61BL3313 Population and Community Ecology - PowerPoint PPT Presentation


Title: 61BL3313 Population and Community Ecology


1
61BL3313Population and Community Ecology
  • Lecture 09 Interspecific competition
  • Spring 2013
  • Dr Ed Harris

2
Announcements announcements
3
This time Part II Interspecific
interactions -introduction -early
experiments -Lotka-Volterra -resource
competition -spatial competition and
colonization -evidence of competition in
nature -natural experiments
4
Part II Interspecific interactions
-introduction The niche Elton (1927) -
subdivision within trophic grouping (carnivore,
herbivore, etc.) Grinnell (1917) - distribution
of species across habitat types Krebs (1994) -
the role or profession of an organism in the
environment its activities and relationships in
the community Begon (1986) - the limits, for all
important environmental features, within which
individuals of a species can survive, grow and
reproduce Hutchinson (1957) - N-dimensional
hypervolume
5
Part II Interspecific interactions
-introduction fundamental niche - The
fundamental niche is the largest ecological niche
that an organism or species can occupy -It is
based mostly on interactions with the physical
environment and is always in the absence of
competition realized niche - that portion of
the fundamental niche that is occupied after
interactions with other species that is, the
niche after competition -the realized niche
must be part of, but smaller than, the
fundamental niche
6
Part II Interspecific interactions -early
experiments Tansley competition shapes
communities -closely related plants living in
the same area often were found in different
habitats, e.g., different soils
7
Part II Interspecific interactions -early
experiments Tansley For his experiment he
selected two species of an herbaceous perennial,
bedstraw, in the genus Galium (Rubiaceae). One
species, G. saxatile, is normally found on peaty,
acidic soils, while the second species, G.
sylvestre, is an inhabitant of limestone soils.
Tansley obtained soils from both areas, planted
each species singly in each soil type and then
placed the two species together in each soil.
He found that each species, when planted alone,
was able to survive in both soils.
8
Part II Interspecific interactions -early
experiments Tansley The fundamental niche for
both species includes both acidic, peat-rich soil
and limestone soil. Growth and germination were
best on the soil where the Galium species was
normally found. When grown together on
limestone soil, G. sylvestre overgrew and
outcompeted G. saxatile. The opposite was true
in the acidic peat soil. At this early date,
Tansley had established that competitive
exclusion could be demonstrated, and that the
results differedby environment.
9
Part II Interspecific interactions -early
experiments Gause - "the struggle for
existence" In a series of experiments with yeast
(Gause 1932) and protozoans, Gause found that
competitive exclusion is observed most often
between two closely related species (two species
in the same genus, for example), when grown in a
simple, constant environment When either
Paramecium caudatum or P. aurelia was
introduced alone, each ?ourished and grew
logistically, leveling off at a carrying capacity
When placed together, however, P. caudatum
diminished and eventually went extinct, while P.
aurelia grew to a steady level
10
Part II Interspecific interactions -early
experiments Paramecium grown seperately
11
Part II Interspecific interactions -early
experiments Paramecium grown together
12
Part II Interspecific interactions -early
experiments Lessons 1. two closely related
species were unable to coexist in the simple
test-tube environment 2. even though we declare
P. aurelia the winner, notice that its
steady state of approximately 300 per 0.5ml
sample is less than the carrying capacity of 500
when this species was grown alone 3. recall the
de?nition of competition as a reciprocally
negative interaction, meaning that competition
has a negative effect, even on the winners
13
Part II Interspecific interactions -early
experiments Gause's theorem A Two species
cannot coexist unless they are doing things
differently B No two species can occupy the same
ecological niche Competitive exclusion
principle Species which are complete
competitors, that is, whose niches overlap
completely, cannot coexist inde?nitely
14
Part II Interspecific interactions
-Lotka-Volterra Modeling interspecific
competition Lotka 1925 and Volterra
1926 -Modeling population growth based on the
logistic growth equation -To model competition
between two species, Lotka and Volterra wrote two
simultaneous equations, one for each
species -Each equation is based on the logistic
equation, but includes a new term, the
competition coef?cient (aij), which describes the
effect of one species on another
15
Part II Interspecific interactions
-Lotka-Volterra N1 the number of individuals
of species one N2 the number of individuals of
species two r1 the intrinsic rate of increase
of species one r2 the intrinsic rate of
increase of species two K1 the carrying
capacity of species one K2 the carrying
capacity of species two a12 the competition
coef?cient effect of species two on species
one a21 the effect of species one on species
two t time
16
Part II Interspecific interactions
-Lotka-Volterra
17
Part II Interspecific interactions
-Lotka-Volterra The value of the competition
coef?cient is usually between 0 and 1, for the
