Population Ecology
Study of Populations in
relation to their environment.
Environmental influences on
Pop. DensityDistributionAge structuresize
Density
Number of individuals per unit area or volume.Ex: Diatoms - 5 million/m3
Trees - 5,000/km2
Deer - 4/km2
How do you count a population?
1. actual count
2. Random plots, then extrapolate
3. Indirect indicators: number of nests; burrows; tracks; droppings
4. MARK & RECAPTURE METHOD
Mark and Recapture Method N= Population estimate Trap, capture, mark, release. Later-recapture, determine proportion of those recaptured
that were marked.
N= # marked in first catch X Total # in 2nd catch Number of marked recaptures
Assumptions: 1. the proportion of marked animals in the 2nd trapping is equivalent to the proportion of marked
animals in the total population2. Those marked mix uniformly within the pop.
1. Clumped PatternMost common patternResults form patchy environmental conditions-food, nutrient availability. Ex: Mushrooms under rotting log.
Groups enhance predationSafety. May increase chances for survival.Ex: Schooling behavior Flocks of birds humans
2. Uniform Dispersion
Individuals are evenly spaced
Results from direct antagonistic interactions
Plants secrete chemicals-inhibit nearby germination & competition (alleopathy)
territoriality
3. Random Dispersion
Spacing varies unpredictably
Absence of strong attractions or repulsions between individuals.
RARE pattern in nature
Example: windblown seeds land randomly & germinate.
Demography
Study of vital statistics of a population & how they change over time.
Add
Lose
2 most important featuresAge structure-relative numbers of
individuals of each age group in a population
Sex ratios
1. Life Tables
How long, on average, are individuals of a given age expected to live.Age-specific summary of survival patterns.
Follow the fate of a cohort (indiv. Of same age) from birth to death.
2. Survivorship CurvesGraphic representation of data in Life Tables.
Plot of the proportion or numbers in a cohort still alive at each age.
3 general Curve Types: ( be able to recognize and describe traits; members exhibiting each; example organism)
Type I—humans, elephantsType II-squirrelType III-oysters, clam
Type I
Low early deaths.Steep decline in death rates among those older Produce few offspring, but provide good care.
Ex: HumansOther large mammals
Type IIIntermediate
Constant death rate over life span.Ex: Annual plants small mammals ( grey squirrel),
lizards
Type III High death rates for the young. ( sharp dip in curve initially)
Few live to adulthoodAssociated with:Produce large numbers of offspring-but provide
little or no care.
Ex: Oysters
VariationsCurve type may change between young and adults.
Ex: Nestlings - Type III Adult Birds- Type II
Stair-step curveInvertebrates—crabs; high mortality
during molt, followed by low mortality
Life-History Traits
Life History: Timing of reproduction and death.Highly diverse, but do show patterns
Determines how populations grow.Results from Natural Selection.
Darwinian “fitness”
Reproduction and survival—Life History Traits
1. # of reproductive episodes
# offspring per reproductive episode
Age at first reproduction
2 Common Reproductive Patterns
1. Semelparity: “big-bang” reproduction1 reproduction event
Salmon, agave ( desert plants) , annual plants
2. IteroparityFewer offspring at a timeOver many reproductive seasons
Population Studies & Reproductive Rates
Focusing on females and female offspring.
2 Kinds of Reproductive Strategies identifiedRK
“r" – Selected Species (density independent selection)
Increase fitness by producing as many offspring as possible.
Do this by one of these strategies:Early maturationMany reproductive eventsMany offspring in one reproductive event
“Big bang” reproduction (semelparity) (pink salmon, agaves) is one time reproduction.
r-Selected Result
Maximize reproduction so that at least a few offspring survive to the next generation.
Most offspring die (Type III curve).
“K" – Selected Species (density dependent
selection)Increase fitness by having most offspring survive.
Maintain populations at or near “K”
Do this by:High parental careLate maturationFew reproduction eventsFew offspring.
K- Selected –Results..
Maximize survivorship of each offspring
.
Few offspring, but most survive (Type I curve).
Exponential Growth
Produces a “J-shaped” growth curve.
Ideal conditions
unlimited resources.
Example: Introduce into new or
unfilled environment Rebounding population
Population growth related to Life History Traits
S-shaped” growth curve.Characteristic of “k" species.
Common when resources are limited.
Populations are limited by space, food. That limit is called the CARRYING CAPACITY
The graph shows a logistic population curve.
At what level do the deer reach their CARRYING CAPACITY?
What Limits Population Size?
Density-dependent factors: limited resources- space, food, water, air related to population size
Density-independent factors: random occurrences that can limit population - earthquake, bad weather. not affected by population size
Is disease density dependent, or density independent?
Regular Population Cycles
Cyclic changes in N over time.
Often seen in predator/prey cycles.
Ex: “Boom & Bust Cycle” of Snowshoe
Hare -& Lynx
Predators kill and consume other organisms. Carnivores prey on animals, herbivores consume plants. Predators usually limit the prey population, although in extreme cases they can drive the prey to extinction.
Why predators rarely kill and eat all the prey: 1.Prey species often evolve protective mechanisms such as
camouflage, poisons, spines, or large size to deter predation. 2.Prey species often have refuges where the predators cannot reach
them. 3.Switch its prey as the prey species becomes lower in abundance:
Age Structure Diagrams
Show the percent of a population in different age categories .
Method to get data similar to a Life Table, but at one point in time.
HUMAN POPULATION GROWTH
Grown since history
1A.D. 300 million
Since 1650 ( 500 million), growing exponentionally.
Over 6 billion today
Ecological Footprint
Model to predict “K”
Relative to The Available Ecologic capacity
Land usage requirements to meet our varied human needs.
SummaryKnow density and dispersion patterns.
Know Life Tables and survivorship curves ( and representative organism for the 3 types).
Know what “K” is
Be able to contrast and compare “r” and “k” strategies ( Suggest making a chart-include Logistic Curve, Exponential growth curve, characteristics of populations).
When/how density independent, and dependent growth factors affect pop size.
Know how (match) Age-structure pyramids relate to either Rapid, Slow, or Zero Pop. Growth; their shapes & what the shape means.