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Hard Substrate Communities Hard substrate = rocks, old shells, pilings, jetties, piers, docks Intertidal – perhaps best know marine community Tides = periodic rise and fall of sea level • Caused by gravitational attraction of sun and moon and centrifugal force generated by the rotation of the earth-moon system around the sun • Periodicity of 24 hours, 50 minutes • gravitational attraction: sun and moon pull at the Earth’s oceans, causing a tidal bulge • the tidal influence of the Moon is about twice that of the Sun • Two classifications of tide based on the orientation of the Sun and Moon. • Spring tide : Sun, Moon and Earth are all in a line (full or new moon), Sun and Moon act together to create one large tidal bulge, and causes the greatest tidal differences because the. • Neap tide: Sun and Moon are at right angles from each other (quarter moon phase), when the, smaller tidal bulge. • distance of Moon: 50K km difference. perigee (nearest), the gravitational pull is ~40% greater than at apogee (farthest). • Approximately two high tides and two low tides each day • when it is high tide at one coastal location, it is low tide along a different coast a quarter of the way around the Earth. • Bay of Fundy, eastern Minas Basin, on the - Moon near perigee @ spring tide, HT up to 16 meters above LT. • The geogr. location, shape of the shoreline, water depth key reasons for tidal variations. Result of the tides – • Exposure in the air for periods every day – Greater temperature differences – Desiccation/water loss – time of exposure changes as tide advances – Intense solar radiation Waves • Mechanical effect • Creates a “splash zone” • Mixing – chemicals, dissolved gases, suspended particles Salinity changes • Rainfall, freshwater runoff • Tidal pools – evaporation exceeds precipitation Barnacles Barnacles Mytilus californianus Nucella (Thais) Limpets Starfish - Pisaster Sea Anemones Tunicates Bryozoans Sea Urchins Algae Adaptations – Organisms 1. Minimize water loss/desiccation during exposure • • • Hide out in most crack, crevice, under algae Tolerate tissue desiccation, rehydrate at high tide Structural behavioral modifications – close up, clamp down, mucus, gills in cavity Adaptations – Organisms 2. Minimize thermal stress • • • Large size, surface/volume ratio Light/heat reflection Evaporative cooling Adaptations – Organisms 3. Avoid destruction in waves • • • Attach to substrate Thicker shells Clamp to substrate 4. Salinity stress Conspicuous horizontal banding, or zonation of organisms Stephenson & Stephenson 1949 universal scheme to describe • Supralittoral – splash zone – Littorina (periwinkle), encrusting lichens and algae • Midlittoral – barnacle and mussels zone • Infralittoral – algae and more diverse mix of animals • Sublittoral – subtidal zone Why? 1. Period of submergence – “critical tidal level” (Doty 1945) 2. Desiccation – sets upper limits to organisms • • north vs south-facing slopes irregularities 3. Thermal stress 4. Sunlight Joseph Connell 1961 Coast of Scotland: Chthamalus stellatus Balanus balanoides Thais lapillus (now Nucella) Robert Paine 1966 • Observed structure of the food web in rocky • • • • intertidal from Washington to Costa Rica Washington – predatory starfish Pisaster and Nucella, the drill – lots of diversity Costa Rica – no starfish, the drill Nucella is the top predator – less species diversity What was going on?? Went back to Washington and experimented Paine’s experiments: • Removed starfish from rocks and observed results • With Pisaster ~ 15 species • Without ~ 8 species (plots dominated by Mytilus, the best competitor for space) • Pisaster - “Keystone species” – indirectly controls the structure of whole community Connell in California (1970) • Balanus glandula (like Chthamalus) • Balanus cariosus • Refuge in space (Balanus glandula) • Refuge in size (Balanus cariosus) Paul Dayton 1971 Space is key – processes involved in getting, keeping, and using space • Physical stress – desiccation, etc. • Physical disturbance – waves, drift logs • Predation – biological disturbance • Competition • Natural death Dayton’s experiments: • Fucus – whip effect on barnacles vs protection from desiccation – Treatment = removal of Fucus • Limpets – eat algae; disturb and/or eat barnacles – Treatment = “dog dishes” to exclude limpets • Thais – drills mussels and barnacles – Treatment = cages to exclude • Pisaster – eats mussels and barnacles, etc. – Treatment = removal Dayton’s conclusions: • Sessile marine organisms compete for space • Three levels of “biological disturbance” – Grazing by limpets – Predation by Thais/Nucella – Predation by Pisaster • Pattern in nature = mosaic of disturbance and recovery; disturbance maintains patchiness, keeps diversity high Impact of Connell, Paine, and Dayton’s work on field of ecology • Shift in thinking about how communities are • • organized Earlier view – competitive dominant has broadest niche, diversity the result of specialization to avoid competition New view – dominants often less abundant, inferior competitors can have broader niche, predators and disturbance create diversity by preventing competition Rocky Intertidal Paradigm 1. Upper limits of distribution are primarily determined by 2. 3. 4. 5. physical factors Lower limits of distribution are primarily determined by biological interactions (competition for space and predation) Predation or physical disturbance can prevent competitive exclusion (enhancing species diversity) by keeping superior competitors at low densities In benign physical environments, predation is most important in determining community composition In harsh physical environments, predation intensity is reduced and competition is most important in determining community composition Exceptions and Limitations of RIP 1. Upper limits of distribution are primarily determined by physical factors 2. Lower limits of distribution are primarily determined by biological factors (competition for space and predation) • Starvation restricts upper reaches • Larval settlement determines the limits of distribution -- shorter period of inundation means reduced probability of settlement in upper reaches -- larvae chose where they settle -- adult larval interactions affect settlement patterns Exceptions and Limitations of RIP 4. In benign physical environments, predation is most important in determining community composition 5. In harsh physical environments, predation intensity is reduced and competition is most important in determining community composition • Assumes prey have a greater physical tolerance than predator, can find refuges in space • Variations in recruitment determine the level of competition • Importance of predator selectivity and competitive hierarchy • In same environment, both predation and competition are important, both for different life stages