Population Ecology

Chapter 52

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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

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Biosphere

Ecosystem 1

Ecosystem 2

Ecosystem 3

Abiotic Factors

Pop 1

Pop 2Pop 3

Community

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Population Properties

5 Properties1. 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

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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

catchndrecapturesofNumber

catchndcapturedTotalcatchstmarkednumberN

2

)2()1(

50010

10050

N Population Estimate = 500

whippets

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Population Properties (Page 2)

3. Dispersion – Pattern of spacing of individuals within the area the population inhabitsa) 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 resourcesc) Random

-- Spacing occurs in absence of any spatial factors-- Uncommon pattern in nature-- Tree spacing in a forest

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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-- Humansb) Type II-- Constant death rate over entire life span-- Hydra, reptiles, & rodentsc) Type III-- High death rate among young, but then slows-- Fish & Invertebrates (external fertilization)

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Survivorship Curves

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Population Properties (Page 4)

5. Age Structure Diagrams-- Shows relative number of individuals at each age

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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

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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

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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

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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.

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Elephants introduced into protected habitat

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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

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

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Characteristic Overshot

Actual Population Growth Models

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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

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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)

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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)

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Typical Populations

Boom-and-bust cycles are common in populations over time Due to complex interactions between biotic and abiotic

factors

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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

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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

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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

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Demographic Transition

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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