Abstract:

Introduced species can affect community structures by changing the habitat, becoming a predator to a native species, or becoming a competitor with one. Carcinus maenas was introduced to the east coast in the 1800s and Hemigrapsus sanguineus was introduced later in 1988. These two introduced species play an important role as predators in the rocky intertidals. Looking specifically at the rocky intertidal at Northeastern Marine Science Center in Nahant, MA, the relationship between the two species is evaluated to find if the later introduced species has become dominant over C. maenas. Random sampling was used, where each crab was identified and the width and length of the carapace was measured. Through a chi-test and ANOVA tests, I found that H. sanguineus is dominant over C. maenas (at least for the low and mid-intertidals), but the body size is not significantly different between the two.

Background:

   

Species being introduced to a community have major affects on community structures. Sometimes these affects are seen immediately after the species is introduced, and other times these affects are seen much later. Entire communities can be altered by the new species becoming a major predator of the area, becoming a new competitor to a different species, or even changing the habitat. These events can harm the biodiversity of the area as well. An example of the negative affects of introduced species is when the Asian chestnut blight fungus almost completely eliminated the American chestnut (Geerat). This has changed the forest structure in many areas.

Many species are brought to new locations through human influences. An example is ballast water which is when unladen ships take on water that can contain small organisms. Once these ships arrive to its destination, they release the water including any organisms it may contain from its origin. When new organisms get introduced to a new community, the scale that they affect the community varies. Sometimes it can be a quick alteration and sometimes a slow one to the community.

Crab species are examples of species that have been introduced to many locations on the eastern coast of the United States. The Carcinus maenas, the green crab, species was introduced in the early 1800s to the eastern coast. This species originates from Northern European waters. The main cause of this introduction is thought to be ballast water. They have been thought to have disrupted many invertebrate community structures throughout the east coast and even the west coast.

The Asian shore crab, Hemigrapsus sanguineus., was first noticed in New Jersey 1988. They quickly spread across the northeast intertidal areas. These species are more commonly found in the low to mid intertidal zones. They are thought to compete with the C. maenas species (Lohrer,et al). Lohrer has found that the H. sanguineus species has caused a decline of C. maenas in three locations in New England. He found evidence using laboratory experiments that the H. sanguineus crab can prey on C. maenas which may be partially causing the decline. He also found that recruitment was reduced for C. maenas through pressure from H. sanguineus while H. sanguineus recruitment was not affected by the other species. Another important finding that Lohrer has is that even though individual H. sanguineus has less pressure on the Mytillus species through predation, this crab species is found to live in higher densities therefore putting more pressure on the prey. 

A different study found that both species share the same preference to prey to Mytilus edulis, Littorina obtusata and L. saxatilis. They both avoided Littorin littorea (Tyrrell, et al). This creates a competitive overlap between the two so one might expect the more fit species to become more dominant over the other. (image: mytillus)

It is important to understand the affects H. sanguineus has on C. maenas since crab species can greatly affect the community structures of the intertidal. Not only is the east coast being affected by the two introductions, but it is also affecting the west coast (Grosholz, et al). In 2004, a Clark student, Brigham Whitnam, researched the distribution of both of these species in Nahant, MA. Whitnam found that the C. maenas species was still dominant in Nahant. However, I found that while I visited Nahant for the first time, more H. sanguineus may be dominating this intertidal than four years ago. This has many implications if true since it gives an example of how fast an introduction can affect a community, even if it is on another introduced species.

In this study the presence of both crab species was surveyed in Nahant, MA. The purpose is to find which crab species is dominant in this particular intertidal.  If H. sanguineus is more dominant now than it gives a good example of how fast introduced species can change communities. To test this, I identified the species and measured the body size of each crab that I found. I used Chi-tests and ANOVA tests to find if one was found more often and if one species tended to be larger over the other.