following reasons - A competition coef?cient
of zero would mean that there is no competition
between the two species If that were the case,
there is no reason to try to model this
interaction -If the competition coef?cient were
negative, the implication would be that species
two actually bene?ts the growth rate of species
one The interaction between species one and
two would then be mutualistic -Notice that the
number of individuals of both species one and two
decreases the carrying capacity
18
Part II Interspecific interactions -resource
competition Dave Tillman and "mechanistic
competition" -resouce-based competition
theory -the idea that population growth is
constrained by the depletion of critical
resources, i.e., a population increases until the
supply of a single critical resource becomes
limiting -for example, plant growth may continue
until the amount of phosphorus, nitrogen, light,
or soil moisture becomes limiting
19
Part II Interspecific interactions -resource
competition Dave Tillman and "mechanistic
competition" -E.g., if plant growth is
constrained by phosphorus and a farmer
adds phosphorus fertilizer, plant growth will
continue until another resource, such as
nitrogen, becomes limiting -If the farmer adds
nitrogen, then soil moisture may become the
limiting factor
20
Part II Interspecific interactions -resource
competition According to what is now known as
the R-rule, for any given resource (R), if we
determine the R-value for each species when
grown alone, the species with the lowest R
should competitively exclude all other species,
given enough time and a constant environment. In
deriving their version of the R-rule, Hansen and
Hubbell (1980) assumed that two competitors are
grown in a continuous culture with a continuous
input of a nutrient (R) as well as an ef?uent
rate, which is equivalent to a death rate,
m. The growth rates for two competing species
were de?ned as...
21
Part II Interspecific interactions -resource
competition bi maximum cell division
rate ( rmax) R the concentration of the one
limiting resource in the culture Ki half
saturation constant for the limiting resource m
death rate, here due to out?ow Ni
concentration of cells in the culture (population
size)
22
Part II Interspecific interactions -resource
competition If we do an equilibrium analysis,
and set dNi/dt 0, the result is If we
set Ki R, then bi /2 m
23
Part II Interspecific interactions -resource
competition Thus one solution is that growth
stops when the concentration R equals the
half-saturation constant Conclusions (i) all
competitors die out, or (ii) one species
survives while the second species dies out that
is, when competitive exclusion occurs Which
species survives depends on the critical
parameter, R, which we already saw in the
equation above as R mKi /(b - m)
24
Part II Interspecific interactions -resource
competition Example of R calculations based on
Hansen and Hubbell (1980) K,halfsaturation
constant m, mortality rate b, maximal growth
rate ra, actual growth rate b - m. R mKi
/(b - m) mKi /ra
25
Part II Interspecific interactions -spatial
competition and colonization The idea that
multiple species can coexist in a community
without yielding to the superior competitors can
traced to the competitioncolonization trade-off
idea ?rst proposed by Levins Recall that in a
metapopulation, two species can coexist if one is
a superior competitor and the other is a better
colonize
26
Part II Interspecific interactions -spatial
competition and colonization Remember also that
in a metapopulation the increase in the
proportion, P, of sites occupied by a species was
based on the colonization rate, cP, times the
proportion of sites occupied and available (1 -
P), minus the local extinction or mortality rate,
eP When the equation below is set equal to zero
and we solve for P we have the proportion
of habitat sites occupied at equilibrium
27
Part II Interspecific interactions -spatial
competition and colonization The colonization
rate necessary for equilibrium is
then This basic idea has been generalized
to multi-species situations by Tilman (1994) and
others Termed the spatial-competition
hypothesis, this theory proposes stable
coexistence for inferior competitors in a diverse
community
28
Part II Interspecific interactions -evidence of
competition in nature The classic experimental
demonstration of competition in the ?eld was done
by Joseph Connell (1961) on the barnacle species
Chthamalus stellatus and Balanus
balanoides Balanus is consistently found on
lower rock surfaces, usually near mean tide level
or slightly above Chthamalus, however, is found
on the upper rocks, between mean high neap tide
and mean high spring tide While the adults of
these two barnacle species have non-overlapping
distributions, the larvae of both species settle
over a wide variety of rock surfaces, showing a
great deal of overlap
29
Part II Interspecific interactions -evidence of
competition in nature The question Connell posed
was, is the distribution of adults the result of
competition, or is there a difference in the
fundamental niches of the two species? Connell
performed a variety of experiments in which he
moved the barnacles to different levels of the
intertidal zone. He also experimentally removed
one species or the other where the two were
growing together, and observed the results of
putting the two species together. He found that
whenever he removed Balanus, Chthamalus was able
to survive in the lower regions of the intertidal
zone.
