Rocky Shore Ecology
The rocky shore ecosystem is a wonderful example of how many layers of sub-habitats can intertwine with each other but differ fundamentally, whilst still sharing close proximity to one another. Various environmental pressures such as salinity, water, wind and oxygen play the master roles here in determining where organisms dwell and build communities.
Rocky shores are known to be one of the most biodiverse habitats in the marine environment and can include many different habitat types such as steep rocky cliffs, platforms, rock pools and boulder fields (2). Organisms such as algae, kelp, mussels, crabs, lobsters, snails, and sea urchins are characteristic of rocky shores, as well as a variety of fish.
The hard substrate offers stability and works as a shelter creating a safer environment for seashore communities to form. However, this habitat is under constant bombardment from waves and winds. It also undergoes alternating periods of air exposure and water submersion due to tidal cycles creating a gradient of environmental conditions from a terrestrial to a marine ecosystem. It is this particular dynamic that makes rocky shores such a complex system and that is why rocky shores are home to many different animals and algae. Organisms that live in this area experience daily and seasonal fluctuations in their living environment so they must tolerate extreme changes in temperature, salinity, moisture and wave action to survive.
Rocky shore zonation
Species are adapted to specific environmental conditions. On rocky shores, the rock surface is divided into different horizontal bands/zones called ‘vertical zonation’ (1). Each zone holds a particular group of species. Species able to withstand sun exposure for long periods will be found in the upper parts of the rocky shore. Moisture dependent organisms are not resistant enough to constant wave action and are only found lower on the rocky shore.
The zonation of the rocky shore corresponds to three big zones—from the driest zone to the submerged zone—those being; the supralittoral (or splash zone), the intertidal zone, and the infralittoral (or subtidal zone).
The supralittoral zone
The supralittoral zone is virtually a terrestrial environment, remaining mostly dry aside from the semi-constant splash of waves. To live there, species must be tolerant to high levels of salt and limited input of water. For this reason, this is categorised as the poorest zone of the rocky shore and is generally dominated by periwinkles, lyme grass, thrift, and lichens.
The intertidal zone
The intertidal zone is the area where water may or may not be covered depending on the height of the tides (4). This zone is subdivided into three other bands according to the time it keeps submerged due to the tide: upper, middle and lower. The diversity and abundance of plants and animals in the upper intertidal are low, with mainly periwinkles and seaweeds.
The lower intertidal zone is a very biodiverse area where the regular covering and uncovering of the shore provides a regular income of food and nutrients for plants and animals. Due to the high levels of nutrients, the distribution of many species is limited by competition with other species for space.
Rocky shore fauna
Rocky shore animal populations are dominated by invertebrates and fish (1). Stationary, filter-feeding invertebrates such as barnacles, mussels, and oysters, are often the dominant animal species on the rocky shore. Their abundance depends on competition with each other, the environment of the shore, and predation pressure from predators like starfish and oyster borers.
Limpets, chitons, and various gastropods (snails) belong to another very important group of animals that live on the rocky shore; the mobile grazers. These animals move around the intertidal zone and scrape the rocky shore free of any algae and settled, juvenile shellfish (3). By scraping off algae they maintain the structure and diversity of rocky shore ecosystems by preventing algal growth from dominating the intertidal zone.
Rocky shores are plentiful with algae and algal density increases as you move lower down the shore. Seaweeds are also abundant in the subtidal zone where they often form dense kelp forests and provide habitats for many fish, worms, crustaceans, gastropods, and many more marine animals.
Fish are regular visitors to the rocky shore due to hiding places found in rocky crevices and the presence of marine plants which offer protection and can act as safe refuge for an incredible number of fish species. Known as reef fishes, these colourful creatures have a crucial ecological role in the marine ecosystem. From tiny ones to bigger fish, the biodiversity is immense and due to fishing and loss of habitat some species are constantly under threat. Rock gobies, clingfish, pipefish, butterfly fish, clownfish, lumpsucker, groupers, parrot fish and sharks are part of the fish community on the rocky shores.
The rocky shore is also visited by many birds and marine mammals as a place to rest, warm up and breed (5). Many seabird species, such as penguins, shags, gannets, and albatross, use the rocky shore as a nesting area to dry off and warm up after a cold swim. If you are lucky enough to live or visit an area with seals or sea lions, chances are you will find them basking in the sun on the rocky shore, warming up after a cold swim.
Next time you’re walking along your local rocky shore, remember it isn’t just rocks and water, but a myriad of complex interactions between organisms and their environment.
By Luana Borde
References
1 Satyam, K. and Thiruchitrambalam, G., 2018 Chapter 7 – Habitat Ecology and Diversity of Rocky Shore Fauna in Biodiversity and Climate Change Adaptation in Tropical Islands, pages 187-215.
2 Marko P.M., J.M. Hoffman, S.A. Emme, T.M. McGovern, C.C. Keever, and L.N. Cox. 2010. The ‘Expansion-Contraction’ model of Pleistocene biogeography: rocky shores suffer a sea change? Molecular Ecology 19:146-169.
3 Menge B.A, F Chan, J Lubchenco. 2007. Response of a rocky intertidal ecosystem engineer and community dominant to climate change. Ecology Letters 11:151-162.
4 Przeslawski R., A.R. Davis, and K. Benkendorff. 2005. Synergistic effects associated with climate change and the development of rocky shore molluscs. Global Change Biology 11:515-522.
5 Kendall MA, MT Burrows, AJ Southward, and SJ Hawkins. 2004. Predicting the effects of marine climate change on the invertebrate prey of the birds on rocky shores. Ibis 146:40-47.
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