Competition in animals and plants

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Competition in animals and plants

  1. 1. Competition in Animals and Plants<br />Dr. Mark McGinley<br />Honors College and Department of Biological Sciences<br />Texas Tech University<br />
  2. 2. Biotic Interactions<br />All species live in complex food chains that cause them to interact both directly and indirectly with a large number of other species<br />Competition<br />Predation<br />Predation<br />Herbivory<br />Parasitism<br />Mutualism<br />
  3. 3. Competition (review)<br />Competition occurs <br />Within species (intraspecific competition)<br />Can limit population size<br />Can affect patterns of spatial dispersion<br />Between species (interspecific competition)<br />Can limit population size<br />Can affect patterns of spatial dispersion<br />Can influence patterns of diversity<br />Can act as a selective force on traits<br />
  4. 4. Community Level<br />Competitive Exclusion Principle<br />Both theory and data suggests that two species with exactly the same niche can not coexist.<br />Law of Limiting Similarity<br />There is a limit to how similar the niches of two species can be and still coexist<br />
  5. 5. Competitive Exclusion<br />
  6. 6. Resource partitioning can lead to niche differentiation<br />
  7. 7. Niche Differentiation in Darwin’s Finches<br />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<br />
  8. 8. Community Level<br />Therefore, if community composition is structured by competition<br />Niche differentiation<br />The maximum number of species in a community is equal to the number of niches<br />“equilibrium approach” approach to understanding community structure<br />
  9. 9. Community LevelResource Partitioning<br />Animals can partition their niches by<br />Feeding on different types of food<br />E.g., insects, seeds, rodents<br />Feeding on different sizes of food<br />E.g., large seeds vs small seeds<br />Feed in different places<br />E.g., feed under shrub or in the open<br />Feed at different times<br />E.g., nocturnal foragers eat insects active at night while diurnal foragers feed on insects active during the day<br />
  10. 10. MacArthur’s Warblers<br />Ecologist Robert MacArthur studied how insectivorous warblers differentiated their niches by feeding on insects in different parts of a tree.<br />
  11. 11. Niche Partitioning in Anolis Lizards<br />
  12. 12. Resource Partitioning in Plants<br />All plants rely on the same resources<br />Sunlight, water, soil nutrients<br />Much more difficult for plants to partition resources<br />E.g., you can’t have a plant that specializes on “eating” only light and another that specializes on “eating” only water<br />
  13. 13. Resource Partitioning in Desert Shrubs<br />Desert plants may be able to partition “water” by “foraging” for water in different ways<br />E.g., fibrous root system near surface, deep root system that taps into ground water.<br />
  14. 14. Grasslands<br />Often have “mono-specific” stands<br />Why is the diversity of dominant plants often low in prairies?<br />
  15. 15. Models of Competition<br />Lotka-Volterra Models<br />Phenomenological model of competition<br />dN1/dt = r1N1((K1 – N1 – a12N2)/K1)<br />dN2/dt = r2N2((K2 – N2 – a21N1)/K2)<br />
  16. 16. Tilman’s Model of Competition For Resources<br /><ul><li>Dr. David Tilman, from the University of Minnesota, has developed a number of mechanistic models examining competition for resources
  17. 17. Tilman is the most cited ecologist of the last 20 years so his work has been quite influential</li></li></ul><li>Relationship Between Resource Level and Growth Rate<br />Growth Rate<br />0<br />Resource Level<br />
  18. 18. Relationship Between Resource Level and Population Growth Rate<br />At some resource level the growth rate is positive<br />At some resource levels the growth rate is negative<br />At one resource level the growth rate equals zero<br />This level of resources is R* <br />Called “r star”<br />
  19. 19. R*<br />R* is the resource level in the environment at which the population growth rate is equal to zero<br />
  20. 20. Thought Experiment<br />What happens if we add fish to an pool full of their favorite food (shrimp)?<br />Initial conditions<br />The pool is full of shrimp<br />We add only two fish the pool<br />Over time, what happens to <br />The number of shrimp?<br />The number of fish?<br />
  21. 21. Thought Experiment<br />Shrimp are added to the pool by births and removed from the pool by deaths<br />Adding fish (shrimp predators) to the pool increases the death rate of the shrimp<br />By consuming them<br />Thus, the number of shrimp should decrease over time<br />
  22. 22. Thought Experiment<br />Fish are added to the pool by births and are removed by death<br />Initially because the number of shrimp is large the growth rate of the fishes is positive <br />Population size of the fish increases<br />
  23. 23. What Happens to Resource Level (# of shrimp) and # of Fish Over Time?<br />Resource <br />Level<br />Population <br />Size<br />R*<br />Time<br />Add fish<br />
