Where land meets sea lies the intertidal zone — the strip between the lowest low tide and the highest high tide. Its residents are sometimes underwater and sometimes in open air, and on rocky coasts it is crowded with seaweeds and animals that stay fixed in place.
Boston Harbor Islands National Recreation Area. Photo by the National Park Service
Why ecologists love rocky shores
The tides divide the shore into bands: lower bands spend more time submerged, upper bands more time exposed. Some species live only low, some only high, some in both. But exposure isn't the whole story — predation and competition for food or space also decide who lives where.
Because conditions on rocky shores vary so much, and because their stationary residents are easy to study, they are ideal places to test two of ecology's big questions:
- How do relationships between species decide where each is found?
- How much do local conditions control those patterns?
The study
In the late 1970s, marine ecologists Jane Lubchenco and Bruce Menge took these questions to the rocky shores of the northeastern United States. They asked something no one had tested before: does wave action — which decides whether seaweeds and animals can hold onto rock — shape the community, alongside species interactions?
- Where: five sites from Maine to Massachusetts, chosen for different wave strengths, in the lower intertidal zone.
- One of them: Little Brewster Island in Boston Harbor Islands National Recreation Area — very rocky, with moderate wave action and little human disturbance.
- What they tracked: three species — barnacles, mussels and Irish moss, a seaweed.
- How: for three years they recorded who was present, who competed with whom, who ate whom, and how waves changed it all. They used metal mesh cages to keep predatory starfish and plant-eating snails out of some plots.
Little Brewster Island. Photo by the National Park Service
What they found
Wave strength decided the winner — through the animals.
| Waves | What happened | Who dominated |
|---|---|---|
| Strong | starfish and snails couldn't hold on to the rock | mussels, the best competitors for space |
| Weak | starfish ate mussels (and barnacles), snails ate a seaweed that competes with Irish moss | Irish moss |
Mussels outgrew barnacles and Irish moss whenever they were present. But where waves were calm enough for predators to work, those predators cleared the way for Irish moss.
Why it mattered
The study confirmed earlier findings that competition and predation shape these communities — and added something new: that wave action can control them too. It set the stage for later work on how the environment structures rocky-shore life.
Scientists are still studying the rocky intertidal zone at Boston Harbor Islands and elsewhere in the Northeast. With sea level rising and storms growing stronger, knowing how its species respond to their surroundings matters more than ever.
Sources
Based on "Rocky Intertidal Research in the Boston Harbor Islands National Recreation Area," by Anna Manyak Davis, a University of Maryland graduate student, from the series Parks in Science History, National Park Service; rewritten in hubnx's own words.
In these publicationsBoston Harbor Islands National Recreation Area
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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