30
Part II Interspecific interactions -evidence of
competition in nature However, in the presence
of Balanus, Chthamalus was overgrown and
eventually displaced. In the upper regions of
the intertidal zone, however, Balanus was unable
to survive the long exposures to air during low
tides. Since Chthamalus was able to survive
this exposure, it survives in the upper
intertidal zone. Thus the two species occupy
mutually exclusive microhabitats due to a
combination of competition and differences in
their fundamental niches.
31
Part II Interspecific interactions -evidence of
competition in nature
32
Part II Interspecific interactions -evidence of
competition in nature Competition in
ants Because both worker and soldier ants are
numerous, easy to observe, and usually diurnal,
aggressive interactions among ant species,
demonstrating interference competition, can be
documented throughout the world (Holldobler and
Wilson 1990). Placing a food bait of tuna or
sugar water will provoke competitive interactions
in a matter of minutes to hours. Once bait is
put out in the West Indies, where there are few
ant species, there is a kind of predictable
sequence, reminiscent of ecological succession (a
kind of ant succession).
33
Part II Interspecific interactions -evidence of
competition in nature Competition in ants As
described by Holldobler and Wilson (1990), ?rst
to arrive are workers of Paratrechina
longicornis, known locally as hormigas
locas(crazy ants). These workers are very
adept at locating food and often are the ?rst to
arrive at newly placed baits. They ?ll their
crops rapidly and hurry to recruit nestmates with
odor trails laid from the rectal sac of the
hindgut.
34
Part II Interspecific interactions -evidence of
competition in nature Competition in ants But
they are also very timid in the presence of
competitors. As soon as more aggressive species
begin to arrive in force, the Paratrechina
withdraw and search for new, unoccupied baits.
Paratrechina is an example of an opportunist
species. They are poor com- petitors, but
excellent dispersers.
35
Part II Interspecific interactions -evidence of
competition in nature Competition in
ants Holldobler and Wilson also emphasize that
territorial ?ghting and ant wars are common,
especially among species with large colonies.
Numerous cases have been documented in which
introduced ant species have eliminated other
species over a few years time. For example, on
Bermuda Iridomyrmex humilis has been replacing
Pheidole megacephala since the former was
introduced in 1953, although the two species may
be reaching equilibrium short of extinction of
Pheidole (Lieberburg et al. 1975).
36
Part II Interspecific interactions -evidence of
competition in nature Competition in ants As a
?nal example, the red imported ?re ant
(Solenopsis invicta) has virtually eliminated the
native ?re ant (S. xyloni) from most of its range
in the United States (Holldobler and Wilson
1990).
37
Part II Interspecific interactions -natural
experiments maniplative field experiments have
some drawbacks (i) The outcome of the
experiment often varies from year to year and
season to season since weather and predators are
uncontrolled. (ii) Most ?eld experiments are not
run for enough time. This de?ciency is, however,
being remedied. For example, the National Science
Foundation (NSF) is addressing this problem in
its Long Term Ecological Studies (LTER)
program. (iii) The importance of large temporal
and spatial scales cannot be addressed in
contemporary time and space.
38
Part II Interspecific interactions -natural
experiments maniplative field experiments have
some drawbacks (iv) A manipulation of two
species may incorrectly ignore the importance of
a third species. (v) The kinds of experiments
that might reveal important information, such as
the removal or introduction of a species in an
ecosystem, are often technically impossible,
morally reprehensible and politically forbidden
(Diamond 1983).
