Rocky Intertidal – Land to Shore Communities

1. Rocky Intertidal – Land to Shore Communities The rocky intertidal zone is an area situated in rocky coastal regions. It is often referred to as the littoral zone and covers the area lying between the maximum height water reaches in a specific area and the lowest height. Many various types of intertidal zones exist due to landscape differences (cliffs, beaches, wetlands, etc). These zones are researched extensively due to the fact that they provide insight into biological communities which are forced to inhabit the relatively narrow bands of height and depth which make up the intertidal area. The intertidal areas are divided into 3 zones; high, middle and low (Figure.1), each with their own set of resources and conditions which various populations of organisms have to adapt to. Pairing this with the other populations occupying these same areas at the same time and one can observe a community of interacting organisms trying to survive. The upper intertidal zone is exposed to wave action only during high tide. For this reason, species dominating this area are under temperature stress, levels of high salinity due to evaporation and longer periods of exposure to sunlight compared to lower or sheltered areas. The most tolerant species occupy this area and most of them rely heavily on the presence of tidal pools to prevent desiccation. There are many biotic and abiotic factors that influence species abundance, richness, diversity, etc. in intertidal communities. Competition is the struggle of individuals to get or acquire a resource that is in limited supply (Figure.3), this then often results in a positive - negative effect amongst individuals. Resources vary from space to food. Figure.1 - Zonation of Littoral zone range Competition is present in all levels of all intertidal zones although much less significant on the sandy shores zone due to factors minimizing competition such as the three dimensional nature of the substratum linked with the fact that most organisms here are burrowers and unlike rocky shores, competition by interference (undercutting, overgrowth and crushing) does not occur very often within sandy zones. Most animals on sandy shores are opportunistic feeders which furthermore decreases competition. Figure.2 - Barnacle colony settled on rocks in the upper rocky intertidal zone Figure.3 – Hermit crabs and snails scavenge on Gumboot chiton carcass The middle of the intertidal zone is exposed to intermediate levels of factors that affect other zones. This area is dominated by bivalves and barnacles in most areas. For example, in the central part of Chile the mussel species Perumytilus purpuratus or barnacle species Jehlius cirratus and Chthamalus scabrosus dominate this area of the intertidal zone depending on the abundance of predators like the sea snail Concholepas concholepas. Main predators inhabiting sandy shores are mobile crustaceans and fish. Some animals like shrimps, prawns, juvenile crabs and juvenile flatfish use sandy shores as nursing grounds and they consume large amounts of smaller invertebrates. Other extensive feeders on sandy beaches and mudflats are large fish like flounders, plaice, dabs, and the elasmobranch rays. Competition for food and space are important features that shape the structure of communities in rocky intertidal areas. There are two main modes of competition seen in rocky intertidal areas. Interference competition: is the most common mode of intertidal competition and results in direct and physical prevention of settling by competitive species. This is used to deal with the limited space resource. This is the one resource most sessile living organisms like sea anemones, bivalves and barnacles defend because on rocky shores your position on the rock influences food availability. Exploitation competition involves mutual striving for a non-defensible resource. It occurs when one species attempts to deplete its food/ resource supply to reduce the probability of another species using it. There is no contact between the participants but the more efficient species wins. This competition interaction for food is seen in the mobile species of the rocky intertidal zones like limpets, sea stars, urchins, etc. The lower intertidal zone is submerged under water except during low tides. There is a greater species abundance and richness in this zone than in the other zones due to a large quantity of vegetation in this area in the form of sea grasses and other algae and the protection from large predators provided by shallow waters, rocky shelters and high levels of wave action. For this reason inter-specific interactions such as, complex food webs, competition and other relationships between species, play a much larger role here than in the other intertidal areas. Sea stars (Figure.4) and other echinoderms are the top predators in this area followed by snails that feed on any substances which contain nutritional value such as dead fish, algae (grazers), and waste. Figure.4 – Dominant Lower Zone Predator starfish Pycnopodia helianthoides