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Competition in animals and plants


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  • 1. Competition in Animals and Plants
    Dr. Mark McGinley
    Honors College and Department of Biological Sciences
    Texas Tech University
  • 2. 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
  • 3. 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
  • 4. 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
  • 5. Competitive Exclusion
  • 6. Resource partitioning can lead to niche differentiation
  • 7. 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
  • 8. 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
  • 9. 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
  • 10. MacArthur’s Warblers
    Ecologist Robert MacArthur studied how insectivorous warblers differentiated their niches by feeding on insects in different parts of a tree.
  • 11. Niche Partitioning in Anolis Lizards
  • 12. 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
  • 13. 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.
  • 14. Grasslands
    Often have “mono-specific” stands
    Why is the diversity of dominant plants often low in prairies?
  • 15. Models of Competition
    Lotka-Volterra Models
    Phenomenological model of competition
    dN1/dt = r1N1((K1 – N1 – a12N2)/K1)
    dN2/dt = r2N2((K2 – N2 – a21N1)/K2)
  • 16. 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
    • 17. 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
    Resource Level
  • 18. 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”
  • 19. R*
    R* is the resource level in the environment at which the population growth rate is equal to zero
  • 20. 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?
  • 21. 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
  • 22. 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
  • 23. What Happens to Resource Level (# of shrimp) and # of Fish Over Time?
    Add fish
  • 24. 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)
  • 25. 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
  • 26. 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
    Both Species A and B
    have positive growth rates
    Resource Level
  • 27. Competition Between Species
    Species B wins in competition
    Species A goes extinct
    Species A
    Species B
  • 28. 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.
  • 29. 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)
  • 30. 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”.
  • 31. Cedar Creek Natural History Area,Minnesota, USA
  • 32. Agrostisscabra
    Weedy grass
    Commonly found in disturbed areaa
  • 33. Poapratensis(Kentucky blue grass)
    Weedy grass
    Introduced to US from Europe
  • 34. Andropogongerardii(Big blue stem)
    Dominant grass in tall grass prairie
  • 35. Schizachyriumscoparium(Little blue stem)
    Dominant species in the tall grass prairie
  • 36. 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.
  • 37. 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
  • 38. Tests of the R* model
    Determined R* for each of the four species by growing species in monocultures in an experimental garden
    Big blue stem and little blue stem had the lowest R*
    Kentucky blue grass had intermediate R*
    Agrostis had the highest R*
  • 39. 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?
  • 40. 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
  • 41. 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
  • 42. Why Do Different Species Have Different R*?
    • Tilman and Wedin found a strong correlation between root biomass and R*
    • 43. 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
  • 44. 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.
  • 45. 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
  • 46. 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