Populations Biology

Describing Populations  Researchers study populations’ geographic range, density and distribution, growth rate, and age structure.

Describing Populations  Population density refers to the number of individuals per unit area.

Describing Populations  A population’s age structure refers to the number of males and females of each age a population contains.  Why is would knowing this information be important?

Population Growth  The factors that can affect population size are the birthrate, death rate, and the rate at which individuals enter or leave the population.

Population growth  A population will grow if individuals move into its range from elsewhere. This process is called……  A population may decrease in size if individuals move out of the population’s range. This process is called…… Immigration Emigration

Exponential Growth  In exponential growth, the larger a population gets, the faster it grows.  Under ideal conditions, with unlimited resources, a population will grow exponentially. Examples?

Human Population Growth is Exponential

Logistic Growth  Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth.

Carrying Capacity  Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support.

Limits to Growth  What determines an ecosystems carrying capacity?  Limiting factors are factors that control the growth of a population

Density-Dependent Limiting Factors  Density-dependent limiting factors operate strongly only when population density-the number of organisms per unit area – reaches a certain level.  Density-dependent factors include:  Competition  Predation  Herbivory  Parasitism  Disease  Stress from overcrowding

Competition  The more individuals living in an area, the sooner they use up the available resources.  Space and food can be related to one another as many grazing animals compete for territories in which to breed and raise offspring.

Predation and Herbivory  Predator – Prey Relationships  Herbivore Effects  Humans as Predators

Parasitism and Disease  Parasites and disease causing organisms feed at the expense of their hosts, weakening them and often causing disease or death.

Stress from Overcrowding  Some species fight amongst themselves if overcrowded. Too much fighting can cause high levels of stress which can weaken the bodies ability to resist disease.

Density-Independent Limiting Factors  Density independent limiting factors affect all populations in similar ways, regardless of population size and density.  Unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density independent limiting factors.