Density Dependence #9/10: Density Dependence, Density Independence, Population Regulation

Question 1

Population Growth Limitations

Answer 1

Population Growth Limitations:
- Reason why populations don’t increase indefinitely
-All populations can increase under favourable conditions, but conditions not always favourable

Density Dependent:
- Resource Limitation (competition involved too)
- Predation (parasitism; pathogen)

Density Independent:
- Weather/Disturbances

Question 2

Density-Independent Factors

Answer 2

Density-Independent Factors: Factors whose effects on birth and death rates independent of population density (N)
- Can have large effects on population size but do not regulate population size

Example:
- Weather conditions (temperature, rainfall)
- Catastrophes (storms, fires, floods)
- Geometric growth
- Salinity effects
- TBT effects

Note: Population growth rates = NOT function of density

Question 3

Density-Dependent Factors

Answer 3

Density-Dependent Factors: Factors that cause birth, death, immigration or emigration rates to change as population density changes
- Regulate population size
- Population growth rates change as population density changes

Example:
- Logistic growth

Note: Can be tested by modifying limiting factors

Question 4

Population Regulation

Answer 4

Population Regulation: Density-dependent factors that cause population to:
- Increase when density low
- Decrease when density high

Question 5

Population Density

Answer 5

Population Density: Average number of individuals per unit of area or volume

Question 6

Carrying Capacity

Answer 6

Carrying Capacity (K): Max population size of species that can be sustained by specific environment, given food, habitat, water, and other resources available
- X-intercept (y = 0) on population growth rate vs density graph
- Where birth and death line cross (b = d) on rate vs population size (n) graph

Question 7

Logistic Growth

Answer 7

Logistic Growth: Population level pattern in which abundance rapidly increases at first and then stabilizes at carrying capacity
- Density dependent
- Population approaches EQ

Question 8

Logistic Growth Equation

Answer 8

Logistic Growth Equation: Modified exponential growth equation that incorporates growth limits and shows how population may stabilize at carrying capacity
- Growth rate (r) ↓ as density (N) ↑
- Better equation for mimicking real population behaviour
- dN/dt = rN(1-N/K)
- If N > K, dN/dt = negative

Note: Exponential growth equation does not apply when resources start to run out

Question 9

r_max

Answer 9

r_max: per capita intrinsic (natural) rate of increase under ideal conditions

Question 10

r_obs

Answer 10

r_obs: per capita intrinsic rate of inc under actual conditions
- Depends on birth and death rates (+ immigration and emigration rates)
- r_obs = b - d

Question 11

Density Dependence Graph

Answer 11

Density Dependence Graph:
- X-axis = density
- Y-axis = population growth rate (r)

Note: Not enough information to tell whether births and/or deaths are density-dependent

Question 12

Birth & Death Rate vs Population Size Graph

Answer 12

Birth & Death Rate vs Population Size Graph:
- K = at centre intersection and referring to population size
- As population size smaller, births bring size forwards to K
- As population size bigger, deaths bring size backwards to K

Note: Most common relationship between population density and death rate = population size inc and death rate inc (resources scarce) (At K, B = D; assume no immigration/emigration)

Question 13

Variable vs Fixed Birth & Death Rates

Answer 13

Variable vs Fixed Birth & Death Rates:
- As K changes over time (function of fluctuating birth and death rates), population size will also fluctuate

Question 14

Population Fluctuation

Answer 14

Population Fluctuation: Reason that no population growth looks same
- All populations fluctuate in size
- Fluctions can be erratic in either increases or decreases

Question 15

Population Extinctions

Answer 15

Population Extinctions: Fluctuation in population growth rate can increase risk of extinction (especially small populations)

Question 16

Allee Effects

Answer 16

Allee Effects: When population grow rate decreases as population density decreases
- Population growth rate increases as population density increases
- Density dependent

Example:
- Important in animals such as bluefin tuna (Thunnus thynnus), which form schools whose protective systems function poorly at small population sizes
- Important in species where individuals have difficulty finding mates at low population densities (ex. kakapos; Strigops habroptilus)