Vital for biodiversity, complexity suffers along our growing concrete coastline

May 18, 2021

A concrete seawall along the Puget Sound juts out in the water. (AP Photo/Elaine Thompson)

For thousands of years, humans have built seawalls, laid down rubble and created other barriers between water and land. New research on both natural and artificial coastlines in Wales has shown that such barriers can be up to 50% less complex than natural landscapes, potentially imperiling the biodiversity enabled by more structurally complex environments.

Artificial shorelines are becoming more common around the world as humans develop the coast and seek protection from storms and coastal flooding, which may only intensify in coming years because of climate change. But such structures' low complexity levels must be addressed, because they do not support biodiversity in coastal ecosystems as well as natural shorelines with many natural ridges, crevices and other complex features, U.K. researchers said in a new paper published Wednesday in Proceedings of the Royal Society B: Biological Sciences.

The team visited 24 coastal sites in Wales, 12 artificial and 12 natural, capturing the surface complexity using three state-of-the-art remote sensing methodologies: handheld photography, used at the fine scale, captured the surface complexity that is important for larvae and barnacles; laser scanners, used at the medium scale, were important for monitoring colonization by seaweed and limpets, a kind of marine mollusk; and drones, used at the large scale, detected the sort of complexity that helps crabs and shrimp thrive. All three of these remote sensing methods generated three-dimensional models called point clouds, the authors said, which they processed and analyzed using a software platform. They found that human-made structures generally simplified coastlines, likely impacting their biodiversity.

Peter Lawrence, first author of the paper and a postdoctoral researcher at Bangor University, told The Academic Times that it was important for him and his co-authors to measure the complexity of the coastal sites at different scales — fine, medium and large — to identify where interventions are most needed to improve existing artificial shorelines. "The coastline is particularly interesting, as the smallest changes in temperature, shelter or chemistry [have] a large impact on the diversity," he said.

Habitats that are more structurally complex and less uniform provide a better living environment for a wide variety of species and increase biodiversity within an ecosystem. In natural rocky habitats, crevices, ridges and holes provide shelter from physical stressors and predators, for example.  

"Seawalls, rock armor and groynes are among the most common forms of sea-defense that are already extremely common in coastal urban cities," Lawrence said. "They are often formed of concrete or granite sections [that] protect farmland and urban infrastructure." Rock armor is made up of broken stones piled up against a shoreline to protect it, while groynes are long walls perpendicular to the shore that are designed to limit the movement of sediment.

"These structures are important; if projections are correct, only about 30% of the world's population lives close to the coast, but this could nearly double by 2060," he continued. "However, as useful as these structures seem, they often do not provide as much biodiversity."

There has been a growing movement of ecologically inspired engineering design, or eco-engineering, intended to increase the complexity and biodiversity value of artificial coastal structures, the authors said. This involves adding or removing material to change a structure's surface and create habitat-forming topographic features, such as rock pools and pits. The current paper is a part of the Ecostructure Project, which is meant to produce research that raises awareness of eco-engineering solutions in Wales and Ireland.

"Surface complexity is much the same as the phrase topography," Lawrence said. "We defined surface complexity as the bumpiness of the surface. By creating bigger and bigger circles from the size of sand up to a small car, we assessed how bumpy the human-made structures were in comparison to natural shorelines."

Lawrence and his colleagues wanted to compare how common different-sized habitats are in natural and artificial shorelines, and what effect such habitat variations have on the life they house. "We felt it was important to cover a range of sizes, as without all these habitats, a diverse community would struggle to colonize — from larva to barnacles, seaweed, mollusks, crabs and even juvenile fish," he said.

In their study, the researchers expected to find that, "Distinct differences exist between artificial structures and natural rocky shores, with natural shores providing a more structurally complex and structurally diverse habitat," Lawrence said, which they found to be largely true.

The study's senior author, Andrew J. Davies of the University of Rhode Island, told The Academic Times the paper was one of only a few that leveraged multiple new platforms to generate three-dimensional models of the structures they wanted to understand. "Using these systems, we show the potential of the next generation of virtual ecological research, where we can capture and preserve systems in unprecedented digital detail and come back time and time again to ask new research questions on our computer," he said.

Lawrence said their results show that the types of structures commonly used to defend a coast are far less complex than natural shorelines, a concern for biodiversity goals. Even more troublingly, this compromised complexity can be seen at most spatial scales, including those known to be important to organisms that live along the coast. 

Seawalls were approximately 20%-40% less complex than natural shorelines at all scales. Rock armor, on the other hand, varied. At the fine scale, both rock armor and seawalls were as much as 29% less structurally complex than natural coastlines. At the medium scale, seawalls were up to 41% less structurally complex than natural shores, but rock armor was roughly on par with natural shorelines. And at the large scale, seawalls were up to 43% less complex and rock armor was almost 50% less complex than natural coastlines.

"In a coastal environment, it can be just a few [centimeters] difference between being caught by a bird, drying out or finding shelter for an upcoming storm," Lawrence said. "To a limpet, larva or shrimp, what a human perceives as nothing more than scrape or crack is the same as the Grand Canyon. Some creatures live in this small space for their entire life, and others use the fringes as hunting grounds."

Eco-engineering efforts must balance the usual costs and impacts of an artificial coastal structure with space to preserve habitats, the authors said. Current approaches modify insufficient existing structures by drilling in pits, grooves and ridges to act as habitats, which are primarily on the fine and medium scales. Large-scale modifications are more difficult and costly, because they may result in an increased physical footprint and environmental impact of the structures. But with coastline biodiversity under threat, the team thinks such changes may be essential.

"It is the hope of this project to help encourage more ecologically sensitive design in the future of sea defense, as with sea-level rise, ever more defense is needed if we are to protect towns and cities on the coast," Lawrence said.

The study, "Artificial shorelines lack natural structural complexity across scales" published May 17 in Proceedings of the Royal Society B: Biological Sciences, was authored by P. J. Lawrence, T. Jackson-Bué, S. R. Jenkins and G. J. Williams, Bangor University; A. J. Davies, Bangor University and the University of Rhode Island; A. J. Evans and P. J. Moore, Aberystwyth University; P. R. Brooks and T. P. Crowe, University College Dublin; and A. E. Dozier, University College Cork.

Correction: A previous version of this article misspelled A.J. Davies' name. The error has been corrected.

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