Population Ecology Chapter 45 Population Ecology Certain ecological

Population Ecology Chapter 45

Population Ecology Certain ecological principles govern the growth and sustainability of all populations--including human populations

Limits to Growth u. A population’s growth depends on the resources of its environment u Moose/Wolf study on Isle Royal

Human Population Problems u Over 6 billion people alive u About u Most 2 billion live in poverty resources are consumed by the relatively few people in developed countries

Population u A group of individuals of the same species occupying a given area u Can be described by demographics – Vital statistics such as size, density, distribution, and age structure

Population Age Structure u Divide population into age categories u Population’s reproductive base includes members of the reproductive and pre-reproductive age categories

Density & Distribution u u Number of individuals in some specified area of habitat Crude density information is more useful if combined with distribution data clumped nearly uniform random Figure 45. 2 Page 808

Determining Population Size u Direct counts are most accurate but seldom feasible u Can sample an area, then extrapolate u Capture-recapture method is used for mobile species

Capture-Recapture Method u Capture, mark, and release individuals u Return sample u Count later and capture second the number of marked individuals and use this to estimate total population

Assumptions in Capture-Recapture u Marking has no effect on mortality u Marking has no effect on likelihood to being captured u There is no immigration or emigration between sampling times

Changes in Population Size u Immigration adds individuals u Emigration subtracts individuals u Births add individuals u Deaths subtract individuals

Zero Population Growth u Interval in which number of births is balanced by number of deaths u Assume no change as a result of migration u Population size remains stable

Per Capita Rates u Total per individual number of events in a time interval divided by the number of individuals u Per capita birth rate per month = Number of births per month Population size

r u Net reproduction per individual per unit time u Variable combines per capita birth and death rates (assuming both constant) u Can be used to calculate rate of growth of a population

Exponential Growth Equation G = r. N u. G is population growth per unit time u r is net reproduction per individual per unit time u N is population size

Exponential Growth u Population size expands by ever increasing increments during successive intervals u The larger the population gets, the more individuals there are to reproduce Figure 45. 4 Page 810

Effect of Deaths u Population grows exponentially as long as per capita death rates are lower than per capita birth rates 25% mortality between divisions Figure 45. 5 Page 811

Biotic Potential u Maximum rate of increase per individual under ideal conditions u Varies u In between species nature, biotic potential is rarely reached

Limiting Factors u Any essential resource that is in short supply u All limiting factors acting on a population dictate sustainable population size

Carrying Capacity (K) u Maximum number of individuals that can be sustained in a particular habitat u Logistic growth occurs when population size is limited by carrying capacity

Logistic Growth Equation G = rmax N (K-N/K) u G = population growth per unit time u rmax = maximum population growth rate per unit time u N = number of individuals u K = carrying capacity

Logistic Growth u u As size of the population increases, rate of reproduction decreases When the population reaches carrying capacity, population growth ceases

Logistic Growth Graph initial carrying capacity new carrying capacity Figure 45. 6 Page 812

Overshooting Capacity u u Population may temporarily increase above carrying capacity Overshoot is usually followed by a crash; dramatic increase in deaths Reindeer on St. Matthew’s Island Figure 45. 6 Page 812

Density-Dependent Controls u Logistic growth equation deals with density-dependent controls u Limiting factors become more intense as population size increases u Disease, competition, parasites, toxic effects of waste products

Density-Independent Controls u Factors unaffected by population density u Natural disasters or climate changes affect large and small populations alike

Life History Patterns u Patterns of timing of reproduction and survivorship u Vary among species u Summarized in survivorship curves and life tables

Life Table u Tracks age-specific patterns u Population is divided into age categories u Birth rates and mortality risks are calculated for each age category

Survivorship Curves Graph of age-specific survivorship Figure 45. 8 Page 815

Predation and Life History u Guppy populations vary in life history characteristics and morphology u Differences have genetic basis u Variation seems to be result of directional selection by predators

Human Population Growth u Population now exceeds 6 billion u Rates of increase vary among countries u Average annual increase is 1. 26 percent u Population continues to increase exponentially

Side-Stepping Controls u Expanded into new habitats u Agriculture increased carrying capacity; use of fossil fuels aided increase u Hygiene and medicine lessened effects of density-dependent controls

Future Growth u Exponential growth cannot continue forever u Breakthroughs in technology may further increase carrying capacity u Eventually, density-dependent factors will slow growth

Fertility Rates u Worldwide, average annual rate of increase is 1. 26% u Total fertility rate (TFR) is average number of children born to a woman u Highest in developing countries, lowest in developed countries

Age Structure Diagrams Show age distribution of a population Figure 45. 14 Page 821 Rapid Growth Slow Growth Zero Growth Negative Growth

Population Momentum u Lowering fertility rates cannot immediately slow population growth rate u Why? There already many future parents alive u If every couple had just two children, population would still keep growing for another 60 years

Slowing Growth in China u World’s most extensive family planning program u Government rewards small family size, penalizes larger families, provides free birth control, abortion, sterilization u Since 5. 7 1972, TFR down to 1. 8 from

Effects of Economic Development u Total fertility rates (TFRs) are highest in developing countries, lowest in developed countries u When individuals are economically secure, they are under less pressure to have large families

Population Sizes in 2001 Asia 3. 7 billion Europe 727 million Africa 816 million Latin America 525 million North America 316 million Oceania 31 million

Resource Consumption u United States has 4. 7 percent of the world’s population u Americans have a disproportionately large effect on the world’s resources u Per capita, Americans consume more resources and create more pollution than citizens of less developed nations