Rocky Shore Ecology – Interactions, Disturbance & Theory

Course & Logistical Reminders

• Class Representative

  • Goldie has volunteered for Hamilton campus.

  • Tauranga still needs a volunteer → email lecturer or speak to Megan.

• Field Trips (compulsory, two 1-day trips)

  • Dates: 26 & 27\text{ July} (Week 3).

  • Worth ≈ 40\% of total paper (data collection + two assignments).

  • One trip = Rocky Shore; one = Soft-sediment.

  • Bring printed protocol (will be uploaded soon).

  • Bad weather ≠ cancellation → dress for cold, rain, mud.

  • Rocky shore: sturdy, grippy footwear.

  • Soft sediment: wetsuit boots/old shoes, spare dry pair, towel.

  • Space on bus for extra gear.

Lecture Objectives

• Show how biological interactions (competition, grazing, predation, recruitment) determine rocky-shore species distributions/abundances.

• Illustrate how classic rocky-shore manipulations underpin general ecological theory.

• Provide foundational case studies (1960s–1980s) still cited in modern marine ecology.

Recommended Reading

• J. S. Levinton – Marine Biology (Chapter 14; multiple editions).

• Texts by Connell, Underwood, Paine, Dayton, Sanford (see reference list).

• No e-versions in library; physical copies in Hamilton & Tauranga.

Physical Template of a Rocky Shore

• Strong vertical gradients in:

  • Desiccation, temperature, salinity, wave action.

• Leads to banded zonation BUT also patchiness created/maintained by biology.

• Organisms generally small, slow ⇒ ideal for manipulative field experiments.

  • Analogy: easier to move whelks than polar bears!

Four Key Biological Processes

1. Competition (space & food).

2. Grazing (limpets scraping algal films).

3. Predation (e.g.

   • Whelks, sea stars eating sessile fauna).

4. Recruitment (larval settlement into available space).

Competition Case Study – Connell’s Scottish Barnacles (1961)

• Species: Chthamalus stellatus (high shore) vs Semibalanus balanoides (mid-low).

• Larval settlement occurs over the whole shore, yet adult zones segregate.

• Removal/transplant experiments showed:

  • Semibalanus grows fast → overgrows & crushes Chthamalus.

  • Chthamalus more desiccation-tolerant (thicker shell) → survives higher.

• Conclusion: Upper limit set by physical stress; lower limit set by interspecific competition.

Competition + Grazing – Underwood’s Ulva Exclusion (NSW, 1980s)

• Question: Why does green alga Ulva stop at mid-shore line?

• Hypotheses tested:

  1. Grazing (limpets).

  2. Desiccation.

  3. Space competition with encrusting algae.

• 8 treatment combinations (caged roof, fenced sides, cleared/not cleared).

• Results:

  • Limpet exclusion ⇒ Ulva cover ↑ rapidly to \approx100\%.

  • Clearing space accelerated recovery.

  • Fully caged plots never reached full cover (≤60\%) → shading artifact decreased photosynthesis.

• Message: must include experimental controls for artifacts.

Predation & Keystone Concept – Paine, Dayton et al. (US Pacific NW)

• Community gouache: mussels, barnacles, algae, whelks, sea stars (Pisaster ochraceus), limpets.

• Keystone predator definition: species whose trophic effect is disproportionate to its biomass.

  • e.g. Pisaster limits mussel dominance, maintaining diversity.

• Diversity vs predation intensity shows a unimodal (hump-shaped) curve: \text{Diversity} \;\sim\; f(\text{predation})

  • Low predation → monocultures (mussels).

  • Intermediate → maximum heterogeneity.

  • High → prey eradication, diversity drop.

  • Framework = Intermediate Predation / Disturbance Hypothesis.

Multi-factor Predator–Grazer Study (Paine & Levin)

• Treatments isolated effects of limpets (bulldozers), whelks (selective predators), & both combined.

• Observations (relative survival):

  • Limpets → greater damage to larger, thin-shelled Balanus; smaller Chthamalus fare better.

  • Whelks prefer Chthamalus first; switch to Balanus once small prey depleted.

  • Combined effects even out, leading to similar survival ratios.

• Demonstrates complex, non-additive interactions among consumer guilds.

Disturbance & Patch Dynamics

Patch Size Matters (Menge & Sutherland; Sousa)

• Small gaps (<≈1\text{ m}): adjacent adult mussels simply close space ⇒ monoculture persists.

• Large gaps (≥4–8\text{ m}):

  • Sea-weeds & bay mussel (Mytilus trossulus) larvae recruit.

  • Leads to alternative stable states: algal canopy vs mixed mussel/barnacle matrix.

• Predator reach limitation: Pisaster seldom crosses >2\text{ m} of bare rock → prey refuge forms inside large clearings.

• Heat/desiccation assessed; found secondary to biological controls.

Alternative Stable States – Maine Canopy Cycle

1. Initial disturbance (ice scour, storm logs) clears canopy.

2. Rapid algal colonisation (kelp, fucoids) retains moisture, shades rock.

3. Canopy shelters gastropod predators → grazing on new barnacles/mussels.

4. Dense canopy winnows larval settlement (physical whipping & shade).

5. Another large disturbance can flip back to mussel/barnacle state.

• Transition threshold determined by interaction of disturbance magnitude, season (larval supply), predator density.

Physiological Context – Temperature & Upwelling (Sanford 1999)

• El Niño upwelling → cold nutrient-rich water (drop from 13^{\circ}\text{C} to <10^{\circ}\text{C}).

• Observed: cooler temps ⇒ Pisaster metabolism ↓ ⇒ predation rate ↓.

• Result: temporary mussel expansion & changed zonation.

• Highlights how climate oscillations modulate biotic interactions.

Key Take-Home Points

• Rocky shores are natural laboratories: small, sessile, slow fauna → elegant field manipulations.

• Upper distribution limits largely physical; lower limits usually biological (competition, predation).

• Diversity peaks at intermediate disturbance/predation; too little or too much reduces heterogeneity.

• Patch size, disturbance frequency, larval supply & consumer mobility jointly determine succession & alternative stable states.

• Experiments must control for artifacts (e.g. shading by cages).

• Temperature shifts (seasonal or climatic) can rapidly alter consumer behaviour and thus community structure.

Glossary

• Recruitment – settlement & survival of larvae to benthic life stage.

• Gregarious settlement – mass larval settlement event.

• Monoculture – area dominated by one species.

• Keystone predator – consumer with outsized community impact.

• Alternative Stable State – distinct community configuration that persists until substantial disturbance.

Classic References for Deep Dive

• Connell, J. H. (1961) – Barnacle zonation & competition.

• Underwood, A. J. (1980, 1981) – Ulva upper-shore limitation experiments.

• Paine, R. T. (1966, 1974) – Pisaster keystone predation.

• Dayton, P. K. (1971); Menge & Sutherland (1987).

• Sousa, W. P. (1984) – Disturbance & patch dynamics.

• Sanford, E. (1999) – Upwelling, temperature & Pisaster feeding rates.

“Every major idea in community ecology has been tested – and often discovered – on a rocky shore.”