Transcript Fundamental and realized niches

Today…Competition
Competition occurs both within and between populations.
Competition within a population (intraspecific competition)
is the interaction that limits logistic growth because resources
are limited.
Here we are interested in competition among different species
for similar resources. This interaction is called interspecific
competition.
Interspecific competition is important because it can affect:
1. survival
2. reproduction
3. growth in individual body size
4. population size (and potentially population extinction)
Definition:
“Competition is any use or defense of a resource by one
individual that reduces the availability of that resource to
other individuals” (Ricklefs 2007, p. 365)
Competition has long been the subject of intensive research…
Papers on competition have constituted 5-6% of the
the ecological literature for over 60 years. (Jackson 1981)
A more recent count (Keddy 1989) found papers about
interactions in this ratio:
competition:predation:mutualism
8:
5:
1
Competition occurs when:
Two or more species (or individuals) use (share) the same
resources…
and
Those resources are limited in abundance or availability (at
least some of the time).
To continue a theme…
We know exotic predators can cause extinction of native
prey.
Can exotic competitors eliminate native species through
intense competition? Keep this question in the back of your
mind as we learn about competition and its effects.
The degree to which resources are shared often determines
the intensity of competition.
We describe the way in which species use resources as a key
element of their niche. Species that share resources have
similar ecological niches. Sharing implies niche overlap.
Definition of the niche (from Ricklefs):
“The ecological role of a species in the community; the
ranges of many conditions and resource qualities within
which the organism or species persists, often conceived of as
a multidimensional space.”
Among the components we would need to evaluate to fully
determine the niche of a species are:
•
a habitat component
•
environmental components, and
•
a food component
We measure the relative abundance of a population or
species along axes that describe each of these aspects (and
others) of the niche…
The resulting model, if it has only 3 dimensions, could be
plotted. If it has more than 3 dimensions, it becomes a multidimensional niche as described by Hutchinson.
In one dimension … we measure how well a species does as
some environmental condition varies. Where the species
persists, it is tolerant of the conditions.
The Law of Tolerance
Optimum zone
Zone of
stress
Zone of
intolerance
Low
High
Gradient
Here’s what it might look like for a bird that mostly chooses
to live among shrubs…
E stands for ecotone
And here is real data - Niches in one dimension: optimum
temperature ranges for net photosynthesis in species
originating from various altitudes in the Alps (Pisek et al.
1973)
Note local
adaptive
differences
in the niches
of B. pendula
from different
elevations.
However, one dimension is rarely enough to adequately
describe a niche. Consider a number of bird species sharing
the habitat space you saw just a moment ago. Now let’s add a
niche dimension reflecting use of different food types…
These niches don’t overlap. There would be no competition
among these 3 species, because those that feed on the same
food do so in different habitats..
When there is niche overlap, then competition can occur if
the resource(s) in question are limiting. In this diagram,
species 1 and 2 may compete…
The intensity with which populations or species compete is
often related to the amount of their niches that is in overlap.
These species may compete strongly…
Humidity
y
Here’s how we can depict three-dimensional niches…
x
Temperature
3 axes (= 3 variables)
There are an infinite number of dimensions possible
Typically, differences in competitive ability lead species to
occupy different regions of their niche when living with
competitors.
Fundamental Niche (entire hypervolume: no competitors)
Realized Niche (where a species occurs with competitors)
Fundamental Niche:
- niche of a species in the absence of competitors
- a niche that the species could potentially occupy
Realized Niche:
In the presence of competitors, the species is restricted to
the actual set of conditions under which an organism
exists.
Here’s an example from the beginning of the chapter that
demonstrates how realized niches differ from fundamental
niches…
British ecologist Tansley
(1871-1955) was the 1st
to experimentally show
competition between two
closely related species
The Tansleys visiting the
University of Chicago
Two perennial, herbaceous plants (bedstraw) used in
a common garden experiment
Galium saxatile: normally
found on acid soils
in peat bogs
G. sylvestre: normally found
on limestone hills
& pastures. Limestone
makes the soils basic.
Two species were grown alone
and in a mixture with the
other species on both soil
types.
Galium saxatile
RESULTS
Ricklefs (2001,
Fig. 19.1)
G. saxatile alone
grows best in
acid soils as
observed
in nature
G. sylvester alone
grows best in
limestone soils
as observed
in nature
When 2 are
grown together,
G. saxatile prevails
on acid soils
When 2 are
grown together,
G. sylvester prevails
on limestone soils
Limestone
Acid
Soils
Tansley’s Conclusions:
• the presence or absence of a species can be
determined by competition with other species
• conditions of the environment affect the outcome
of competition
• competition may be felt broadly throughout
the community
• the present segregation of species may have
resulted from past competition ….
“the ghost of competition past”
What happens to population size when species compete?
The result depends on their tolerances, their efficiency in
using a resource they share and its abundance …
In the previous slide, species B reached its carrying capacity,
but the population of species A would be depressed to a size
significantly below its K. When both competing species
persist, the result is called competitive coexistence.
