POPULATION BIOLOGY NOTES
What is a population? Groups of individuals that belong to the same species and live in the same area.
3 characteristics of a population:
1. Geographic distribution: the area inhabited by a population.
2. Population density: the number of individuals per unit area.
3. Population growth:
· Affected by the # of births, the # of deaths and the # of individuals entering (immigration) or leaving (emigration) the population.
· Populations grow if the birthrate is greater than the deathrate.
· Under ideal conditions with unlimited resources, a population will grow exponentially. This is called exponential growth.
· This is represented by a J-shaped curve.
Carrying Capacity: the largest # of individuals that a given environment can support indefinitely. Standard B1.37
· Exponential growth does not occur in natural populations for long. Eventually resources (food and space) become less available and grow slows down. This is called logistic growth. Standard B1.45
· The J-shaped curve levels off as the population reaches carrying capacity (K).
· This is represented by an S-shaped curve.
· In an undisturbed environment (no human intervention), populations will fluctuate around carrying capacity (equilibrium).
What limits population growth and maintains balance (equilibrium) in an ecosystem? Density-dependent and density-independent limiting factors
A. density-dependent limiting factors: factors that depends on the population size.
· The impact increases as the population size increases
· Some organisms avoid competition by migrating seasonally to areas where climate is better and more food is available
2. Predation and herbivory: controls numbers of predators and prey and plants
a. Predator-prey interactions: cause both populations to cycle up and down and maintains a stable ecosystem.
b. Herbivore effects: also causes both populations to cycle up and down and maintains a stable ecosystem.
c. Humans as predators: humans tend to over hunt and over fish which in turn disrupts the stable ecosystem.
3. Parasitism and Disease: crowding leads to an increase in parasitism and resistance to disease.
4. Stress from overcrowding: when species are overcrowded, this leads to fighting and stress. Stress can cause resistance to disease, neglect of young or emigration.
B. density-independent limiting factors: affects all populations in similar ways, regardless of the population size
1. Unusual weather: such as extreme hot or cold temperatures
2. Natural disasters: such as hurricanes, tornados, floods, or droughts.
3. Human activity: such as damming a river or clear-cutting a forest.Standard B1.41