Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 54.2 Diversity and Trophic Structure Characterize Biological Communities
Ecological communities vary in their species makeup and feeding relationships. These characteristics—species diversity and trophic structure—determine how communities function, respond to change, and resist disturbance or invasion.
1. Species Diversity
- Species richness: number of species
- Relative abundance: proportion of each species present
- Shannon diversity index (H) combines both
- Higher H = higher diversity
- Example: Equal abundance → H = 1.39; Dominance by one species → H = 0.71
2. Measuring and Identifying Diversity
- Large sampling required to detect rare species
- DNA barcoding identifies visually similar species
- Microbial diversity assessed using PCR and rRNA sequencing
- Example: Highest bacterial diversity at neutral pH
3. Diversity and Community Stability
- Higher diversity leads to:
- More biomass production
- Greater year-to-year stability
- Faster recovery from stress
- Resistance to invasion
- Example: Diverse grass plots were more productive and resilient
4. Trophic Structure and Food Chains
- Trophic structure: feeding relationships among species
- Primary producers → herbivores → carnivores
- Food chain: linear energy flow
- Food web: interconnected food chains
- Example: Antarctic web includes krill, whales, squid, etc.
- Consumers may feed at multiple levels
5. Limits on Food Chain Length
- Most chains ≤5 links
- Energetic hypothesis: ~10% energy transfer
- 100 kg plants → 10 kg herbivores → 1 kg carnivores
- Supported by tree-hole experiments
- Carnivore size can limit chain length
6. Species With Large Impacts
- Foundation species: abundant and structure habitat (e.g., kelp)
- Keystone species: low in number but critical role (e.g., sea stars)
- Ecosystem engineers: modify environment (e.g., beavers)
7. Bottom-Up vs Top-Down Control
- Bottom-up: nutrients and producers regulate higher levels
- Top-down (trophic cascade): predators regulate lower levels
- Example: fish removal → more zooplankton → less algae
- Used in Lake Vesijärvi restoration
8. Disturbance and the Intermediate Disturbance Hypothesis
- Disturbance: events that change communities (e.g., fire, flood)
- Intermediate disturbance → highest diversity
- Too frequent → stress excludes species
- Too rare → dominant species outcompete others
- Example: New Zealand stream invertebrate diversity
- Both small- and large-scale disturbances can enhance diversity
In a Nutshell
Species diversity and trophic structure shape how communities function and respond to change. A few species may have large impacts, and interactions—both feeding and environmental—create dynamic ecosystems. Stability, productivity, and resilience are strongly influenced by diversity and the nature of disturbances.