In many other cases, both species in a competitive interaction
are depressed to a population size lower than they would
have reached if growing alone. First, if growing alone…
Now, what do we observe when they grow together? This is
one possibility.
both species persist
Another is that the species that is ‘weaker’ in competition is
driven extinct by the interaction.
one species driven extinct
After theoretical predictions; there is experimental evidence:
Gause did experiments on competition between two
Paramecium species. His observations were that one species,
P. aurelia, outcompeted and drove the other, P. caudatum, to
extinction. This result is called competitive exclusion.
Bacteria are the limiting food resource for Paramecia. The
species that ‘wins’ draws the level of food available down to
an abundance insufficient for the ‘losing’ species.
Another example of competitive exclusion comes from your
text (p.371). Here is a picture of citrus scale. It a serious and
damaging pest in California citrus orchards.
It was an example of biological control. Three different
parasitoids were introduced to control it. They show clear
evidence of geographical competitive exclusion…
An aside:
Remember Leibig’s law of the minimum (Justus von
Leibig, 1840). It says that there is, in the end, some single
resource, the limiting resource, whose supply no longer
meets the population’s need for it.
Does this law always apply? No! We came up with an
alternative before. Here the evidence of competition brings
the simple interpretation of the ‘law’ into question.
Frequently resources interact in affecting population
growth. Your text provides an example: Peace and Grubb
(1982)…
Two resources may enhance the growth of the
consumer more than the sum of each resource.
Peace & Grubb 1982 New Phytol. 90:127-150.
• Designed a 2 (fertilized) x 4 (light level)
factorial experiment to examine growth
of Impatiens
• Both factors are resources need by plants:
N & P for protein synthesis
C assimilated by photosynthesis
for carbohydrates
• Interesting interaction effects
http://www.botany.com/
impatiens.html
The two fertilizers used were nitrogen and phosphorus. Both,
considered separately, enhanced plant growth (dry weight),
but together they increased growth compared to controls
much more than either alone. The two fertilizers are
synergistic.
Next, add the effect of light intensity…
Fertilized plants use light resources
more efficiently than unfertilized plants
Ricklefs (2001, Fig. 19.4)
You may think (logically) that the intensity of competition
should lessen as resources become more abundant. This is
the view of some prominent ecologists, e.g. P.J. Grubb and
David Tilman.
However, an opposing view, that the intensity of
competition increases with resource abundance, also has
important adherents, including J.P. Grime and Paul Keddy.
The explanation of that counterintuitive view is that at low
resource abundance (lack of water or mineral nutrients)
plants are more widely spaced, and that will limit the
intensity of competition both for soil resources and light.
Opposing views can be tested experimentally…
Here’s evidence that competition can increase with resource
abundance (in a desert annual grass)…
Water is the resource
controlled in this
experiment.
As standing crop increased (evidence of greater water
availability, which would seemingly reduce competition in a
desert environment) the intensity of competition instead
increased. Why? (Hint: competition occurred above ground)
The opposing view was tested by David Tilman in studies of
competition among three species of prairie grasses. Plots
were fertilized with ammonium nitrate at three levels: low,
medium, and high. The intensity of competition did not vary
much. What changed was the nature of the competition…
At low nutrient level, the competition among below ground
parts (roots) was fierce for the limiting nitrogen.
At high nutrient level, the competition was high among the
much larger above ground plant parts (stems and leaves). The
limiting factor became sunlight and space.
Back to the main theme – competition…
The competition between Paramecium species for food is
called exploitative competition or exploitation.
Definition:
A competitive interaction in which each species
consumes the shared resource, thus reducing the amount
remaining to be used by the other species.
It occurs when individuals compete indirectly through their
mutual effects on shared resources. They don’t ‘fight’ with
each other; they use resources to their individual best ability.
There is an alternative form of interspecific competition that
is called interference competition.
Definition:
Interference occurs when one species sequesters or defends a
resource without consuming it. It interferes with access to the
resource by the other species.
Usually the interference occurs by means of antagonistic
behaviour. This is direct interaction. It is only likely when it
is profitable for one species to defend resources. There is no
evidence that it occurs spitefully.
Previous examples (competition between P. aurelia and P.
caudatum or between species of Galium or bedstraw) are
occurrences of exploitative competition.
An example of interference competition:
Competition for space frequently occurs by means of
interference. Allelopathy among plants provides classic
examples. The text describes the protection of space around
individual shrubs of sage (Salvia spp.) in California.
Allelopathy occurs by leaves producing volatile terpenes (or
other inhibitory compounds) that wash and evaporate off into
the air and soil beneath and around the plants.
The result is a bare zone and an inhibition zone around the
plants.
sage
bare zone
suppressed zone
A Review before going on:
Interspecific competition occurs when two species share
resource(s) and those resources are available only in short
supply.
Resource sharing is evident in niche overlap among
competing species.
Competitive interactions can take the form of exploitative
or interference interactions.
The classic experiments demonstrate occurrence of
competitive exclusion or the persistence of both species in
competitive coexistence.