23: Species Community

Tophat

  • You are studying a plant species that has a fungus growing in its roots. You remove the fungus and evaluate what happens to the plant. Which outcome indicates a mutualistic relationship?

    • The plant becomes less toxic to herbivores.

  • How does an ecologist determine the presence of an ecotone?

    • Measure the abundance of plant species over space and abrupt changes in species composition.

  • The mathematical models for competition between species are based on the _____ equation.

    • logistic population growth

Species abundance and diversity

Community: all populations of species living together in a particular area

Definitions

  • Species richness: the number of species in a community

  • Relative abundance: the proportion of individuals in a community represented by each species

  • Diversity: the number of different species in a particular area and their relative abundance

  • Log-normal distribution: a normal, or bell-shaped, distribution that uses a logarithmic scale on the x-axis

  • Rank-abundance curve: a curve that plots the relative abundance of each species in a community in rank order from the most abundant species to the least abundant species

  • Species evenness: A comparison of the relative abundance of each species in a community.

The use of indices to characterize communities

  • Diversity Index: a quantitative measure of the number of species in a community

    • useful to compare communities

    • account for: richness, evenness, dominance

  • Taxonomic diversity index:

    • Simpson index, Shannon-Wiener index

  • Scale (landscape ecology)

    • Alpha diversity

      • diversity within a particular area

    • Beta diversity

      • diversity between ecosystems or sites

    • Gamma diversity

      • overall diversity within a region

Simpson Diversity Index

  • Developed by Simpson (1949)

  • It is a measure of probability: the less diversity, the greater the probability that two randomly selected individuals will be the same species

Shannon-Wiener Diversity Index

  • Shannon (1948)

  • Shannon’s diversity index

  • Based on uncertainty

  • In a community with low diversity, we can be fairly certain of the identity of an organism we might choose by random (high certainty or low uncertainty)

  • If a community is highly diverse and we choose an organism by random, we have a greater uncertainty of which species we will choose (low certainty or high uncertainty)

Rarefaction Curves

  • A technique to assess species richness from the results of sampling

  • The larger the number of samples, the greater number of species that will be found

  • Rarefaction allows the calculation of species richness for a given number of individual samples, based on the construction of so-called rarefaction curbes

  • Rarefaction curves are necessary for estimating species richness

  • If a curve hasn’t yet reached its aymptote, there are additional species in that habitat still to discover