39
Part II Interspecific interactions -natural
experiments In order to solve these problems,
Diamond (1983) extolled the virtues of natural
experiments and other kinds of data gathered
from ?eld observations as opposed to
experiments. According to Diamond, natural
experiments have three advantages First, they
permit an ecologist to rapidly gather data. As an
example, he described the work of Schoener and
Toft (1983). They surveyed spider population on
92 small Bahamian islands, 48 of which lacked
lizards and 26 of which were occupied by at least
one species of lizard. They found that spiders
were ten times more abundant on the islands
without lizards. The explanation was that lizards
are both competitors with and predators on
spiders.
40
Part II Interspecific interactions -natural
experiments Diamonds point, however, was that
this natural experiment (lizards present on some
islands, absent on others), would have been very
dif?cult and time consuming to set up, and we
would have waited a very long time (up to several
years) before the spider populations reached new
equilibrium values. Using the natural
experiments, Schoener and Toft completed their
?eldwork in 20 days!
41
Part II Interspecific interactions -natural
experiments Second, natural experiments allow
ecologists to examine situations they would not
be allowed to set up experimentally. It is
likely, for example, that the Bahamian government
would have objected to having lizards removed
from 48 islands. In another example, Brown
(1971) has shown that two species of chipmunk
(genus Eutamias) divide the forest by altitude
when they are sympatric on mountains in the
Sierra Nevada range. But on several mountains,
probably due to chance colonization or extinction
events, only one species is present.
42
Part II Interspecific interactions -natural
experiments When only one species occupies the
mountain, without its competitor, it is found at
all elevations. A ?eld experiment, in which one
species or the other was eliminated from an
entire mountain, would never have been approved
by the US Fish and Wildlife Service or by any
granting agency. Yet this natural experiment is
an elegant demonstration of the phenomenon known
as ecological release.
43
Part II Interspecific interactions -natural
experiments Ecological release In ecological
release, a species occupies a broader niche or
geographical area in the absence of a closely
related competitor. An example is the
distribution of two species of Planaria in
streams.
44
Part II Interspecific interactions -natural
experiments Ecological release When found
alone in a stream (allopatric distribution) each
species occupies a wide range of stream
temperatures. When both species are found in
the same stream (sympatric distribution),
however, the distribution of both species is
restricted. P. montenegrina is found from 5 to
about 13.5C, whereas P. gonocephala occupies the
warmer portions of the stream from 13.5 to
approximately 23C (Beauchamp and Ullyott 1932).
45
Part II Interspecific interactions -natural
experiments Niche partitioning In niche
partitioning, two or more species coexist while
sharing one or more resources in such a way that
the niche overlap apparently violates the
competitive-exclusion principle. Upon closer
investigation, the resources, though shared, are
used with different frequencies or are used in
different ways so as to allow coexistence.
46
Part II Interspecific interactions -natural
experiments Niche partitioning For example, the
root systems of coexisting annual plants can be
shown to partition the soil by depth, thereby
avoiding direct resource competition (Wieland and
Bazzaz 1975). In his classic study, MacArthur
(1958) showed that ?ve species of Dendroica
warblers coexisted by foraging in different
portions of trees in a coniferous
forest. Although there was overlap, each species
spent the majority of its foraging time in a
unique portion of the trees.
47
Part II Interspecific interactions -natural
experiments Character dispalcement Character
displacement is de?ned as a situation in which
two species, when living in separate geographical
ranges (allopatric distributions), have nearly
identical physical characteristics (i.e., beak
sizes in birds, overall body sizes in lizards and
snails, canine sizes in the cat family). When
sympatric, however, these physical or
morphological characteristics diverge in one or
both species. This divergence minimizes
competition for food and allows the two species
to coexist. Brown and Wilson (1956) appear to
have introduced this idea.
48
Part II Interspecific interactions -natural
experiments Character dispalcement When
examining the overall size and beak lengths of
specimens of the eastern (Sitta tephronota) and
western rock nuthatches (S. neumayer), they found
that the allopatric populations were almost
identical in both average size and in the range
of sizes. However, these two species become
sympatric in Iran. In sympatry, the eastern
rock nuthatch is larger, while the western
species has become smaller. In this sympatric
zone their beak and body sizes are completely
non-overlapping. This allows them to feed on
different-sized prey items and therefore
coexist.
