Transcript Ecological niche and gradients

Ecological niche and gradients
Why are there so many species?
• How is it that so many species can co-exist?
• Why are some species common and others
rare?
• Why don’t you find mangroves in
freshwater?
• Niche
Eltonian definition of niche
• Niche = doing a job in
a small village.
• Only room for one
baker, one butcher,
etc.
• One species, one
niche, one place, one
time.
Global plant richness
• What patterns are
there here?
• And why do they
exist?
Bird endemic areas
Endemic =
range
restricted to a
relatively
small area
(50,000 km2)
in this case.
Birdlife International recognizes 21 endemic bird
areas (regions with an unusual concentration of
endemic species)
Factors that influence
biodiversity
• Historical factors: e.g. evolutionary history,
response to ice ages.
• Modern environmental gradients: e.g.
temperature, precipitation, soil moisture
deficit, salinity, disturbance, soils.
Environment and competition
• Winner of competition between two competing
species of fish is different at different temperatures.
Species response to
Environmental gradient
Within the range of tolerance
there may be further subdivisions
• Generally better
conditions are needed
for reproduction than
for growth and both of
these need better
conditions than simple
survival.
• Thus reproduction will
limit the actual
occurrence of the sp.
Niche and gradients
• The niche defines the total ecological space in which a
species could survive.
• Ecological conditions vary from one place to another,
e.g. Warmer, drier, higher pH, salinity, better drained,
lack of competitor/predator.
• Often we can think of these as environmental gradients.
Adaptation
• Adaptations are evolved to allow niche specialization.
Environental factors will affect
growth response
• Generation times may be different through the
year.
• As climate changes growth responses will change.
Environmental
gradients as axes
• Organism’s
distribution can be
defined by its range on
an environmental axis
In this example:
Temperature is axis 1
Salinity is axis 2
With 3 gradients/dimensions
• Example corals
– Respond predictably to
light, salinity and
temperature.
• As niche is
represented by a cube
we can think of it as a
volume.
What about all the other
dimensions?
• How many dimensions are there….n
• So we can define a niche as an n-dimensional
hypervolume, first introduced by G.E. Hutchinson (1959).
• This definition is more useful than the “job” definition of
niche offered by Elton as it is predictive and can be
quantified.
• The maximum possible range of a species defined in n
dimensions is its fundamental niche (i.e. in the absence of
competition).
With competition
the niche space
may be reduced
• Competition prevents
occupation of all of
fundamental niche. The
portion actually
occupied in the presence
of competition is the
realized niche
Fundamental
niche sp1
Realized niche with
1 competitor
With competition
the niche space
may be reduced
Fundamental
niche sp1
• Competition prevents
occupation of all of
fundamental niche. The
portion actually
occupied in the presence
of competition is the
realized niche
And now
with 2
competitors
Niche and competition
• Species that compete for the same resource must minimize
competition…..15% difference hypothesis for coexistence.
• Spatial, temporal or physiological separation.
• Large zone of competition, means resource harder to acquire ->
less efficient ->less reproductive success -> lower fitness ->
extinction of lineage.
Back to ecological gradients
• We can predict that
along an ecological
gradient a species will
rise to an optimum,
decline and be
replaced by another
species rising to an
optimum and so on.
One sp. response to a gradient
• Occurrence of
Eragrostis (a
sedge) from
submerged
(left) to high
ground (right).
• Quadrat 2 is at
waterline
Whittaker’s (1960) classic study of the Siskyou Mtns
Demonstrated
individualistic
response to
gradients.
Strongly
Gleasonian
outcome
Coenocline from Boomer Pond
Coenocline: a figure representing the distributions of all species as a function of
environmental gradients. (i.e. all species response curves combined)
r-K strategies
r
K
Unstable environment, density
independent
Stable environment, density
dependent interactions
small size of organis m
energy used to make each individu al
is low
large size of organism
energy used to make each individu al
is high
many offspring are produced
early maturity
few offspring are produced
late maturity, oft en after a prolonged
period of parental care
short lif e expectancy
long lif e expectancy
each individu al reproduces only once individu als can reproduce more than
once in their lifetim e
type III survivorship pattern
type I or II survivor ship pattern
in which most of the ind ividuals die
in which most indiv iduals live to
within a short time but a few live
near the maximum l ife span
much long er