Limiting Factors

Logistic model of population growth:
     - Valid in many natural populations.
     - Is a simplification of real-world population dynamics.
     - Implicit in model: carrying capacity does not change.
The carrying capacity varies.
     - Annual: Carrying capacity during winter is lower, higher in summer.
     - Catastrophic natural events (earthquakes, volcanoes, fires, etc.) alter carrying capacity.
Populations do not exist in isolation.
     Intraspecific Competition:
     Interspecific Competition:

Population Regulation

Two main groups of factors regulating population growth:
Density-Dependent Factors: density (number of individuals per unit area) of population affects growth rate and mortality rate.
Density-Independent Factors: population density does not affect growth rate or mortality rate.

Density-Dependent Factors

Density of population affects growth/mortality.

1. Competition
Limited space and resource availability.
High density populations show greater competition.
Fewer resources per individual can lead to starvation and therefore a higher mortality rate, lower growth rate and migration.

2. Predation
Predation limits population growth.
High density populations offer many opportunities for predators.
Predator and prey populations form a cycle
     - As prey population increases, predators have more to eat.
     - Predator population increases.
     - Increased predator population reduces prey population.
     - As prey population declines, less food is available for predators.
     - Predator population then declines.
     - This then leads to prey population increasing.

3. Parasitism
One organism, the parasite, lives off of another organism.
Examples:
High density populations easily transmit parasites, due to high levels of contact.
Parasites thrive in densely packed host populations.
If virulent, parasites can decimate a population.
Decline of host population will reduce parasite population.
Transmission of parasites occurs at lower rate in low density population.

4. Disease
Similar to Parasitism factor
Bacteria, viruses and fungi
Diseases spread quickly through high density populations.
     - Disease vectors as carriers
     - What will disease do to growth rates and mortality rates?
Low density populations have less contact.
Less opportunity for disease to spread from one host to another.

Density-Independent Factors

Factors affecting a population regardless of density.
Chances of survival are same whether population density is high or low.
Typically due to extreme physical processes.

1. Unusual or Extreme Weather
Heat Waves
Drought
Hurricanes
Tornadoes
Extreme Cold
Excessive Snowfall
Excessive Rainfall (groundwater saturation, leaching of nutrients)

2. Seasonal Cycles

3. Catastrophic Events
Volcanic Eruptions
Flooding
Earthquakes
Asteroid Impact
Forest Fires

4. Human Activities
Deforestation
Domesticated Agriculture
Fertilizers
Biocides (Insecticides, Herbicides, Poisons, Antibiotics)
Hunting (legal and illegal)
Pollution

5. Oxygen Levels
Warm temperatures (reduces O₂ content)
Algal blooms (cuts off oxygenation)

6. Salinity

7. Soil Acidity
Acid Rain

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