2Unit Overview Questions What is the history of human population growth, and how many people are likely to be here by 2050?How is population size affected by birth, death, fertility, and migration rates?How is population size affected by percentages of males and females at each age level?How can we slow population growth?
3Unit Overview Questions (cont’d) What success have India and China had in slowing population growth?What are the major impacts of human activities on the world’s natural ecosystems?
4Population ecologyCharacteristics that help predict future dynamics of a populationDensity, distribution, sex ratio, age structure, birth and death rates
5Population size Number of individuals present at a given time May increase, decrease, change cyclically, or remain stableImmigration, emigration also affect population size in addition to birth and death rates
6Population Density and Population Change: Effects of Crowding Population density: the number of individuals in a population found in a particular area or volume.A population’s density can affect how rapidly it can grow or decline.e.g. biotic factors like diseaseSome population control factors are not affected by population density.e.g. abiotic factors like weather
7Population characteristics High densities make it easier to find mates, but increase competition, and vulnerability to predationLow densities make it harder to find mates, but individuals enjoy plentiful resources and space
8Population characteristics Population distribution (dispersion) = spatial arrangement of organisms within an areaRandom – haphazardly located individuals, with no patternUniform – individuals are evenly spaced due to territorialityClumped – arranged according to availability of resourcesMost common in nature
9Population characteristics Sex ratio = proportion of males to femalesIn monogamous species, a 50/50 sex ratio maximizes population growthAge Structure = the relative numbers of organisms of each age within a populationAge structure diagrams (pyramids) = show the age structure of populations
10Birth and death ratesCrude birth/death rates = rates per 1000 individualsSurvivorship curves = the likelihood of death varies with ageType I: More deaths at older agesType II: Equal number of deaths at all agesType III: More deaths at young ages
11Four factors of population change Natality = births within the populationMortality = deaths within the populationImmigration = arrival of individuals from outside the populationEmigration = departure of individuals from the populationGrowth rate formula =(Crude birth rate + immigration rate) - (Crude death rate + emigration rate) = Growth rate
12Exponential population growth Steady growth rates cause exponential population growthSomething increases by a fixed percentGraphed as a J-shaped curveExponential growth cannot be sustained indefinitelyIt occurs in nature with a small population and ideal conditions
13Limiting factors restrain growth Limiting factors = physical, chemical and biological characteristics that restrain population growthWater, space, food, predators, and diseaseEnvironmental resistance = All limiting factors taken together
14Carrying capacityCarrying capacity = the maximum population size of a species that its environment can sustainAn S-shaped logistic growth curveLimiting factors slow and stop exponential growthCarrying capacity changesHumans have raised their carrying capacity by decreasing the carrying capacity for other species
16Population density affects limiting factors Density-dependent factors = limiting factors whose influence is affected by population densityIncreased risk of predation and competition for mates occurs with increased densityDensity-independent factors = limiting factors whose influence is not affected by population densityEvents such as floods, fires, and landslides
17Biotic potential and reproductive strategies vary Biotic potential = the ability of an organism to produce offspringK-selected species = animals with long gestation periods and few offspringHave a low biotic potentialStabilize at or near carrying capacityGood competitorsr-selected species = animals which reproduce quicklyHave a high biotic potentialLittle parental care
19Population changes affect communities As population in one species declines, other species may appearHuman development now displaces other species and threatens biodiversityAs Monteverde dried out, species from lower, drier habitats appearedBut, species from the cloud-forest habitats disappeared