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Title: 61BL3313 Population and Community Ecology


1
61BL3313Population and Community Ecology
  • Lecture 09 Interspecific competition
  • Spring 2013
  • Dr Ed Harris

2
Announcements announcements
3
This time Part II Interspecific
interactions -introduction -early
experiments -Lotka-Volterra -resource
competition -spatial competition and
colonization -evidence of competition in
nature -natural experiments
4
Part II Interspecific interactions
-introduction The niche Elton (1927) -
subdivision within trophic grouping (carnivore,
herbivore, etc.) Grinnell (1917) - distribution
of species across habitat types Krebs (1994) -
the role or profession of an organism in the
environment its activities and relationships in
the community Begon (1986) - the limits, for all
important environmental features, within which
individuals of a species can survive, grow and
reproduce Hutchinson (1957) - N-dimensional
hypervolume
5
Part II Interspecific interactions
-introduction fundamental niche - The
fundamental niche is the largest ecological niche
that an organism or species can occupy -It is
based mostly on interactions with the physical
environment and is always in the absence of
competition realized niche - that portion of
the fundamental niche that is occupied after
interactions with other species that is, the
niche after competition -the realized niche
must be part of, but smaller than, the
fundamental niche
6
Part II Interspecific interactions -early
experiments Tansley competition shapes
communities -closely related plants living in
the same area often were found in different
habitats, e.g., different soils
7
Part II Interspecific interactions -early
experiments Tansley For his experiment he
selected two species of an herbaceous perennial,
bedstraw, in the genus Galium (Rubiaceae). One
species, G. saxatile, is normally found on peaty,
acidic soils, while the second species, G.
sylvestre, is an inhabitant of limestone soils.
Tansley obtained soils from both areas, planted
each species singly in each soil type and then
placed the two species together in each soil.
He found that each species, when planted alone,
was able to survive in both soils.
8
Part II Interspecific interactions -early
experiments Tansley The fundamental niche for
both species includes both acidic, peat-rich soil
and limestone soil. Growth and germination were
best on the soil where the Galium species was
normally found. When grown together on
limestone soil, G. sylvestre overgrew and
outcompeted G. saxatile. The opposite was true
in the acidic peat soil. At this early date,
Tansley had established that competitive
exclusion could be demonstrated, and that the
results differedby environment.
9
Part II Interspecific interactions -early
experiments Gause - "the struggle for
existence" In a series of experiments with yeast
(Gause 1932) and protozoans, Gause found that
competitive exclusion is observed most often
between two closely related species (two species
in the same genus, for example), when grown in a
simple, constant environment When either
Paramecium caudatum or P. aurelia was
introduced alone, each ?ourished and grew
logistically, leveling off at a carrying capacity
When placed together, however, P. caudatum
diminished and eventually went extinct, while P.
aurelia grew to a steady level
10
Part II Interspecific interactions -early
experiments Paramecium grown seperately
11
Part II Interspecific interactions -early
experiments Paramecium grown together
12
Part II Interspecific interactions -early
experiments Lessons 1. two closely related
species were unable to coexist in the simple
test-tube environment 2. even though we declare
P. aurelia the winner, notice that its
steady state of approximately 300 per 0.5ml
sample is less than the carrying capacity of 500
when this species was grown alone 3. recall the
de?nition of competition as a reciprocally
negative interaction, meaning that competition
has a negative effect, even on the winners
13
Part II Interspecific interactions -early
experiments Gause's theorem A Two species
cannot coexist unless they are doing things
differently B No two species can occupy the same
ecological niche Competitive exclusion
principle Species which are complete
competitors, that is, whose niches overlap
completely, cannot coexist inde?nitely
14
Part II Interspecific interactions
-Lotka-Volterra Modeling interspecific
competition Lotka 1925 and Volterra
1926 -Modeling population growth based on the
logistic growth equation -To model competition
between two species, Lotka and Volterra wrote two
simultaneous equations, one for each
species -Each equation is based on the logistic
equation, but includes a new term, the
competition coef?cient (aij), which describes the
effect of one species on another
15
Part II Interspecific interactions
-Lotka-Volterra N1 the number of individuals
of species one N2 the number of individuals of
species two r1 the intrinsic rate of increase
of species one r2 the intrinsic rate of
increase of species two K1 the carrying
capacity of species one K2 the carrying
capacity of species two a12 the competition
coef?cient effect of species two on species
one a21 the effect of species one on species
two t time
16
Part II Interspecific interactions
-Lotka-Volterra
17
Part II Interspecific interactions
-Lotka-Volterra The value of the competition
coef?cient is usually between 0 and 1, for the
following reasons - A competition coef?cient
