Chapter 52 Population Ecology Population ecology is the
- Slides: 40
Chapter 52 Population Ecology
Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, population size and 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 Determining the density of natural Immigration Births Population size populations is difficult (mark & recature) 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 Deaths Emigration
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. A uniform dispersion is one in which individuals are evenly distributed It may be influenced by social interactions such as territoriality In a random dispersion, the position of each individual is independent of other individuals 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 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, survivorship curves and reproductive rates help demographers show change in populations over time
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 Plots of the individuals that are alive at the start of each year Number of survivors (log scale) representing the data in a life table The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate 1, 000 Females 100 Males 10 1 0 2 4 6 Age (years) 8 10
LE 52 -5 Number of survivors (log scale) Survivorship curves can be classified into three general types: Type I, Type II, and Type III 1, 000 I 100 II 10 III 1 0 50 Percentage of maximum life span 100
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 (reproductive age varies) Allocation of limited resources Number of reproductive episodes per lifetime Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die Agave and salmon Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly Lizzards IF survival rate is LOW (unpredictable environments) semelparity is favored (increases survivorship) IF survival rate is HIGH (more stable environments) Iteroparity is favored
LE 52 -8 a Selective pressures mandate trade-offs between investment in reproduction and survival Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce 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. In animals, parental care of smaller broods may facilitate survival of offspring
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 (ZPG) 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: d. N d t r. N N = population size r = increase in growth rate K = carrying capacity t = time interval
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: d. N dt rmax. N N = population size rmax = intrinsic rate of increase K = carrying capacity t = time interval Exponential population growth results in a J-shaped curve
2, 000 Population size (N) d. N = 1. 0 N dt 1, 500 d. N = 0. 5 N dt 1, 000 500 0 0 5 10 Number of generations 15
The J-shaped curve of exponential growth characterizes some rebounding populations Elephant population 8, 000 6, 000 4, 000 2, 000 0 1900 1920 1940 Year 1960 1980
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 per capita rate of increase that The reduces logistic growth equation includes K, as the. Ncarrying increases capacity (K N ) d. N rmax N dt K When r is equal/less than 0, populations are growing logistically
The logistic model of population growth produces a sigmoid (S-shaped) curve 2, 000 Population size (N) d. N = 1. 0 N dt 1, 500 Exponential growth K = 1, 500 Logistic growth 1, 000 d. N = 1. 0 N dt 1, 500 – N 1, 500 0 0 5 10 Number of generations 15
The growth of laboratory populatio ns of parameci a fits an S -shaped curve Number of Paramecium/m. L The Logistic Model and Real Populations 1, 000 800 600 400 200 0 0 5 10 15 Time (days) A Paramecium population in the lab
Number of Daphnia/50 m. L Some populations overshoot K before settling down to a relatively stable density 180 150 120 90 60 30 0 0 20 40 60 80 100 120 140 160 Time (days) A Daphnia population in the lab
The Logistic Model and Life Histories The logistic model fits few real populations but is useful for estimating possible growth 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 Sickness/disease, competition, territoriality, health birth rates fall and death rates rise with population density Density-dependent birth and death rates are an example of negative feedback that regulates population growth r-selection, or density-independent selection, selects for life history traits that maximize reproduction Natural disasters birth rate and death rate do not change with population density
LE 52 -15 Competition for Resources 4. 0 10, 000 100 Average clutch size Average number of seeds per reproducing individual (log scale) In crowded populations, increasing population density intensifies intraspecific competition for resources 3. 8 3. 6 3. 4 3. 2 3. 0 2. 8 100 10 1 Plants per m 2 (log scale) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. 0 10 40 50 60 70 Females per unit area 20 30 80 Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.
Territoriality (density dependent, k-selection) Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries In many vertebrates and some invertebrates, territoriality may limit density Oceanic birds exhibit territoriality in nesting behavior
Health (density dependent, k-selection) Population density can influence the health and survival of organisms In dense populations, pathogens can spread more rapidly Predation (density dependent, k-selection) As a prey population builds up, predators may feed preferentially on that species Toxic Wastes (density dependent, k-selection) Accumulation of toxic wastes can contribute to density- dependent regulation of population size Intrinsic Factors (density dependent, k-selection) 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
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 60 Number of breeding females Song sparrow populations on a cluster of small islands make up a metapopulation. Immigration keeps the linked populations more stable than the isolated population on the larger island. 50 40 Mandarte Island 30 20 10 0 Small islands 1988 1989 1990 Year 1991
Snowshoe hare 160 120 9 Lynx 80 6 40 3 0 1850 1875 1900 Year 1925 0 Lynx population size (thousands) Many populations undergo boom-andbust cycles Boom-andbust cycles are influenced by complex interactions between biotic and abiotic factors Hare population size (thousands) LE 52 -21
Concept 52. 6: Human population growth has slowed after centuries of exponential increase No population can grow indefinitely, and humans are no exception
The human population increased relatively slowly until about 1650 and then began to grow exponentially 6 5 4 3 2 The Plague 1 8000 B. C. 4000 B. C. 3000 B. C. 2000 B. C. 1000 B. C. 0 1000 A. D. 0 2000 A. D. Human population (billions) The Global Human Population
LE 52 -23 2. 2 Annual percent increase 2 1. 8 Though the global population is still growing, the rate of growth began to slow about 40 years ago 1. 6 2003 1. 4 1. 2 1 0. 8 0. 6 0. 4 0. 2 0 1950 1975 2000 Year 2025 2050
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
LE 52 -24 Birth or death rate per 1, 000 people 50 40 30 20 10 Sweden Birth rate 0 1750 Mexico Birth rate Death rate 1800 Death rate 1850 1900 Year 1950 2000 2050
The demographic transition is associated with various factors in developed and developing countries Family planning Volunteer contraception Delayed marriage and reproduction Education
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
Estimates of Carrying Capacity The carrying capacity of Earth for humans is uncertain At more than 6 billion people, the world is already in ecological deficit
Ecological Footprint The ecological footprint concept summarizes the aggregate land 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 person) LE 52 -27 16 Ecological deficit 14 12 New Zealand 10 USA Germany Netherlands Japan Norway 8 6 UK Spain 4 World China India 2 0 Australia Canada Sweden 0 2 6 4 10 12 8 Available ecological capacity (ha person) 14 16
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