Chapter 6
Populations
Population Properties
 The numbers or population of an
organism is limited by their
environment
 Population- all the members of a species
living in the same area at the same time
 Ex: all the oak trees in Houston County
Properties
 Can be described by:
 Population size
 Population Density
 Patterns of dispersion
 Demographics
 Population growth
 Limits on population growth
Terms to Know
 Population size- number of individuals present
living in a defined area
 Population Density- number of individuals in a
population per unit area
 Population dispersion or distribution- the spatial
arrangement of organisms within a particular
area
 Sex ratio- proportion of males to females
Distribution Patterns
 Populations may be distributed in different
ways:
 1. Random- individuals located haphazardly in
no particular pattern (minimal interaction)
 Individuals are independent of others
 Can occur when resources are found
throughout an area
Distributions
 2. UNIFORM DISTRIBUTION
 Individuals are evenly spaced, same distance
apart
 Can occur when individuals hold territories or
compete for space
 Ex: plants in the desert need certain amount of
“root” space to get water they need
Distributions
 3. CLUMPED DISTRIBUTION
 Most common in nature
 Organisms arrange themselves according to
the availability of resources
 Organisms interact and are attracted to
certain areas
 Ex: Bees and flowering plants, breeding
pools, humans to urban areas
Distribution Patterns
Age Distribution/Structure
 Describes the relative numbers of organisms
of each age within a population
 Can make predictions based on age structure
 Ex: Population past reproductive age will
decline over time
 Population pre-age or at age of reproductionwill likely increase
 Even age distribution – remain stable
Age Structure Diagrams
 Survivorship: percent of members likely
to survive
 Growth rate: birth & death rates, change
in size
 Growth rate= birth rate – death rate
 Rate can be zero (no increase); negative
(decrease); or positive (exceeds death
rate)
Age Structure Diagram
Graphs
Survivorship Factors
 Determined by 4 factors:
 1.) Births (natality)
 2.) Deaths (mortality)
 3.) Immigration – arrival on individuals outside the
population
 4.) Emigration – departure of individual from the
population
Survivorship Types
K-selected
r-selected
 Devote large amounts of
energy and resources to
caring for a few
offspring
 Devote their energy and
resources to producing
as many offspring as
possible in a short time
 Tend to stabilize over
time at or near their
Carrying Capacity
 Survivorship is left to
chance
 K is abbreviation for
Carrying Capacity
 Exponential Growth
Exponential Growth
 Occurs when a
population or
anything else,
increases by a fixed
percentage each year
 Occurs when pop. has
plenty of food, space
and limited
competition or
predators
Limiting Factors
 Restrain population growth
 Are: physical, chemical and biological
characteristics
 Carrying Capacity- the maximum pop. size of
a species that a given environment can
sustain
Carrying Capacity Graph
r-selected vs K-selected
Exponential vs Logistic
Logistic Growth (Curve)
(Carrying Capicity)
 limiting factors =
environmental
resistance
 Space, food,
water, shelter,
disease,
predators, temp.
mates/breeding
sites
 Plants: sunlight,
moisture, soil
 Aquatic: salinity,
sunlight, temp,
dissolved
oxygen, and
pollutants
Limits to Pop. Growth
 Density Dependent:
depends on size of
the population
 Can help find
mates, but
increases:
 Factors that occur
with the size of the
population
 Competition
 Increases and
decreases with
population density
 Disease and
Parasitism
 Predation
Limits to Pop. Growth
 Density Independent
 Examples:
 Limiting factors that
affect the population
regardless of size
(density)
 Climate/Temperat
ure extremes
 Can eliminate large #’s
of individuals without
regard to their density
 Natural/Catastrop
hic events or
disasters
 Human Activity
Chapter 6: Communities
Species Interactions and Community Ecology
Organisms Niche
 Includes species habitat use, its role in the
community, consumption of foods, use of
resources, role in food chain/food web, and
it’s interactions with other organisms
 Summary of everything an organism does in
its environment
Types of Niches
 Fundamental Niche
 Realized Niche
 The full niche of a
species
 Plays only a part of its
(species) role
 No competitors
 Forced to use fewer resources
due to competition or other
species interactions
 Fulfills all its roles or
uses all resources it
can and capable of
using
 Competitors restrict what an
organism can do or what
resources it can use
 Chart – Fig. 6.2, pg. 143
Species Relationships
When 2 organisms living in close association
with each other interact = Symbiotic
relationship
In symbiosis, at least one of the organisms
usually benefits from the relationship
Species Interactions
 Types:
 Competition
 Predation
 Parasitism
 Herbivory
 Mutualism
 Commensalism/Amensalism
Competition
Occurs when more than one species is
seeking or attempting to use the same
limited resource
Can take place in 2 ways:
Intraspecific Competition – competition
among members of the same species
Interspecific Competition – competition
among members of two or more different
species
Consequences of
Competition
 Competition Exclusion –
when one species excludes
another species from
resource use entirely
 Species Coexistence –
when neither species fully
excludes the other, live in
equilibrium;

*use resources at different
times of the day or
different levels
 Resource Partitioning – the
species partition or divide the
resources they use in common
 Character displacement –
evolve physical characteristics
that reflect their portion of
resource use, natural selection;
*Ex: birds eat same type of seed
– one eats the smaller, the other
the larger seeds
 Larger seeds = bigger beak;
Smaller seeds = smaller beak
(Darwin’s Finches)
Predation
 Process in which a (predator) hunts, captures, kills,
and eats another organism(prey);
*one benefits/one harmed
 The primary organization forces and influence in
community ecology
 These interactions structure/influence food
chains/webs, community make up, numbers and
abundance of the predator and prey; creates cycles
in populations
Predation
Adaptations of Predation
 Camouflage: blending in with environment
 Warning coloration: black stripes or red, orange
and yellow
 Mimicry: imitates another organisms warning
coloration
 Protective covering: quills, shells, exoskeleton
 Odors/Poisons/Inks: skunks, snakes, octopus
 Flying
Parasitism
 Relationship in which one organism (parasite)
depends on another (host) for nourishment, while
doing the host harm
 Some parasites cause little harm, while others kill
 Some parasites live in close contact with the host;
ticks, tapeworm, and lampreys
 Others are free-living and come into contact with
the host infrequently
Herbivory
* When animals feed on the tissues of plants
Insects that feed on plants are the most
widespread type
Some plants recruit animals as allies to help
in defense
Mutualism
Relationship in which 2 or more
species benefits from the interaction
with each other
Bacteria in our intestines
Acacia trees and ants – trees provide
shelter, the ants defend and protect the
trees
Commensalism and
Amensalism
 Commensalism: one species benefits, the other is
neither harmed or helped (unaffected);
*Sharks and remora’s: remora’s attach to sharks and
feed on scraps of food; clown fish/sea anemome
 Amensalism: one species is harmed and the other
is neither harmed or helped (unaffected);
*Ex: black walnut tree that secretes chemical that
kills neighboring plants, penicillin/bacteria
Ecological Communities
 Energy passes through Trophic Levels
 As organisms feed on one another, energy moves through
the community
 Producers = 1st level
 Primary consumers = 2nd level; herbivores
 Secondary consumers = 3rd level; eat primary consumers
 Tertiary consumers = 4th and up; eat 2nd and 3rd
Energy/Biomass
 Food webs: show relationships and energy flow
 Keystone species: strong impact on community;
secondary and tertiary consumers
 Community that resists change and remains stable;
shows resistance
 A community that changes in response to
disturbance then returns to original state; shows
resilience