POPULATIONS AND COMMUNITIES
LS Ch 21
LEVELS OF ECOLOGICAL ORGANIZATION
• Individual– one organism
• Population– Members of a single species who exchange genes (interbreed)
• Community
– Interacting species (same place at the same time)
• Ecosystem
– Biotic + Abiotic factors
• Biosphere
– Portion of Earth that supports life
THE BIOSPHERE
• The portion of Earth that supports life
– From highest that birds fly to lowest bacteria/archaea can survive
POPULATION ECOLOGY
• A population is a group of organisms of the same species, in the
same place at the same time.
– Genes exchanged
• Deer of Minnesota vs. Deer of Mexico
POPULATION GROWTH
• When conditions are ideal, populations grow exponentially
– 24
– 48
– 816, etc.
• Biotic potential-The number of organisms in a population that
would be possible if conditions were ideal
– Spoiler Alert: Conditions are not!
THOUGHT EXPERIMENT
• You wash your countertop with antibacterial cleaner and kill all but 1
bacteria. 24 hours later, how many bacteria would there be if you do not
clean it again before then if the bacteria split every 5 minutes?
– At 5 minutes: 2 bacteria
– At 1 hour: 162,755 bacteria
– At 6 hours: 10,000,000,000,000 bacteria
– At 12 hours: 100,000,000,000,000,000,000,000,000
– At 24 hours: 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 (52 zeros)
– At 36 hours, there would be more bacteria than there are atoms in the universe
EXPONENTIAL GROWTH
• The rate of growth grows each generation
• J-shaped curve
• Where have you seen this before?
THOMAS MALTHUS!
POPULATIONS CANNOT GROW EXPONENTIALLY
LONG-TERM
• Starts exponentially, then factors limit the size of the population
– Carrying capacity (occurs at Malthus’s “point of crisis”)
• The number of organisms that can be supported by an environment
• Changes our graph into an “S-shaped curve”
LIMITING FACTORS
• Any factor that limits the number of organisms in a population
– Many are abiotic:
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Amount of sunlight
Availability of shelter
Amount of fixed nitrogen in the soil
Availability and quality of water
Etc.
– Some are biotic
• Competition between organisms
• Predation
• Availability of food
REAL-LIFE POPULATION GROWTH
• Organisms often overshoot carrying capacity
– Called overpopulation
• Too many for environment to support
• Deaths will outnumber births-population will drop below carrying capacity
• Below carrying capacity, births again will outnumber deaths
– Will repeat the cycle
FACTORS THAT AFFECT POPULATION SIZE
• Populations will grow if:
– Birthrate+immigration>deathrate+emigration
• Birthrate- number of offspring produced over a time period
• Deathrate-number of organisms that die over a time period
• Immigration-entering the population
• Emigration-leaving the population
WHEN POPULATION DECREASE BECOMES A
CONCERN
• Natural and human-caused events can decrease the population to
critical levels
• Extinction can occur if the last individual dies
– Populations can be replenished, but not species
• If a species is nearing extinction, it is considered endangered
• If a species is at risk of becoming endangered, it is considered
threatened
JUST A FEW SPECIES WE HAVE DIRECTLY CAUSED
THE EXTINCTION OF (GOOGLE FOR MANY MORE)
• We have caused the extinction
of at least 300 species in the
past 500 years, including:
– African bear (hunting~100 years
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ago)
Aurochs (hunting 1600’s)
Bali tigers (hunting 1937)
1 species of lion (hunting 1960s)
Bush wrens (rats brought by
humans 1970s)
Dodos (hunting 1600s)
Carrier pigeons (hunting 1900s)
SPECIES LIKELY TO GO EXTINCT SOON
• Critically endangered:
– Black Rhino (2000 left, hunting)
• All other rhinos endangered
– Russian Leopards (60 left, hunting)
• All other leopards endangered
– Cross river gorilla (300, hunting, habitat
loss)
• Nearly all apes endangered
– South China Tiger (none seen in 20
years, hunting)
• All other tigers endangered
– Sumatran elephant (1500 left, hunting,
habitat loss)
• All other elephants critical or endangered
– Pandas (1500 left, habitat loss)
– Atlantic Right Whales (300 left, hunting)
• All other whales endangered
BIOLOGICAL COMMUNITIES
• Interacting populations
– Does not include abiotic factors
• Within a community, each population has evolved to survive in
certain places
– Called a habitat
• All organisms also have roles in the ecosystem
– Called the niche
• The job of the organism
RELATIONSHIPS IN A COMMUNITY
• Predator-prey
• Movement of energy
– Food webs
• Matter Cycles
PREDATOR-PREY INTERACTIONS
• Availability of prey is a limiting factor (for the predator)
– So is predation (for the prey)
• As the population of predators increases, they greatly decrease
the population of prey
– This lowers the amount of food available for the predators, driving their
population down
• This allows the population of prey to grow because not as many are being eaten
– Which allows the population of predators to increase
•
Which lowers prey
– Which lowers predators
•
Which increases prey
– Which increases predators
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Which decreases prey… and so on
SYMBIOSIS (SYM=TOGETHER, BIO=LIFE)
• Organisms of 2 species in a long-term
relationship (living together)
• Mutualism (+/+)
– Both species gain benefit
• Bee and flower
• Human and bacteria
• Commensalism (+/0)
– One species gains benefit, the other is unaffected
• Bird on tree
• Parasitism (+/-)
– One species gains benefit, the other is harmed or even
killed
• Mosquitoes, tapeworms, etc.
• http://www.animalplanet.com/tv-shows/monsters-inside-
me/videos/pork-tapeworm/