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Population Ecology

Population Ecology. Chapter 52. Ecology. Def – Study of the interactions of organisms with their physical environment & with each other Population – Group of individuals of one species living in one area who are able to interbreed and interact with each other

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Population Ecology

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  1. Population Ecology Chapter 52

  2. Ecology Def – Study of the interactions of organisms with their physical environment & with each other Population – Group of individuals of one species living in one area who are able to interbreed and interact with each other Community – All organisms living in one area Ecosystem – all the organisms in a given area & the abiotic (non-living) factors with which they interact Abiotic (non-living) – temperature, water, sunlight, wind, rocks, & soil Biosphere – global ecosystem

  3. Biosphere Ecosystem 1 Community Ecosystem 2 Pop 1 Abiotic Factors Pop 3 Pop 2 Ecosystem 3

  4. Population Properties • 5 Properties 1. Size– Total number of individuals in a population -- Typically represented by N 2. Density – The number of individuals per unit area/volume -- Here we are measuring not population size but how closely packed they are into an area -- For example: which population is more dense: Manhattan’s or Weston’s? -- Sampling techniques used to estimate the number of organisms living in an area -- One sampling technique is called mark & recapture

  5. Mark & Recapture Method • Sampling technique used to estimate population size • Organisms are captured, tagged, then released • Then at some future time, the process is repeated • Example: 1st catch: 50 whippets tagged 2nd catch: 100 whippets captured, but only 10 tagged Population Estimate = 500 whippets

  6. Population Properties (Page 2) 3. Dispersion – Pattern of spacing of individuals within the area the population inhabits a) Clumped -- Most common pattern of dispersion -- Pack animals b) Uniform -- Animals that defend their territories -- Certain plants which secrete toxins that keep away other plants that compete for the same resources c) Random -- Spacing occurs in absence of any spatial factors -- Uncommon pattern in nature -- Tree spacing in a forest

  7. Population Properties (Page 3) 4. Survivorship Curve (Mortality Curve) -- Show the size & composition of a population -- 3 basic types a) Type I -- Low death rates in young & middle age -- High mortality in old age -- Humans b) Type II -- Constant death rate over entire life span -- Hydra, reptiles, & rodents c) Type III -- High death rate among young, but then slows -- Fish & Invertebrates (external fertilization)

  8. Survivorship Curves

  9. Population Properties (Page 4) 5. Age Structure Diagrams -- Shows relative number of individuals at each age

  10. Age Structure Diagrams • Curve I – Afghanistan • Pyramidal shape • Bottom heavy or majority of population is young • May indicate future population explosion • Alternatively, may indicate population pressure • The majority of population will die young • Curve II – USA • Stable population • Experiencing little or zero population growth • Birth & death rates are equal = numbers in each age group is the same

  11. Population Growth • Biotic Potential • Maximum rate at which a population could increase under ideal conditions • Maximum reproductive capacity of a population under optimum environmental conditions. • As should be evident, this is probably never true • If true, it will only exist briefly • Used more for comparison to the actual situation

  12. Biotic Potential Factors 1. Age of reproductive opportunity onset 2. Reproductive Life span 3. Number of reproductive periods during lifetime 4. Number of offspring produced in each reproductive event • Also called Variables of Life History -- The variables that affect an organism’s schedule of reproduction and survival

  13. Exponential Population Growth • Simplest model for population growth is exponential growth • Represents unrestrained growth • Unlimited resources • No predation/parasitism • No competition • No immigration/emigration • Example: introduction of a foreign organism into a stable ecosystem • Humans have experienced exponential growth for ~300 yrs.

  14. Elephants introduced into protected habitat

  15. Exponential Growth is Constrained • Populations grow exponentially at first, but then grows logistically • S-shaped curve • Highest growth rate is at intermediate population size • An example of constrained optimization • There is an environmental limit to population size • Carrying Capacity – the population size that an environment will permit • Referred to as K

  16. It is no longer dirty to say: “hey baby, let’s go study my growth curve”

  17. Actual Population Growth Models Characteristic Overshot

  18. Factors influencing Population Growth • 2 main factors • Density-dependent factors – factors that increase directly as the population density increases • K-selection (K = carrying capacity) • Associated with logistic growth • Density-independent factors – factors that are independent of the density of a population • r-selection (r = growth rate) • Associated with exponential growth

  19. Different Life History Strategies 1. r-strategists • Reproduce rapidly • Small but numerous offspring • Little or no parenting • Insects 2. K-strategists • Large but few offspring • Intensive parenting • Mammals (Chimps)

  20. Regulation of Population Growth • Density-dependent factors • Competition for resources • Territoriality • Predation • Infectious Disease • Density-independent factors • Naturally occurring disasters, such as fires, floods, earthquakes, & hurricanes (not The “U” type of hurricane, though they were a disaster last year)

  21. Typical Populations • Boom-and-bust cycles are common in populations over time • Due to complex interactions between biotic and abiotic factors

  22. Predator-Prey Relationships --Exponential Growth then crash -- Lynx population follows the hare pop. -- Cycles in hare due to food availability (grass overgrazing), disease, or predation -- Cycles in Lynx due to hare availability

  23. Human Populations • Until 1650, human populations were very low growth • Humans have experienced high growth rates since about 1650 • Since 1970s, population growth has decreased over time

  24. Demographic Transition • Individual country populations vary widely in size and growth rates, but economic development leads to a demographic transition • From: high birth rates – high death rates = 0 growth • To: Low birth rates – Low death rates = 0 growth • Typically, death rate falls  Fast population growth • Due to medical care & Santiation • Too many people, so birth rates fall as well

  25. Demographic Transition

  26. Ecological Footprint • Proxy measure of carrying capacity • Total land + water area needed for all the resources 1 person consumes in a population • 1.7 hectares per person = sustainable usage • In the US, 10 hectares per person is typical

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