MODULE 8

• Community


  Association of interacting species inhabiting some defined area
• Guild


  Group of organisms that all make their living in a similar way
• Guilds


  • Seed-eating animals in an area of a desert
  • Mammals, birds, and ants in Sonoran Desert
  • Fruit-eating birds in a tropical rain forest
  • Pigeons in many South Pacific islands
  • Filter-feeding invertebrates in a stream
• Abundance


  • Total number of individuals, or biomass, of a species in a specified area
  • number of individuals per species
• Density


  Number of individuals per unit area
• Distribution


  Size, shape, and locations of the area it occupies
• Richness


  Number of species in an area
• Relative Abundance


  One of the most fundamental aspects of community structure, evenness of distribution of individuals among species in a community
• Species Diversity


  Combination of the number of species and their relative abundance

  Ecologists define on the basis on two factors:

  • number of species in the community (species richness)
  • relative abundance of species (species evenness)

  
• Species diversity is distributed unevenly across space following latitudinal gradient, with increasing diversity from the poles toward the equator
• Alpha diversity


  Diversity on a local scale, describing the species diversity (richness) within a functional community
• Beta diversity


  • Amount of differentiation between species communities, number of species that are unique to each system
  • ratio between gamma and alpha diversity (β = γ/α)
  • commonly used measure is the Sørensen dissimilarity index
• Gamma diversity


  Overall species diversity across communities within a larger geographic area
• Alpha diversity is usually expressed by the number of species (i.e., species richness) in that ecosystem
• Gamma diversity is a measure of the overall diversity within a large region, "Geographic-scale species diversity"
• Beta diversity is a comparison of diversity between ecosystems, measured as the amount of species change between the ecosystems
• Species evenness


  • Measure of relative abundance of different species that make up the richness, distribution of abundance across the species in a community
  • higher when species are present in similar proportions
• More diverse ecosystem tend to be more productive
• Greater species richness and productivity makes an ecosystem more sustainable and stable
• More diverse the ecosystem, greater is the ability to withstand environmental stresses like drought or invasive infestations
• Species richness makes an ecosystem able to respond to any catastrophe
• Rich diversity is important for the survival of mankind
• In species-rich communities, each species can use a different portion of resources available as per their requirement
• Healthy biodiversity has innumerable benefits like nutrients storage and recycling, soil formation and protection from erosion, absorption of harmful gases, climate stability
• Humans get lots of product from nature like fruits, cereals, meat, wood, fiber, raisin, dyes, medicine, antibiotics, etc.
• Amazon forest is estimated to produce 20% of total oxygen in the earth's atmosphere through photosynthesis
• Pollinators, symbiotic relationships, decomposers, each species perform a unique role, which is irreplaceable
• Diversity in large numbers help in large scale interaction among organisms such as in the food web
• In the nitrogen cycle, bacteria, plants have a crucial relationship, earthworms contribute to soil fertility
• Apart from these, there are other benefits such as recreation and tourism, education, and research
• Habitat Loss and Fragmentation is a major cause of loss in species diversity and driving plants and animals extinct
• Over-exploitation of natural resources leads to the extinction of many species
• When alien species are introduced deliberately or unintentionally, some of them become invasive, leading to the extinction of indigenous species
• When a species becomes extinct, the species that are associated with it also becomes extinct
• The graph suggests that if more cores (effort) were to be taken, further species would be found and the line is clearly still rising

  
• When sufficient sampling effort is expended, the curve is asymptotic (gradually levels off)
• The order in which samples were taken alters the shape of a species accumulation curve both because of random error and sample heterogeneity
• Simpson's index


  Combines both species richness and species evenness in one number, ranges from 0 to 1, where 0 means no diversity and 1 means infinite diversity
• Shannon-Wiener Function (H)


  Assumes that individuals are randomly sampled from an infinite population and that all taxa are represented in the sample, determines the amount of information in a code
• Berger-Parker Dominance Index


  • Expresses the proportional importance of the most abundant type
  • is highly biased by sample size and richness, does not make use of all the information available from sample
  • increasing d values -> increasing diversity