What Is a Population?

• All the members of a species living in the same place at the same time– a reproductive group– Refers to the group in general and also to the

size of the population

Properties of Populations

• Size• Density• Dispersion

Properties of Populations

• Size– can be measured by

• Mark & recapture• Sampling

Properties of Populations

• Density– Number of individuals in a given area– People in Luzerne County:

• 320,918 people / 907 sq. mi.• Population density: 853.8 people per sq. mi.

– Whitetail deer in Pennsylvania:• 1.6 million whitetail / 46,055 sq. mi.• Population density: 34.7 deer per sq. mi.

Properties of Populations

• Dispersion– The relative distribution or arrangement of its

individuals within a given amount of space– Even, clumped, or random

How Does a Population Grow?

• Growth rate– A change in the size of a population over a

given period of time– Growth rate = birth rate minus death rate– Can be positive, negative, or zero

• To be zero, the average number of births must equal the average number of deaths

How Fast Can a Population Grow?

• Reproductive potential– Maximum number of offspring that each

member of the population can produce– Limits the biotic potential (fastest rate at

which the population of a species can grow)– Increases when individuals produce more

offspring at one time, reproduce more often, and reproduce earlier in life

How Fast Can a Population Grow?

• Exponential Growth– Growth in which numbers increase by a

certain factor in each successive time period– Occurs in nature only when populations have

plenty of food and space, and have little or no competition or predators

What Limits Population Growth?

• Carrying Capacity– The maximum population that the ecosystem

can support indefinitely

What Limits Population Growth?

• Resource Limits– A species reaches its carrying capacity when it

consumes a particular natural resource at the same rate at which the ecosystem produces the resource (LIMITING RESOURCE)

What Limits Population Growth?

• Competition Within a Population– Members of a population will compete with

each other as the population approaches its carrying capacity

– Members may compete indirectly for social dominance or for a territory

Two Types of Population Regulation

• Population size can be limited in ways that may or may not depend on the density of a population– Density dependent: deaths occur more quickly in a

crowded population than in a sparse population• Predation, disease, limited resources

– Density independent: a certain proportion of a population dies regardless of the population’s density• Severe weather, natural disasters

What jobs are there in your community?• Select a job.• Answer the following questions concerning that job:

– What they do for the community– How they provide the service– What resources are used by them in providing the service– Where they live and work– The times during which they work– What other professions they are dependent upon for the

functioning of their profession– What special adaptations (skills, tools, behaviors) they use

or they are required to have– What other professions they compete with, if any

How Species Interactwith Each Other

• The unique role of a species within an ecosystem– Includes:

• Its physical home• The environmental factors necessary for the

species’ survival• All the species interaction with other organisms

• Habitat: location• Niche: an organism’s pattern of use of its

habitat (its job in an ecosystem)

Now. . .• Imagine a forest habitat.• Think of all the members of this natural

community.• Choose one.• Think about the niche it fills using the same

questions used for human jobs.

– What they do for the community– How they provide the service– What resources are used by them in providing the

service– Where they live and work– The times during which they work– What other professions they are dependent upon

for the functioning of their profession– What special adaptations (skills, tools, behaviors)

they use or they are required to have– What other professions they compete with, if any

Your Assignment:

• Create a “Help Wanted” ad– Develop an ad for “recruiting” individuals into

given ecological niches, using special contributions, advantages, etc., as points to highlight.

Types of Species Interaction

• Based on whether each species causes benefit or harm to the other species

Symbiosis

CommensalismMutualismParasitism

PredationCompetition

Competition

• Relationship in which different individuals or populations attempt to use the same limited resource

• Each has less access to the resource, and so is harmed

• Can occur both within and between species– Same species: both are occupying the same

niche– Different species: niches overlap

• Use some of the same resources

Competition

• Indirect competition– Using the same resource without coming into

direct contact– One insect species feeds on a certain plant

during the day and another insect species feeds on the same plant at night.

• Adaptations to competition– Niche restriction: when each species uses

less of the niche than it is capable of using

Predation

• Interaction between 2 organisms in which one organism, the predator, kills and feeds on the other organism, the prey

• Examples:– Snakes eating mice– Bats eating insects– Whales consuming krill

Symbiosis

• Relationship that exists when two species of organisms live in close physical contact with each other

• At least one of the organisms directly benefits from the association

Parasitism

• Relationship between 2 species in which one species, the parasite, benefits from the other species, the host, which is harmed

• Examples: ticks, fleas, tapeworms, mistletoe

• Unlike predators, parasites usually don’t kill their host

Mutualism

• Close relationship between 2 species in which each provides a benefit to the other

• Example:– Bacteria in your intestine

Commensalism

• Relationship in which one species benefits and the other species is neither harmed nor helped

• Examples:– Certain orchids and trees– Birds nesting in trees