Ecology Introduction

1
Lecture 21

• Chapter 35
• Ecology Introduction

2
Learning Objectives

• Calculate
• Population density
• Lincoln Index population estimate
• Recognize exponential and logistic growth
  equations
• Contrast exponential logistic growth models
• Give examples of organisms that fit each

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Population Density

• A number, describing how many individuals are in
  a given area
• Pop. Density indiv./area

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Examples

• You go to an 100 hectare island and estimate
  there are 50 owls.
• Density 50 owls / 100 ha 0.5 owls per ha

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Examples

• An estimated 75 squirrels live in a park
• Park is 20 acres
• What is density of squirrels?

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Answer

• Density 75 squirrels / 20 acres 3.75
  squirrels per acre

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Population Counting Difficulties

• Some organisms can be counted directly
• Trees (dont move, big)
• People (pay taxes, file census forms)
• Some cannot
• Rodents
• Small, fast
• Hard to see in grass
• Fish
• Too many to count directly
• Move constantly

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Estimating the Population

• Sometimes, a population estimate is best we can
  do
• Too many to count
• Difficult to see all
• Lincoln index is basic population estimate

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Estimation Logic

• Imagine we put 100 white marbles in a bucket
• Pull out 50 and put a red dot to mark
• Put back in bucket, releasing them to population
• 50 of marbles are marked

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Percentages Should be Constant

• 50 of marbles are marked in population, so
• 50 should be marked in any sample
• In other words, PERCENTAGES should be the same

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Deriving the Lincoln Index

• Percentage of marked individuals should be same
  in population samples
• ( released to pop. / pop.) ( recaptures in
  sample / sample)

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Solve for Population

• Can use algebra to solve for population size (N)

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Meadow Vole Example

• Voles are rodents
• Researcher eartags 50 voles
• Releases into population
• 2 weeks later, captures 100 voles
• 5 have eartags
• What is size of population?

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Mark-Recapture

• Lincoln index used in mark-recapture studies
• Different animals need different markers
• Plastic dots on bees
• Eartags on rodents
• Coded wire or clipped fin for fish

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7th Inning Stretch
16
Population Growth Models

• Population ecologists use mathematical models to
  describe natural phenomena
• Exponential growth
• Logistic growth
• In both, G growth rate

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Exponential Growth

• Population increases by a constant factor, every
  generation
• Be able to recognize
• G rN
• Example bacteria doubling
• Increase by factor of two every generation

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Conditions for Exponential Growth

• Occurs under ideal conditions
• Infinite resources
• Food
• Space
• Territories
• No limitations on numbers
• No predators
• No pollution

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Bacteria Grow Exponentially
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Exponential Growth Rate

• Larger population faster growth

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Question

• How do you feel about exponential growth?
• Realistic for bacteria?
• Can it apply to wildlife populations?
• Can it apply to human population?

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Problems with Exponential Growth Model

• Can only be applied to small set of circumstances
• Infinite resources
• No limitation on numbers
• Most populations dont meet these criteria
• If they do, only for limited time

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Logistic Growth Model

• Like exponential growth, describes a population
• Unlike exponential growth, includes limitation of
  resources
• When resources plentiful, population grows
• When resources are limited, population stops
  growing, or shrinks

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Carrying Capacity

• Carrying capacity (k) is the number of
  individuals an area can support
• May be determined by
• Food
• Territories
• Water
• Nutrients

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Fur Seals

• St. Paul Islands, AK
• Before 1925 population low
• Unlimited hunting
• 1925 to 1935 population grew dramatically
• Hunting heavily regulated
• After 1935, population reached carrying capacity
  of island, stopped growing

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Fur Seals

• Carrying capacity reached after 1935

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Logistic Growth Equation

• Be able to recognize
• G rN (K-N)/K

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Comparison

• Exponential is G rN
• Logistic G rN (K-N)/K