Interspecific competition Flashcards
definition
Organisms of two different species competing for the same limiting resource
2 types of interspecific competition
Interference competition – direct competition for resources involving antagonistic displays and direct physical conflict
Exploitative competition– indirect – when one species consumes or uses up a shared resource more efficiently than the other depleting it for competitors
This is all underpinned by the competitive exclusion principle introduced by Gause (~1930’s)
That two species with an identical niche cannot coexist indefinitely
competitive exclusion principle Gause (~1930’s)
two species with an identical niche cannot coexist indefinitely
the role of interspecific competition
A factor in evolution of sympatric species
Competition regulates diversity – maintaining biodiversity and stability in the environment
This prevents homogenous communities with low biodiversity which are unhealthy and unstable
Ecosystem balance
As a unit the community is more resilient to abiotic/biotic change
Interspecific competition creates evolutionary pressure e.g. lions and cheetahs inhabit different areas of the environment
Modelling a single species: Verhulst’s logistic population method
Verhulst logistic population growth equation
Lotka-Voltera modelled interspecific competition into the population growth equation
^Competition coefficient meaning interspecific competition relative to intraspecific competition
If dn/dt= 0 population is not changing = 0 growth
Modelling interspecific interaction: Lotka-Volterra interspecific competition coefficient method
The effect an individual of species 2 has on the population growth of species 1, also written as α12
The effect an individual of species 1 has onthe population growth of species 2, also written as α21
Interspecific competition - relative to intraspecific
relative to intraspecific:
i.e. how many individuals of species 2 is equivalent toone individual of species 1
- one cheetah is the equivalent of ¾ a lion
- therefore α = 0.75*
- Then multiplied by total cheetah population to calculate overall cheetah effect on the lion population
Zero growth isocline model
e.g. There are both lions (species 1) and cheetahs (species 2) in the Maasai Mara. They compete for some, but not all, of their resources.
K1 is 70 (x-axis)
α is 0.75
therefore K1/α = 93.3 (y-axis)
K2 is 55 (y-axis),β is 0.60
therefore K2 /β = 91.7 (x-axis)
^ resulting in crossed lines and 4th outcome - stable coexistance
4 possible outcomes:
1) Competitive exclusion of species 2 by species 1.
2) Competitive exclusion of species 1by species 2.
^ in 1 and 2 Non-crossing isocline sresults in competitive exclusion of the species with the lower K-value.
3)Coexistence of both species, but only when both populations are experiencing zero growth(where isoclines cross.)It is unstable and vulnerable to change.
Eventually either species 1 or 2 will outcompete the other.
4) Coexistence of both species, regardless of initial population size. Stable equilibrium.
Interspecific < Intraspecific competition
^ in 3 and 4 Coexistence of two species is only possible when two isoclines cross.
Limitations of isocline model
Limitations: makes assumption that carrying capacity rate of growth and competition coefficient constant and all individuals have the same impact, assumes the environment is homogenous
Alternative modelling: Tilman
Tilman competition model: if multiple species are competing for a single limiting resource, then whichever species can survive at the lowest equilibrium resource level (i.e., the R*) can outcompete all other species.
definition of resource by Tilman:
“any substance or factor which is consumed by an organism, and which can lead to increase growth rates as its availability in the environment increases”
3 components;
Resource: requirements, consumption and supply
see Tilman graphs
Methods
- Laboratory / Greenhouse
or - Field
‘An interspecific competition experiment: Manipulation of the abundance of one or more hypothetically competing species’ (Schoener, 1989)
Laboratory example
Altering species proportion to test pair-wise competition e.g.
a) Pairwise competition design to measure competition between two species(Woo, Jiang, 2019
b)Mutual invasion design to measure competition two species (Woo, Jiang, 2019)
Result used to quantify competition coefficients
Once you know if they are in competition you can check for mutual invasion to test if an introduced species can take over an environment with one species already at carrying capacity
+ Test for significance
Lab results using the Lotka-Volterra model
- Use experimental data to calculate carrying capacity, competition coefficient, and growth rate* Zero Net Growth Isocline graphs can then be used to determine competitive status of species
- Niche difference and relative fitness difference values are also commonly calculated from experimental methods
- Resulting non-crossing isoclines show competitive exclusion occurs
Field experiment example
Connells Barnacle experiment found that two species of barnacles occupied different areas of the intertidal, competitive exclusion appeared to cause C to only exist higher up the shore line and removing B confirmed this as in their absence the C barnacles exploited the whole area – confirming the theory
This can be made quantitative by calculating reproductive output and competition coefficient from this.
Unanswered questions
What ecosystems will look like in future:
If an invasive species benefits from anthropogenic landscape change it can have a competitive advantage over native species this is called ecological release – population boom due to being freed from ecologically regulating factors
Intensified interspecific competition due to novel species interactions: food, territory, predation, disease and biotic homogenisation (either due to cross breeding or replacement of the native species with an invasive species)
Human degredation of the environment is affecting species distribution changing biotic and competitive interactions
