Transcript 61BL3313 Population and Community Ecology

61BL3313
Population and Community Ecology
Lecture 09 Interspecific competition
Spring 2013
Dr Ed Harris
Announcements
announcements
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This time
Part II: Interspecific interactions
-introduction
-early experiments
-Lotka-Volterra
-resource competition
-spatial competition and colonization
-evidence of competition in nature
-natural experiments
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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
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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
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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
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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.
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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.
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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
flourished 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
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Part II: Interspecific interactions -early experiments
Paramecium grown seperately
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Part II: Interspecific interactions -early experiments
Paramecium grown together
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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 definition of competition as a reciprocally negative interaction,
meaning that competition has a negative effect, even on the winners
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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 indefinitely
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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 coefficient (αij), which describes the effect of one species on another
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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
α12 = the competition coefficient: effect of species two on species one
α21 = the effect of species one on species two
t = time
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Part II: Interspecific interactions -Lotka-Volterra
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Part II: Interspecific interactions -Lotka-Volterra
The value of the competition coefficient is usually between 0 and 1,
for the following reasons:
- A competition coefficient 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 coefficient were negative, the implication would be that
species two actually benefits 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
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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
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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
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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 effluent rate, which is equivalent to a death rate, m.
The growth rates for two competing species were defined as...
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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 outflow
Ni = concentration of cells in the culture (population size)
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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
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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)
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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
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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 competition–colonization trade-off idea
first 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
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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, εP
When the equation below is set equal to zero and we solve for P:
we have the proportion of habitat sites occupied at equilibrium:
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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
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Part II: Interspecific interactions -evidence of competition in nature
The classic experimental demonstration of competition in the field 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
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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.
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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.
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Part II: Interspecific interactions -evidence of competition in nature
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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”).
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Part II: Interspecific interactions -evidence of competition in nature
Competition in ants
As described by Holldobler and Wilson (1990), first 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 first to arrive at
newly placed baits.
They fill their crops rapidly and hurry to recruit nestmates with odor trails laid
from the rectal sac of the hindgut.
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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 competitors, but excellent dispersers.
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Part II: Interspecific interactions -evidence of competition in nature
Competition in ants
Holldobler and Wilson also emphasize that territorial fighting 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).
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Part II: Interspecific interactions -evidence of competition in nature
Competition in ants
As a final example, the red imported fire ant (Solenopsis invicta) has virtually
eliminated the native fire ant (S. xyloni) from most of its range in the United
States (Holldobler and Wilson 1990).
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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 field experiments are not run for enough time. This deficiency 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.
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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).
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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 field 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.
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Part II: Interspecific interactions -natural experiments
Diamond’s point, however, was that this natural experiment (lizards present on
some islands, absent on others), would have been very difficult 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 fieldwork in 20
days!
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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.
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Part II: Interspecific interactions -natural experiments
When only one species occupies the mountain, without its competitor, it is found
at all elevations.
A field 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.
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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.
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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 13.5°C, whereas P. gonocephala
occupies the warmer portions of the stream from 13.5 to approximately 23°C
(Beauchamp and Ullyott 1932).
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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.
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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 five 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.
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Part II: Interspecific interactions -natural experiments
Character dispalcement
Character displacement is defined 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.
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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 nonoverlapping.
This allows them to feed on different-sized prey items and therefore coexist.
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