Limiting Factors

Students know that:

· ecosystems are complex, interactive systems that include both biological communities (biotic) and physical (abiotic) components of the environment. Organisms and populations of organisms are dependent on their environmental interactions both with living and nonliving factors. As with individual organisms, a hierarchal structure exists; groups of the same organisms (species) form populations, different populations interact to form communities, communities live within an ecosystem, and all of the ecosystems on Earth make up the biosphere. Like individual organisms, ecosystems are sustained by the continuous flow of energy.

ecosystems are dynamic in nature; their characteristics can vary over time. Disruptions to any physical or biological component of an ecosystem can lead to shifts in all its populations. Biodiversity describes the variety of species found in Earth’s terrestrial and oceanic ecosystems. The completeness or integrity of an ecosystem’s biodiversity is often used as a measure of its health.

· a population is a group of organisms belonging to the same species that live in a particular area. Populations can be described based on their size, density, or distribution.

· population density measures the number of individual organisms living in a defined space. Regulation of a population is affected by limiting factors that include density-dependent, density-independent, abiotic and biotic factors.

o Density-dependent factors Limiting factors that are density-dependent are those that operate more strongly on large populations than on small ones. Density-dependent limiting factors include competition (for food, water, shelter & space), predation, parasitism, and disease. These limiting factors are triggered by increases in population density (crowding).

o Density-independent factors Limiting factors that are density-independent are those that occur regardless of how large the population is and reduce the size of all populations in the area in which they occur by the same proportion. Density-independent factors are mostly abiotic (such as weather changes), human activities (such as pollution), and natural disasters (such as fires).

o Abiotic and biotic factors Limiting factors can change within an ecosystem and may affect a population.

§ Abiotic factors are nonliving things in an ecosystem and may be chemical or physical. Some examples are water, nitrogen, oxygen, salinity, pH, soil nutrients and composition, temperature, amount of sunlight, and precipitation.

§ Biotic factors include all of the living components of an ecosystem. Some examples are bacteria, fungi, plants, and animals. A change in an abiotic or biotic factor may decrease the size of a population if it cannot acclimate or adapt to or migrate from the change. A change may increase the size of a population if that change enhances its ability to survive, flourish or reproduce.

Teacher Note: It is not essential for students to calculate population growth patterns or population density.