Trophic structure

1. Introduction & Context

• Lecturer notes connection to career pathology and invites students interested in shark ecology, pathology, or fieldwork to reach out for contacts or advice.

• Today’s focus: trophic structure, energy flow, and how sharks fit into marine food webs.

• Sharks are used as a case study for examining energy transfer, biomass distribution, and ecological conundrums like inverted trophic pyramids.

2. Trophic Structure – The Basics

• Definition: How organisms obtain and use energy; their position on the energetic ladder of life.

• Common categories:

  • Primary producers (e.g., phytoplankton)

  • Primary consumers (herbivores)

  • Secondary consumers (carnivores feeding on herbivores)

  • Tertiary consumers (apex predators)

• Classic trophic pyramid:

  • Energy decreases up the pyramid due to the 10% rule — only ~10% of energy is transferred between trophic levels; ~90% lost as heat, waste, metabolism.

  • Hence, biomass is expected to decrease as trophic level increases.

  • Thermodynamics dictates less biomass at the top.

3. The Shark Biomass Paradox

• Sharks are often apex predators, so classical theory predicts low shark biomass relative to prey.

• But field surveys on remote reefs show the opposite — sometimes more predator biomass than prey → inverted trophic pyramids.

Case Study 1 – Sandin et al. (2008)

• Surveys of remote Central Pacific reefs found higher shark biomass than prey fish biomass.

• Initially controversial — challenged thermodynamic expectations.

• Suggested possible energy inversion in pristine systems.

Case Study 2 – Subsequent NOAA Surveys

• 15+ years of NOAA Coral Reef Division work across the Pacific (Hawaii, Mariana, American Samoa, Pacific Remote Islands).

• Pattern repeated:

  • Remote, uninhabited reefs → high shark biomass.

  • Populated, exploited reefs → fewer sharks.

• Implies human exploitation reshapes trophic structure.

4. The “Fishing Down the Food Web” Hypothesis

• Overfishing top predators flattens trophic pyramids → classic upright shape seen in exploited systems.

• Remote/unexploited reefs can show inverted trophic pyramids.

• Suggests our “normal” understanding of ecosystems is based on disturbed systems.

Key Insight:
Most ecology textbooks reflect heavily exploited systems; pristine systems may naturally show inverted pyramids — but still must obey thermodynamics.

5. Energy Flow Problem

• The paradox: How can there be more biomass at the top if energy is lost up the chain?

• Two possible explanations:

  1. Counting bias – sharks are overcounted (curious behaviour → approach divers).

  2. Energy import – sharks feed on prey outside the measured ecosystem (spatial energy subsidies).

6. Measuring Shark Diets

A. Stomach Contents Analysis

• Direct method: Identify recent prey from stomachs.

• Can be done non-lethally via “throw-up technique” (induce regurgitation, then release).

• Data used to:

  • Identify diet composition by species, sex, size, or life stage.

  • Construct short-term trophic profiles.

Example:
Study of Hawaiian sharks:

• Sandbar sharks → mainly fish/squid (higher trophic levels).

• Tiger sharks → varied diet, including birds, reptiles, mammals (broad trophic range).

Limitations:

• Represents only 12–24 hours of feeding (short time window).

• Captures opportunistic feeding, not long-term preference.

• Sampling bias: Sharks caught near certain areas or seasons might not reflect typical diet.

(Lecturer’s humorous analogy: judging human diet from a 2 a.m. kebab — snapshot, not preference.)

B. Stable Isotope Analysis (SIA)

• Used to infer long-term diet and trophic position.

• Based on ratios of stable isotopes in tissues:

  • δ¹⁵N (Nitrogen) → increases with each trophic level (~2.5–4‰ per step).
    → Indicates trophic position.

  • δ¹³C (Carbon) → varies by habitat or source of primary production (inshore vs offshore).
    → Indicates feeding habitat or carbon source.

Advantages:

• Integrates diet over weeks to months.

• Non-lethal (small tissue sample, e.g., fin clip or muscle biopsy).

Limitations:

• Requires baseline isotope data from ecosystem producers.

• May blur fine-scale diet differences.

Recommended Reading:

• Hussey et al. (2012+) – major reviews on isotope use in shark ecology.

7. Combining Methods – Example: Hammerheads & Dusky Sharks (South Africa)

Hussey et al. study

• Combined stomach content and isotope analysis to assess diet and trophic shifts by sex & size.

Findings:

• Hammerheads:

  • Large males feed on smaller sharks → higher δ¹⁵N.

  • Clear increase in trophic level with body size.

• Dusky sharks:

  • Opposite pattern → as they grow, feed on lower trophic level prey.

Comparison:

• Trophic level estimates differ depending on method (SIA gives higher values).

• Suggests SIA gives a truer long-term average, while stomach contents give a short-term snapshot.

8. Resolving the Inverted Pyramid – Macaulay et al. (Palmyra Atoll Study)

Question:
How can sharks have more biomass than prey?
Are they feeding on prey not counted in reef surveys?

Method:

• Combined stable isotope analysis (carbon + nitrogen) with spatial ecology at Palmyra Atoll.

• Compared carbon signatures across three habitats:

  1. Lagoon

  2. Fore reef

  3. Open ocean (pelagic)

Baseline Species Used:

• Species restricted to each habitat → used to calibrate δ¹³C gradients:

  • Lagoon species (e.g., snappers): ~–8.7‰

  • Fore reef species (reef fishes): ~–14.9‰

  • Offshore tuna: ~–16‰

Findings:

• δ¹⁵N (trophic level) ≈ constant → all predators high-level consumers.

• δ¹³C varied widely → indicates source of carbon differs by shark species.

Results:

• Blacktip reef sharks → feed mostly on reef-based prey (carbon signature matches lagoon/fore reef).

• Grey reef sharks → feed mostly offshore (carbon signature matches pelagic zone).

Conclusion:

• Many sharks obtain energy from outside the reef (offshore prey), explaining why reef surveys show “more predators than prey.”

• When accounting for external energy sources, trophic structure becomes thermodynamically consistent.

Implication:
Inverted pyramids are artefacts of spatial bias — not true energy inversions.

9. Broader Implications

• Energy flow in marine systems is not confined to a single habitat — top predators often integrate energy across ecosystems.

• Stable isotopes are essential for tracing such cross-boundary flows.

• Shows the importance of integrating behavioural, ecological, and biochemical data.

10. Final Key Points & Skills Takeaway

• Stomach contents = short-term, direct evidence.

• Stable isotopes = long-term, integrated diet signal.

• Combining both gives the best dietary resolution.

• “Inverted trophic pyramids” are apparent, not actual, caused by:

  • Energy subsidies from other ecosystems.

  • Temporal or spatial biases in sampling.

Analytical Skills

• Ecological research now heavily depends on quantitative analysis, data interpretation, and statistical modelling.

• Lecturer encourages students to strengthen data handling and coding (R, statistics) — critical for ecology and conservation.

🧠 Summary