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FROM Wednesday - end of lecture on
comparative life histories:
A laboratory evolution experiment - effects
of different rates of adult mortality on life
history traits in Drosophila (fruit flies)
Question: In environments where there is
high adult mortality (e.g. high predation)
what happens to other aspects of
Drosophila life history?
Experimenters set up selection lines - in some
removed adults 2 x weekly (“High adult
mortality treatment”), in others didn’t (“Low
adult mortality treatment”) kept all other
factors constant
Kept selection up from 1993-1998
Effect of high adult mortality on
amount of early reproduction:
Some of their predictions: When adult
mortality is high, populations should
evolve
Which treatment reproduces earlier? Do
they do it from the start of the
experiment?
Higher fecundity early in life
Shorter development times
Smaller adult size
High mortality
Low mortality
Effect of high adult mortality on larval
development time:
Effect of high adult mortality on body
size:
Which treatment has a shorter larval
development time?
Which treatments’ adults are smaller?
High
mortality
Low
mortality
High mortality
Low mortality
Months
Months
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Population regulation and
competition
Summary
Life history evolution studies show some
mechanisms by which animal and plant
populations diversify in response to their
environment.
Some patterns found are general to many
organisms. For example, the manipulative
selection experiment in Drosophila shows
a pattern seen in other descriptive
experiments:
High adult mortality leads to earlier
reproduction, shorter development time,
and smaller body size
I. Population regulation
Density-dependent and densityindependent factors
II. Intraspecific competition
III. Introduction to species interactions
Types of interactions
The ecological niche
IV. Interspecific competition
Competitive displacement, exclusion,
and dominance
In 1983, a couple of naturalists
returned to the same village…
A naturalist observed swifts in his village in
England...
"I am now confirmed in the opinion that we
have every year the same number of pairs
invariably; at least the result of my inquiry has
been exactly the same for a long time past.
The number that I constantly find are eight
pairs, about half of which reside in the church,
and the rest in some of the lowest and
meanest thatched cottages. Now as these
eight pairs - allowance being made for
accidents - breed yearly eight pairs more,
what becomes annually of this increase?"
Fewer thatched cottages, no swifts in the
church, but a total of twelve pairs overall. A net
change of 4 pairs of swifts in 205 years...
White 1778
We are likely to focus on dramatic
changes in population densities:
So, what regulates
populations? i.e. keeps them
within certain limits?
increases such as locusts, gypsy
moths, lemmings, or humans
decreases such as buffalo,
condors, passenger pigeons
But most populations are, like the
swifts, remarkably constant
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We can categorize the factors that
influence populations as
What’s a density-dependent factor?
density-dependent factors
and density-independent factors
What’s a density-dependent factor?
A factor that increases in intensity
as density (population size per unit
area) increases.
Specifically, this means the
proportion of the population
affected increases with density.
Is flooding a density-dependent
mortality factor acting on the
grass?
A (made-up) example.
We want to know if flooding acts as
a density-dependent mortality
factor of a certain grass growing in
a wash.
‘91
‘95
From 1989 to 1995 we go out each
year after the monsoon, count the
number of grass plants that are
alive, and the no. dead from
flooding. Then we plot our data.
‘89
No.
dead
‘94
‘92
‘90
‘88
Density (population size)
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Because we expect a larger number
to die as the population size
becomes large. If we change the y
axis to proportion dead...
If a factor is density-dependent
it should affect the population in
this way...
We can see there is no
increase in proportional
mortality with an increase
in density.
Prop.
dead
‘88
‘90
‘94
‘89
Prop.
dead
‘95
‘92
‘91
Density (population size)
Density (population size)
Why do we care about densitydependent factors?
Factors which may act in a densitydependent manner?
Only density-dependent factors can
regulate populations. Other factors
can have effects on population
dynamics, but don’t predictably
‘rein in’ populations at high
densities.
Competition intraspecific (within species)
interspecific (between species)
Factors which may act in a densitydependent manner?
Factors which may act in a densitydependent manner?
Predation and herbivory
predators may be more likely
to find abundant prey types
Disease caused by pathogens
Contagious diseases are better
transmitted at high density
predator reproduction greater
when abundant food
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Density-independent mortality factors
can be important in a population’s
dynamics, even if they are not
regulating. Example: winter mortality of
herons
Factors which are generally density
-independent?
Weather - rain, wind, drought,
freezes
Fire
5000
No. of
nesting
pairs
= Severe winters
‘28
‘32
‘36
‘40
‘44
‘48
‘52
‘56
‘60
‘64
‘68
Year
Experimental demonstration of densitydependent parasitism: Goldenrod gall fly
Goldenrod gall fly - an experiment
Adult lays eggs in stems of goldenrod
Larval fly induces the formation of
a gall
Fly overwinters
in gall
All the galls in several fields
gathered at the end of a season,
put in an unheated shed for the
winter
In the spring, galls put at one of two
densities back into fields
Most important
mortality agent a
parasitic wasp
that also spends
the winter in the
gall
High density
fields
Goldenrod gall fly - an experiment
Low density
fields
Goldenrod gall fly - an experiment
Experimenter measured the parasitism
of the new season galls in the high and
low density fields
If parasitism density-dependent, what
would pattern would you expect to see?
