Rocky Shore Ecosystems – Physical Drivers, Zonation & Adaptations (copy)

Lecturer & Course Context

• Lecturer: Dr Megan (new Marine Science & Mātauranga Māori Research Fellow, University of Waikato)

  • PhD on managing crown-of-thorns starfish outbreaks to help mussel restoration in partnership with local iwi

  • Personal research focus: integrating Indigenous knowledge with marine science for better ecosystem management

• Two-part Rocky-Shore block

  1. Today – physical/environmental drivers + organismal adaptations

  2. Next week (Hazel) – biological interactions (predation, competition, experiments)

• Recommended readings

  • Marine Biology: An Ecological Approach – Nybakken & Bertness, Ch 6

  • Marine Biology: Function, Biodiversity & Ecology – Levinton, Ch 14

Why Rocky Shores Are Model Systems

• High accessibility: walkable at low tide; no SCUBA or corers needed

• Two-dimensional habitat: organisms live on substrate, not buried in it

• Sharp environmental gradients occur over centimetres–metres → easy to detect zonation

• Amenable to manipulation: organisms can be removed/added, cages installed, etc.

  • Classic experiments:

  • Paine’s keystone-predator (starfish removal → mussel overgrowth)

  • Disturbance/succession (logs scouring surfaces, post-disturbance colonisation)

• Generalisable theory: ideas developed here (keystone species, disturbance, succession, grazing control) now applied in terrestrial ecology

Core Physical Drivers of Community Structure

1. Tides (Primary Driver)

• Generated by gravitational pull of Moon & Sun + centrifugal force from Earth’s rotation

• Lunar day ≈ 24\,\text{h}\,50\,\text{min} ➔ successive high tides shift ≈ +50\,\text{min} daily

• Semi-diurnal regime (NZ): ≈ 2 high + 2 low tides each lunar day

• Spring vs Neap Tides

  • Spring (Sun–Moon–Earth aligned, full/new moon) → larger range

  • Neap (Sun & Moon at 90^{\circ}) → smaller range

  • Tauranga example:
    \text{Range}{spring}\,\approx\,2\,\text{m} \qquad \text{Range}{neap}\,\approx\,1.3\,\text{m}

2. Wave Exposure

• High-energy coasts: water pushed farther up shore → wider zones but lower diversity (mechanical stress)

• Sheltered sites: narrower zones, higher species richness

3. Substrate Topography & Microhabitats

• Cracks, crevices, tide-pools retain water & provide shade ⇒ local refuges that extend upper limits of species

4. Salinity, Temperature, UV, Dissolved O₂

• Strongest gradients in mid-intertidal; more stable in subtidal

Universal Zonation Pattern (Biological Zonation)

“Same functional groups appear in the same order worldwide despite differing species”

Community Components & Their Roles

Primary Producers

• Macroalgae (Seaweeds)

  • Provide habitat more than nutrition (low caloric value)

  • Zonation within algae:

  • Subtidal: Ecklonia, Macrocystis, Durvillaea (bull kelp)

  • Lower intertidal: Carpophyllum (with float bladders)

  • Mid intertidal: tough turf/crusts (Ulva, Corallina, Homosira)

• Microalgae

  • Benthic film (microphytobenthos) → food for limpets, chitons

  • Pelagic phytoplankton → filtered by suspension feeders

Suspension / Filter Feeders (Sessile “Encrusters”)

• Bivalves: mussels, oysters

  • Behaviour: close valves to trap water; live in clumps → mutual moisture retention

• Barnacles (crustaceans)

  • Cement shell to rock; open operculum under water and rake with cirri

  • Occupy high-intertidal where few competitors survive

• Serpulid Polychaetes, some crustaceans

Mobile Grazers

• Limpets & Chitons

  • Use hard radula to scrape benthic algal film

  • Need bare rock; avoid dense mussel cover

  • Foot forms vacuum seal during emersion

  • Classic cage experiments show: grazing → \approx100\% algae removal, frees space for recruits → ↑ biodiversity

• Top shells, periwinkles (upper zones)

