Transcript Understanding populations

Understanding populations
Which of the following is a
population?
1. Green sea turtles nesting on a beach
2. A flock of cardinals, chickadees, titmice,
and nuthatches
3. Coho salmon migrating upstream
4. The forest behind PVHS (just the trees)
5. Aquatic insects in the Peckman River
Properties of populations
1. Size
2. Density = # of individuals/area
3. Dispersion
Regular
Random
– How members of
a population are
spread in space
– Examples?
– Most common?
Clumped
How does a population grow?
• What adds to a population?
– Births
– Immigration
• What subtracts from a population?
– Deaths
– Emigration (note: just one ``m’’)
• Change in population size = (B+I)-(D+E)
EXAMPLES/PRACTICE
• All numbers are per 1,000 people:
– e.g. 6 births per 1,000 people would give a birth
rate = 6/1,000
– Why report births per thousand?
How fast can a population grow?
• Fastest rate possible = biotic potential
• Which has greatest biotic potential?
– Humans or fruit flies?
• Fruit flies
– Humans or sparrows?
• sparrows
– Humans or sea turtles?
• Sea turtles
• In general, the more offspring an organism can
have at one time, the greater its biotic potential.
How fast can a population grow?
• Organisms with high biotic potential:
– Tend to:
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•
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•
Be small
Produce lots of offspring at one time
Have short generation times
Become sexually mature early
Populations sometimes grow
exponentially
• Simply put: a population that grows at a
fixed percentage each year.
• = constant rate
Example of exponential growth
• Money in an account earning a fixed rate of interest. e.g.
$100 invested at 5% interest:
• Year
1
2
3
4
5
6
7
8
9
10
15
20
21
balance
change (from previous year)
100
105
110.25
115.76
121.55
127.63
134.01
140.71
147.75
155.13
197.99
252.70
265.33
-5
5.25
5.51
5.79
6.08
6.38
6.70
7.04
7.38
12.63
Human population growth
•
=1 billion
year
What limits population growth?
• Populations don’t grow exponentially
forever.
– Why not? LIMITING FACTORS
– = Limiting resources: one or more natural
resources that are not abundant enough to
support continued growth.
Populations have a carrying
capacity
•
Carrying capacity
is the size of a
population that can
be sustained over
time.
Populations are regulated in 2
ways
• Density dependent: when deaths of
population members are more common in a
crowded population than in a sparse
population. EXAMPLES?
• Density independent: when deaths are
equally likely in a crowded or sparse
population. EXAMPLES?
Species interactions
• 5 main kinds
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–
–
–
Competition
Predation
Parasitism
Mutualism
Commensalism
-/+/+/+/+
+/0
Competition
• What is it?
– Different individuals attempt to use the same
limiting resource(s)
– Such as:
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•
•
•
•
Food
Water
Shelter
Mates
Nesting sites
Competition
• KEY POINT: Both individuals harmed in
competition, because both have reduced
access to a limiting resource, even if one
individual ultimately gets the resource.
• 2nd key point: Competition can be both
intraspecific and interspecific
Competition
• Competition can be indirect
– Two individuals might never come in contact
with each other and still compete
– Examples?
Competition
• How do species avoid interspecific
competition?
– All species have a NICHE.
• NOT ``habitat’’
• But includes habitat, and more
• The way a species uses its resources
Competition
• KEY POINT: The more similar species are
in their niches, the greater the competition
between them.
• To avoid competition, species sometimes
shift their niche
Predation
• One organism feeds upon another, and kills
it.
• Specialists vs generalists
– Specialist examples? Lynx eats mostly
snowshoe hares
– Generalist examples? Praying mantis
Predation
• Specialists
•
Predation
Generalists
Above: praying mantis
Right: crab spider
Both: sit-and-wait predators.
Eat whatever comes along that they
can catch.
Predation
• Prey may adapt to better avoid predation.
• Predators may adapt to better catch prey.
• ``arms race’’
Predation
• Prey adaptations
–
–
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Camouflage
Thorns (physical defense)
Speed
Bad taste/poison (chemical defense)
Predation
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Parasitism
• Like predation, is +/• BUT, unlike predation, parasite doesn’t kill
host.
– Why not?
Parasitism
• Parasites may be
– Internal: e.g., round worms, bacteria,
protozoans, insect larvae
or
– External: e.g., ticks
Parasitism
•
Parasitism
• Special case: parasitoid
– Usually a wasp or fly that lays an egg in an
insect host, which eventually kills the host
• show movie
Mutualism
When two species benefit each other
• +/+ interaction
• Sometimes, the two species (or one of them,
at least) can’t live without the other.
• One example: bacteria in our guts (they
can’t live without us; we might get ill or
lose weight without them)
Mutualism
• Ant-acacia in Central
America
• Many acacias have
chemical defenses
• Some don’t, and these
have evolved a
mutualistic
relationship with
stinging ants, which
live inside the thorns
• Ants defend the tree
agains herbivores; tree
provides food
Commensalism
• One species benefits
• NO EFFECT on the other species
• This is very difficult to prove
Commensalism
• Examples
– Cattle egrets
– Barnacles