DC_Population_Ecology

Chapter 52:

Chapter 52 Population Ecology

Population Ecology:

Population Ecology To understand human population growth we must consider the general principles of population ecology Population ecology is the study of populations in relation to environment Includes environmental influences on population density and distribution, age structure, and variations in population size

Population Ecology:

Population Ecology 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: A Dynamic Perspective:

Density: A Dynamic Perspective Density is the number of individuals per unit area or volume Determining the density of natural populations is possible, but difficult to accomplish in most cases, it is impractical or impossible to count all individuals in a population

Slide 5:

Density is the result of a dynamic interplay Between processes that add individuals to a population and those that remove individuals from it Figure 52.2 Births and immigration add individuals to a population. Births Immigration PopuIation size Emigration Deaths Deaths and emigration remove individuals from a population. Density: A Dynamic Perspective

Patterns of Dispersion:

Patterns of Dispersion Dispersion is the pattern of spacing among individuals within the boundaries of the population Environmental and social factors influence the spacing of individuals in a population Three main types of dispersion Clumped Uniform Random

Clumped dispersion:

Clumped dispersion Is one in which individuals aggregate in patches May be influenced by resource availability and behavior Figure 52.3a (a) 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 dispersion:

Uniform dispersion Is one in which individuals are evenly distributed May be influenced by social interactions such as territoriality Figure 52.3b (b) 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 dispersion:

Random dispersion Is one in which the position of each individual is independent of other individuals Figure 52.3c (c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

Demography:

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 best constructed by following the fate of a cohort

Slide 11:

The life table of Belding’s ground squirrels Reveals many things about this population Table 52.1

Slide 12:

The survivorship curve for Belding’s ground squirrels Shows that the death rate is relatively constant Figure 52.4 1000 100 10 1 Number of survivors (log scale) 0 2 4 6 8 10 Age (years) Males Females Survivorship Curves

Slide 13:

Survivorship curves can be classified into three general types Type I, Type II, and Type III Figure 52.5 I II III 50 100 0 1 10 100 1,000 Percentage of maximum life span Number of survivors (log scale)

Reproductive Rates:

Reproductive Rates A reproductive table, or fertility schedule is an age-specific summary of the reproductive rates in a population Table 52.2

Life History Traits:

Life History Traits Life history traits are products of natural selection Life history traits are evolutionary outcomes Reflected in the development, physiology, and behavior of an organism semelparity , or “big-bang” reproduction, r eproduce a single time and die iteroparity , or repeated reproduction, p roduce offspring repeatedly over time

“Trade-offs” and Life Histories:

“Trade-offs” and Life Histories Organisms have finite resources Figure 52.7 Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.) EXPERIMENT The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents. CONCLUSION 100 80 60 40 20 0 Reduced brood size Normal brood size Enlarged brood size Parents surviving the following winter (%) Male Female Which may lead to trade-offs between survival and reproduction RESULTS

Slide 17:

Some plants produce a large number of small seeds Ensuring that at least some of them will grow and eventually reproduce Figure 52.8a (a) 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. “Trade-offs” and Life Histories

Slide 18:

Other types of plants produce a moderate number of large seeds That provide a large store of energy that will help seedlings become established Figure 52.8b (b) 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. “Trade-offs” and Life Histories

Per Capita Rate of Increase:

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 The population growth equation can be expressed as dN dt  rN

Exponential Growth:

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 dN dt  r max N Figure 52.9 0 5 10 15 0 500 1,000 1,500 2,000 Number of generations Population size ( N ) dN dt  1.0 N dN dt  0.5 N

Slide 21:

The J-shaped curve of exponential growth Is characteristic of some populations that are rebounding Figure 52.10 1900 1920 1940 1960 1980 Year 0 2,000 4,000 6,000 8,000 Elephant population

Logistic Growth:

Logistic Growth 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:

The Logistic Growth Model The per capita rate of increase declines as carrying capacity is reached The logistic growth equation includes K , the carrying capacity dN dt  ( K  N ) K r max N Table 52.3

Slide 24:

The logistic model of population growth Produces a sigmoid (S-shaped) curve Figure 52.12 dN dt  1.0 N Exponential growth Logistic growth dN dt  1.0 N 1,500  N 1,500 K  1,500 0 5 10 15 0 500 1,000 1,500 2,000 Number of generations Population size ( N )

Slide 25:

Some populations overshoot K Before settling down to a relatively stable density Figure 52.13b 180 150 0 120 90 60 30 Time (days) 0 160 140 120 80 100 60 40 20 Number of Daphnia /50 ml (b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.

