Ch 36 Population EcologyGoals• Define population, density, dispersion, and demography.• Explain how age structure, generation time, & sex ratio are important in determining the growth & decline of a population.• Explain what a survivorship curve shows & describe the 3 types.• Define life history & explain why trade-offs must be made between putting energy into reproduction & survival.• Discuss 3 basic factors that determine an organisms life history.• Distinguish between exponential growth & logistic growth. (no math please)• Explain how density – dependent & density independent factors regulate population growth.• State the current human population & explain why it’s so high and still growing.• Explain why different countries have different population growth rates.
Ch 36 Population EcologyPopulation –• single species• rely on same resources• influenced by similar env factors• high chance of interacting2 important characteristics - density & spacing (dispersion)• Clumped, uniform, random
Demography - study of factors that affect the growth & decline of pops• Additions - thru births or immigration• Eliminations –thru deaths or emmigration
Most important factors that determine growth & decline of pop:• Age structure – the # of individuals of each age.• Ex: a pop with large % of individuals at prime reproductive age will grow faster than one with a large % of individuals who are older. (baby boomers)• sex ratio – proportion of individuals of each sex• # of females - good indicator of births• # of males - less sig b/c they can mate w/ several females.•Ex: deer hunting regs let you kill more bucks than does
Also important in determining growth & decline:• generation time – ave span between the birth of individuals & the birth of their offspring• small orgs have short gen times resulting in faster pop growth
Survivorship Curves• a plot of # in a pop still alive @ each age• 3 types:• Type I – humans & large mammals that have few offspring but take good care of them• Type III – fishes & marine invertebrates• Type II – mortality is constant over the lifespan – rodents, birds
Life history – the traits that affect an orgs schedule of reproduction & death• Limited resources mandate tradeoffs b/t investments in reproduction & survival• Remember – Darwinian fitness is measured by how many offspring survive to produce their own offspring• Trade – offs are involved – “should I invest E in my survival or in my reproduction?”• Experiments have been done that show if less eggs are layed, the female lives longer – this shows the trade – off.
3 basic questions determine an orgs life history:1. # rep events per lifetime• Just once?• Ex:• many times?• Ex: perenial plants, mammals2. # of offspring per rep episode• if chances of parent surviving to reproduce again are low, then…• if chances of parent surviving to reproduce again are high, then …• if chances of offspring survival are low, then …. Ex:• if chances of offspring survival are high, then ... Ex:3. Age @ 1st rep• if delay first reproduction, then benefits are -• if delay first reproduction, drawbacks are –
3 basic questions determine an orgs life history:1. # rep events per lifetime• just once /many – when cost to parents of staying alive between broods is great• Ex: mosquito & pacific salmon• many times /few each time – when parents are likely to survive but immature individuals are not• Ex: perenial plants, mammals2. # of offspring per rep episode• if chances of parent surviving to reproduce again are low, than have many• if chances of parent surviving to reproduce again are high, then have few cause need to put E into own self survival• if chances of offspring survival are low, then many. Ex: insects & dandelions• if chances of offspring survival are high, then few. Ex: mammals3. Age @ 1st rep• if delay first reproduction, then benefits are - don’t need to put E into courtship, nest building, gamete production, or migration to breeding areas• if delay first reproduction, drawbacks are – may die before producing any offspring at all.
2 population growth models:• Exponential growth – when reproductive rate is greater than 0. Makes a J shaped curve. – Carrying capacity not reached• Logistic growth – when limiting factors restrict size of the population to carrying capacity (K) of the habitat. Makes an S shaped
POPULATION LIMITING FACTORSDensity dependent factors - regulate pop growth by varying with density.• As density increases, the density – dependent factor intensifies, regulating the pop growth even more.• Sometimes the density – dependent factor determines the carrying capacity (K)• Examples of how density – dependent factors regulate pop growth:Density independent factors - are unrelated to pop size – they affect the same % of individuals regardless of pop density.• Examples of density – independent factors• A mix of density – dependent & density – independent factors probably limits the growth of most pops
HUMAN POP GROWTHgrowing almost exponentially for centuries. Why so high?• birthrates increased & deathrates decreased when we became farmers instead of hunters & gatherers.• Industrial revolution decreased death rates, esp infant• Better nutrition , medical care, & sanitationWhat factors may limit our growth?• Food –technology has helped us keep up. Do better if all vegetarians• Space? - seem to be few limits on how closely humans can be crowded• Limited resources –metals & fossil fuels• own wastesDiff countries have diff pop growth rates – Why?• age structures• Sweden – uniform age distribution• Mexico – many more of reproductive age• US – fairly even except baby boomers from post WW2 (1945)What makes our fate in terms of pop growth diff from other orgs is that we can consciously control it. It’s up to us!