Keystone species and the rocky shore. A bit of Community Ecology

The Rocky Shoreline of Mount Desert Island Maine.

Summer is here, and that means AIO students are exploring Maine’s rocky intertidal zone of Mount Desert Island. The intertidal region, between low and high tide, has been studied in depth, particularly in the field of community ecology. Community ecology is the study of how species living together in the same place interact with one another and with their shared environment, and how those interactions shape the composition, structure, and dynamics of ecological assemblages. 

Or more simply and directly:

Species living in the same place affect each other in ways that determine who thrives, who gets crowded out, and what the community ultimately looks like. Non-native species and the influence they have on a system is right up the alley of community ecology.

Beyond non-native species, influential ideas have sprung from studying communities of the rocky intertidal. One of the big ideas is the concept of keystone predators.

An oversimplified idea of what a keystone predator does is that it keeps the system balanced; perhaps by eating the species that would otherwise take over and dominate the area. If this domination occurred, it would reduce biodiversity, which, in turn, reduces energy flow and nutrient cycling, key health measures of an ecosystem. 

No, not Robert Redford, Robert Paine: Though there is some resemblance.

Robert Paine figured this out in his series of experiments within the US West Coast rocky intertidal zone. By removing ochre sea stars, Pisaster, he altered the rocky shore community. Mussels became overly abundant, outcompeting other rock bound organsims like limpets, and chitins. The biodiversity of the system was reduced.

The mussels were competitively dominant and did not allow co-occurrence of others in their space; this only occured when it got the chance to take over (the chance being a lack of predators). In his words: “When the top predator is artificially removed or naturally absent, the systems converge toward simplicity.”

He also removed other beasties, a few murex snails, Acanthina brevidentata, and Stramonita biserialis, which are also predators in the intertidal. He saw influences, but nothing to the extent of our classic keystone predator, Pisaster.

Note: The official name of the second snail,  Stramonita biserialis, has changed since Robert Paine’s time, when it was Thais biserialis. This is similar to name changes on the East Coast; the Dog Whelk, now Nucella lapillus, was once considered to be in the same genus, Thais.

Are species other than predators considered keystone species? Keystone species are typically defined by their disproportionate influence on ecosystem structure relative to their biomass. Most examples are predators, because a base of the food web organism will have a high biomass, and thus not fit the classical definition of a keystone species.

However, if you remove a species and it has a massive influence on the system, we want to be able to think about that, study that, and stop that from happening, whether it’s a traditional keystone or not. Thus, we need a terminology change.

Top-down control is when a predator disproportionately influences a system, while base of the food web organisms that influence the system are termed bottom-up control.

Zooxanthellae, the color-causing symbionts living inside coral, are functionally keystone species in the coral reef system. If they die off, the coral dies off, and the reef-building system is gone. The entire reef framework collapses. Not being a traditional keystone species, this is better referred to as bottom-up control. This is occurring now due to climate change, and why we are so concerned with coral bleaching (when they expel their zooxanthellae, exposing their white skeletons). Coral reef systems are incredibly biodiverse, and when the coral goes, so does this biodiversity.

What are the major system influencers on the rocky intertidal shore of Maine?

A sea star, genus Asterias, on the Maine rocky shore showing signs of disease.

The predators most akin to Robert Paine’s sea star are, one would think, the Asterias sea stars. Two species are common: the Northern Sea Star, Asterias rubens, and the Forbes Sea Star, Asterias forbesi. These species have been decimated in the last handful of years due to disease. Since the disease, the Maine coast might be expected to see an overabundance of mussels, similar to what Paine observed on the West Coast. This did not occur. Has the keystone predator, top-down control, spot of the sea stars been taken.

Their spot may be taken by the Dog Winkle/Dog Whelk, Nucella lapillus. However, as was found by Paine, these snail species are unlikely to exert the same magnitude of influence. Do other predators step in to fill the role? Seagulls have some influence on the intertidal, grabbing mussels on occasion. But they are more likely to grab other things, thus their influence, like the predatory snails, would appear to be small. Here’s the predator that may be saving the biodiversity of the intertidal in Maine…are you ready? I’m not sure you are.

Green Crabs, Carcinus maenas!

These aggressive predators, oft maligned as invasive species, though they have been here for more than 100 years, may be the answer. 

They are fiesty, which led to their scientific name specific epithet, maenas, meaning frenzied. They do eat mussels, likely more smaller ones, the larger ones being too difficult to open.

But green crabs are opportunistic feeders, eating whatever is around in abundance. As young, they feed on detritus and tiny fauna in the sand and mud. As adults, they eat at least 158 different types of things, including algae. Could these opportunist omnivores exert enough control on the intertidal system to control the biodiversity? They do have a preference for mollusks, so the possibility exists. 

They likely exert top-down predation to a degree enough to maintain the biodiversity of the rocky intertidal of Maine.

Green crabs, whether good, bad, or ugly, are now, and have been for some time, a substantial part of the ecosystem of Maine shorelines. The current system, I’m convinced, is made more resilient due to their presence. We could go down the resilience and functionality gaps (a gap which green crabs fill), but that is best saved for another time, and that rabbit hole may take up more space than I (or you) can deal with in these blogs.

Note: For those not in the know, AIO is the Acadia Institute of Oceanography. A magical teaching habitat on Mount Desert Island.

Sources and Further Readings:

Paine RT. 1966. Food web complexity and species diversity. The American Naturalist 100(910): 65-75.

Power ME, Tilman D, Estes JA, Menge BA, Bond WJ, Mills LS, ... & Paine RT. 1996. Challenges in the quest for keystones. BioScience 46(8): 609-620.

Young AM, and Elliott JA. 2019. Life history and population dynamics of green crabs (Carcinus maenas). Fishes 5(4): doi:10.3390/fishes5010004