What is a population

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What is a population? Definition A group of
interbreeding or potentially interbreeding
organisms of the same species occupying the same
space at the same time. (Hint Know this
definition, it will likely be on a test!!)
Spatial - Individuals in a population must be
able to interbreed and must share the same space
Temporal - A population is defined by a moment
in time (populations are dynamic and vary with
time)
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Properties NOT associated with individuals
1.) Characterized by density ( of individuals /
area) 2.) Age Structure (can be described by
descriptive statistics) 3.) Temporal
Variation Births and immigrants are additions,
deaths and emigrants are losses.
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Population Ecology


Demography
Study of the vital
Population genetics
The genetic
statistics of a population

composition and breeding structure
1.)

Age Structure

of a population.

2.)

Density

1.)

Variation w/in population

3.)

Births

2.)

Population
n size

4.)

Deaths

3.)

Life History

5.)

Growth


6.)

Reproduction



Because a population is composed of interbreeding
individuals each is a genetic unit
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Problems associated with defining a population
(the gray areas)

• 1.) Defining the boundaries of a population
• The salmon example

• 2.) Where is the boundary between populations and
  
• when do they become separate populations?
• Organisms do not recognize political boundaries
• The idea of subpopulations, metapopulations, and
• source-sink dynamics

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Metapopulations within patchy environments,
smaller populations exist. Dispersal between
subpopulations (demes) keeps them connected.
Individual demes may/may-not be self-sustaining.
Source
Sink
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Problems associated with defining a population

• 3.) Defining the individual
• simple for animals but not always so easy with
  plants
• Modules trees in a forest may appear to be
  individuals
• but in reality may be produced by the same root
• structure (e.g. aspen, ring fungi)

4.) Sampling Error and lack of knowledge into
plant or animal life history (particularly
movement and dispersal)
Population is an often misused term.
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Density and Dispersion
Crude Density (commonly Density) The number
of individuals per some specific area
(geographic, political, etc.) Ecological
Density Number of individuals per amount of
suitable habitat area
Not all area within a specific area is suitable
habitat, however accurately determining what is
suitable habitat is difficult. This is why most
populations are patchily distributed.
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Why is density an important parameter?
It interacts to determine
1.) Energy flow 2.) Resource availability and
utilization 3.) Physiological stress 4.)
Dispersal 5.) Productivity 6.) Reproductive
capability 7.) Other ideas??
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Uniform
Random
Clumped

• More regular than would
• be by chance
• interspecific competition
• for space

Response to habitat, environmental change,
reproductive patterns, social behavior

• Locations are independent
• from one another.
• Uniform environment
• Very rare

Densities are all equivalent
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The Concept of Population Grain
The Grain refers to the distribution of an
individual relative to its neighbors.
Fine-grained The likelihood that an
individuals neighbor will belong to another
species in proportion to its relative abundance
(Remember Chi-Square from statistics?)
Coarse-grained If an individual is more likely
to have its neighbor be an individual of its
own species.
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Temporal Dispersion
Populations are distributed not only over space,
but also over time. A density is a snapshot
in time and will change due to

• Circadian Rhythms (due to daily changes in light
  levels
• Temperature
• Predator-prey interactions
• Seasonally
• Annually
• Can you think of other factors affecting
  dispersion?

Examples 1.) Bay Anchovies in the Hudson
River 2.) How many bats have you seen during the
day?? 3.) Elk in the Rocky Mountains
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Dispersal Movements
Most animals move during some stage of their
life cycle. Why, especially considering the
costs of dispersal?

• Reduce competition for resources (population
  density)
• Reduce inbreeding
• Deteriorating habitats (temporal shifts in
  habitat quality)
• Reproduction finding a mate
• Seasonal changes in habitat needs (e.g.
  over-wintering in fish)
• Migration

What some of the costs associated with dispersal?
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Dispersal Movements

• Evolution and dispersal isolation of a small
  group of
• colonizers leads to locally adapted populations
  (genetic drift)
• Dispersal can lead to the colonization of new,
  suitable areas,
• expansion of the species, gene flow, etc.
• The costs of dispersal are increasing your
  susceptibility to
• predators, what if you dont find sufficient
  habitat, a mate,
• etc.,

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Types of Dispersal
1.) Immigration A one-way trip into a new
habitat 2.) Emigration A one-way trip out of
a habitat 3.) Migration Dispersal with a
return trip to the place of origin
One areas emigrant is another areas immigrant
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Migrations may be of 3 types
1.) Seasonal Movements are based on seasons.
Most visible examples are migratory waterfowl
which typically breed in the spring, raise young
during the summer, migrate south during the
fall, and overwinter in the south before
returning North to breed.
2.) Single Migration Such as Pacific Salmon
which return to natal streams to breed (and
die!) after living 2-5 years in the ocean.
3.) Generational Migration Monarch butterfly is
the best example, adults migrate and do not
return themselves but their offspring do.
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Active vs. Passive Dispersal
Active dispersal by mobile organisms, they
disperse under their own power.
Passive dispersal by sessile organisms (and their
seeds) which are moved by water, wind, gravity,
animals, etc. More common in plants than
animals.
Hybrid methods small animals may use both
active and passive methods (e.g. fleas on dogs)
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Age Structure
Age Categories 1.) Actual Age i.e.) years,
months, days, etc. 2.) Life History
stages a.) Prereproductive, reproductive, post
reproductive in many mammals or birds b.) Egg,
pupa, larvae, and instars in insects 3.) Size
classes (often in trees or fish)
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Age Structure in Animals
Stable Age Distribution (S.A.D.) What all
populations theoretically tend towards. The
ratio of each age class remains the same if
age-specific birth and death rates do not
change.
Disruption will cause changes but the population
will eventually return to a S.A.D.
Stationary Age Distribution A Stable Age
Distribution where the population is at
equilibrium with a constant population and the
age structure does not change. A stationary age
distribution is a S.A.D. but a S.A.D. is not
necessarily a stationary age distribution
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Growing Population
Normal Population
6436 Growing- Stable
7624 Growing
4
4
8
8
Age Classes
12
24
76
64
Percentage of Population
Poor Production
Very Poor Production
4
2476 Aging
4852 Aging
4
8
8
12
12
16
24
16
48
20
24
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Population Regulation