of zero would mean that there is no competition
between the two species If that were the case,
there is no reason to try to model this
interaction -If the competition coef?cient were
negative, the implication would be that species
two actually bene?ts the growth rate of species
one The interaction between species one and
two would then be mutualistic -Notice that the
number of individuals of both species one and two
decreases the carrying capacity
18
Part II Interspecific interactions -resource
competition Dave Tillman and "mechanistic
competition" -resouce-based competition
theory -the idea that population growth is
constrained by the depletion of critical
resources, i.e., a population increases until the
supply of a single critical resource becomes
limiting -for example, plant growth may continue
until the amount of phosphorus, nitrogen, light,
or soil moisture becomes limiting
19
Part II Interspecific interactions -resource
competition Dave Tillman and "mechanistic
competition" -E.g., if plant growth is
constrained by phosphorus and a farmer
adds phosphorus fertilizer, plant growth will
continue until another resource, such as
nitrogen, becomes limiting -If the farmer adds
nitrogen, then soil moisture may become the
limiting factor
20
Part II Interspecific interactions -resource
competition According to what is now known as
the R-rule, for any given resource (R), if we
determine the R-value for each species when
grown alone, the species with the lowest R
should competitively exclude all other species,
given enough time and a constant environment. In
deriving their version of the R-rule, Hansen and
Hubbell (1980) assumed that two competitors are
grown in a continuous culture with a continuous
input of a nutrient (R) as well as an ef?uent
rate, which is equivalent to a death rate,
m. The growth rates for two competing species
were de?ned as...
21
Part II Interspecific interactions -resource
competition bi maximum cell division
rate ( rmax) R the concentration of the one
limiting resource in the culture Ki half
saturation constant for the limiting resource m
death rate, here due to out?ow Ni
concentration of cells in the culture (population
size)
22
Part II Interspecific interactions -resource
competition If we do an equilibrium analysis,
and set dNi/dt 0, the result is If we
set Ki R, then bi /2 m
23
Part II Interspecific interactions -resource
competition Thus one solution is that growth
stops when the concentration R equals the
half-saturation constant Conclusions (i) all
competitors die out, or (ii) one species
survives while the second species dies out that
is, when competitive exclusion occurs Which
species survives depends on the critical
parameter, R, which we already saw in the
equation above as R mKi /(b - m)
24
Part II Interspecific interactions -resource
competition Example of R calculations based on
Hansen and Hubbell (1980) K,halfsaturation
constant m, mortality rate b, maximal growth
rate ra, actual growth rate b - m. R mKi
/(b - m) mKi /ra
25
Part II Interspecific interactions -spatial
competition and colonization The idea that
multiple species can coexist in a community
without yielding to the superior competitors can
traced to the competitioncolonization trade-off
idea ?rst proposed by Levins Recall that in a
metapopulation, two species can coexist if one is
a superior competitor and the other is a better
colonize
26
Part II Interspecific interactions -spatial
competition and colonization Remember also that
in a metapopulation the increase in the
proportion, P, of sites occupied by a species was
based on the colonization rate, cP, times the
proportion of sites occupied and available (1 -
P), minus the local extinction or mortality rate,
eP When the equation below is set equal to zero
and we solve for P we have the proportion
of habitat sites occupied at equilibrium
27
Part II Interspecific interactions -spatial
competition and colonization The colonization
rate necessary for equilibrium is
then This basic idea has been generalized
to multi-species situations by Tilman (1994) and
others Termed the spatial-competition
hypothesis, this theory proposes stable
coexistence for inferior competitors in a diverse
community
28
Part II Interspecific interactions -evidence of
competition in nature The classic experimental
demonstration of competition in the ?eld was done
by Joseph Connell (1961) on the barnacle species
Chthamalus stellatus and Balanus
balanoides Balanus is consistently found on
lower rock surfaces, usually near mean tide level
or slightly above Chthamalus, however, is found
on the upper rocks, between mean high neap tide
and mean high spring tide While the adults of
these two barnacle species have non-overlapping
distributions, the larvae of both species settle
over a wide variety of rock surfaces, showing a
great deal of overlap
29
Part II Interspecific interactions -evidence of
competition in nature The question Connell posed
was, is the distribution of adults the result of
competition, or is there a difference in the
fundamental niches of the two species? Connell
performed a variety of experiments in which he
moved the barnacles to different levels of the
intertidal zone. He also experimentally removed
one species or the other where the two were
growing together, and observed the results of
putting the two species together. He found that
whenever he removed Balanus, Chthamalus was able
to survive in the lower regions of the intertidal
zone.
