Chapter 52 Population Ecology

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

Concept 52.1: 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 and Dispersion 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: A Dynamic Perspective 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

LE 52-2 Births Immigration Population size Emigration Deaths

Patterns of Dispersion Environmental and social factors influence spacing of individuals in a population

Video: Flapping Geese (clumped) In a clumped dispersion, individuals aggregate in patches A clumped dispersion may be influenced by resource availability and behavior Video: Flapping Geese (clumped)

LE 52-3a 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.

Video: Albatross Courtship (uniform) A uniform dispersion is one in which individuals are evenly distributed It may be influenced by social interactions such as territoriality Video: Albatross Courtship (uniform)

LE 52-3b 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.

Video: Prokaryotic Flagella (Salmonella typhimurium) (random) In a random dispersion, the position of each individual is independent of other individuals Video: Prokaryotic Flagella (Salmonella typhimurium) (random)

LE 52-3c 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

Life Tables 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

Number of survivors (log scale) 1,000 100 Number of survivors (log scale) Females 10 Males 1 2 4 6 8 10 Age (years)

Survivorship curves can be classified into three general types: Type I, Type II, and Type III

Number of survivors (log scale) Percentage of maximum life span 1,000 I 100 Number of survivors (log scale) II 10 III 1 50 100 Percentage of maximum life span

Reproductive Rates 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

Concept 52.2: Life history traits are products of natural selection Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

Life History Diversity Life histories are very diverse Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly

“Trade-offs” and Life Histories Organisms have finite resources, which may lead to trade-offs between survival and reproduction

Parents surviving the following winter (%) LE 52-7 100 Male Female 80 60 Parents surviving the following winter (%) 40 20 Reduced brood size Normal brood size Enlarged brood size

Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce

LE 52-8a 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.

Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established

LE 52-8b 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.

In animals, parental care of smaller broods may facilitate survival of offspring

It is useful to study population growth in an idealized situation Concept 52.3: 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

Per Capita Rate of Increase If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

Zero population growth occurs when the birth rate equals the death rate Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time: dN dt  rN

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

Equation of exponential population growth: dN dt  rmaxN

Exponential population growth results in a J-shaped curve

Population size (N) 5 10 15 Number of generations LE 52-9 2,000 dN = 1.0N dt 1,500 dN = 0.5N dt Population size (N) 1,000 500 5 10 15 Number of generations

The J-shaped curve of exponential growth characterizes some rebounding populations

Elephant population 1900 1920 1940 1960 1980 Year 8,000 6,000 Elephant population 4,000 2,000 1900 1920 1940 1960 1980 Year

Concept 52.4: 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

The Logistic Growth Model 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) LE 52-11 Maximum Per capita rate of increase (r) Positive N = K Negative Population size (N)

The logistic growth equation includes K, the carrying capacity dN dt  (K  N) K rmax N

The logistic model of population growth produces a sigmoid (S-shaped) curve

Population size (N) 5 10 15 Number of generations LE 52-12 2,000 dN = 1.0N Exponential growth dt 1,500 K = 1,500 Logistic growth Population size (N) 1,000 dN 1,500 – N = 1.0N dt 1,500 500 5 10 15 Number of generations

The Logistic Model and Real Populations The growth of laboratory populations of paramecia fits an S-shaped curve

Number of Paramecium/mL LE 52-13a 1,000 800 600 Number of Paramecium/mL 400 200 5 10 15 Time (days) A Paramecium population in the lab

Some populations overshoot K before settling down to a relatively stable density

Number of Daphnia/50 mL 20 40 60 80 100 120 140 160 Time (days) LE 52-13b 180 150 120 Number of Daphnia/50 mL 90 60 30 20 40 60 80 100 120 140 160 Time (days) A Daphnia population in the lab

Some populations fluctuate greatly around K

Number of females 1975 1980 1985 1990 1995 2000 Time (years) LE 52-13c 80 60 Number of females 40 20 1975 1980 1985 1990 1995 2000 Time (years) A song sparrow population in its natural habitat

The logistic model fits few real populations but is useful for estimating possible growth