• Populations are regulated through both biotic and
  abiotic
• processes
• Biotic?
• Abiotic?

Stochastic Populations - Population structured by
chance characterized by large fluctuations
Deterministic Populations - Population NOT
structured by chance populations size regulated
by biological factors i.e.) competition, carrying
capacity
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Sources of Population Disturbances
Disturbances can change age structure,
population density, sex ratios, fecundity, etc.

• Harvest
• Catastrophes
• Emigration
• Immigration
• Reproductive failure
• Long-term trends (e.g. global temperature change)
• Competition (interspecific/intraspecific)

• Weather (flood, drought)
• Chemical (spill)
• Disease (Avian pox)
• Starvation (acorn failure)

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Suppression
Due to competition and shading, even aged stands
can appear to have distinct size (age!) classes
when all plants are the same age.
(modified from Smith 1996)
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Changes in Age Distribution alone can not be
relied upon to indicate if a population is
increasing or decreasing! Why?? Are size
(age) classes more reliable in plants or animals?
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Sex Ratios
Primary Sex Ratio Male to Female ratio at
conception Secondary Sex Ratio The ratio at
birth

• 1º Sex Ratio is generally 11, the 2 º Sex Ratio
  is often
• weighted towards males
• Sex ratios tend to shift towards females in older
  age groups

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Shifts in Sex Ratio

• Depends upon life history of the species
• Elk in Western Canadian Parks showed large shift
  towards
• females as age increased
• Birds, especially Galliformes, show shift towards
  males due
• to increased predation of nesting females
• Male fish that guard nests are more susceptible
  to anglers
• than female fish since they spend more time at
  the nest
• site

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Mortality

• Mortality is often age specific, acting more on
  younger
• individuals
• Crude death rate deaths / 1000 individuals in
  a given
• time period
• Probability of Surviving of survivors /
  alive at t0
• Life Expectancy The average number of years
  that an
• individual within a population will live

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Natality (Births)
Realized Natality the amount of successful
reproduction that occurs over a specified period
of time. Crude Birth Rate number of births
relative to the total population such as 500
births per 1000 individuals Specific Birth Rate
birth rate relative to a specific criteria, such
as age-specific birth rate (for fish in
particular, the of young produced per female
generally increases with age.)
e.g. A female carp may lay 200,000 eggs per
season for 10-12 seasons. Only 2 eggs need to
survive to maturity to maintain stable population
(0.0001 Survival Rate!!)
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The Life Table

• A life table is a summary of all the attributes
  of survivors
• and provides a picture of mortality in a
  population and is
• useful to determine
• probabilities of survivorship of individual in a
  population
• ages most vulnerable to mortality
• predict population growth
• Life Tables are used extensively by insurance
  companies to
• determine rates based on probabilities of
  occurrence of an
• event.

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A cohort

• Definition A group of individuals within a
  population all
• born at the same time.
• Examples are
• A year class of Striped Bass
• All the woodducks born within a breeding season
• Annual plants of the same species within a field
• Cohorts formations is typically an annual event
  but may be
• more or less frequent depending upon the breeding
  system.

Most life tables are based on cohorts and each
cohort is followed through its life.
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Example Grant and Grant 1992
Mean expectancy of further life (ex)
alive each year (Nx)
Prop. surviving at start of age x (sx)
Prop. Dying from x to x-1 (dx)
Mortality rate (qx)
Age Class (x)
0
82
1.000
0.488
0.488
2.63
1
0.404
3.64
42
0.512
0.207
2
25
0.305
0.098
0.301
4.78
0.207
0.036
3
17
0.174
5.79
4
14
0.171
0.000
5.93
0.000
0.171
4.93
5
14
0.012
0.070
6
13
0.159
0.000
0.000
4.27
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Life Table Terms
dying during interval
Mortality Rate qx
alive at beginning of the interval
For the interval from age 0 to age 1 qx
(82-42) / 82 0.4888
sx (1-qx)
Survivorship
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Calculation of Life Expectancy
To calculate ex we need Lx and Tx
Lx is the average of years lived by all
individuals in each category in the
population Lx ?( alive at tx and tx1) /
2 Tx is the of time units left for all
individuals to live from age x onward To
determine sum all Lxs from the bottom up to
the age class in question Life Expectancy (ex)
Tx / sx