Rocky Intertidal Research in the Boston Harbor Islands National Recreation Area

By Anna Manyak Davis, University of Maryland College Park

This article, and others in the series "Parks in Science History", was written by a graduate student at the University of Maryland. The articles highlight the roles that national parks have played in the history of science and, therefore, the world's intellectual heritage. More articles and videos will be produced in the future.
A white lighthouse on a small rocky island surrounded by blue ocean water
Boston Harbor Islands National Recreation Area

NPS Photo

Along coastlines, where the land meets the sea, is an area known as the intertidal zone -- the region between the lowest low tide and the highest high tide. Organisms that live there are sometimes under water and sometimes exposed to air. On rocky coasts, the intertidal zone is home to many different stationary seaweeds and animals.

The rise and fall of the tides in these areas creates zones with different conditions. Lower areas spend more time under water and upper areas spend more time in the air. Seaweeds and animals are distributed in distinct ways across these zones: some are only found in the lower zones, others only in the upper zones, and still others occur in both.

But it’s not only the physical environment – exposure to air or water – that determine what lives where. Interactions between species, like predation or competition for food or space, can affect where species are able to live in the intertidal zone.
Ecologists have always been interested in how environmental factors and the relationships between species affect which organisms live together in a community. Because rocky shores experience such variability in environmental conditions and are dominated by stationary species that are easy to study, these areas provide an ideal location to answer some of ecology’s biggest questions. In particular, how do relationships between organisms affect where different species are found, and how important are local environmental conditions in controlling the distributions of species?

In the late 1970s, Drs. Jane Lubchenco and Bruce Menge, two marine ecologists, set out to address these questions using the intertidal zone along the northeast coast of the United States as their study system. They were particularly interested in how wave action, in addition to relationships between species, could affect rocky intertidal communities. Wave action is important on rocky coasts because it can affect whether seaweeds and animals can attach to the rocks. However, studies on how wave action might influence the community had never been conducted before. They chose five coastal sites from Maine to Massachusetts that differed in wave intensity and focused on seaweeds and animals present in the lower intertidal zone.
A rocky coast surrounded by blue ocean water with with sea birds and boat in the distance
Little Brewster Island

NPS Photo

One site that the researchers selected was Little Brewster Island, one of the islands in the Boston Harbor Islands National Recreation Area. Little Brewster has a very rocky coastline. The site they used on Little Brewster had an intermediate amount of wave action and little human influence and was a perfect place to conduct one part of their experiment.

The researchers focused on the abundance of three species in their study: barnacles, mussels, and a type of seaweed known as Irish moss. For three years, they were busy. They identified what types of seaweed and animals were present at each site. They looked to see which species were competing for food and space. They looked to see who was eating whom. And, of course, they examined how different wave conditions influenced what species were present. They even used metal mesh cages to keep predatory starfish and herbivorous snails out of certain areas to see how their presence (or lack thereof) affected the community.
What did they discover? Rocky intertidal zones are complex! There’s so much more going on than meets the untrained eye.

They found that species presence or absence was influenced by how much wave action the sites experienced. Mussels were the most abundant species in the presence of strong waves and Irish moss was most abundant under calmer conditions. Many factors influenced this pattern, but interactions between species were very important. First, mussels were the best competitors; when they were present, they were more abundant than barnacles or Irish moss because they were better able to grow and occupy space. Second, predation and herbivory also played a role. Starfish preferred to eat mussels and barnacles over other animals, and snails preferred to eat a type of seaweed that competed with Irish moss.
But what controlled the presence of starfish and snails? It turns out wave action was key here as well. When waves were powerful, starfish and snails could not attach to the rocks, and mussels could therefore dominate. When wave action was weak, starfish and snails would remove mussels and seaweed, allowing for Irish moss to dominate. This explained the pattern that Lubchenco and Menge had first observed.
A close-up image of barnacles, mussels, and seaweed
Researchers focused on the abundance of three species in their study: barnacles, mussels, and a type of seaweed known as Irish moss.

COURTESY ANNA MANYAK DAVIS

Lubchenco and Menge’s study helped expand knowledge at the time about what factors are important in determining the distribution of species in rocky intertidal communities. Their findings on the importance of competition and predation in controlling what species were present reinforced findings from prior studies, but their finding that wave action could control communities was not previously known. Their research set the stage for future studies that continued to address the complexity of the rocky intertidal zone and how the environment plays a key role in structuring communities.

Today, studies on rocky intertidal communities are still occurring in the northeastern United States. Even in the Boston Harbor Islands National Recreation Area, scientists continue to study the species in the rocky intertidal zone to better understand the patterns we see and how they might change over time. Particularly with current concerns about sea level rise and increasing storm intensity, understanding how species in the rocky intertidal are influenced by the environment continues to be an important area of research.

Last updated: April 20, 2020