Population
Ecology & Biomes
What is Ecology?
● The study of interactions among and
between organisms and their environment
○ Living AND nonliving things!
● Organism → Population → Community →
Ecosystem → Biosphere
● Individual → Planet
Why it’s tricky
● It can be hard to figure out where the limits
are
○ How do you break the planet down into smaller
pieces?
● What do you think?
The earth is “patchy”
● The environment isn’t uniform
○ Many factors contribute
What are a few of these factors?
The earth is “patchy”
● This patchiness creates different habitats
and environments in different areas
The Environment
● Organisms and nonliving surroundings
○ Birds AND rocks
● Biotic Factors: ____________
● Abiotic Factors: ________________
The Environment
● Organisms and nonliving surroundings
○ Birds AND rocks
● Biotic Factors: The living parts of the
environment
○ Examples: plants, animals, soil!
● Abiotic Factors: The nonliving parts of the
environment
○ Examples: Sunlight, water, oxygen, wind, rock,
topography, weather, and so on...
Why Ecology Matters
● In the real world, nothing happens in
isolation
● Understanding how the ecosystem works
helps people make decisions about
environmental issues
Biomes
● Biome: A group of ecosystems that share
similar climates and organisms
● What makes this happen?
○ Think about the maps we just looked at
The Major Biomes
Tropical Forest
Savanna
Desert
Chaparral
Temperate grassland
Temperate deciduous forest
Coniferous forest
Tundra
Biomes
Biomes
Biomes
● Biome: A group of ecosystems that share
similar climates and organisms
Main Factors:
● Temperature
● Precipitation (rain)
Also
● Sunlight - Latitude!
● Elevation (above/below sea level)
Major Biomes
Community Interactions
● First things first
● Every organism has a niche: its place in the
environment/ecosystem
● Niche: Includes habitat, food source, time of
day most active, predators, etc
○ The organism’s “job” in the ecosystem
Niches
● Fundamental Niche: The FULL range of
environmental conditions and resources and
organism could possibly occupy/use
○ No limiting factors
● Realized Niche:
The fraction of the
fundamental niche
an organism
ACTUALLY
occupies
Fundamental v Realized Niches
● What’s going on here?
Competition (-,-)
● Two organisms competing for the same
niche, or for the same resources
● Intraspecific: Competition within the same
species
● Interspecific: Competition between different
species
Competitive Exclusion & Resource
Partitioning
● Competitive Exclusion: If two species are
so similar that the same resources limit both
species’ growth, then competition means
that one will “beat” the other and exclude
the other from the resource
● Resource Partitioning: When species use
the same resource at different times of
day/year
Resource Partitioning
Predation (+,-)
● One organism capturing and eating another
organism
● Usually involves the death of the captured
organism
Symbiosis
● Two species interacting closely
● Three major categories
○ Mutualism
○ Commensalism
○ Parasitism
● Categories are based on whether one or
both of the species benefits or is harmed
Mutualism (+,+)
● “You scratch my back, I’ll scratch you”
● Both species benefit!
● Examples??
Mutualism (+,+)
● Clownfish/Anemone
● Hummingbirds/Plants
● Intestine/Bacteria
Commensalism (+,0)
● “Helping you is no trouble to me”
● One organism benefits, and doesn’t affect
the other
● Examples??
Commensalism (+,0)
● Buffalo/Egrets
● Epiphytes/Trees
● Hitchhiking
Parasitism (+,-)
● “I will benefit at your expense”
● One organism benefits, and the other is
harmed
● May or may not result in the death of the
“host”
● Examples??
Parasitism (+,-)
● Ticks/Dogs
● Mosquitoes/People
● Intestines/Parasites
Predator-Prey Relationships
Idealized Predator-Prey Coupled
Dynamics
● Basically, predator and prey populations
affect each other
● Usually more complex than this
○ Multiple species
○ Environmental Factors
Predator-Prey Interactions
● Different levels
● Individual: Predator (+), Prey (-)
● Population:
○ Can actually help the prey population
○ Weak, sick, and old are killed first
○ Remaining prey have more access to food
○ Improves genetic stock of prey
Questions
1. Why are predator-prey cycles cyclical?
2. What keeps predators from eating all the
prey?
Lynx & Hares
Lynx & Hares
● 10 year cycle
○ Lynx on a 2 year lag
Why?
● Prey numbers increase, so predator
numbers increase
● Once there are a lot of predators, they start
bringing the prey population down
● Once there aren’t many prey, the predator
population starts to come down
Population Growth
What is a population?
Members of the same species living in the
same geographic area
How Do Populations Change?
Some change a lot over time
Some are more stable
Influenced by food, space, weather
conditions, breeding patterns…
Exponential Growth
Like in the previous
bacteria example
When a population
multiplies by a
constant factor at
constant time
intervals
Forms a J-shaped
curve
Logistical Growth
When these
factors limit
growth, the
population has
reached carrying
capacity (S-curve)
Remember:
limiting growth
indicates amount
coming in =
leaving
Carrying Capacity
Population may start to grow
exponentially, but eventually its growth
will be limited by a limiting factor
Limiting factors can include disease, food
availability, space, competition, etc.
Types of Limiting Factors
o Density-Dependent
• Factors that become limiting only at high
population densities
- Competition, Predation, Parasitism, food, space,
disease, etc.
o Density-Independent
• Factors that limit population growth regardless
of density
• Weather (e.g. - drought, storms, hurricane,
temperature), natural disturbances (e.g. - fire, tree
falls), human changes, etc.
Population Growth Cycles
Patterns of “Boom-and-Bust”
Population Growth
Immigration
Births Population Size Deaths
Emigration
Reproduction = Growth
A populations ability to grow depends on
its rate of reproduction
Bacteria are the fastest at reproducing
Their population can double every 20 minutes if
conditions of unlimited food, space, and water are
available
This means that after 36 hours, the entire planet
would be covered with a layer of bacteria ½ meter
deep
Population Density
Often used to describe a population
Defined as the number of individuals per
unit of area or volume
10 bacteria/mL
300 deer/square meter
Helpful measurements for comparing
populations in multiple locations
What kind of growth do humans show?
