Invasive species are known to grow, reproduce, and spread aggressively; this gives them the ability to thrive in new biological communities (Jahoda et al. 2009). Invasive species are species that are intentionally or unintentionally are out of their native range (Jahoda et al. 2009). In these environments, invasive species cause disturbance to the habitat (Jahoda et al. 2009). These disturbances increased competition and disease; they also alter nutrients availability and the habitats’ physical structure (Jahoda et al. 2009).
The Asian shore crab (Hemigrapsus sanguineus), native to the east coast of Asia, was introduced in 1988 via international shipping; to the Atlantic coast of the United States at Cape May, NJ (Bloch 2015).
Asian shore crab’s prey on invertebrate species such as Mytilus edulis L. (Blue Mussel) and Littorina littorea L. (Common Periwinkle). The green crab (Carcinus maenas L.) native to eastern Atlantic, northern Africa, and Norway, has ingrained in western and eastern North America, and South Africa (MacDonald 2018; Grosholz 1996). The green crab, an invasive species, to North harbor, has unfavorable effects on native invertebrate species such as bivalves (Hamilton 2018).
Green Crabs are also natural predators governing native crabs and feeding on mussels (Hardy 2008).
In this study, tidal action, Habitat Heterogeneity, and environmental stressors will be examined when studying the abundance of Blue Mussels and Common Periwinkle. Both these species are essential to New England’s intertidal environments (Jahoda 2009). Tidal action brings in nutrients, food, flushes out debris, and maintains proper salinity. Habitat Heterogeneity can increase species diversity due to environmental conditions (Tews environmental resources et al.
2003). In terrestrial habitats, species diversity can either increase or decrease depending on a species ability to adapt to an environment (Tews, 2004). Environmental stressors include air and sunlight exposure, salinity, and water temperature. Tidal action affects all these factors causing an environmental gradient. Tolerance to tide action navigates where species will be found in these different intertidal zones. Species with higher tolerance will be found closer to low tide zones. Low tide zones have more rocks providing more shelter for species, leading to more diversity. I hypothesize that the relative abundance of Asian Shore Crabs and Green Crabs is greater in the lower and mid tidal zones, because of greater habitat heterogeneity in the low and mid intertidal zones; providing both crab species more shelter and food.
The purpose of this study is to examine differences in species abundance between Asian Shore Crabs and Green Crabs. To understand the relationship between what drives the species’ abundance to alter over the years. And to see the long-term effects, invasive species have on native species in different intertidal zones.
Figure 1: Town Neck beach in Sandwich Massachusetts, with the different intertidal zones.
The study was conducted at Town Neck Beach in Sandwich MA on the Cape Cod pine barren and sand dunes. Shorelines are subjected to residential development. The low tidal zones are heavily concentrated with large rocks and pebbles. The concentration of these rocks decreased as proximity to the high-tide line increased. The high tide zones were concentrated with sand and a few larger rocks. At the time of the data collection, the day was sunny with clear skies.
Continuous lines of seaweed distinguished the High tidal zone. From the high tide line with a metric tape, I measured three transects. The High tide zone was measured at 15 m from the high tide line. Mid tide zone is 30m, and the Low tide zone is 45 m. I placed three circular plots that had an area of 45.6m in each intertidal zone. In each zone, I collected Asian Shore Crabs (Hemigrapsus sanguineus), and Blue Mussels (Mytilus edulis). Green Crabs (Hemigrapsus sanguineus), and Common Periwinkle (Littorina littorea) were also collected. Barnacles in each sampling area were examined using a three by four grid. Barnacles that touched the line of the grid were recorded. At each tidal zone, I measured ten individuals from each species to the nearest 0.1 mm with a dial caliper. A dial caliper is a tool used for measuring in millimeters come with a handle making it easy and portable to use. A light touch method was used.
When sampling each plot, species are counted for and sampled. Those numbers I used to make a bar graph to represent the mean relative abundance of both the Green Crabs and Asian Shore Crabs in each of the three intertidal zones. The error bars were made using 95% confidence intervals around the mean. These data analyses were done using Excel, 2016.