Lecture 19 Flashcards
Outcome of competition
hurts both species
Outcome of predation
benefits predators, but hurts prey
Outcome of host-parasite and plant herbivore interactions
same as predation - positive and negative
Outcome of mutualism
helps both species
Interactions between species are often classified by
their outcome (+ or -)
Two main foci of study in ecology and evolution of species interactions
- population dynamics and effects on community structure (how species interactions affect these two things)
- evolutionary dynamics (adaptation and co-evolution)
Intra-specific competition
competition among the members of the same species (i.e. among conspecifics) for resources
Inter-specific competition
competition among members of different species (ie among heterospecifics) for resources
Scramble/exploitative competition
depletion of a shared resource
Contest/interference competition
direct interactions, such as battles over territory
Give an example of interference competition
- Invasive Argentine ants fight a harvester ant in California
- Invasive ants (superior competitors) often drive down populations of native ants
Exploitative competition
- two species do not need to directly interact or even to be active at the same time to compete
- if one consumes a resource, leaving less resource for the other, then they compete
Example of exploitative competition
squirrels and birds, and bird feeders
- squirrel eats food from the feeder and leave no seeds left from the birds
- squirrel is competing successfully with birds by consuming a lot of shared resources
model for inter-specific competition for resources
Lotka-Volterra equations for two species competing for resources
- is a simple outgrowth of logistic equation
- logistic already has a breaking term for intra-specific competition
- Just add a second braking term for
inter-specific competition
give the 4 steps for arriving at the Lotka-Volterra model from a logistic model
- Start with the logistic model for population growth
- Rewrite the logistic model with subscripts to indicate species 1
- Add a term to show effect of species 2 on species 1
- Write matching equation for species 2
give the equation for Lotka-Volterra model
α(ij) =
α(ji)
per-capita effect on i by j
per capita effect on j by i
=> competition coefficient
describe the competition coefficients (α’s)
- fixed for a particular pair of species
- α(12)N(2) converts individuals of species 2 into an equivalent number of individuals of species 1
- eg a squirrel can eat a lot more seeds than a sparrow; a measures how many sparrows-worth of seeds a single squirrel eats
four possible equilibria outcomes of Lotka-Volterra competition
- the two species may stably coexist
- species 1 may always win (N1 = K1, N2 = 0)
- species 2 may always win (N2 = K2, N1 = 0)
- identity of winner may depend on starting N’s
meaning of Equilibrium for Lotka-Volterra competition
N’s are no longer changing
what do the outcomes of the Lotka-Volterra competition depend on?
values of K’s and α’s
coexistence requires
both species to inhibit their own growth more than they inhibit each other’s
define equilibrium
- for a population: size not changing over time (dN/dt = 0)
- for a community: a community not changing over time (in a strict sense: all populations in a community at equilibrium. more generally: constant species composition)
define stability
the ability of a system to return to equilibrium following a perturbation or disturbance
define coexistence
occurs when two or more species have non-zero population sizes at equilibrium
Principle of competitive exclusion
- Lotka-Volterra predicts that for two species to coexist, competition between species must be weaker than competition within a species
- in other words, two species can’t compete too intensely (i.e. overlap too much in resource use/niche space), or one will outcompete the other
- This idea is very old: “As a result of
competition two similar species scarcely ever
occupy similar niches” (Gause 1934) - Or: “Complete competitors cannot coexist”
(Hardin 1960)
Character displacement
coexisting similar species evolve differences to minimise effects of competition on their fitness
- eg Darwin’s finches and beak size
- when finches live on same island, beak size becomes different so that they can eat different sized seeds
Paradox of the plankton
The principle of competitive exclusion
seems false for phytoplankton
- Hutchinson wrote “The problem that
is presented by the phytoplankton is how is it
possible for a number of species to coexist in a relatively … unstructured environment all
competing for the same sorts of materials”
- Phytoplankton need only light, CO2 , N, P, and micro-nutrients to grow
Paradox of the tropical forest
- hundreds of species of trees living in very small areas, despite having the same niche
- how is this possible?
- either every species has a distinct niche or something prevents competitive
exclusion from driving species extinct - this is subject of intense study/debate
how do Lotka-Volterra models relate to the real world?
Experiments by Gause (1930’s)
studied competition among protozoa
in artificial culture vessels; saw both stable
coexistence and competitive exclusion
Gause’s famous competition experiments with Paramecium species in lab culture - draw graphs
How are competitive effects manifested in nature compared to the lab?
- in nature, competitive exclusion is less likely to go to completion
- nonetheless, competition can drastically affect abundances and alter distributions in space
- Biological effects interact with physical
effects: different outcomes in different
environments
Connell, 1961: Field experiments with two barnacle species in the marine intertidal zone
- Zone upper limits set by desiccation
- Lower limits set by competition for space on the rock
- Competition is asymmetrical
- Remove Balanus, Chthamalus extends its distribution down (distribution limited by competition)
- Remove Chthamalus, Balanus does not extend upwards; not competition, but simply can’t tolerate the conditions at the top of the rocks
Resolving the paradox of the
plankton
Lotka-Volterra models too simple, ignore too
much reality, including:
- Most real communities are not at a
competitive equilibrium
- Real populations are kept below carrying
capacity by weather, disease, predators
- Real conditions fluctuate, favouring different species at different times (or in different places)
Scaling up from two populations
to ecological communities
- Competition can affect which and how many
species occur in an ecological community,
which ecologists call community composition
and species richness, respectively - Competition is generally expected to
decrease species diversity (e.g., if a superior
competitor excludes other species) - It is a real challenge to scale up from simple,
species-poor systems (e.g., two Paramecium
in lab cultures) to complex, species-rich
systems (e.g., a whole tropical rainforests)
