An Introduction to Zonation

Rocky Intertidal Ecology Rocky - hard substrate Intertidal - area between high tide and low tide marks Ecology - the study of the interactions that determine the distribution and abundance of organisms. * Interactions can be between organisms or between organisms and their environment

Intertidal zone This area is is exposed to the air usually twice a day as the tides ebb and flood Tides create a partly aquatic and partly aerial environment Locally the tidal range is about 3 meters Rocky, sand, estuaries, mangroves, coral reefs, etc. Patterns are similar worldwide

Vertical zonation Banding pattern on the rocky shore as a result of the rise and fall of the tides The distribution of organisms is in horizontal bands Upper, middle and low zones Caused by differing tolerances of organisms to stresses, both biotic and abiotic Examples?

Limits of organisms Upper limits are set by abiotic factors such as desiccation, temperature, salinity, food and oxygen availability, light Lower limits are set by biotic factors such as competition, predation, herbivory

Abiotic factors

1. Desiccation = water loss; drying out Occurs at low tide due to air exposure Can increase with wind Some species are more tolerant to being in the air (e.g. limpets, Fucus, Porphyra) and can live higher in the intertidal zone Adaptations: - close up e.g. barnacles, mussels - lose water then rehydrate e.g. Fucus

2. Temperature Water temperature varies only slightly over the year (7-12oC) Air temperature, however, can vary drastically (0-30oC) Organisms out of the water are subject to greater variations in temperature –Adaptations: –Evaporative cooling by mussels –Eurythermal enzymes

3. Salinity Salinity can increase with evaporation or decrease with fresh water input (e.g. rain) Euryhaline - tolerant to wide variations in salinity e.g. mussels, barnacles Stenohaline - can only tolerate a narrow range of salinities e.g. sea stars Adaptations: Close up e.g. barnacles, mussels Ion pumps e.g. crabs

4. Food & oxygen availability Many marine organisms get their food from the water (as plankton!) they can only feed when underwater Also, many marine organisms obtain oxygen from the water Organisms living high in the intertidal zone have a limited time in which they can feed and acquire oxygen Adaptations: - scaleless fish e.g. clingfish - feed whole time they are underwater

5. UV light Marine organisms can get sunburns too Especially when out of the water –Adaptations: –Sun screen - natural compounds that absorb UV –Protective shell

Biotic factors

1. Competition = use or defense of a resource that reduces its availability to other individuals Interference - direct interaction Exploitative - differential use of resource Interspecific - between species Intraspecific - between individuals of the same species

2. Predation Animals eating other animals Predators can be generalists (eat a wide range of food) or specialists (eat a single prey type) Refuges from predation: Size e.g. some mussels are too big to be eaten by Pisaster Space e.g. limpets living on vertical surfaces to avoid predation by birds Height in the intertidal e.g. Pisaster / mussel interactions (Paine experiments)

3. Herbivory Animals eating plants Similar to predation interactions Refuges from herbivory: Chemical defenses e.g. sulphuric acid in Desmarestia, bromine in Prionitis Physical defenses e.g. calcium carbonate in coralline algae Height in the intertidal e.g. Ulva and Porphyra live very high to avoid being eaten

A reminder… Upper limits are set by abiotic factors such as desiccation, temperature, salinity, food and oxygen availability, light Lower limits are set by biotic factors such as competition, predation, herbivory