Limits on Populations and Communities in Ecosystems Who Eats Who part 5 Limits on Populations and Communities in Ecosystems

Biotic Potential Biotic potential – is the maximum number of offspring that a species could produce with unlimited resources. There are 4 factors that influence biotic potential: Birth potential Capacity for survival Breeding frequency Length of reproductive life Poccket INB

Birth Potential The maximum number of offspring per birth. Ex. Whooping cranes lay two eggs per year and usually only one chick survives.

Capacity for Survival The number of offspring that reach reproductive age. Ex. The female sea turtle lays many eggs, but only a few of her offspring even reach the sea and fewer still reach maturity.

Breeding Frequency The number of times that a species reproduces each year. Ex. Elk mate only once per year, during the fall.

Length of Reproductive Life The age of sexual maturity and the number of years the individual can reproduce. Ex. African elephants reach sexual maturity at about 15 years of age, but may reproduce until they are 90.

Limiting Factors Factors that cause a population to increase   Factors that cause a population to increase Factors that cause a population to decrease Abiotic favourable light too much or too little light favourable temperature too cold or too warm favourable chemical environment unfavourable chemical environment Biotic sufficient food insufficient food low number of predators high number of predators few or weak diseases and parasites many or strong disease and parasites ability to compete for resources inability to compete successfully for resources

Carrying Capacity Populations commonly fluctuate because of an interaction of the many biotic and abiotic limiting factors. However, the ecosystems are able to remain in equilibrium, when none of the populations exceed the carrying capacity. Carrying capacity – the maximum number of individuals of a species that can be supported by an ecosystem.

Limits of Tolerance In the mid-1800’s, Justus von Liebig noted that certain substances must be present in order for plants to grow. He found that if any one of these substances is present in low amounts, the growth of the plant is reduced, regardless of the quantity of other substances. This became known as the law of the minimum: the nutrient in the least supply is the one that limits growth.

In 1913, Victor Shelford added to von Liebig’s work by noting that too much of a factor can harm an organism. This principle is called Shelford’s law of tolerance, which states that an organism can survive within a particular range of an abiotic factor.

A species maximum population size is possible when an abiotic factor is at an optimum or an ideal within the range of tolerance. However, many abiotic factors act on a species at any given time. Most species have a broad range of tolerance for some factors and a narrow range for others.

Density-Independent Factors Density-Independent Factor – a factor in an ecosystem that affects members of a population regardless of population density.

Density-Dependent Factors Density-Dependent Factor – a factor in an ecosystem that affects members of a population because of the population density.

Factors that Cause Changes in Populations Density-independent factors Density-dependent factors flood food shortage fire competition for mates, breeding areas spraying with pesticides habitat change in climate or temp. disease destruction of habitat introduction of exotic species drought increased predation   competition for water and other resources