, but instead of waiting 2 weeks, wait longer. Gradually increase this period until eggs are being collected only from flies that survive 10 weeks.

Flies that didn't survive to this point do not contribute eggs (or genes) to the next generation.

Flies that didn't survive to this point do not contribute eggs (or genes) to the next generation.

1

2

3

THE EXPERIMENT

Over many generations of selection for later and later age of reproduction, the average life span of the flies is doubled!

Experiment continues through 90 generations.

2-WEEK GENERATION TIME

Avg. longevity:

THE RESULTS

10-WEEK GENERATION TIME

Avg. longevity:

MalesFemales

Industrialized countries, such as Norway, have age pyramids that appear more rectangular in shape, due to low birth rates and the low death rates in older individuals.

Industrialized countries, such as Norway, have age pyramids that appear more rectangular in shape, due to low birth rates and the low death rates in older individuals.

Developing countries, such as Kenya, have age pyramids that appear triangular in shape, due to high birth rates and the high death rates in older individuals.

Developing countries, such as Kenya, have age pyramids that appear triangular in shape, due to high birth rates and the high death rates in older individuals.

NORWAY KENYA

MalesFemales

UNITED STATES OF AMERICA

Baby boomers

Because birth rates were unusually large for a brief time about 50–65 years ago, those age classes appear as a bulge moving up the age pyramid. And the younger age classes may not be large enough to support them.

Bir

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Po

pu

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Birth rate

The demographic transition is a pattern of population growth that is experienced as a country industrializes. It is characterized by slow growth, then fast growth, and then slow growth again.

Death rate

Population growth rate

1 SLOW GROWTH• High birth rates and high death rates• Populations with inefficient systems of food production and distribution, along with a lack of reliable medical care, usually have high birth rates and death rates, leading to slow population growth.

2 FAST GROWTH• High birth rates and low death rates• As industrialization begins, both food production and health care improve, leading to a reduction in the death rate. The birth rate remains relatively high, and so the country’s population grows rapidly.

3 SLOW GROWTH• Low birth rates and low death rates• As industrialization continues, the birth rate decreases as a result of higher levels of education and employment, along with improved health care. The new, lower birth rate then slows the population’s growth.

1

2

3

8000 B.C. 5 million

2011 7 billion

1999 6 billion

1987 5 billion

1850 1 billion

1930 2 billion

1960 3 billion

1975 4 billion

Current birth rates exceed death rates by so much that we add 80 million people to the world population each year!

One reason that the carrying capacity for the human population is difficult to estimate is that we can increase it in a variety of ways.

Dubai, United Arab Emirates Juehnsdorf, Germany New York City, United States

EXPANDING INTO NEW HABITATSWith fire, tools, shelter, and efficient food distribution, we can survive almost anywhere on earth.

INCREASING THE AGRICULTURAL PRODUCTIVITY OF THE LANDWith fertilizers, mechanized agricultural methods, and selection for higher yields, fewer people can now produce much more food than was previously thought possible.

FINDING WAYS TO LIVE AT HIGHER DENSITIESPublic health and civil engineering advances make it possible for higher and higher densities of people to live together with minimal problems from waste and infectious diseases.

1. What is the purpose of this graph?1. What is the purpose of this graph?

2. Why does the light green line become flat after initially increasing?2. Why does the light green line become flat after initially increasing?

3. When the population reaches carrying capacity it appears to persist indefinitely. How is that possible? Do organisms stop dying?

3. When the population reaches carrying capacity it appears to persist indefinitely. How is that possible? Do organisms stop dying?

4. Are data presented in this graph? If so, what are they? If not, what is presented?4. Are data presented in this graph? If so, what are they? If not, what is presented?

5. What data would improve this graph? Why? How might they be incorporated into the graph?

5. What data would improve this graph? Why? How might they be incorporated into the graph?

6. Create an alternative graph showing the relationship between population size (on the x-axis) and growth rate (on the y-axis). Label carrying capacity and the point of maximum sustainable yield on your graph.

6. Create an alternative graph showing the relationship between population size (on the x-axis) and growth rate (on the y-axis). Label carrying capacity and the point of maximum sustainable yield on your graph.

7. Describe two assumptions about population growth that are implicit in this figure?7. Describe two assumptions about population growth that are implicit in this figure?