Population EcologyPopulation EcologyThe study of the growth, abundance, and The study of the growth, abundance, and

distribution of populationsdistribution of populations

Population DynamicsPopulation Dynamics

Population growthPopulation growth is the increase in is the increase in population size over time. It can be population size over time. It can be exponential at times, and is controlled by exponential at times, and is controlled by limiting factors that determine the carrying limiting factors that determine the carrying capacity of the environment.capacity of the environment.

Factors Which Affect Factors Which Affect Population Growth and SizePopulation Growth and Size

• Immigration – movement of individuals into a population from another population

• Births• Emigration – movement of

individuals out of a population and into another population

• Deaths

Exponential GrowthExponential Growth• Occurs when a population size

increase dramatically over a period of time

• The graph of a growing population first resembles a J-shaped curve.

• As a population gets larger, it also grows faster, and can result in a population explosion.

• Often occurs when a species moves to a previously uninhabited area or one in which there is little competition.

ExponentialExponential Population GrowthPopulation Growth

Exponential GrowthExponential Growth

Logistic GrowthLogistic Growth• A population begins with a period of

slow growth followed by a brief period of exponential growth before leveling off at a stable size.

• Takes the form of an S-shaped curve.• Abundant resources are available at

first, but resources may begin to be depleted and growth may start to slow.

Logistic GrowthLogistic Growth

Limiting FactorsLimiting Factors

• Affect the amount of organisms in a population that an environment can support

• Limiting factors include any biotic or abiotic factor that restricts the existence, numbers, reproduction or distribution of organisms.

Limiting FactorsLimiting Factors

• Two types of limiting factors:

a. Density dependent- disease, competition, parasites, predator-prey relationships and food; as population increases these have an increasing effect on the population; a population has to reach a certain density ( ex. a disease spreads more quickly in a dense population than in a less dense one)

Density-Dependent FactorsDensity-Dependent Factors

b. Density independent – affect

populations regardless of their density and include temperature, storms, floods, droughts and human activities such as habitat disruption ( no matter how many mosquitoes there are, a cold winter will kill them)

• Factors that limit one population can also have an indirect effect on another population.

Example – A drought can cause a decrease in grass growth which can affect on the number of grass seeds available for a mouse population. The change in the mouse population can then affect the number of hawks in a habitat.

Carrying CapacityCarrying Capacity

• The number of organisms of one species that an environment can support is its carrying capacity.

• When populations are under the carrying capacity, births exceed deaths until the c.c. is reached. If the population temporarily overshoots the carrying capacity, deaths exceed births until the population levels are once again below carrying capacity.

Consider this illustration representing muskrats in a marsh. The barrel represents the marsh habitat-the amount of food, water,and cover for a fixed number of muskrats. The water in the barrelis the number of muskrats the habitat can support. The pipe pouringwater into the barrel represents the new muskrats that are born inthe marsh or wander in from other habitats. The water spilling outis the number of muskrats that die each year due to starvation, disease, predators or other factors.The barrel can only hold so much water. That is, there is a limit to the number of muskrats that can survive here from year to year unless the habitat(the size of the barrel) is changed in some way.Every habitat has a different carrying capacityfor every kind of wildlife that lives there. A pristine cattail marsh would be a deep barrel for muskrats, while a dune forest would hold few, if any.

Dynamic EquilibriumDynamic Equilibrium

• Dynamic Equilibrium – Wildlife populations are never truly static…… they tend to fluctuate as part of an overall “balance of nature” in response to a variety of stimulating and limiting factors. Nature is never really in balance or equilibrium. Ecological systems are involved in a process of continual change.