Population Ecology: population dynamics

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Population Ecology population dynamics

• BS 111
• Leanne Hepburn

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Learning objectives

• describe the basic features of population growth
  and mortality
• describe the basic features of a life table and
  the three basic types of survivorship curve

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What is a Population?

• Population groups of organisms of same species
  which occupy a given area.
• One species
• One area
• Isolated from other areas
• Able to interbreed

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Populations

• Population size no. of inds. in a given area
• For colonial/modular organisms, e.g. plants,
  corals, situation is more complex.
• The number of pieces (ramets) or number of
  shoots (modules) may give more meaningful
  indication of abundance.

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Populations and population change
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Population change

• BI DE pop. in equilibrium
• BI gt DE pop. Will increase (gains gtlosses)
• BI ltDE pop. Will decrease and may cease to
  exist
• Rate at which mature adults replaced in a pop.
  Depends on life span of organisms.
• 100s yrs low death rate but succ. replacement
  of adults also low.
• Possible birth rate high but young die before
  maturity, e.g. bracken many spores released but
  probably only few prothalli produce established
  young ferns

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Dispersal of organisms

• Leave a population and join another dispersed
  (emigration)
• If emigration is high then so too might be
  immigration
• Plants seeds most remain within same pop. But
  some travel large distances
• Special structures e.g. feathery attachments,
  sticky, hooked seeds so attach to animals or
  float
• Marine organisms, e.g. crabs, corals, barnacles
  have larval young which join plankton and are
  carried by ocean currents
• Mammals and birds juveniles emigrate as they
  mature

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Characteristics of a Population

• What features can we measure of a population?
• Features
• Size
• Age structure
• Sex ratios
• Effective population size
• Birth rate
• Death rate
• Immigration
• Emigration

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The study of populations

• Many ways to study pops. and changes within them
• Collection of information about number/ages of
  organisms
• Study of pop. dynamics demography
• Aim quantify changes in a pop. By finding out
  number of births , deaths, immigrants and
  emigrants.
• Changes calculated by births immigrations to
  original pop. at time t (Nt) and number deaths
  emigrants to give new pop. Size at time t (Nt
  1).
• Nt1 Nt B I D - E
• But what affects no. of births? What causes
  deaths? Why do organisms leave or join pop.?
• MORE TO IT THAN THIS SIMPLE EQUATION!!!!

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Populations age and structure

• Work out age structure of pop. At one point in
  time
• Need size of pop. And ages of ind.
• Easy for trees, more diff. for small, mobile
  animals
• MARK, RELEASE, RECAPTURE
• Estimation of pop. Size calculated by this method
  
• Lincoln index

P28 ECOLOGY MMU
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Assumptions of Lincoln index

• Marked animals, released after 1st capture, mix
  in with the pop. After release and have same
  chance of being recaptured as any unmarked animal
• Markings do not wear off or are destroyed
• The marks do not lower the survival chance of the
  animal so that it is more likely to be spotted
  and captured by a predator before recapture
• Estimate age of individual

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Estimating density mark release recapture

• Trapping, marking trapped individuals, releasing
  them and trapping a second time.
• The prop. of marked to unmarked inds. in traps
  prop. of marked to unmarked in the whole
  population, assuming a random prop. of the pop.
  is trapped. Total number of marked animals known
  an estimate of total population can be obtained
  from
• total number in population
• number captured x number originally marked
• /number recaptured

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Cohort/age at death studies

• Clearly defined, e.g. mice on an island
• More often complex
• The age structure describes the no. of inds. in
  each age class as a ratio of one to another
• Age classes (cohorts) yrs/months or life history
  stages, e.g. eggs, larvae, pupae.
• Cohort studies following group of individuals
  through all cohorts to death not suitable for
  long-lived species
• Age at death study of remains, only method for
  palaeoecologists

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Long-term population studies

• Valuable results in population dynamics
• Several cohorts successive yrs followed through
  life span
• Diffs. In mortality and movements between pops.
  Become more obvious
• Inds. Recognised from natural marks, e.g. tigers
  pattern of stripes/marks/tags added by
  researchers

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Cont.

• E.g. Red deer (Cervus elaphus) living on isolated
  island, Rhum, Scotland (1957 onwards) now known
  as individuals and detailed info. About
  behaviour, reproductive success and pop. Dynamics
• Clutton-Brock et al. (1982) Red Deer behaviour
  of two sexes

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Life tables

• Numerical data collected during pop. Study
  presented as table of figures
• Usually represent data for a cohort (cohort life
  tables) but also using age structure data
• Include nos. of orgs. surviving or dying in a
  given time for various age classes and survival
  and death rates.
• Often Difficult to extract important info.
  Quickly
• So represented in graphic form population
  pyramids, survivorship curves

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Chorthippus brunneus life table
Instant notes
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Population pyramid
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Births, deaths and population growth

• Natality birth of new individuals
• Mortality rate number of individuals dying
  during a given time divided by the average
  population size over that time interval.
• Survivorship converse of mortality. Often
  expressed as life expectancy average number of
  yrs to be lived in the future by population
  members of a given age.
• Often represented by Survivorship curves

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Survivorship curves

• Graph showing proportion of survivors on a log
  scale through each phase of life

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Survivorship curves

• Type I relatively constant death rate throughout
  life. Death, e.g. hunting, disease. E.g. corals,
  squirrels, many reptiles
• Type II high survival young, live most of
  expected life span and die in old age, e.g.
  humans
• Type III large numbers of offspring most die
  before maturity, extensive early mortality but a
  high rate of subsequent survival, e.g. plants,
  oysters, sea urchins.

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Evolutionary strategies r- and k-strategies

• Survivorship curves can indicate what
  evolutionary strategy a species has and how pop.
  numbers are maintained
• r- and K- come from the logistic equation for
  pop. Growth (later)
• r- selected pop. Type III max. rate of increase
  imp., can take adv. Of favourable situation by
  having ability to increase pop. Size rapidly.
  (e.g. plants, barnacles)
• K-selected pop. Type I few, well-cared for
  young (e.g. humans)

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Evolutionary strategies
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Human population dynamics

• Longest and most detailed record of pop. change
• Archeology, interp. Historic records, censuses,
  anthropology pop. from 2 mya
• Growth rate for humans estimated as 0.001 until
  Holocene (10 mil)
• Neolithic rapid increase to 50 mil
• Increase in pop. Growth up to 0.1 coincides with
  change from hunter-gatherer to agriculture

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Human pop. Dynamics cont.

• Log-log scale 3 steps
• 1. development of tools by palaeolithic,
• 2. hunter gatherer settled agriculture
• Industrialisation of 18/19 00s
• CURRENTLY NEARLY 7 BILLION PEOPLE!!!!

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Internet site

• http//www.globalchange.umich.edu/globalchange2/cu
  rrent/lectures/human_pop/human_pop.html

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Summary

• Population groups of organisms of same species
  which occupy a given area
• Ways to measure pops. mark-recapture, estimating
  abundance, long-term studies, life tables, pop.
  Pyramids
• Survivorship curves
• Evolutionary strategies (r K)
• Human population dynamics