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
