Digging Deep: Burrowing Animals are Just One Element of a Healthy Mudflat

Dissanayake NG, Frid CLJ, Drylie TP, Caswell BA (2018) Ecological functioning of mudflats: global analysis reveals both regional differences and widespread conservation of functioning. Marine Ecology Progress Series 604: 1-20.

At first glance, a mudflat looks a bit like a barren wasteland. Appearing only with low tide, mudflats, as their name suggests, have soft, muddy sediment and an absence of plants. They exist in the in-between, a realm not quite land but also not quite sea. While tide pools can feel like a small window into the ocean, the wanderer looking to better understand a mudflat will find their ankles sucked down as the sea begins to reclaim the land. But don’t be fooled by the mudflat’s daily disappearance beneath the water. These environments are incredibly important, both ecologically and economically. One estimate places the global value of intertidal systems, including salt marshes, at 5.2 trillion USD per year in 2007. These areas provide habitat for juvenile fish and support fisheries, provide a home for birds, and serve as a buffer between the coast and the sea. Despite their importance, these ecosystems are under attack. Humans are constantly trying to claim this land from the sea, developing it, dredging it, and at times polluting it even as climate change and sea level rise threaten ecosystems globally.

A group of fiddler crabs in at India River Lagoon in Florida. While these fiddler crabs are on sand, fiddlers burrow into the soil and fulfill an important role in many mudflats. Photo by Andrew Westmoreland at Wikimedia Commons.

With so much at stake, researchers at Griffith University, the University of Waikato, and the University of Hull decided to take a closer look at what makes mudflats tick. They found that in order for mudflats to remain healthy, 4 important roles must be filled by the animals living in them.

To get to this conclusion, the researchers combed through 163 datasets on what they determined were “pristine” mudflats, looking at the types of animals living in each environment. In total, they looked at 448 taxa of animals, 10 biogeographic areas, and 4 different climates, which included temperate, tropics, subtropics, and polar. They decided to focus on larger animals, or those that would be caught in a 0.5mm net sieve at least. They also assigned biological traits to each organism, essentially describing how each animal functions. The kinds of traits they looked at included the body length and shape, how the animals moved, ate, lived, and how deep they travel into the mud and if they dig burrows, among other traits. When each taxa in each mudflat environment had been described, they compared the taxa and the traits between the mudflats.

The data showed that globally, the kinds of animals living in mudflats vary. For example, animals living in mudflats closer to the equator are smaller in general, don’t live as long, and are more likely to have an exoskeleton, or a protective, hard outer shell. They also found that there are more animals that will burrow into the soil in temperate mudflats, helping with the movement of important nutrients through the mud and the turnover of sediment. In comparison, in the tropics, there are more animals that live in hard tube-shaped structures that they create to protect themselves.

A mudflat. Photo by Bert Kaufmann at Flickr.

However, when the scientists compared variability in biological traits to the variability in the kinds of animals in mudflats, they found something much more interesting: the traits of the animals in each mudflat did not vary as much as the animals themselves. In fact, what they found was that in order for the ecosystem to function properly, certain roles needed to be filled. Although each animal might do it in a different way, there were 4 necessary jobs that must be done.

Firstly, there need to be animals that will move the soil around and help break down old, dead material and turn it into nutrients. This ensures that oxygen is transferred down into the deeper layers of the mud for things living down there and that nutrients can be reintroduced into the food web.

Secondly, there need to be animals that create habitat, such as tube dwellers, or animals that build a hard tube around their bodies. These kinds animals basically change the environment around them. One example is how mussels can create mussel reefs; after a few mussels grow, their bodies form a hard substrate for other mussels to latch on to and grow.

Thirdly, there needs to be a changeable method of cycling carbon through the ecosystem. Carbon is the building block of life, so if carbon isn’t moving, nothing is changing or growing. Carbon is transferred when an animal eats another, so in an ecosystem where there are a lot of hungry predators, carbon cycling is faster. When those predators die, they are broken down by worms, crabs, and microbes, and thus the carbon in their bodies starts back at the lowest level and is reintroduced into the system.

An oystercatcher grabbing a meal in a mudflat. Photo from Pixabay.

Finally, there needs to be food for predators. Many taxa of animals are important in mudflats because they are food for fish or crabs, which in turn helps our fisheries.

One animal might fulfill more than one of these functions, but each one of these things must happen for a mudflat to continue to function. A mudflat may be able to sustain the loss of one particular species, but if that species is the only one that burrows, for example, the whole ecosystem could fail. Just as a human society can’t function without people who build or people who grow food, a mudflat also has non-negotiable jobs that must be done. To protect the future, we must also protect these workers.

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