Species Diversity Lab Notes

Community

  • A community is a group of individuals of different species all living and interacting in the same area.

Clements’s Definition of Community

  • Predictable community development over time.
  • Community = super-organism.
  • Each community is a tightly linked group of species, so the same species always occur together in each community.
  • NOT supported by the data.

Gleason’s Definition of Community

  • Community = individualistic.
  • Each community is an emergent property of the populations & the patterns of population distribution and abundance.
  • A given community can vary in species composition (i.e., not always the same species).
  • Accepted by ecologists.

Species Richness

  • Total number of species in an area.

Problems with Measuring Richness

  • #1: Total number of species correlates with sample size.
    • Why? You are more likely to find a rare species if you sample more!
    • Therefore, you can NOT compare diversity between communities that did not have the same sampling intensity.
  • #2: The number of individuals representing a species may not represent the importance of that species to the community
    • Keystone Species: may only have a few individuals in a community but play a big role in structuring the richness & abundances of species in that area
    • What happens if you remove a top predator? Lose diversity.

Solving Problems in Measuring Species Richness

  • Plot data in Species-Area Curve &/or Sampling Effort Curve.
    • Purpose: identify when we have exerted a sufficient sampling effort.
    • Both curves determine whether species richness is increasing with your sampling or has leveled off, but each curve is appropriate for a different type of sampling.

In-Class Practice

  • Cumulative: sum increases by successive addition.
  • Example:
    • Sample Values: 1 (5), 2 (7), 3 (3)
    • Cumulative Sum: 5, 12, 15
    • Calculation: 5, 5 + 7 = 12, 5 + 7 + 3 = 15 (or 12 + 3 = 15)

In-Class: Calculate by Hand

  • Given data:
    • Total Area (m^2): 1, 2, 3, 4, 5
    • Cumulative Number of Species (S): 3, 5, 7, 7, 7
    • Cumulative Number of Individuals (N): 27, 47, 71, 78, 84
  • Species Found:
    • 1 - EM, 2 - SM, 3 - MC, 4 - LK, 5 - GT, 6 - LB, 7 - DS
  • Results:
    • Species richness (total number of species)? 7 species
    • Total number of individuals sampled? 84 individuals

Curve 1: Species-Area Cumulative

  • Independent Variable = Sample Number
    • How much total area or time or surveys have you sampled?
  • Dependent Variable = Total number of species found (cumulative)
    • How many species have you found in this community?
  • Line increases steeply at first, then levels off in asymptote.
    • No additional information about new species.
  • Note: total number of species in community determines how large a sample is required to reach the asymptote/optimum number of samples; number of rare species present also affects this point.
  • Sufficient sampling effort occurred around ~ 3 m^2 in the example graph.

Curve 2: Sampling Effort Cumulative

  • Independent Variable = Total number of individuals found (cumulative).
  • Dependent Variable = Total number of species found (cumulative).
    • How many species have you found in this community?
  • Sufficient sampling effort occurred around ~ 70 individuals in the example graph.

Species Diversity Index

  • For a community that accounts for both richness and abundance of individual species.
  • Does a community have an even number of each species, or is one species dominant and other species are rare?
  • Comparing Communities:
    • Do these communities differ in species richness? No – both 4 spp.
    • Do these communities differ in relative abundance of each species? Yes! Community 1 is EVEN; in Community 2, species A is DOMINANT.

Simpson’s Index (D)

  • Measure of dominance representing the likelihood that two randomly chosen individuals will be the same species.
  • Emphasizes common species & is therefore affected very little by rare individuals.
  • Formula: D=1/Σpi2D = 1 / \Sigma pi^2
    • Where: pi=ni/Ntotalpi = ni / Ntotal
      • nini = number of individuals for a given species (i)
      • NtotalNtotal = total number of individuals sampled
      • pipi = proportion of individuals for a given species (i)
  • D ranges from 1 to the total number of species found (S).
    • 1 = community dominated by a single species
    • Higher D = more even community
  • Community 1 = high D; even abundance of each species
  • Community 2 = low D; Species A is dominant
  • If D = 3.1 for example: Not very even; ~3 common species dominate; Diversity is low because it’s closer to 1 than 7

Shannon-Wiener Index (H)

  • Formula: H=−Σ[pi∗ln(pi)]H = - \Sigma [pi*ln(pi)]
  • H ranges from 0 to 5
    • <1.5 = low diversity
    • 1.5 to 3.5 = moderate diversity
    • >3.5 = high diversity
  • For example, if H = 2.3: Diversity is moderate because it’s between 1.5 – 3.5 This index gives more weight to rare species.

Comparing Indices

  • Example comparing Simpson’s (D) and Shannon-Weiner (H) indices before and after Hurricane Irma:
    • Before Irma: Simpson’s (D) = 5.9, Shannon-Weiner (H) = 2.3
    • After Irma: Simpson’s (D) = 10, Shannon-Weiner (H) = 2.6
  • Observations:
    • Increased diversity after Hurricane Irma according to the Simpson’s Index (D); more even.
    • Similar moderate diversity according to the Shannon-Weiner Index (H).
  • Why the change in evenness? Plants that dominated before Irma may have been negatively affected, such as being ripped up by roots.

Rapid Assessment Program (RAP)

  • Goal: quickly assess species diversity in regions of conservation concern.
  • Scientists that specialize in different species quickly move through a region to document species & individuals found in that region.
  • Not accurate measures of area, but accurate number of individuals… which curve do they use? Sampling effort.