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Lecture 17: Community Structure

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  1. Lecture 17: Community Structure EEES 3050

  2. What is a community? • Any assemblage of populations of living organisms in a prescribed area or habitat.

  3. Community Structure • How to calculate biodiversity? • How does biodiversity change as the environment changes, i.e. along a gradient?

  4. “Historical” debate • Are communities a kind of superorganism, a category? • Ideas developed by Fredrick Clements and Arthur Tansley • Or, are species more individualistic causing continuous change through time and space? • Idea pushed by Henry Gleason.

  5. Two models • Categorical (Clements) • Organized system • Emergent properties • Essential components • Rivet Hypothesis* • Continuous (Gleason) • Haphazard collection of species • Any classification is because of convenience not an inherent property. • Redundancy hypothesis*

  6. Rivet vs. Redundancy • Rivet Hypothesis (Paul and Anne Ehrlich) • Each species (rivet) is necessary. • Begin removing rivets and the community collapses. • Redundancy Model (Walker) • Most species are not tightly connected. • Loss of species will effect little. • Many ecological processes of a community are redundant.

  7. What would these two ideas look like? • Categorical. • Hypothesis: • Species are closely related and constrained and there are distinct changes in species composition. • Continuous: • Hypothesis: • No distinct boundaries are found between groups of species.

  8. One Example: Wales • Observation: • In Northern Wales the rush Juncus effusus is considered a weed. • It is found in several “community types”. • Goal: • Hypothesis: • Even though Juncus effusus is found in several community types, these communities can be classified by their species composition.

  9. One Example: Wales • Methods: • …that is, they chose locations that looked ideal in their mind as three distinct, homogenous community types and sampled the plant species. • The three communities were: • Rough grazing • Enclosed grassland • Bog

  10. One Example: Wales • Results

  11. One Example: Wales • Results

  12. One Example: Wales • Remember: • …that is, they chose locations that looked ideal in their mind as three distinct, homogenous community types and sampled the plant species. • Despite setting up the study to find distinct communities, they found lost of other interactions.

  13. Categories vs. Continuous today…

  14. Categories vs. Continuous today… • Categories: • Easy to understand. • Convenient for management. • Continuous: • Could only measure 1 species at a time. • Analysis is complex.

  15. Measures of Structure • Measures of Taxonomic Structure • Richness • Diversity • Rank-Abundance

  16. Richness • How many taxa? • Rarefaction: adjusting for different pool sizes ... • Number of individuals • Space examined • Time spent • Observer-hours Number of Species

  17. Diversity • Diversity indices are abstractions of: • Richness and rarity (or evenness) • Shannon-Wiener diversity index: • Simpson's diversity index: • Relative Abundance

  18. Shannon-Wiener Index • Examples: Two species • Example 1: 99 of Species A, 1 of Species B • -(0.99*ln(0.99) + 0.01*ln(0.01)) • -(-0.014 – 0.066) = - (-0.081) = 0.081 • Example 2: 50 of Species A, 50 of Species B • -(0.5*ln(0.5) + 0.5*ln(0.5)) • -(-0.5 – 0.5) = - (-1) = 1 H = -(∑Pi·log2Pi)

  19. Simpson’s Index • Examples: Two species • Example 1: 99 of Species A, 1 of Species B • 1 – [(0.99)2 + (0.01)2] • 1 – [(.9801)+(.0001)] = 0.02 • Example 2: 50 of Species A, 50 of Species B • 1 – [(0.5)2 + (0.5)2] • 1 – [(.25)+(.25)] = 0.5 D = 1-∑Pi2

  20. Relative Abundance • Plots relationship between number of species and rank abundance • note: useful for comparisons WITHIN a particular measurement type

  21. How do things change across space or the environment? • Diversity gradients • Gradient analysis • distributions of species usually are gradual • choice of gradient is subjective

  22. Ordination • Logical means of ordering communities according to environmental parameters

  23. Classification • Dividing groups based on particular attributes • ordination and classification can be complimentary

  24. Diversity Gradients • Example Ant Species: • Brazil: • 222 • Trididad: • 123 • Cuba: • 101 • Utah: • 63 • Iowa: • 73 • Alaska: • 7 • Arctic Alaska: • 3

  25. Why does diversity change across space or the environment? • Diversity gradients • History • Habitat Heterogeneity • Competition • Predation • Climate • Climatic variability • Productivity • Disturbance

  26. How do things change across space or the environment? • History • More time permits more colonization and evolution • Habitat Heterogeneity • Physically or biologically complex habitats provide more niches.

  27. How do things change across space or the environment? • Competition • Competition affects niche partitioning • Predation • Predation retards competitive exclusion After removal of Pisaster: …”eventually the experimental area will be dominated by Mytilus…”

  28. How do things change across space or the environment? • Climate • Fewer species can tolerate climatically unfavorable conditions • Climatic variability • Fewer species are adapted to tolerate variable environments. • Evapotranspiration: sum total of water lost from the land by evaporation and transpiration

  29. How do things change across space or the environment? • Productivity • Richness is limited by the partitioning of production or energy among species. • Disturbance • Moderate disturbance retards competitive exclusion

  30. Disturbance

  31. Disturbance • What are some examples of disturbances? • Fire, hurricanes, insect outbreaks, volcanoes, floods, wind, drought… • Definition • Disturbances are relatively discrete events in time that disrupt ecosystem, community or population structure and change resources, substrate availability or the physical environment.

  32. Intermediate disturbance hypothesis.