The Logistic Model and Life Histories Life history traits favored by natural selection may vary with population density and environmental conditions K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density r-selection, or density-independent selection, selects for life history traits that maximize reproduction

The concepts of K-selection and r-selection are somewhat controversial and have been criticized by ecologists as oversimplifications

Concept 52.5: 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

LE 52-14 Density-dependent birth rate Density-dependent birth rate Density- independent death rate Density- dependent death rate Birth or death rate per capita Equilibrium density Equilibrium density Population density Population density Density-dependent death rate Density- independent birth rate Equilibrium density 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

Competition for Resources In crowded populations, increasing population density intensifies intraspecific competition for resources

Average number of seeds per reproducing individual LE 52-15 4.0 10,000 3.8 3.6 Average number of seeds per reproducing individual (log scale) 1,000 Average clutch size 3.4 3.2 3.0 100 2.8 1 10 100 10 20 30 40 50 60 70 80 Plants per m2 (log scale) Females per unit area Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Territoriality In many vertebrates and some invertebrates, territoriality may limit density

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

Oceanic birds exhibit territoriality in nesting behavior

Health Population density can influence the health and survival of organisms In dense populations, pathogens can spread more rapidly

Predation As a prey population builds up, predators may feed preferentially on that species

Toxic Wastes Accumulation of toxic wastes can contribute to density-dependent regulation of population size

Intrinsic Factors For some populations, intrinsic (physiological) factors appear to regulate 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

Moose population size 1960 1970 1980 1990 2000 Year LE 52-18 2,500 2,000 Steady decline probably caused largely by wolf predation 1,500 Moose population size 1,000 Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population 500 1960 1970 1980 1990 2000 Year

Extreme fluctuations in population size are typically more common in invertebrates than in large mammals

Commercial catch (kg) of LE 52-19 730,000 100,000 Commercial catch (kg) of male crabs (log scale) 10,000 1950 1960 1970 1980 1990 Year

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

Number of breeding females 60 50 40 Mandarte Island Number of breeding females 30 20 Small islands 10 1988 1989 1990 1991 Year

Many populations undergo boom-and-bust cycles Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors

LE 52-21 Snowshoe hare 160 120 Lynx 9 Hare population size (thousands) Lynx population size (thousands) 80 6 40 3 1850 1875 1900 1925 Year

Concept 52.6: Human population growth has slowed after centuries of exponential increase No population can grow indefinitely, and humans are no exception

The Global Human Population The human population increased relatively slowly until about 1650 and then began to grow exponentially

Human population (billions) LE 52-22 6 5 4 Human population (billions) 3 2 The Plague 1 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 1000 A.D. 2000 A.D.

Though the global population is still growing, the rate of growth began to slow about 40 years ago

Annual percent increase LE 52-23 2.2 2 1.8 1.6 2003 1.4 Annual percent increase 1.2 1 0.8 0.6 0.4 0.2 1950 1975 2000 2025 2050 Year

Regional Patterns of Population Change 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

Birth or death rate per 1,000 people 50 40 30 Birth or death rate per 1,000 people 20 10 Sweden Mexico Birth rate Birth rate Death rate Death rate 1750 1800 1850 1900 1950 2000 2050 Year

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

LE 52-25 Rapid growth Afghanistan Slow growth United States Decrease Italy Male Female Age Male Female Age Male Female 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8 Percent of population Percent of population Percent of population

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

Infant mortality (deaths per 1,000 births) Life expectancy (years) LE 52-26 60 80 50 60 40 Infant mortality (deaths per 1,000 births) Life expectancy (years) 30 40 20 20 10 Developed countries Developing countries Developed countries Developing countries

Global Carrying Capacity How many humans can the biosphere support?

Estimates of Carrying Capacity The carrying capacity of Earth for humans is uncertain

Ecological Footprint 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

Ecological footprint (ha per person) Available ecological capacity 16 14 12 New Zealand 10 USA Germany Ecological footprint (ha per person) 8 Australia Netherlands Japan Canada Norway 6 Sweden UK 4 Spain World 2 China India 2 4 6 8 10 12 14 16 Available ecological capacity (ha per person)

At more than 6 billion people, the world is already in ecological deficit