• Community ecology is concerned with the interaction of populations
  • Communities and ecosystems are described with regards to the interactions between organisms and environment, biodiversity, and species diversity and composition (identity of species)

Species Diversity in Ecosystems

  • “Number of species in an ecosystem or across the biosphere”

Species Diversity depends on:

  1. Species Richness: number of different species
  2. Relative Abundance:
  • Benefits of species diversity: ecosystem with higher species diversity can better resist environmental change
    • Increased productivity and stability of communities (can survive better), produce more biomass (total mass of all organisms in a habitat), more resistant to introduced species

Structure of Communities:                                                                                                             

  • Simpson’s Diversity Index: measures species diversity
    • Higher index value = more diverse
  •   Based on random samples of environment

Interactions in Communities

  • Competitive Exclusion principle (Gause’s principle): Two species cannot occupy the same niche (when resources are limiting)
    • When two species compete for the same resources or occupy the same niche, one is probs gonna be more successful and 2nd is eliminated
      • Ex: when two species of Paramecium competed for same resources, one outcompeted other and grew more rapidly
    • Competition is an interaction that can affect how populations access energy and matter
      • Can result in change in community structure
    • One form of interaction is interspecific competition (competition between different species). Different ways to resolve competition & enable similar species to coexist in a community
  1. Resource partitioning: organisms divide resources by pursuing slightly diff resources or getting them in slightly diff ways,
  2. Realized niche: the actual space an organism inhabits as a result of competition → allows two species to coexist
  • Fundamental niche: niche that an organism occupies w/o competing species
  • Ecological niche: specific set of biotic and abiotic resources that an organism uses in its environment
    • Ex: temp tolerance range, habitat, time of eating
  • Species can partition their niches in space & time
  1. Character displacement (niche shift): natural selection favours a divergence of characteristics when two similar species inhabit the same environment
  • Ex: finches with diff beaks (thru evolution) suited to diff food → minimize competition


  • Form of community interaction when an animal hunts another organism
  1. True predator kills and eats other animals
  2. Parasite feeds and lives on host’s tissues, weakening it
  3. Parasitoid is an insect that lays its eggs in a host
  1. Herbivore is an animal that eats plants                                             

Predator-Prey Interactions

  • Increase/decrease in predator occurs slightly after increase/decrease in prey
  • Increase in predator → decrease in prey
  • Trophic cascade: negative effect of removal of key species


  • When two species live together in close contact during a portion of their lives; examples of interactions among populations include
  • Positive
  1. Mutualism: when both species benefits (+,+)
  • Lichens: symbiotic associates of fungi and algae
    • Algae produce sugar and fungi provide water & protection
      • Can also be parasitic
    • Mycorrhizae: mutualistic association of fungi with roots of plants
      • Plants provide sugar and filaments of fungi increase surface area of roots, facilitate absorption of water and minerals (especially phosphorus)
  1. Commensalism: one species benefits, 2nd unaffected (+, 0)
  • Birds build nests in trees
  • Negative Interactions
  1. Parasitism: parasite benefits, host is harmed (+, -)
  • Tapeworms in digestive tract of animals
  1. Predator-Prey (+/-)
  2. Competition: Different species compete for the same resource that limits the survival and reproduction of both species (-/-)
  • Ex: weeds compete for nutrients
  • Although negative for one species, might benefit another species by providing new niches or freeing up resources


  • Disturbances: events that change a community by removing organisms from it and altering resource availability.
    • Ex: Humans have altered much of Earth’s surface by replacing natural terrestrial communities with urban and agricultural one
  • Some organisms depend on periodic disturbances
    • Ex: pine that depend on burning for reproduction
  • Disturbances that threaten stability include fires, floods, disease, and human effects
  1. El Nino: trade winds and upwelling that promote bottom up effect stop; algae declines → then consumers → collapse in food webs
  2. Meteor Impacts and volcanic eruptions: increase amount of matter → reduce solar radiation → less primary production
  3. Plate tectonics (continental drift): describes movement of land masses (plates) over surface of earth
  • Plates collide and move to new latitudes → earthquakes, create volcanoes, and form mountains
    • Environmental conditions change → create new niches for speciation

Characterizing Disturbance

  • Vary in frequency and severity among communities
    • High level of disturbance = frequent or intense disturbance
    • Low disturbance levels = low frequency or low intensity of disturbance.
  • Intermediate disturbance hypothesis: moderate levels of disturbance → open up habitats for less competitive species → greater species diversity than high or low levels of disturbance
    • Small-scale disturbances can create patches of different habitats across a landscape, which help maintain diversity in a communitys


  • The tit-for-tat evolution in one species in response to adaptation in another species
    • Results from natural selection of characteristics that promote most successful predators and most elusive prey leads to coevolution of predator and prey
  1. Secondary Compounds: toxic chemicals produced in plants that discourage herbivores; some herbivores have adaptations that allow them to tolerate them
  2. Camouflage (cryptic coalition): physical characteristics or behavior that allows prey or predator to hide
  3. Aposematic coloration (warning coloration): pattern or coloration that warns predators that prey are to be avoided
  • Associate yellow and black body of bees with danger
  1. Mimicry : when species resemble one another in appearance. 2 kinds
  • Mulelerian mimicry: animals with similar defense mechanism share same coloration
    • Mutualistic: effective because a single pattern shared among many animals is more easily learned by a predator than would be a different pattern for each species
      • Ex: 2 diff species of butterflies, toxic dart frogs
    • Batesian mimicry: harmless animal mimics the coloration of an animal that does have a defense
      • Parasitic relationship: mimic benefits while mimicked is negatively affected
        • Ex: harmless beetle mimics noxious beetle
  1. Pollination: of flowers result of coevolution of traits between the flowers and their pollinators
  • Red, tubular flowers coevolved with hummingbird who attracted to red & have long beaks