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).

Standard B1.40



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


1. Competition: when the demand for resources such as food, water, space, and other essentials exceeds the supply.

    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