Community Ecology studies the interaction of species and populations in a particular geographic area

• Ecological interactions:   Table 55.1
  • Mutualism (+/+) : animal / oxpecker ,   Rhizobia / legume plants,   cow / rumen bacteria,   coral / algae
  • commensalism (+/0) : decorator crab , S. cerevisiae releases ribflovin which is taken up by L. casei, Cattle Egrets
  • amensalism (-/0): antibiotic released by an organism, algal mat shading corals
  • predation/parasitism (-/+) : Predation: one organismis consumed by another organism Parasitism: the parasite lives on or inside the host, but the host is not immediately killed
  • competition (-/-) : interspecies, intraspecies for food , space, mates

• Resources : any factor that contributes to the growth of a population and its availibility is reduced through the continued use.
  A limiting resource is a resources which has become scarce and can limit population growth. For terrestrial oganisms oxygen (nitrogen ) is not a resource, however in an aquatic environment oxygen ( nitrogen) can be a limiting resource . In many ecosystems space is a limiting resource and not nutrients

• ecological niche : includes all resources and physical conditions that are required for a species to live and reproduce in a particular range. A niche can be modeled as an imaginary multidimensional space , in which each dimension or axis represents the range of some environmental condition or resource. Niches do not exist before animals occupy them, only a range of potentialities exists. The animal defines the niche through all its features, it is the total range of conditions in which an organism thrives. A niche can change depending on environmental change or as a result of competition.

• Competition: when resources and space become limiting factors competition among individual organisms occurs.
  • intraspecies competition (among members of the same species)
  • interspecies competition (among members of different species).
  • Exploitative (resource) competition: common resource for two or more species however members of the two species do not come into contact with each other
  • interference competition: member of the two species come in contact with each other and fight over the resources.
  • competition when niches overlap :
    Plant competition Fig. 55.2-1,2
    Barnacle competion Fig. 55.01 : Study of two barnacle species, Chthamalus stellatus and Semibalanus balanoides in Scotland (Joseph Connell)
    Barnacles (belong to the crustaceans) are free-swimming in their larval state. In their adult form they attach themselves to rocks and secrete calcite platelets as protective shield. Barnacles compete intensely for limited space on rocks in the intertidal zone. (they are not competing for food). Barnacle Chthamalus is more tolerant of desiccation and is a more successful competitor in the upper intertidal zone. Barnacle Semibalanus is less tolerant of desiccation and is more successful in the lower intertidal zone image
    
  • the logistic growth dN/dt = r [(K - N)/K] N
    can be modified to include interspecies competition:
    dN₁ /dt = r₁ [(K₁ - N₁ - aN₂ )/K₁ ] N₁
    a = factor which measures how effiecient species 2 uses the common resources

    Conclusions about competition:

    • Competition is a negative interaction for competing species.
    • Species that share a resource will compete if the resource is limited.
    • Species which inhabit the same ecological niche will not be able to coexist for longer periods of time. The overlap of their niches has to be minimum for the species to coxists.

• Predation : benefits one participant (predator) harms the other (prey)
  • Predators and prey influence each other's evolution. The prey will be selective for traits which help not being caught by the predator and the predator will show traits that enhance the ability to capture the prey. schooling
  • Effect of predation on prey and predator:
    Numerical Response: Increase in predator population with increasing prey population
    Cycling: Population density of the predator lag slightly behind population density of their prey. predators eat prey and reduce the numbers of prey. Because of lack of food predator's numbers drop. With fewer predators, the remaining prey survive better and prey numbers h increase again, completing the cycle
    Lynx , predatory mite /pest mite

  • Functional response : the relationship between a preditor's consumption rate and prey (food) density
    Prey consumption is controlled by : search time (looking for prey and capturing prey) and by the time required to eat the prey.
    • Type I functional response : Predator's feeding rate depends on prey density. The resonse is linear until saturation is reached. (e.g. mechanical limitation at high prey density) Herbivore / plant interactions, and invertebrate predator/ prey interactions
      Examples : Foraging Lemmings ; passive predators like spiders (the number of flies caught in the net is proportional to fly abundance); Marine organisms that remove nutrients from water through filtration .

    • Type II functional response : Predation rate increases slower than the prey density. Eventually the predation rate remain constant even if the prey density increases (saturation) The curve is hyperbolic because of invariant prey handling and consumption times. : Typical of invertebrates: bean bettle

    • Type III functional response : Predation rate lags behind as initial prey density increases. The response is linear for a small range of prey densities, then decline asymptotically to saturation. The type III occurs either where the predator has to develop a search behavior (which doesn't occur well at low densities) or ignores low density prey in favour of another prey that may be at higher density.

  • Regulation of prey density via Functional response
    Regulation requires that prey mortality rates increase with prey density.

  • Extinction: Nymphs can eliminate tadpoles Fig. 55.7

• Keystone species: a species that has an important influence on the diversity of an ecosystem. It's loss would cause a significant change in the populations of other species. Examples :
  • plants and other vegetation ,
  • microorganisms
  • Predators as keystone species
    Pisaster
    Stratification of the western coastal line: At tidepool zone (exposed during low tide) brown algae. Further below are Blue mussels (Mytilus edulis), giant barnacles. California mussels (Mytilus californianus) begin to dominate further down. Deeper: Semibalanus balanoides , purple sea urchins and sea cucumbers . In the subtidal zone, increasing numbers of aquatic animals (dungeness crab sand dollar, red abalone and giant Pacific octoposus.
    Mytilus is competitive only in the upper rocky intertial zone ( boundary to the brown algae) . The lower limit of mytilus population is determined by predation by the seastar (pisaster).
    In absence of pisaster the mussel bed will extend down into the subtidal zone. This will lead to the extinction of other species, which populated the subtidal zone. Also brown algae will gradually disappear.

    Sea otters:
    Kelp forest beds off the West coast are critical habitat for many species. The feeding of Sea urchins on the kelp foothold weakens kelp and make it vulnerable to currents and storms. At large urchin populations, kelp is eliminated. Otters feed on urchins, kelp forests are large and healthy. Otters were killed by humans in large numbers beginning 1800 until the 1950 ies

• Community succession : change in populations over time
  
  r-strategists: disturbed or transitionary habitats are typically populated by rapidly growing species. (r-strategists). Seeds (preennials, grasses, wildflowers) are usually the first to colonize a habitat. r-strategists: short life spans, short generation times, large numbers of offspring, have efficient means of dispersal. Given time other slower growing organisms will follow.
  K-strategists
  When a habitat is populated with a diverse collection of organisms competing with one another the advantage shifts K-strategists. K-strategists have stable populations that are close to K. The species will benefit most by a close adaptation to the conditions of its environment. K-strategists : found in stable habitats, long life span, long generation time, small numbers of offspring, take care of their young.
  Fig. 55.2 Succession in a glacial moraine