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Population Ecology

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1Population ecologyDefinitions• Habitat• Population• Community• Ecosystem• Biotic Factors• Abiotic FactorsCharacteristics of Populations• Size • Age Structure • Density• Distribution45.1Age Structure DiagramsShows age distribution of a populationRapid GrowthSlow GrowthZero GrowthNegative Growth• Different species occupying the same area compete…– These interactions influence the density and dispersion of individuals and populations.Distribution45.1Distributionclumped• Clumped populations:– Suitable physical, chemical, and biological conditions are patchy, not uniform.– Social groups– Offspring not mobile45.12Distribution• Rare in nature• Result of fierce competition for very limited resourcesuniform45.1Distribution• Uniform environmental conditions • Members are neither attracting nor repelling each otherrandom45.1Determining population size• Direct counts are most accurate but seldom feasible• Sample an area, then extrapolate• Capture-recapture method is used for mobile species45.2Capture-Recapture MethodTotal captured in sampling 2Marked individuals in sampling at time 2Total population sizeMarked individuals in sampling at time 145.2Survivorship Curves• represent age-specific patterns of death for a given pop in a given environment• Each species has a characteristic curve.• Three types of curves are common in nature.45.5Fig. 45-10a, p.8053Fig. 45-10b, p.805Fig. 45-10c, p.805Life History Patterns• Patterns of timing of reproduction and survivorship• Vary among species• Summarized in survivorship curves and life tablesReproductive Strategies• Different environments and population densities can favor different reproductive strategies• r-selection favors traits that maximize number of offspring• K-selection favors traits that improve offspring qualityLife History and Reproductive Strategies--Certain combinations of traits tend to occur together:Trait r-selected K-selectedLifespan Short LongGrowth rate Fast Slow1stReproduction Early LateReproductive effort High LowParental care Less MoreOffspring Many, small Few, largeReproductive episodesall at once more than onceKind of environment Variable Stabler- or K- selected?• Mouse• Mushroom• Oak tree• Human• Elephant• Dandelion4Population Growth45.3r• Net reproduction per individual per unit time• Can be used to calculate rate of growth of a population45.3r = b - dExponential Growth45.3Population growth per unit time(G)Per capita growth rate(r)Total population(N)x=G=r x NExponential Growth: Any quantity that is growing at a rate proportional to its size.TIMEPOPULATION SIZE45.3Biotic Potential• Biotic potential: Maximum rate of increase per individual (r) under ideal conditions• Varies between species• In nature, biotic potential is rarely reached45.3Limits on Growth of PopulationsLimiting factor: any essential resource that is in short supply5Actual rate of increase is influenced by environmental conditions:• nutrient supply• competition for space• Pollution• floods• etc. 45.4Carrying Capacity (K)• Maximum number of individuals that can be sustained in a particular habitat• Logistic growth occurs when population size is limited by carrying capacity45.4Logistic Growth45.4Logistic GrowthPopulation growth per unit time(G)Net reproduction per indiv. per unit time(r)# Individuals(N)x=Proportion of resources not yet usedx45.4TIMEPOPULATION SIZEFastest growth of pop.Births > DeathsResources abundantResources become limitingBirths = DeathsCarrying capacity45.4Overshooting Capacity• Population may temporarily increase above carrying capacity• Overshoot is usually followed by a crash; dramatic increase in deathsReindeer on St. Matthew’s IslandFig. 45-9, p.8036Density-Dependent Factors• Logistic growth equation deals with density-dependent controls• Limiting factors become more intense as population size increases45.4• Factors unaffected by population density.• Natural disasters, climate changes, etc.Density-Independent factorsHuman Populations World population densityHuman Population Problems• World pop reached 6 billion in 1999current population• About 2 billion live in poverty• Most resources are consumed by the relatively few people in developed countries Side-Stepping Growth Controls• Expanded into new habitats • Agriculture increased carrying capacity; use of fossil fuels aided increase• Hygiene and medicine lessened effects of density-dependent controls7Human Population GrowthFuture Growth• Exponential growth cannot continue forever• Breakthroughs in technology may further increase carrying capacity • Eventually, density-dependent factors will slow growthFertility Rates • Worldwide, average annual rate of increase is 2.6%• Total fertility rate (TFR) is average number of children born• Highest in developing countries, lowest in developed countriesFig. 45-17b,


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