Explaining Co-existance in plant communities- R* and resource ratio Flashcards
Competition for resources
All species compete for limiting resources
Gause’s principle of competitive exclusion (1934) – species can’t coexist if share same niche -> competitive exclusion
Spacial patitioning: MacArthur’s warblers (1958) – niche separation in where on coniferous tree they eat insects
Temporal paritioning: Owls and Hawks both rely on similar resources (e.g. small mammals), but hunt at different times of day.
it’s much harder to understand the niches of -> Feeding niche?
R* theory
The level in which concentration of a limiting resource is reduced by an equilibrial monoculture of a species is called R* -> minimum resource concentration required for +ve population growth
The species with lower mimum conc requirments will outcompete the other species
Monoculture experiments used to predict interaction outcomes.
Assumption 1: species compete for a single resource
Assumption 2: the resource is labile (fluctuating not constant- it can be depleted) and the system is well mixed.
Example: Tilman’s atoms
- Photosynthetic, planktonic organisms competing for silica
Example: Grasses in minesota
- He was able to establish the competitive hierarchy by growing these grasses in monoculture and calculating their R* and making predictions.
However:
- Few communities are dominated by one species.
- Only considers one limiting resource while plants have many non-substitutable resources (light, K, N, P)
- Tinbergen’s Resource ratio hypothesis
Assumes 2 non-substitutable vectors.
ZNGIs
- Each species has minimum requirements for two resources R1 and R2 called zero net growth isoclines (ZNGIs)
- These show minimum conc of the 2 resources a species needs to have +ve pop’n growth
**Supply rate **
- the level at which resources are supplied varies and is depicted on the graph
- This effects where vectors hit the ZNGIs
- Only get co-existance when S lies between the two consumption vectors.
COnsumptions vector
- Relative requirement of species for the two limiting resources (which does it need more of to put on 1 unit of growth)
- shows vector of how will draw down resources
- Different species have different consumption vectors.
**Equilibrium **
Where the vector line meets the ZNGI is the equilibirum for that species.
Monoculture equilibrium
- Due to different rates of consumption of each resource, one Rmin is reached first, and another resource is left in excess
Stable co-existance
ZNGIs overlap
STABLE coexistence if species are limited by resource they need the most of (the other species needs less off than them)
The ZNGIs overlap. The resource that limits each species does is above the minimum level required in the other species.
The dots are on the lower lines.
More intraspecific comp > than interspecific competition
No coexistance
If both ZNGI lines of one species are above the other then species with higher ZNGIs will go extinct and be competitively excluded
Species A depletes both resources below a level that species B can survive or vice versa
unstable coexistance
ZNGIs overlap
Unstable coexistence if species are not limited by resource they need the most of (the other species needs less off than them)
The ZNGIs overlap. The resource that limits each species does is below the minimum level required in the other species.
The species that is able to edge agead will outcompete the other and push them to negative growth.
The dots are on the upper lines. ZNGIs overlap
mutual invasibility
When there is stable co-existance there is also mutual invasibility
monocultures of each species can be invaded by each other
When A is in monoculture, the equilibrium availability of resources still allows positive population growth of species B – hence B can invade a monoculture of A and vice versa
Votka- Volterra model
stable co-existance (intra> inter) linkes to votka volterra competition model
a12/ a21/ a11/ a22 describes interaction.
Coexistence occurs when:
K1>K2 α12 AND K2>K1 α21
K1 = K2, then coexistence occurs when the interspecific competition coefficients are < 1 (i.e. interspecific competition is weaker than intraspecific competition).
Co-existance of more than 2 species
More than 2 species can only coexist if resource supply rates vary (env heterogeneity) across spave with each species having different ZNGIs
Only 2 species co-exist, but different combinations co-exist in different regions.
Little evidence
A very famous theory that has not really been tested that well.
Model has only been 5 tests in terrestrial environments.
Overview
Different models to explain plant co-existance:
- R* model
- Tinbergen’s Resource ratio hypothesis
Tinbergen’s Resource ratio hypothesis
- ZNGIs
- Supply rate (must be within vectors)
- consumption vector
3 scenarios
- stable co-existance
- unstable co-existance
- Competitive exclusion
Multiple species can co-exist if there is resource heterogeniety and differet pairs of species can co-exist in different regions.
Limitations of Tinbergen’s Resource ratio hypothesis
- Assumes only 2 limiting resources required by a species -> not the case
- Neglect of non resource factors in co-existance (e.g. space)
- Hard to empiracally test -> few tests have been carried out.
