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 Interact
with 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
Competition
Predation
Symbiosis
Parasitism
Mutualism
Commensalism
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