Competition

• Threespot damselfish


  • Aggressively guard small territories of <1 m2 (space, protection and algae)
  • Territory can be claimed by other threespot damselfish in minutes, and some are cocoa damselfish
• Interference competition


  Direct aggressive interaction between individuals
• Resource limitation


  Limited supply of suitable space territories
• Types of competition


  • Intraspecific competition
  • Interspecific competition
• Competition in a forest can be as intense as on a coral reef. However, much of the competition in a forest takes place underground, where the roots of plants compete for water and nutrients
• Competition


  Two species share a requirement for a limited resource à reduces fitness of one or both species
• Intraspecific competition


  Individuals compete through their dependence on same limited resource like space
• In plants, competition is reflected in terms of water, light and nutrients through the process called self-thinning
• Resource competition


  Competition involving the use of limited resources
• Interspecific competition


  Niche - reflects the environmental requirements of the species
• The niche concept has had prominent place in the study of interspecific competition because of the competitive exclusion principle (Gause, 1934) stating that two species with identical niches cannot coexist indefinitely
• Intraspecific competition


  Contributes to K (carrying capacity)
• Interspecific competition


  Affects N (population size)
• As the seeds of annual plants germinate, their population density often numbers in the thousands per square meter. However, as the season progresses and individual plants grow, population density declines. This same pattern occurs in the development of a stand of trees. As the stand of trees develops, more and more biomass is composed of fewer and fewer individuals. This process is called self-thinning

  
• Self-thinning appears to be the consequence of intraspecific competition for limited resources
• At the highest density, P. marginata showed reduced survivorship, decreased body length, and increased developmental time. These signs of intraspecific competition were probably the result of reduced food quality at high leafhopper densities, since plants heavily populated by planthoppers show reduced concentrations of protein, chlorophyll, and moisture

  
• Supplementing food had no effect on survival by P. scaber, indicating that food was not limiting survival. However, survival was lower at the higher population density. Grosholz attributed lower survival at the higher density to cannibalism, a common occurrence in terrestrial isopods
• Complete competitors cannot coexist
• Competitive exclusion is reached more slowly with higher resource abundances
• Gause's competitive exclusion principle states that two species with identical niches cannot coexist indefinitely
• Stable coexistence requires niche differentiation, such that members of each species compete more strongly among themselves than with members of the other species
• Niche


  Set of conditions within which an organism can maintain a viable population
• As seeds were depleted, the birds ate the smallest and softest seeds first, leaving the largest and toughest seeds. Consequently, at the end of the drought, the G. fortis population on Daphne Major was dominated by larger individuals with larger, stronger beaks that had survived by feeding on hard seeds

  
• Wasps were much more likely to be found developing in smaller caterpillars while flies were more likely to be found developing in larger ones. These differences were not based on caterpillar species but rather on the caterpillar's size (a close proxy for developmental stage). It appeared that wasps and flies reduced competition with each other through niche partitioning

  
• An alternate hypothesis was that the observed differentiation by size was simply a function of phenology (seasonal life cycle timing): perhaps wasps simply emerged and attacked earlier in the season when caterpillars were small, whereas flies were more active later when the hosts had grown to be bigger

  
• Wasps and flies were avoiding competition for caterpillar hosts by partitioning the niche of their shared host based on size

  
• Competition coefficients (a's)

  Show per capita competitive effect of each species on the other
• When a's are less than 1, stable coexistence is possible
• Gause (1934) conducted a classic interspecific competition experiment with two species of Paramecium
• Niche


  1. dimensional hypervolume, where n equals the number of environmental factors important to the survival and reproduction by a species
• Fundamental niche


  Depends on physical (abiotic) conditions
• Realized niche


  Depends on biotic as well as abiotic conditions
• The niche of a species may contract in the presence of a competitor species. This phenomenon leads to resource (niche) partitioning and coexistence among functionally similar species
• When the dominant competitor is removed, the niche of the inferior competitor can expand by competitive release
• Theoretically, natural selection may lead to divergence in the niches of competing species, a situation called character displacement