Competition in Animals and Plants

Dr. Mark McGinleyHonors College and Department of Biological

SciencesTexas Tech University

Biotic Interactions

• All species live in complex food chains that cause them to interact both directly and indirectly with a large number of other species– Competition– Predation• Predation• Herbivory• Parasitism

– Mutualism

Competition (review)

• Competition occurs – Within species (intraspecific competition)• Can limit population size• Can affect patterns of spatial dispersion

– Between species (interspecific competition)• Can limit population size• Can affect patterns of spatial dispersion• Can influence patterns of diversity• Can act as a selective force on traits

Community Level

• Competitive Exclusion PrincipleBoth theory and data suggests that two species with exactly the same niche can not coexist.

• Law of Limiting Similarity– There is a limit to how similar the niches of two

species can be and still coexist

Competitive Exclusion

Resource partitioning can lead to niche differentiation

Niche Differentiation in Darwin’s Finches

• Species that have similar niches when they are the only species living on an island have evolved to differentiate their niches on islands where they live together

Community Level

– Therefore, if community composition is structured by competition• Niche differentiation• The maximum number of species in a community is

equal to the number of niches– “equilibrium approach” approach to understanding

community structure

Community LevelResource Partitioning

• Animals can partition their niches by– Feeding on different types of food• E.g., insects, seeds, rodents

– Feeding on different sizes of food• E.g., large seeds vs small seeds

– Feed in different places• E.g., feed under shrub or in the open

– Feed at different times• E.g., nocturnal foragers eat insects active at night while

diurnal foragers feed on insects active during the day

MacArthur’s Warblers

• Ecologist Robert MacArthur studied how insectivorous warblers differentiated their niches by feeding on insects in different parts of a tree.

Niche Partitioning in Anolis Lizards

Resource Partitioning in Plants

• All plants rely on the same resources– Sunlight, water, soil nutrients

• Much more difficult for plants to partition resources– E.g., you can’t have a plant that specializes on

“eating” only light and another that specializes on “eating” only water

Resource Partitioning in Desert Shrubs

• Desert plants may be able to partition “water” by “foraging” for water in different ways– E.g., fibrous root system near surface, deep root

system that taps into ground water.

Grasslands

• Often have “mono-specific” stands– Why is the diversity of dominant plants often low

in prairies?

Models of Competition

• Lotka-Volterra Models– Phenomenological model of competition• dN1/dt = r1N1((K1 – N1 – a12N2)/K1)

• dN2/dt = r2N2((K2 – N2 – a21N1)/K2)

Tilman’s Model of Competition For Resources

• Dr. David Tilman, from the University of Minnesota, has developed a number of mechanistic models examining competition for resources

• Tilman is the most cited ecologist of the last 20 years so his work has been quite influential

Relationship Between Resource Level and Growth Rate

Growth Rate 0

Resource Level

Relationship Between Resource Level and Population Growth Rate

• At some resource level the growth rate is positive

• At some resource levels the growth rate is negative

• At one resource level the growth rate equals zero– This level of resources is R* • Called “r star”

R*

• R* is the resource level in the environment at which the population growth rate is equal to zero

Thought Experiment

• What happens if we add fish to an pool full of their favorite food (shrimp)?

• Initial conditions– The pool is full of shrimp– We add only two fish the pool

• Over time, what happens to – The number of shrimp?– The number of fish?

Thought Experiment

• Shrimp are added to the pool by births and removed from the pool by deaths– Adding fish (shrimp predators) to the pool

increases the death rate of the shrimp• By consuming them

– Thus, the number of shrimp should decrease over time

Thought Experiment

• Fish are added to the pool by births and are removed by death– Initially because the number of shrimp is large the

growth rate of the fishes is positive • Population size of the fish increases

What Happens to Resource Level (# of shrimp) and # of Fish Over Time?

Add fishTime

Resource Level

Population Size

R*

The System Eventually Reaches an Equilibrium

• Equilibrium population size of fish– Occurs when birth rate equals the death rate

• Equilibrium population size of shrimp (equilibrial resource level)– Occurs when the rate of supply of the resource

(birth rate) equals the consumption rate (death rate)

What happens if we have two species of fishes competing for shrimp in the same pond?

