1. Chapter 40
Population Ecology and Distribution of Organisms – Part 2

2. Population characteristics
Density- # of individuals per unit of area
•counts •sample size estimate •indirect indicators
•mark-recapture
Dispersion- pattern of spacing •random- unpredictable, pattern less spacing (a)
•clumped- patchy aggregation (b)
•uniform- even spacing (c)

3. 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
N = population size
t = time
r = b (births) – d (deaths) = (rate of growth)
Pos. r = population growth
Neg. r = population decline
R = 0 = ZPG (Zero Population Growth)

4. Survivorship Curves (plot of numbers still alive at each age)
Type I Curve - Low death rate early in life (humans).
Type II Curve – Constant Mortality (squirrels).
Type III Curve – High death rate early in life (oysters/zebra mussels).

5. Population life history “strategies”
r-selected (opportunistic)
exponential
Short maturation & lifespan
Many (small) offspring; usually 1 (early) reproduction; no parental care
High death rate
K-selected (equilibrial)
logistic
Long maturation & lifespan
Few (large) offspring; usually several (late) reproductions; extensive parental care
Low death rate

6. Population Growth Models
Exponential model (blue)
• idealized population in an unlimited environment (J-curve);
r-selected species
(r=per capita growth rate)
Organisms exhibit high rates of reproduction and occur in variable environments in which population densities fluctuate well below K
Logistic model (red)
•carrying capacity (K): maximum population size that a particular environment can support (S-curve);
K-selected species
Organisms live and reproduce around K and are sensitive to population density

7. Exponential Growth Population-Whooping Crane

8. Logistic Models (Paramecium, Daphnia)
Vs.
Population Affected by Density – Independent Limiting Factors (severe winters)

9. Population limiting factors
Density-dependent factors •competition for resources
•predation
•parasites and disease transmission
•stress/crowding
•waste accumulation
Density-independent factors •weather/climate
•periodic disturbances
Population Cycles
- Fluctuations in Population size in response to varying effects of limiting factors (Hares & Lynx)