Community Ecology: Diversity, Trophic Structure, and Species Impact

Diversity & Trophic Structure

• Biological communities are characterized by their diversity and trophic structure.

Species Diversity

• Species diversity includes the number (richness) and relative abundance of species in a biological community.
• Two components:
  • Species richness: the number of different species in the community.
  • Relative abundance: the proportion each species represents of all individuals in the community.

Comparing Diversity: Shannon Diversity Index

• Diversity can be compared using a diversity index.
• Shannon diversity index (H) (sometimes called Shannon-Wiener index): $H = – (p<em>A ln p</em>A + p<em>B ln p</em>B + p<em>C ln p</em>C + …)$
  • Where A, B, C… are the species.
  • $p$ is the relative abundance of each species.
  • $ln$ is the natural logarithm.
• Example:
  • Community 1: A: 25%, B: 25%, C: 25%, D: 25%
    $H = – (0.25 ln 0.25+ 0.25 ln 0.25 +0.25 ln 0.25 +0.25 ln 0.25) = 1.39$
  • Community 2: A: 80%, B: 5%, C: 5%, D: 10%
    $H = –(0.8 ln 0.8 + 0.05 ln 0.05 +0.05 ln 0.05+ 0.1 ln 0.1) = 0.71$

Challenges and Tools

• Determining species richness and abundance is challenging.
• Molecular tools can help identify species and determine diversity.
• Species hard to identify with morphology may be separated by “DNA barcode”.
• Molecular tools allow for mass collection of “environmental DNA”.

Diversity and Community Stability

• Communities with higher diversity are more productive, producing more biomass.
• They are more stable in their productivity.
• They are more resistant to invasive species.

Trophic Structure

• Trophic structure: feeding relationships between organisms in a community.
• Food chains: link trophic levels from producers to top carnivores.
• The position an organism occupies in a food chain is called its trophic level.

Food Webs

• Food web: group of food chains linked together forming complex trophic interactions.

Limits on Food Chain Length

• Energetic hypothesis: food chain length is limited by inefficient energy transfer.
• Only about 10% of the energy stored in organic matter at each trophic level is converted to organic matter at the next trophic level.

Species with Large Impacts

• Certain species have a very large impact on community structure.
• Dominant species: most abundant or have highest biomass (often these are invasive species).
  • Does removing the dominant species have catastrophic consequences?
  • Why is any given species dominant?
    • Hypothesis 1: dominant species are most competitive in exploiting resources.
    • Hypothesis 2: they are most successful at avoiding predators, herbivores, or disease.
• Keystone species: exert strong control on a community by their ecological roles.
• Ecosystem engineers (or “foundation species”): cause physical changes in the environment that affect community structure (e.g., beaver dams).

Bottom-Up and Top-Down Controls

• Bottom-up model: unidirectional influence from lower to higher trophic levels (N -> V -> H -> P).
  • N = nutrients, V = vegetation, H = herbivores, P = predators
• Top-down model: control of a trophic level comes from the level above (predators limit herbivores, herbivores limit plants, and plants limit nutrient levels) (V <- H <- P).