• First we need to examine what happens if each species lives alone.

ResourceLevel

Time

ResourceLevel

Time

R*A

R*B

Species A Species B

Competition Between Two Species

Resource Level

0 R*B R*A

Both Species Aand Species B have negative growthrates

Species B has positive growthrate. Species Ahas negativegrowth rate

Both Species A and B have positive growth rates

Competition Between Species

Time

Population Size

Species ASpecies B

Species B wins in competitionSpecies A goes extinct

Rule

• When two species are competing for the same single limiting resource the species with the lowest R* always wins– It is able to drive the second species to extinction

by lowering the resource availability so low that the second species has a negative growth rate.

Test of the R* Model in Grasslands

• Dave Tilman and Dave Wedin• Chose to study competition among 4 species

of prairie grasses– Agrostis scabra (no common common name)– Poa pratensis (Kentucky blue grass)– Andropogon gerardii (Big blue stem)– Schizachyrium scoparium (Little blue stem)

Cedar Creek Natural History Area, Minnesota

• Very sandy soil, so it was one of the last parts of the upper midwest of the United States to be colonized.

• Because the soil was not very fertile, many farms were abandoned

• Researchers, led by Dr. David Tilman, have been studying succession in old fields for a number of years.

• By having fields that have been abandoned for different numbers of years can study changes over time using a “chronosequence”.

Cedar Creek Natural History Area,Minnesota, USA

Agrostis scabra

• Weedy grass• Commonly found in

disturbed areaa

Poa pratensis (Kentucky blue grass)

• Weedy grass• Introduced to US from

Europe

Andropogon gerardii (Big blue stem)

• Dominant grass in tall grass prairie

Schizachyrium scoparium (Little blue stem)

• Dominant species in the tall grass prairie

R* Model only applies to systems where species are competing for one limiting resource

• Determined limiting factors by adding a large number of macro and micro nutrients alone and in combination and examining resulting plant growth.

Cedar Creek Nutrient Addition Experiments

• Determined that the only limiting factor was the level of soil nitrogen

• Thus, conditions for applying the R* model were met

Tests of the R* model

• Determined R* for each of the four species by growing species in monocultures in an experimental garden

• Results– Big blue stem and little

blue stem had the lowest R*

– Kentucky blue grass had intermediate R*

– Agrostis had the highest R*

Predictions

• R* model predicts that if two species are competing for a single limiting resource then the species with the lowest R* should win

• Tilman and Wedin did a series of pairwise “battles” between different species– Seeds vs seeds– Seeds vs adult plants– Adult plants vs adult

plants

Who should win in competition between Agrostis and Big blue stem?

Results

• In every case, the species with the lowest R* eventually won in competition

• In some cases it took up to 5 years for this result to occur

• Good support for the R* model

Can We Generalize Across Grasslands?

• If there is often a single limiting resource in grassland ecosystems, then Tilman’s model may help us to understand how competition regulates plant community structure

• Still lots more research needs to be done in a variety of grasslands both in US and elsewhere

Why Do Different Species Have Different R*?

• Tilman and Wedin found a strong correlation between root biomass and R*– Species that produced more roots (big blue stem

and little blue stem) had much lower R*s than species that produced fewer roots (Agrostis)

Plant Strategies

• Plants may have different “resource allocation strategies”– Plants make “decisions” about how to “allocate”

their resources• Roots, shoots & leaves, reproduction

Plant Strategies

• At Cedar Creek, some plants (big blue stem and little blue stem) invest a large a amount of resources to producing roots and a much smaller fraction of their resources to producing seeds– Allows them to be effective competitors for

nitrogen, but does not make them very good at colonizing new habitats.

Plant Strategies

• Other species (Agrostis) invests very little in roots but invests a large proportion of resources into reproduction– Not very good at competition for resources but

are good at colonizing new habitats– Weedy species- good at colonizing disturbed

habitats and then moving on before competition for resources gets to severe

Resource Allocation Trade-Offs

• Because resources can not be allocated to two tissues simultaneously, plants must “decide” how to allocate their resources

• Patterns of resource allocation might strongly influence the “strategy” of a species