Nine years ago, I was standing on the jumble shoreline of saline swamps, an hour's drive north of Boston. I pulled out a small crab with very large claws out of a hole. It's a fiddler crab that won't be found north of Cape Cod, let alone north of Boston. Little did I know at the time that the swamp I was in was about to undergo major changes. I have seen first-hand the effects of climate change. Located in the Gulf of Maine, the Everglades is part of the Atlantic Ocean that stretches from Cape Cod, Massachusetts to Nova Scotia. The marshes along the coast of the bay are important breeding grounds for many birds. However, water temperatures in this region are rising almost faster than anywhere else on the planet. As the water temperature rises, so do the warmer-loving species.
Maryland blue crabs and black bass (lú) in the south are currently being caught in Maine**. In addition, fiddler crabs, known for their attractive male large claws to attract the opposite and repel competitors, are crawling toward the east coast of the United States. This rapid migration is largely influenced by their descendants. When adult fiddler crabs swim in the mud, their juveniles swim in the water and are carried away by the current. Warm waters facilitate the completion of their life cycles, and ocean currents carry the next generation farther north.
Violinist friends become enemies.
As a marine ecologist with many years of experience in the Everglades, my research focuses on climate migration, that is, species that change their habitat as a result of climate change, and I want to understand the impact of these migrations on the ecosystems they enter. I was amazed at the discovery of fiddler crabs in the Everglades, and even more surprising was their impact on the Everglades. The salt marshes are basically grasslands that are flooded by the sea every day. Think of the prairies of the Midwest, but in a seaside setting. Decades of research in southern Cape Cod have shown that the presence of fiddler crabs increases grass yields. The feces and burrows of these crabs release nutrients that provide nutrients for plant growth. Essentially, they play an earthworm-like role in the salt marshes, aiding in the growth of plants.
In the Everglades, however, things are not going as expected. Fiddler crab excavation activities resulted in a 40% reduction in leaf biomass and a 30% decrease in root biomass in the summer of 2020 and 2021. This is the opposite of the expected summer growth pattern. To my surprise, these crabs coexist with Spartina alterniflora species in the Everglades south of Cape Cod. So, why the different effects. One reason for this is that, although they belong to the same species, the plants of the Everglades did not evolve with fiddler crabs in the same way that their counterparts in the southern regions. Fiddler crabs do not eat grass, but their digging activities destroy the root system of the plant. Plants in the southern regions have adapted to and benefited from this disruption, but plants in the northern regions have not adapted to this situation.
The ripple effect of the ecosystem.
This disturbance can extend beyond the grass and affect the entire food chain of the Everglades. Insects, spiders, snails, and small crustaceans all depend on grass for their livelihoods. These creatures, in turn, become food for fish, shrimp, and crabs. Declining plant biomass may lead to a decline in fish and shrimp populations. The numerous bird species that breed and migrate in the marshes depend on this interconnected food chain. For plants, this harmful interaction with crabs continues indefinitely. Spartana has demonstrated the ability to adapt to a new environment over the course of a few decades. Over time, plants in the Everglades, as well as throughout the Maine Gulf, may adapt to the presence of fiddler crabs.
At the same time, fiddler crabs could exacerbate the effects of climate change in the region. As temperatures rise and sea levels rise, the Everglades are under threat of inundation. Historically, people built to defend against rising sea levels, just as you need to fortify your house to defend against water intrusion. Plants promote the formation of marshes by absorbing the sediment brought by each tide. The reduction of grass can lead to a decrease in the area of marshland, making it susceptible to flooding. In addition, fiddler crabs have weakened the Everglades' ability to store carbon. The salt marsh is essentially a huge piece of compost that would take centuries to decompose. Every gardener knows that temperature and oxygen are key factors in the decay process. That's why you're often going to turn over the compost.
There are climate migrants all over the world.
Every year, the roots of dead plants are buried in oxygen-deficient soil. As a result, the decomposition process slows down considerably, which helps with "composting" and the accumulation and storage of carbon. This unique property makes salt marshes play a vital role in storing carbon, preventing it from entering the atmosphere, and contributing to climate change. However, the caves created by fiddler crabs disrupt this process by promoting decomposition. Dead plants begin to decay, and carbon that was previously buried is released. Fiddler crabs are just one of the many climate migrations observed globally. While ecosystems are able to adapt to the arrival of these migrants, they may undergo irreversible changes that may never be restored.
In Australia, the situation took a turn for the worse when a plant-feeding sea urchin expanded its territory southward. Once-thriving seaweed forests have been stripped away, leading to a dramatic decline in plant and animal diversity. In California, a predatory nudibranch, also known as a sea slug (kuò) slug (yú), has thrown the species out of balance by reducing the number of other nudibranchs in the area during its northward migration. Meanwhile, in the cold of Antarctica, krill are migrating south. Since krill are the main food for whales, penguins and seals, this shift could disrupt Antarctica's food chain. However, not all climate migration brings doom and gloom. For example, in the southern United States, when mangroves replace swamps, they end up storing more carbon. Climate migration also has the potential to have a positive impact on fisheries.
In my research lab, we study the famous blue crab, especially when it landed at the Chesapeake Bay Pier in 2022, bringing in more than $200 million in local revenue. Now with an interesting twist, these blue crabs appear in Maine lobster cans, hinting at the possibility of fishing in northern Massachusetts, New Hampshire, and Maine. The big question, however, is to what extent blue crabs and lobsters can coexist in this new situation. In Virginia, warm waters have led to the emergence of large numbers of white shrimp, which in turn has led to the development of a new type of fishery. And anglers in Florida's Big Bend in the Gulf of Mexico have something to celebrate – an exciting sport fish, the Snook, has expanded to the region.
There will be more immigrants in the future.
In 2023, the world's oceans experienced an unprecedented heatwave, breaking records. As greenhouse gas emissions continue to increase, the warming trend is expected to continue. While climate migrants are not invasive species, they have the potential to alter ecosystems, with changes in the Everglades being a clear example. The key is to understand the mechanisms behind these changes and determine whether ecosystems can adjust as species continue to migrate.
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