Transcript Understanding populations

Which of the following is a
population?
Green sea turtles nesting on a beach
2. A flock of cardinals, geese, flamingos, and
sparrows
3. Aquatic insects in the Coosa River
1.

A population is all the members of a
species living in the same place at the
same time.
Properties of populations
1. Size = # of individuals
2. Density = # of individuals/area
3. Dispersion - How members of a population are
spread in space
Regular
Random
Clumped
How does a population grow?
 What adds to a population?
 Births
 Immigration
 What subtracts from a population?
 Deaths
 Emigration
 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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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:
 Yearbalance
1
2
3
4
5
6
7
8
9
10
15
20
21
100
105
110.25
115.76
121.55
127.63
134.01
140.71
147.75
155.13
197.99
252.70
265.33
change (from previous year)
-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.
 Density independent: when deaths are equally likely in
a crowded or sparse population.
How species interaction with
each other
 5 main kinds
 Competition
 Predation
 Parasitism
 Mutualism
 Commensalism
-/+/+/+/+
+/0
Competition
 What is it?
 Different individuals attempt to use the same limiting
resource(s)
 Such as:





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 within and
between species
 When members of different species compete, we say
that their niches overlap.
Competition
 Competition can be indirect
 Two individuals might never come in contact with each
other and still compete
 Examples?
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 mantises and spiders
both eat any vulnerable insect
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.
Predation
 Prey adaptations
 Camouflage
 Thorns (physical defense)
 Speed
 Bad taste/poison (chemical defense)
Predation

Parasitism
 Like predation, is +/ BUT, unlike predation, parasite doesn’t kill host.
 Why not?
Parasitism
 Parasites may be
 Internal: e.g., round worms, bacteria,, protozoan insect
larvae
 External: e.g., ticks
Parasitism
 Schistosomiasis is
disease caused by
parasitic blood flukes
Parasitism
 Special case: parasitoid
 Usually a wasp or fly that lays an egg in an insect
host, which eventually kills the host
QuickTime™ and a
decompressor
are needed to see this picture.
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
against herbivores;
tree provides food
Commensalism
 One species benefits
 NO EFFECT on the other species
Commensalism
 Examples
 Cows and Cattle egrets
 Sharks and remoras
Symbiosis and Coevolution
 A relationship in which two organisms live in close
association is called symbiosis.
 Examples: Honey bees and flowering plants
 Over time, these two organisms may evolve
adaptations that reduce the harm or improve the
benefit of the relationship.