30
Part II Interspecific interactions -evidence of
competition in nature However, in the presence
of Balanus, Chthamalus was overgrown and
eventually displaced. In the upper regions of
the intertidal zone, however, Balanus was unable
to survive the long exposures to air during low
tides. Since Chthamalus was able to survive
this exposure, it survives in the upper
intertidal zone. Thus the two species occupy
mutually exclusive microhabitats due to a
combination of competition and differences in
their fundamental niches.
31
Part II Interspecific interactions -evidence of
competition in nature
32
Part II Interspecific interactions -evidence of
competition in nature Competition in
ants Because both worker and soldier ants are
numerous, easy to observe, and usually diurnal,
aggressive interactions among ant species,
demonstrating interference competition, can be
documented throughout the world (Holldobler and
Wilson 1990). Placing a food bait of tuna or
sugar water will provoke competitive interactions
in a matter of minutes to hours. Once bait is
put out in the West Indies, where there are few
ant species, there is a kind of predictable
sequence, reminiscent of ecological succession (a
kind of ant succession).
33
Part II Interspecific interactions -evidence of
competition in nature Competition in ants As
described by Holldobler and Wilson (1990), ?rst
to arrive are workers of Paratrechina
longicornis, known locally as hormigas
locas(crazy ants). These workers are very
adept at locating food and often are the ?rst to
arrive at newly placed baits. They ?ll their
crops rapidly and hurry to recruit nestmates with
odor trails laid from the rectal sac of the
hindgut.
34
Part II Interspecific interactions -evidence of
competition in nature Competition in ants But
they are also very timid in the presence of
competitors. As soon as more aggressive species
begin to arrive in force, the Paratrechina
withdraw and search for new, unoccupied baits.
Paratrechina is an example of an opportunist
species. They are poor com- petitors, but
excellent dispersers.
35
Part II Interspecific interactions -evidence of
competition in nature Competition in
ants Holldobler and Wilson also emphasize that
territorial ?ghting and ant wars are common,
especially among species with large colonies.
Numerous cases have been documented in which
introduced ant species have eliminated other
species over a few years time. For example, on
Bermuda Iridomyrmex humilis has been replacing
Pheidole megacephala since the former was
introduced in 1953, although the two species may
be reaching equilibrium short of extinction of
Pheidole (Lieberburg et al. 1975).
36
Part II Interspecific interactions -evidence of
competition in nature Competition in ants As a
?nal example, the red imported ?re ant
(Solenopsis invicta) has virtually eliminated the
native ?re ant (S. xyloni) from most of its range
in the United States (Holldobler and Wilson
1990).
37
Part II Interspecific interactions -natural
experiments maniplative field experiments have
some drawbacks (i) The outcome of the
experiment often varies from year to year and
season to season since weather and predators are
uncontrolled. (ii) Most ?eld experiments are not
run for enough time. This de?ciency is, however,
being remedied. For example, the National Science
Foundation (NSF) is addressing this problem in
its Long Term Ecological Studies (LTER)
program. (iii) The importance of large temporal
and spatial scales cannot be addressed in
contemporary time and space.
38
Part II Interspecific interactions -natural
experiments maniplative field experiments have
some drawbacks (iv) A manipulation of two
species may incorrectly ignore the importance of
a third species. (v) The kinds of experiments
that might reveal important information, such as
the removal or introduction of a species in an
ecosystem, are often technically impossible,
morally reprehensible and politically forbidden
(Diamond 1983).