  24. 24. The System Eventually Reaches an Equilibrium<br />Equilibrium population size of fish<br />Occurs when birth rate equals the death rate<br />Equilibrium population size of shrimp (equilibrial resource level)<br />Occurs when the rate of supply of the resource (birth rate) equals the consumption rate (death rate)<br />
  25. 25. What happens if we have two species of fishes competing for shrimp in the same pond?<br />First we need to examine what happens if each species lives alone. <br />Species B<br />Species A<br />R*A<br />Resource<br />Level<br />Resource<br />Level<br />R*B<br />Time<br />Time<br />
  26. 26. Competition Between Two Species<br />Species B has <br />positive growth<br />rate. Species A<br />has negative<br />growth rate<br />Both Species A<br />and Species B have <br />negative growth<br />rates<br />Both Species A and B <br />have positive growth rates<br />0<br />R*B<br />R*A<br />Resource Level<br />
  27. 27. Competition Between Species<br />Species B wins in competition<br />Species A goes extinct<br />Population <br />Size<br />Species A<br />Species B<br />Time<br />
  28. 28. Rule<br />When two species are competing for the same single limiting resource the species with the lowest R* always wins<br />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.<br />
  29. 29. Test of the R* Model in Grasslands<br />Dave Tilman and Dave Wedin<br />Chose to study competition among 4 species of prairie grasses<br />Agrostisscabra(no common common name)<br />Poapratensis(Kentucky blue grass)<br />Andropogongerardii(Big blue stem)<br />Schizachyriumscoparium(Little blue stem)<br />
  30. 30. Cedar Creek Natural History Area, Minnesota<br />Very sandy soil, so it was one of the last parts of the upper midwest of the United States to be colonized.<br />Because the soil was not very fertile, many farms were abandoned<br />Researchers, led by Dr. David Tilman, have been studying succession in old fields for a number of years.<br />By having fields that have been abandoned for different numbers of years can study changes over time using a “chronosequence”.<br />
  31. 31. Cedar Creek Natural History Area,Minnesota, USA<br />
  32. 32. Agrostisscabra<br />Weedy grass<br />Commonly found in disturbed areaa<br />
  33. 33. Poapratensis(Kentucky blue grass)<br />Weedy grass<br />Introduced to US from Europe<br />
  34. 34. Andropogongerardii(Big blue stem)<br />Dominant grass in tall grass prairie<br />
  35. 35. Schizachyriumscoparium(Little blue stem)<br />Dominant species in the tall grass prairie<br />
  36. 36. R* Model only applies to systems where species are competing for one limiting resource<br />Determined limiting factors by adding a large number of macro and micro nutrients alone and in combination and examining resulting plant growth.<br />
  37. 37. Cedar Creek Nutrient Addition Experiments<br />Determined that the only limiting factor was the level of soil nitrogen<br />Thus, conditions for applying the R* model were met<br />
  38. 38. Tests of the R* model<br />Determined R* for each of the four species by growing species in monocultures in an experimental garden<br />Results<br />Big blue stem and little blue stem had the lowest R*<br />Kentucky blue grass had intermediate R*<br />Agrostis had the highest R*<br />
  39. 39. Predictions<br />R* model predicts that if two species are competing for a single limiting resource then the species with the lowest R* should win<br />Tilman and Wedin did a series of pairwise “battles” between different species<br />Seeds vs seeds<br />Seeds vs adult plants<br />Adult plants vs adult plants<br />Who should win in competition between Agrostis and <br />Big blue stem?<br />
  40. 40. Results<br />In every case, the species with the lowest R* eventually won in competition<br />In some cases it took up to 5 years for this result to occur<br />Good support for the R* model<br />
  41. 41. Can We Generalize Across Grasslands?<br />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<br />Still lots more research needs to be done in a variety of grasslands both in US and elsewhere<br />
  42. 42. Why Do Different Species Have Different R*?<br /><ul><li>Tilman and Wedin found a strong correlation between root biomass and R*
  43. 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)</li></li></ul><li>Plant Strategies<br />Plants may have different “resource allocation strategies”<br />Plants make “decisions” about how to “allocate” their resources<br />Roots, shoots & leaves, reproduction<br />
  44. 44. Plant Strategies<br />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<br />Allows them to be effective competitors for nitrogen, but does not make them very good at colonizing new habitats.<br />
  45. 45. Plant Strategies<br />Other species (Agrostis) invests very little in roots but invests a large proportion of resources into reproduction<br />Not very good at competition for resources but are good at colonizing new habitats<br />Weedy species- good at colonizing disturbed habitats and then moving on before competition for resources gets to severe<br />
  46. 46. Resource Allocation Trade-Offs<br />Because resources can not be allocated to two tissues simultaneously, plants must “decide” how to allocate their resources<br />Patterns of resource allocation might strongly influence the “strategy” of a species<br />

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