Results: Greater proportion parasitized in
high density fields than in low density
fields
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Intraspecific competition increases
death rates. Example: Winter moth
Proportion surviving
II. Intraspecific competition
Competition reduces the
contribution of the competitors to
the next generation through
increased death rates or decreased
birth rates
Numbers of eggs in spring
Not a winter moth
Intraspecific competition reduces birth
rates - example in longhorn
cattle
III. Species interactions - an
introduction
Species interactions can be
classified according to the net
effect on the species involved,
positive or negative
1.0
Young
per
female 0.8
0
110
Density of longhorns
Mutualisms are +/+ interactions both partners benefit
In commensalism one partner
benefits, and the other is unaffected
(+/0)
Example: Pollinators and flowers
more examples to come in next
lecture...
Example - cattle
egrets feed on
insects that cattle
dislodge. Cattle are
unaffected by the
egrets.
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In amensalism one partner is harmed,
and the other is unaffected (-/0)
In predation and parasitism one partner
benefits, and the other is harmed (+/-)
Examples - Large animals may negatively
influence plants near water holes. Larger
herbivores may incidentally consume
smaller herbivores like insects, etc.
Predation
In competition, both partners are
harmed (-/-)
Parasitism
Exploitative vs. interference
competition
Exploitative competition:
competition through exploitation of
a common resource. Winners of
competition appropriate more of
the resource.
For example, gila
woodpeckers and
starlings compete for nest
cavities
Exploitative competition: Example
Competition between trees for light
*
*
C. Exploitative vs. interference
competition
Seedlings
Slight disadvantage
of some seedlings
(e.g. late germination,
slower growth rate)
becomes exaggerated
as lose access to light.
Interference competition:
Competitors directly interfere with
each other. The victor is the one
that wins the contest.
Mature
trees
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Interference competition. Example:
Interactions with other species may
affect a species’ ecological niche.
Young larvae of
parasitic wasps are
often equipped
with large mandibles
used to kill competitors
in the same insect host.
What’s a niche?
The ecological niche
Many definitions in the literature
Two kinds of niches…
A simple one: The niche is the sum
of the ecological requirements of a
particular species. E.g., habitat
requirements, food requirements,
climatic requirements, etc.
The fundamental niche - that which
a species could use in the absence
of predation or competition
Two kinds of niches…
Two kinds of niches…
The fundamental niche - that which
a species could use in the absence
of predation or competition.
Which is likely to be bigger? The
fundamental niche or the realized
niche?
The realized niche - what the
species uses when predators and
competitors are present
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Niches in the presence of
interspecific competition - a classic
example from the intertidal region
on the coast of Scotland
Interspecific competition in
barnacles
Intertidal region area between high
and low tides sometimes under
water, sometimes
dry
vs.
Barnacles (crustaceans, Arthropoda) start life as
free-swimming larval forms. They then settle on
rocks and grow in size by molting, but do not move.
Researcher found two
species:
Results of manipulation
Balanus - covered by
water most of the time
Effects on Balanus - the one covered by
water most of the time
Chthamalus - exposed
most of the time
Distribution didn’t change when
Chthamalus removed - couldn’t colonize
area ordinarily occupied by Chthamalus
because too dry
From different areas he
removed all adults of one
or other species, and
continued to remove their
settling larvae.
Results of manipulation
So what does this have to do with
niches?
Effects on Chthamalus - the one exposed
most of the time
When Balanus removed, Chthamalus
colonized the entire region. Ordinarily
Balanus would out-compete Chthamalus
in the lower regions by growing over them.
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So what does this have to do with niches?
So what does this have to do with niches?
Chthamalus
Balanus
The niche occupied by Balanus is unaffected by
Chthamalus
The niche occupied by Chthamalus is reduced in the
presence of Balanus
The barnacle study provides an
example of competitive
displacement: one species niche
was contracted in the presence of
the other.
What do you think would have
happened to Chthamalus (the
exposed species) if both species
had the same fundamental niche?
What do you think would have
happened to Chthamalus (the
exposed species) if both species
had the same fundamental niche?
The principle of competitive
exclusion: when two species
occupy the same fundamental
niche, they cannot coexist.
Chthamalus would have become
extinct wherever Balanus was.
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Competitive exclusion: An example
from the Sonoran desert
Competitive exclusion: An
example from the Sonoran desert
In plots where the kangaroo rats were
excluded:
Kangaroo rat
Graduate student at the U of A
looked at competition for seeds
among seed feeders
Copyright (c) Grolier Interactive Inc.
1) Competition among rodents
There were more species of small
rodents, and they were more abundant
Put fences around areas with holes
that excluded kangaroo rats,
allowed smaller rodents to pass
So, kangaroo rats competitively exclude
other rodents
Competitive dominance. Another
example of competition for seeds in
the desert.
2) Are ants competing with rodents
for seeds?
Competitive dominance. Perhaps
most commonly, competition leads
simply to the greater abundance of
one species than the other.
Harvester ant
with large seed
2) Are ants competing with rodents
for seeds?
2) Are ants competing with rodents
for seeds?
Removed ants from some plots,
rodents from others, kept some as
control plots.
Rodents
removed
Ants
removed
Control
plots
No. ant
Colonies
543
0
318
No. of
Rodents
0
144
122
Yes. Both kinds of seed predators
are more abundant when the other
kind is removed.
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What species dominates in competition
may depend on the environment. An
example with flour beetles.
Experimenter put the same 2 species of
beetles together at 32°C - A and B
Experimenter put 2 species of beetles
together at 29°C - A and B
B
No.
beetles
At 32°C, B wins
At
At29
29°C, A wins
A
A
No.
beetles
Days
B
Why the reversal in outcome? A grows faster than
B at 29°C, and B grows faster than A at 32°C
Days
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