Community Ecology

• Community


  Populations of organisms living in a defined habitat + interacting with each other
• Types of Interactions


  • Competition
  • Predation
  • Symbiosis
  • Mutualism
  • Commensalism
  • Amensalism
• Predation


  Feeding interactions of herbivores, carnivores + detrivores with their food/prey to obtain nutrients
• All food chains /webs rely on predation energy+ materials passed on as each organism provides food for next organism in sequence
• Predators maintain balance of organisms in ecosystem: grazers keep grass from growing out of control
• Detrital Food Chain


  • dead rat > blowfly > maggot > ground beetle > frog > Viper
• Grazing Food Chains


  • 4 consumer > 3° consumer > Terrestrial carnivore/detrivore > Aquatic carnivore/detrivore > 2° consumer > Carnivore > 1° consumer > herbivore > zooplankton > 1° producer > plant > phytoplankton
• Prey population


  Predator population
• Predator-prey cycle: prey population increases, then predator population increases, then prey population decreases, then predator population decreases, repeating the cycle
• Parasitism


  Symbiotic relationship where one species (parasite) benefits at the expense of the other (host)
• Parasitoid


  • Parasite for only part of life cycle, as a juvenile up till metamorphosis, when it becomes adult + kills host
• Mutualism


  Interspecific symbiotic relationship in which both species benefit
• Mutualistic relationships


  • N-fixing Bacteria + Leguminous Plants
  • Lichens
• Commensalism


  Interspecific symbiotic association with benefits to 1 species while the other species is unaffected
• Commensal relationships


  • Epiphytes
  • Clown Fish + Sea Anemones
  • Cattle Egrets
• Amensalism


  Interspecific symbiotic interaction where 1 species is inhibited + other species (amensal) is unaffected
• Interspecific Interactions


  • Predation
  • Parasitism
  • Mutualism
  • Commensalism
  • Amensalism
  • Competition
• Ecological Niche


  The role and position a species has in its environment, including how it meets its needs for food, shelter, survival, and reproduction, and its interactions with biotic and abiotic factors
• Niche Breadth


  The range of conditions a species can tolerate, both abiotic and biotic, measuring the specialization of the organism
• Niche Breadth


  • Narrow niche (Specialist)
  • Broad niche (Generalist)
• Gause's Competitive Exclusion Principle: No 2 species can co-exist if they occupy the same niche, leading to elimination of one of the competing species
• Resource Partitioning


  Different species partition/divide their resources by differentiation of ecological niches when living together to reduce interspecific competition so that species can co-exist in community
• Resource Partitioning


  • Warblers in spruce forests foraging in different parts of the tree
  • Owls and hawks feeding on rodents at different times
  • Woodpeckers eating insects while finches eat nuts
• Fundamental Niche


  The niche a population of species has in the absence of competition from other species, showing all resources which could be used
• Realised Niche


  The niche a population of species has in the presence of competitor species, showing resources actually used
• For co-existence of species in communities, fundamental niche is reduced to realised niche by resource partitioning
• Over time, species evolve to occupy only realised niches, an adaptation to competition by using slightly different resources or shared resources in different ways
• Displacement Character


  Physical characteristics that evolve among competing species to reflect specialized realised niches
• Ecological Succession


  Orderly process of change over time in a community involving gradual replacement of one plant community with another, changing the species it supports
• Types of Succession


  • Primary Succession
  • Secondary Succession
• Primary Succession


  1. Occurs on newly formed habitats that have not previously supported a community

  2. Starts with pioneer species

  3. Soil starts forming due to action of lichens and weather

  4. Simple plants grow in soil

  5. Simple plants die, adding more organic matter

  6. Soil layer becomes deeper, grasses and flowering plants begin taking over

  7. Plants die and decompose, adding nutrients to soil and preparing terrain for shrubs and low trees to grow

  8. Insects, small birds and mammals move in at each stage

  9. With time and if conditions permit, forest/climax community forms
• Secondary Succession


  1. Occurs on sites which previously supported a community

  2. Occurs at a faster pace

  3. Starts with arrival of seeds of annual and perennial plants

  4. Tall herbaceous plants follow

  5. Climate and terrain permitting, large trees/climax community colonise area
• Sere


  Entire sequence of ecological communities or steps/stages in succession which successively occupying area from initial/pioneer to climax
• Types of Seres


  • Hydrosere (starts in shallow lake)
  • Xerasere (starts in dry place)
  • Lithosere (starts on bare rock)
• Species Density


  Number of individuals of a particular species per unit area
• Species Cover


  Proportion of ground covered by aerial parts of plants of one or more species, expressed as a percentage
• Species Frequency


  Probability that an individual of a particular species is present in a randomly placed quadrat
• Species Richness


  Number of different species present in a sample/area
• Species Evenness


  Measure of relative abundance of different species making up the richness of an area
• Simpson's Reciprocal Diversity Index


  Measure of biological diversity that considers both richness (number of species present) and evenness (abundance of each species)