Slide 26:

Some populations Fluctuate greatly around K Figure 52.13c 0 80 60 40 20 1975 1980 1985 1990 1995 2000 Time (years) Number of females (c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.

The Logistic Model and Life Histories:

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

Population Change and Population Density:

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 Figure 52.14a–c Density-dependent birth rate Density-dependent death rate Equilibrium density Density-dependent birth rate Density-independent death rate Equilibrium density Density-independent birth rate Density-dependent death rate Equilibrium density Population density Population density Population density Birth or death rate per capita (a) Both birth rate and death rate change with population density. (b) Birth rate changes with population density while death rate is constant. (c) Death rate changes with population density while birht rate is constant.

Competition for Resources:

Competition for Resources In crowded populations, increasing population density Intensifies intraspecific competition for resources Figure 52.15a,b 10 0 100 100 0 1,000 10,000 Average number of seeds per reproducing individual (log scale) Average clutch size Seeds planted per m 2 Density of females 0 70 10 20 30 40 50 60 80 2.8 3.0 3.2 3.4 3.6 3.8 4.0 (a) Plantain. The number of seeds produced by plantain ( Plantago major ) decreases as density increases. (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Territoriality may limit density:

Territoriality may limit density Cheetahs are highly territorial Using chemical communication to warn other cheetahs of their boundaries Figure 52.16

Other Density Dependent Factors:

Other Density Dependent Factors Pathogens can spread more rapidly As a prey population builds up predators may feed preferentially on that species The accumulation of toxic wastes can contribute to density-dependent regulation of population size Intrinsic (physiological) factors appear to regulate population size

Stability and Fluctuation:

Stability and Fluctuation Long-term population studies Have challenged the hypothesis that populations of large mammals are relatively stable over time Figure 52.18 The pattern of population dynamics observed in this isolated population indicates that various biotic and abiotic factors can result in dramatic fluctuations over time in a moose population. Researchers regularly surveyed the population of moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from the effects of immigration and emigration. FIELD STUDY Over 43 years, this population experienced two significant increases and collapses, as well as several less severe fluctuations in size. RESULTS CONCLUSION 1960 1970 1980 1990 2000 Year Moose population size 0 500 1,000 1,500 2,000 2,500 Steady decline probably caused largely by wolf predation Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population

Stability and Fluctuation:

Stability and Fluctuation Extreme fluctuations in population size Are typically more common in invertebrates than in large mammals Figure 52.19 1950 1960 1970 1980 Year 1990 10,000 100,000 730,000 Commercial catch (kg) of male crabs (log scale)

Slide 34:

High levels of immigration combined with higher survival Can result in greater stability in populations Figure 52.20 Mandarte island Small islands Number of breeding females 1988 1989 1990 1991 Year 0 10 20 30 40 50 60

Population Cycles:

Population Cycles Many populations Undergo regular boom-and-bust cycles Figure 52.21 Year 1850 1875 1900 1925 0 40 80 120 160 0 3 6 9 Lynx population size (thousands) Hare population size (thousands) Lynx Snowshoe hare

The Global Human Population:

The Global Human Population The human population Increased relatively slowly until about 1650 and then began to grow exponentially Figure 52.22 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 1000 A.D. 0 The Plague Human population (billions) 2000 A.D. 0 1 2 3 4 5 6

Slide 37:

Though the global population is still growing The rate of growth began to slow approximately 40 years ago Figure 52.23 1950 1975 2000 2025 2050 Year 2003 Percent increase 2.2 2 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 1.8

Age structure:

Age structure Age structure Is commonly represented in pyramids Figure 52.25 Rapid growth Afghanistan Slow growth United States Decrease Italy Male Female Male Female Male Female Age Age 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 Percent of population Percent of population Percent of population 80–84 85  75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19 80–84 85  75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19

Ecological footprints for 13 countries:

Ecological footprints for 13 countries Show that the countries vary greatly in their footprint size and their available ecological capacity At more than 6 billion people t he world is already in ecological deficit Figure 52.27 16 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14 16 New Zealand Australia Canada Sweden World China India Available ecological capacity (ha per person) Spain UK Japan Germany Netherlands Norway USA Ecological footprint (ha per person)