Predators / Keystone Species

• Asteroids (sea stars)

  • Consume mussels; removal → mussel monoculture (Paine 1966)

• Carnivorous gastropods (whelks = oyster borers)

  • Drill through shells with radula, suck out tissue

• Role: regulate dominants, maintain species balance

Adaptations to Intertidal Stressors

Water-Loss / Desiccation

• Morphological

  • Bivalve shells close; mussel clumping

  • Limpet shell crenulation ↑ surface area → better convective cooling

  • Light-coloured shells reflect solar radiation

  • Chitons tolerate up to 75\% body-water loss

• Behavioural

  • Retreat to crevices, tide-pools (crabs, gastropods)

  • Anemones embed among mussels/algae

Wave & Mechanical Stress

• Macroalgae: shorter, tougher forms on exposed coasts; develop larger holdfasts

  • Exhibit phenotypic plasticity (e.g.
    bladders in sheltered sites only)

• Sessile animals: heavy cementation (barnacles) or byssal threads (mussels)

Physiological

• Seaweeds = osmoconformers (internal salinity tracks ambient)

• Some animals produce heat-shock proteins, mucous coatings etc. (implicitly referenced)

Experimental & Theoretical Insights Originating from Rocky Shores

• Keystone Predation (Paine)
  \text{Starfish removal} \rightarrow \text{Mussel dominance} \rightarrow \text{↓ diversity}

• Disturbance/Succession: log scour events create gaps → predictable recolonisation sequence

• Grazer Control: Exclusion cages show limpets/chitons regulate algal cover & promote community heterogeneity

• Concepts extrapolated to forests, grasslands (e.g.
  browsing ungulates ≈ limpets)

Geographic Variation in Tidal Range & Exposure

• Micro-tidal tropics (<0.5\,\text{m}) → weak zonation

• Meso-tidal NZ (East ≈ 2\,\text{m}, West ≈ 3\,\text{m})

• Macro-tidal polar/boreal (Bay of Fundy up to 11\,\text{m}) → huge zones

• Wave-exposure case study (Whangarei Heads)

  • Site C (exposed): wide zones, low diversity

  • Sites A & B (sheltered): narrow zones, high diversity

Key Take-Home Questions (Self-Test)

• What three physical factors most strongly structure rocky-shore communities?
  \text{Answer: Tides}\;{\rightarrow}\;\text{desiccation},\;\text{Wave energy},\;\text{Substrate microtopography}

• Map the typical vertical order of functional groups from subtidal to splash zone.

• Give two behavioural and two morphological adaptations that mitigate water loss.

• Explain how limpet grazing can increase species richness.

• During which lunar phases do spring tides occur and why?
  (Hint: full & new moons; Sun–Moon–Earth alignment maximises combined gravitational pull.)

Ethical, Cultural & Practical Notes

• Integrating Mātauranga Māori provides holistic perspectives on stewardship, emphasising collaborative management with iwi for sustainable shellfish restoration (linking lecture content to real-world conservation).

• Rocky-shore experiments offer low-impact, high-insight opportunities; but ethical removal/manipulation should respect indigenous values and local regulations.

Reference Cheat-Sheet (Equations & Data)

• Lunar-day length: T_{lunar}=24\,\text{h}+50\,\text{min}

• Tidal amplitude: A = H{high} - H{low}

• Tauranga ranges: A{spring}\approx2\,\text{m},\;A{neap}\approx1.3\,\text{m}

• Water-loss tolerance example: \text{Chiton}\;\Delta H_{2}O \le 75\%

Quick Summary

• Rocky shores are natural laboratories: easy access, sharp gradients, manipulable communities.

• Desiccation stress tied to tides is the master variable; wave force & microhabitats modulate patterns.

• Despite geographic idiosyncrasies, a universal banding of functional groups occurs worldwide.

• Organisms display remarkable behavioural, morphological & physiological adaptations to survive alternating marine & terrestrial conditions.

• Foundational ecological theories (keystone predation, disturbance, grazing control) emerged from simple but elegant rocky-shore experiments and now inform broader ecosystem management.