Populations

A population is all the individuals of a species that live in the same area.

Characteristics of Populations Four characteristics:  Geographic distribution  Density  Growth rate  Age structure

Geographic Distribution The area inhabited by a population.  The range can vary in size from a few cubic centimeters to millions of square kilometers.

Population Density The number of individuals per unit area.  The number can vary depending on species and ecosystem. Ex. Saguaro cactus in the desert plant community has a low density. Sage brush has a high density.

Population Dispersion Population dispersion is the way in which individuals of a population are spread in an area or volume. Clumped dispersion  Individuals may live close together in groups in order to facilitate mating, gain protection, or access food resources. Uniform dispersion  Territoriality and intraspecies competition for limited resources lead to individuals living at specific distances from one another. Random dispersion  Individuals are spread randomly within an area or a volume.

Survivorship Curves A survivorship curve is a generalized diagram showing the number of surviving members over time from a measured set of births.  By measuring the number of offspring born in a year and following those offspring through until death, survivorship curves give information about the life history of a species.  There are three basic patterns of animal survivorship curves.

Survivorship Curve Type I  This represents a life history that is common among large mammals.  It shows a low infant mortality and a population that will generally survive until old age.  A behavior shared by organisms showing Type I survivorship curves is parental care for the young.

Survivorship Curve Type II  Organisms such as birds, small mammals, and some reptiles show a survivorship rate that is roughly equal at all ages of an organism’s life.  At all times, these species have an equal chance of living and dying, whether as a result of predation or from disease.

Survivorship Curve Type III  These organisms have a very high birth rate and also a very high infant mortality rate.  These species are generally invertebrates, fish, amphibians, and plants.  Many of their offspring will die from predation, but inevitably, a few will survive to adulthood and pass on their genes to the next generation.

Population Growth Natural populations stay the same size from year to year. Three factors affect population size:  Number of births  Number of deaths  Number of individuals that enter or leave the population.

Population Growth A population can grow when its birthrate is greater than its death rate.

Population Growth If the birthrate equals the death rate, the population stays the same size.

Population Growth If the death rate is greater than the birthrate, the population shrinks.

Population Growth Immigration  The movement of individuals into an area.  Causes population growth. Emigration  The movement of individuals out of an area.  Causes population decline.

Exponential Growth If a population has abundant space and food, and is protected from predators and disease, then organisms in that population will multiply and the population size will increase.

Exponential Growth  Occurs when the individuals in a population reproduce at a constant rate.  It will grow slowly at first, then faster and faster.  Under ideal conditions with unlimited resources, a population will grow exponentially.

Exponential Growth The current population of African elephants is 470,000 to 690,000 individuals (down from 1.3 million)  An elephant can produce offspring only every 2 to 4 years (they’re pregnant for 22 months!)  The baby’s take 10 years to mature and cannot reproduce until ~13 years of age. If all the offspring of a single pair of elephants survived and reproduced for 750 years there would be nearly 20 million elephants!

Logistic Growth As resources become less available, the growth of a population slows or stops. Logistic growth  Occurs when a population’s growth stops or slows down after a period of exponential growth.

Logistic Growth Will occur when:  Death increases  Emigration increases  Birthrate decreases  Immigration decreases

Logistic Growth Carrying Capacity (K)  The environment can only support so many individuals.  The number that represents the largest amount of individuals an environment can support at any one time is the carrying capacity.

Limits to Growth

Limiting Factors Limiting Factor  Any factor that causes population growth to decrease Competition Predation Parasitism and disease Drought and other climate extremes Human disturbances

Density-Dependent Factors Density-Dependent Factor  A limiting factor that depends on population size. Factors become limiting only when a population has reached a certain size. Factors operate most strongly when a population is large and dense. They do not affect small, scattered populations as greatly.  Competition, predation, parasitism, and disease are all density-dependent factors.

Density-Dependent Factors Competition  When populations become crowded, organisms compete with each other for food, water, space, sunlight, and other essentials.  Can occur between members of the same species  Can occur between members of different species Is a major source of evolutionary change

Density-Dependent Factors

Predation  Predator-prey relationships are the best known mechanisms of population control.  Ex. Wolves and Moose on Lake Superior When the moose population increases, the wolf population increases shortly afterward. As the wolf population increases, the moose population decreases (due to predation), followed shortly by a decrease in wolf population.

Density-Dependent Factors Parasitism  Parasitic organisms can range in size from microscopic to 30 centimeters or more in length (tapeworm)  Parasites are similar to predators They take nourishment at the expense of their hosts, weakening them and causing disease or death as a result.

Density-Dependent Factors Disease  The closer packed a population is, the faster disease will spread. The Black Plague killed about 75 million people in 1340.

Density-Independent Factors  Affect all populations in similar ways, regardless of the population size. Unusual weather, natural disasters, seasonal cycles, certain human activities (damming rivers, clear- cutting forests).

Density-Independent Factors Regardless if the population has 5,000,000 individuals or 5,000, a hurricane, fire, or drought can kill them all off.