Community ecology

• Biological community


  A group of different species living close enough together for potential interaction
• Interspecific interactions


  • Competition
  • Predation
  • Herbivory
  • Symbiosis (including parasitism, mutualism, and commensalism)
• Interspecific interactions symbolized by

  • Positive (+) or negative (-) effects on survival and reproduction of the two species
  • ( ) indicates a population is not affected
• Interspecific competition


  Occurs when species compete for a resource that limits their growth and survival
• Two species engage in interspecific competition for a limiting resource


  Results in detrimental effects to one or both species (-/-)
• Interspecific competition


  • Grasshoppers and bison in the Great Plains competing for grass
  • Oxygen is rarely in short supply so most species do not compete for it
• Competitive exclusion


  When strong competition between two species leads to the local elimination of one of the two competing species
• Gause's conclusion: two species competing for the same limiting resources cannot coexist in the same place
• Ecological niche


  The sum total of a species' use of abiotic and biotic resources in its environment
• Habitat vs Niche


  Habitat is a species' "address", niche is its "profession" or ecological role
• Competitive exclusion principle: two species cannot coexist permanently in a community if their niches are identical
• Resource partitioning


  The differentiation of niches that enables two similar species to coexist in a community
• Fundamental niche vs Realized niche

  Fundamental niche is the niche potentially occupied by a species, realized niche is the niche a species actually occupies in a particular environment
• Ecologists can identify a species' fundamental niche by testing the range of conditions in which it grows and reproduces in the absence of competition
• Ecologists can test whether a potential competitor limits a species' realized niche by removing the competitor and seeing whether the first species expands into the newly available space
• Character displacement


  The tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species
• Character displacement


  • Variation in beak size between different populations of the Galápagos finches Geospiza fuliginosa and Geospiza fortis
• Predation


  A +/- interaction where the predator kills and eats the prey
• Predator adaptations


  • Acute senses
  • Weaponry such as claws, teeth, fangs, stingers, or poison to catch and subdue prey
• Predator hunting strategies


  • Pursuers are generally fast and agile
  • Ambushers are often camouflaged
• Prey adaptations


  • Behavioral defenses like fleeing, hiding, herding, and alarm calls
  • Adaptive coloration like camouflage and warning coloration
  • Mechanical or chemical defenses
• Batesian mimicry


  A harmless or palatable species mimics a harmful or unpalatable model
• Müllerian mimicry


  Two or more unpalatable species resemble each other
• Herbivory


  A +/- interaction where an herbivore eats parts of a plant or alga
• Herbivore adaptations


  • Chemical sensors to recognize appropriate food plants
  • Specialized dentition and digestive systems to process vegetation
• Plant defenses against herbivory

  • Chemical toxins
  • Spines and thorns
• Symbiosis


  Direct and intimate relationships between species, whether harmful, helpful, or neutral
• Parasitism


  A +/- symbiotic interaction where a parasite derives nourishment from a host, harming the host
• Types of parasites


  • Endoparasites live within the host
  • Ectoparasites live and feed on the external surface of the host
  • Parasitoid insects lay eggs on or in living hosts, with larvae feeding on the host body
• At least one-third of all species on Earth may be parasites
• Many parasites have complex life cycles involving multiple hosts
• Some parasites can change the behavior of their hosts in ways that increase the probability of the parasite being transferred to another host
• Mutualism


  An interspecific symbiosis where both species benefit (+/+)
• Mutualism


  • Nitrogen fixation by bacteria in legume root nodules
  • Cellulose digestion by microorganisms in ruminant guts
  • Nutrient exchange in mycorrhizae
• Obligate mutualism


  At least one species has lost the ability to survive without its partner
• Commensalism


  An interaction that benefits one species but neither harms nor helps the other (+/0)
• Commensalism


  • Barnacles attached to whales
  • Cowbirds and cattle egrets feeding on insects flushed out by grazing herbivores
• Species diversity


  The variety of different kinds of organisms that make up a community
• Components of species diversity

  • Species richness: total number of different species
  • Relative abundance: proportion each species represents of the total individuals
• Shannon diversity index (H)


  A widely used index to quantitatively compare the diversities of different communities