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
• Competition prevents occupation of all of fundamental niche. The portion actually occupied in the presence of competition is the realized niche
Fundamental niche sp1
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
Unstable environment, density independent
K
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 long lif e expectancy short lif e expectancy each individu al reproduces only once individu als can reproduce more than once in their lifetim e type III survivor ship pattern in which most of the ind ividua ls die within a short time but a few live much long er type I or II survivor ship pattern in which most indiv iduals live to near the maximum l ife span