39
Part II Interspecific interactions -natural
experiments In order to solve these problems,
Diamond (1983) extolled the virtues of natural
experiments and other kinds of data gathered
from ?eld observations as opposed to
experiments. According to Diamond, natural
experiments have three advantages First, they
permit an ecologist to rapidly gather data. As an
example, he described the work of Schoener and
Toft (1983). They surveyed spider population on
92 small Bahamian islands, 48 of which lacked
lizards and 26 of which were occupied by at least
one species of lizard. They found that spiders
were ten times more abundant on the islands
without lizards. The explanation was that lizards
are both competitors with and predators on
spiders.
40
Part II Interspecific interactions -natural
experiments Diamonds point, however, was that
this natural experiment (lizards present on some
islands, absent on others), would have been very
dif?cult and time consuming to set up, and we
would have waited a very long time (up to several
years) before the spider populations reached new
equilibrium values. Using the natural
experiments, Schoener and Toft completed their
?eldwork in 20 days!
41
Part II Interspecific interactions -natural
experiments Second, natural experiments allow
ecologists to examine situations they would not
be allowed to set up experimentally. It is
likely, for example, that the Bahamian government
would have objected to having lizards removed
from 48 islands. In another example, Brown
(1971) has shown that two species of chipmunk
(genus Eutamias) divide the forest by altitude
when they are sympatric on mountains in the
Sierra Nevada range. But on several mountains,
probably due to chance colonization or extinction
events, only one species is present.
42
Part II Interspecific interactions -natural
experiments When only one species occupies the
mountain, without its competitor, it is found at
all elevations. A ?eld experiment, in which one
species or the other was eliminated from an
entire mountain, would never have been approved
by the US Fish and Wildlife Service or by any
granting agency. Yet this natural experiment is
an elegant demonstration of the phenomenon known
as ecological release.
43
Part II Interspecific interactions -natural
experiments Ecological release In ecological
release, a species occupies a broader niche or
geographical area in the absence of a closely
related competitor. An example is the
distribution of two species of Planaria in
streams.
44
Part II Interspecific interactions -natural
experiments Ecological release When found
alone in a stream (allopatric distribution) each
species occupies a wide range of stream
temperatures. When both species are found in
the same stream (sympatric distribution),
however, the distribution of both species is
restricted. P. montenegrina is found from 5 to
about 13.5C, whereas P. gonocephala occupies the
warmer portions of the stream from 13.5 to
approximately 23C (Beauchamp and Ullyott 1932).
45
Part II Interspecific interactions -natural
experiments Niche partitioning In niche
partitioning, two or more species coexist while
sharing one or more resources in such a way that
the niche overlap apparently violates the
competitive-exclusion principle. Upon closer
investigation, the resources, though shared, are
used with different frequencies or are used in
different ways so as to allow coexistence.
46
Part II Interspecific interactions -natural
experiments Niche partitioning For example, the
root systems of coexisting annual plants can be
shown to partition the soil by depth, thereby
avoiding direct resource competition (Wieland and
Bazzaz 1975). In his classic study, MacArthur
(1958) showed that ?ve species of Dendroica
warblers coexisted by foraging in different
portions of trees in a coniferous
forest. Although there was overlap, each species
spent the majority of its foraging time in a
unique portion of the trees.
47
Part II Interspecific interactions -natural
experiments Character dispalcement Character
displacement is de?ned as a situation in which
two species, when living in separate geographical
ranges (allopatric distributions), have nearly
identical physical characteristics (i.e., beak
sizes in birds, overall body sizes in lizards and
snails, canine sizes in the cat family). When
sympatric, however, these physical or
morphological characteristics diverge in one or
both species. This divergence minimizes
competition for food and allows the two species
to coexist. Brown and Wilson (1956) appear to
have introduced this idea.
48
Part II Interspecific interactions -natural
experiments Character dispalcement When
examining the overall size and beak lengths of
specimens of the eastern (Sitta tephronota) and
western rock nuthatches (S. neumayer), they found
that the allopatric populations were almost
identical in both average size and in the range
of sizes. However, these two species become
sympatric in Iran. In sympatry, the eastern
rock nuthatch is larger, while the western
species has become smaller. In this sympatric
zone their beak and body sizes are completely
non-overlapping. This allows them to feed on
different-sized prey items and therefore
coexist.
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