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Ecology
Lecture 7
Ralph Kirby
Population (Ecology)
– Group of individuals of the same
species inhabiting the same area
Interbreeding if sexual
Limited in space
Populations have
– Density
– Distribution in time
– Distribution in space
These are defined by
– Rate of birth
– Rate of growth
– Rate of death
Resulting in age & sex structure
What is an individual
– Easy for most large
sexually
reproducing
animals
– Not so easy for
colonies of animals
and many plants
– Ants
– Bees
– Coral
– Trees (Ramets)
– Fungi
Distribution
– Area over which a
species occurs
Abundance
– Numbers of that
species present in
the area
Both vary by
available habitat
– Temperature
Red Maple
– -40oC
Carolina wren
– Northwards -7oC
– Westward>52mm
– When all
environmental
factors within its
range of tolerance,
the organism can
live in its habitat
Horned lark
– Avoids forests
– Available
territory
Density can be a key factor
– Number of individuals per unit space
– Affects
growth rate
Resources
Mates
predation
Thus density controls in part
– Birth rate
– Growth rate
– Death rate
Density is difficult to define or measure
– What area do you use
– What distribution has the species within the
area
– How do you measure the density
– Ecological density
Some use a factor associated with the habitat
Distribution
– Clumped
Normal
– Uniform
High density and competition
– Random
Social behavior
Available nesting sites
Metapopulation
– Multi-Habitat based multiple populations
How do you count
Direct Counts
– Time consuming
– Needs to be sure you can see all individuals in the area
Quadrats
– Organisms must be static
– Needs laying of quadrats to be random
– Statistical analysis essential
Mark and recapture
– Estimate
Assumes equal chance of capture for all
No deaths or births
Marked animals random among unmarked
No loss of marks
No emigration or immigration
Other factors include time of capture, stress of capture, sex, age, etc.
Relative abundance
– Using a factor such as tracks, bird song, etc
– Get a relative measure
Disperal – Active and Passive
Emigration – Escaping high density
Immigration – Moving into empty habitat
Migation – A round trip, perhaps involving mating
Age structure
–
–
–
–
Prereproductive
Reproductive
Postreproductive
Short life span
Rapid increase
– Long Life span
Slow increase
Measuring Age
structure
– Mark and recapture
Accurate but hard
to do
– Special features
Teeth
Horn rings
Tree rings
Age Pyramids
– Show status of
population
Size/Age can affect the
population structure
Sex ratios in populations
shifts with age
– Humans
More males
Males have shorter life span
– War
– High risk activities
Females need to survive giving
birth to children
Therefore sex ratios have
change a lot over the last two
hundred years
– Parturition fever in 19th Century
– First World War in UK
– Second World War in USSR
Probability of survival is age specific
– See death table
– Probability of death in next year
Life Table
– nx
Number in cohort
– lx
Probability of surviving from birth
– qx
Age specific mortality
Plants are more
difficult
– Hard to detrmine
age
– Extremely high
loss fo seedlings
– Use of yield table
Birthrate
– Sex specific
– Age specific
Net reproductive rate
depends on
– Fecundity
– Survivorship
Given
– Age specific mortality
– Age specific birth rate
– Project population
growth/decline
Exponential Growth
– Depends on rate of
reproduction, λ
– If λ less than 1, there
is cline
Population growth is
limited by the
environment
– Carrying Capacity
– See reindeer
Nonconformity
– Change in age
structure
– Immigration
– Emigration
– Changes in birth and
death rates
But stable once it
reaches carrying
capacity
Populations should work via positive and negative feedbacks
Does not work smoothly
– Depends on environment such as overgrazing
Population cycles in simpler systems
– Food availability
Can result in extinctions
– Two types
Change in environment. Start locally
Mass extinctions. External event?
Three Models for regulation of
population density
K represents equilibrium
– Partial density regulation A
Birth Rate (B) independent of
population density
Mortality Rate (M) increases with
density
– Partial density regulation B
Birth Rate (B) declines with population
density
Mortality Rate (M) independent of
population density
– Full density dependent regulation
Birth Rate (B) increases with
deceasing population density
Mortality Rate (M) increases with
increasing population density
Carrying capacity
See effect of density for
tadpoles and white clover
– Maximum number of sustainable
individuals
– Results in intra-specific
competition
– Two types
Scramble competition
– Growth and reproduction depressed
for all individuals
Can result in local extinction or
population crash
Contest competition
– Some individual deny resources to
other individuals resulting in some
growing and reproducing better than
others
Indirect via exploitation of
resources
Direct via interference
Population crash
caused by
Rindepest
Note exponential
increase from low
base
Grass limited in dry
season
Competition related
to rainfall producing
grass
Once in equilibrium,
rainfall most
important
Similat effect
for white clover
Effect of
repotting
Effects can
differ
– Bison is
exponentia
l curve
– Grizzly
bear is
linear
effect
Competition
can affect
reproduction
– harp seals
Increas
ed age
of
sexual
maturity
– maize
Linear
– marsh
herb
Effects of high density on
reproduction
Stress
Pheromones
Disease
Increased mortality of young
Results in dispersal
– Sources Habitat
– Sink Habitat
Can be dangerous
Lack protective cover
High number of predators
– Move no further than necessary
– Move in forays
Home Range
Can overlap
Territory
No overlap
Fight to protect
Dominant male gets
best locations
Males and female
ranges can differ
Male and female
ranges overlap
Social dominance can affect reproduction and dispersion
Fighting and social position. Both male and female hierarchies are possible
Results in limited mating
– Alpha
– Beta
– Omega
Territory is protected
–
–
–
–
–
–
Food
Mating
Nesting site
Attraction of mates
Avoidance of suboptimal habitat
Needs energy
May not be optimal strategy when
resources are low
Other density
independent factors
can overrule density
– Weather
– Rainfall
– Temperature