Chapter 52 ~ Population Ecology

Population characteristics

Density ~ # of individuals per unit of area •counts •sample size estimate •indirect indicators •mark-recapture

Dispersion ~ pattern of spacing •random ~ unpredictable, patternless spacing (a) •clumped ~ patchy aggregation (b) •uniform ~ even spacing (c)

Demography : factors that affect growth & decline of populations

Birthrate (natality, fecundity)~ # of offspring produced

Death rate (mortality)

Age structure ~ relative number of individuals of each age

Survivorship curve ~ plot of numbers still alive at each age

Population Growth Models

Exponential model (blue) • idealized population in an unlimited environment (J-curve); r-selected species (r=per capita growth rate)

Logistic model (red) •carrying capacity (K): maximum population size that a particular environment can support (S-curve); K-selected species

Population life history “strategies”

r-selected (opportunistic)

Short maturation & lifespan

Many (small) offspring; usually 1 (early) reproduction; no parental care

High death rate

K-selected (equilibrial)

Long maturation & lifespan

Few (large) offspring; usually several (late) reproductions; extensive parental care

Low death rate

Population limiting factors

Density-dependent factors [Effect on population changes as the population size changes]

•competition (resources/mates) •predation

•stress/crowding

•waste accumulation

Density-independent factors [Effect on population does not correlate with rise/fall of population]

•weather/climate •periodic disturbances

Density Dependency

Negative feedback limits population growth.

Accumulation of wastes

Spread of disease

Predation affects increased numbers of organisms

Increased competition

May be intrinsically controlled (have fewer offspring in high density situations)

Life History

The traits that affect an organism’s schedule of reproduction and survival.

Results from natural selection

Helps to determine how populations grow.

Reproductive strategies
Determined by survival rates

Repeated reproduction (iteroparity): with onset of sexual maturity, reproduction is repeated.

Small brood sizes

Favored in unstable conditions

Big-Bang reproduction (semelparity): members of the population reproduce once.

Usually large numbers of eggs

Favors rapid increase in population

Basic “decisions” in life history

When to reach reproductive age

How many offspring/how big

How mature at birth

How often to breed

How much post-birth parental nurture.

Predator/Prey Relations

Ecological Footprint

Calculations

For any given population:

r = the rate of natural increase

b = birth rate

d = death rate

N = number of individuals

r = (b-d)/N

I = expected increase

I = r N

Biotic Potential

Biotic potential = maximum rate = rmax

Occurs under ideal conditions:

plenty of space

unlimited resources

no predators or disease

Biotic Potential

Affected by:

number of offspring/reproduction event,

chance of survival until reproductive age,

number of reproductive events/individual,

age of first reproduction

Biotic Potential

Offset by environmental resistance

Density independent factors

Climate and weather

Natural disasters

Requirements for growth

Density dependent factors

Competition

Emigration

Predation

Parasitism/disease

Biotic Potential

Occurs for a short time, if at all.

Bacteria could cover the earth in 48 hours

1 pair of elephants would have 19 million decendants in 750 years