Alien Species

Many species of organisms have been introduced into salt marshes.  They have come as “passengers” on ships either attached to the hull or in water used as ballast.  Some species have been purposefully introduced for food or sport.  Often these alien species have significant impacts on the biology of the salt marsh.  They can change the balance of species interactions causing some species to decline and others to increase.  Some examples of important introduced species are briefly described below.

           Northern Periwinkle, Rough Periwinkle (Littorina littorea).  This species is a snail that is now dominant in many rocky shoreline areas.  It was introduced in the 1860’s, possibly as a human food item.  It eats algae and scrapes the sediment off rocks and has resulted in many rocky areas being denuded of resident algal species.  It can reach densities of several hundred per square meter.  It has often displaced the native mud snail.

           European Green Crab (Carcinus maenus).  An omnivorous crab that was introduced in the late 1700’s.  It has become one of the dominant crabs along Atlantic shoreline intertidal areas.  The crab is found in both the intertidal zone and the low marsh.  The green crab is primarily a scavenger but can also be predatory.    

           Japanese Shore Crab (Hemigrapsus sanguineus).  A small (to 4 cm) greenish to purple crab that was introduced into New Jersey in 1988.  The legs have a distinct banding pattern of alternating light and dark colors.  It is spreading rapidly along the Atlantic coast.  It is found in the intertidal zone.  The crab eats various kinds of algae and a variety of animals.  It is able to move fairly quickly and is one of the more aggressive crab species.  It has probably been responsible for a decline in the number of intertidal snails such as the periwinkle.  The long-term impacts of this species are unknown and are under active investigation.

            Species that are native to the NYC area can be considered invasive aliens in other locations.  For example, smooth cordgrass (Spartina alterniflora) is invading salt marshes in the coastal areas of the Pacific northwest.  There it is being treated as a major problem and threat to the function of local salt marshes.