Competition in Animals and Plants Dr. Mark McGinley Honors College and Department of Biological Sciences Texas 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 Principle Both 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
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.
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)
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? Resource Level Population Size R* Time Add fish
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. Species B Species A R*A Resource Level Resource Level R*B Time Time
Competition Between Two Species Species B has positive growth rate. Species A has negative growth rate Both Species A and Species B have negative growth rates Both Species A and B have positive growth rates 0 R*B R*A Resource Level
Competition Between Species Species B wins in competition Species A goes extinct Population Size Species A Species B Time
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 Agrostisscabra(no common common name) Poapratensis(Kentucky blue grass) Andropogongerardii(Big blue stem) Schizachyriumscoparium(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
Agrostisscabra Weedy grass Commonly found in disturbed areaa
Poapratensis(Kentucky blue grass) Weedy grass Introduced to US from Europe
Andropogongerardii(Big blue stem) Dominant grass in tall grass prairie
Schizachyriumscoparium(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
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