Some of the coastal habitats that are fun to explore include the numerous jetties and groins in most of the towns on the Cape. Several years ago those were built in an effort to control erosion at public beaches and private property. We now know that they have had the opposite effect, and actually increase erosion in those areas by focusing wave energy on a narrower section of the beach.
Still these structures may contain a wide diversity of living organisms that have adapted to the harsh conditions of constant wave, tide, and storm activity. The most conspicuous inhabitant of this habitat is the common rockweed (Fucus vesiculosus), a brown alga that can survive periodic exposure to the air at low tide. Hiding underneath its fronds are bands of small animals such as barnacles, periwinkle snails, and crabs.
However, these plants and animals are not distributed evenly along the jetty. In fact, some of the rocks might be completely devoid of all life, while others have smaller, more randomly distributed populations. This variation may be caused by several physical and biological factors.
The most important physical influence occurs on the side directly facing the waves and tides. Obviously strong wave action will limit the colonization and growth of any critter trying to get established. Even those already present could be removed by strong wave energy from a storm.
Some of the most significant factors are not readily visible, and these include the interactions between the different life forms. Because of the limited surface area on the rocks, there is competition for that valuable real estate. Also, some species may only occupy a small portion or band because they are unable to survive prolonged periods of exposure. Some may seek shelter under the seaweed fronds to avoid predators or to remain moist.
The rockweed can also influence colonization rates. The movement of the water causes the fronds to sway back and forth, and this motion can brush away any larvae attempting to attach to the rock surface. Other plants and animals may be subject to grazing pressures. Some of these habitats on the Cape contain huge numbers of periwinkle snails which normally feed on small diatoms and algae on the rocks and seaweed. But if their numbers get too high, they may even start to consume the rockweed, affecting its growth and that of many other species. These small variations make this as dynamic a system as that seen along the sandy beach. The zones and sections of living organisms on a jetty are constantly changing as one would expect in the marine environment.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.