Known as one of the most biologically diverse beaches in the world, Southern California’s sandy beaches used to be filled with life such as crabs and clams. But that’s now changing, according to new research, which showed that the number of beach animals is in decline.

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For about a third of the beaches between Santa Barbara and San Diego, only a small subset of highly specialized animals remains, and in reduced numbers, researchers at the University of California discovered – an unintended consequence of the quest to maintain the iconic look of the urban beaches.

“This study will force us to make critical choices about whether we value well-groomed beaches or healthy natural ecosystems,” said David Garrison, a program director in the National Science Foundation’s Division of Ocean Sciences, which co-funded the research.

“Shorebirds and other marine life we value are critically dependent on resources provided by thriving ecosystems.”

Cities up and down the coast have flattened dunes — which destroyed native vegetation — and groomed the sand with heavy equipment. All of this, the scientists write in a paper in the journal Ecological Indicators, has massive impacts on the larger beach ecosystem. Furthermore, it could already be having negative effects in terms of erosion, sea-level rise, and the health of the surrounding ocean and coastal ecosystems.

In previous studies, the team found that disturbance from beach grooming caused strong negative impacts to upper intertidal biodiversity on Southern California beaches. The current study took a much wider and deeper look at the diversity of beach ecosystems affected by these management practices.

“We explored how disturbance from these management practices affected ecological communities on different spatial scales,” Nicholas Schooler, the study’s lead author, said. “This includes the littoral cells, which are basically compartments of the coast that contain a sand source, usually rivers, alongshore transport of sand by waves and currents, and a sink where sand exits the system, such as a submarine canyon.”

In comparisons between select urban beaches in Carpinteria, Malibu, Santa Monica, Redondo Beach, Huntington Beach, and Carlsbad, and neighboring minimally disturbed “reference” beaches within the same littoral cells, the scientists found that up to half of the natural inhabitants were missing on the urban beaches. The ones that remained tended to be the same few species across all littoral cells.

Although sandy beach ecosystems are generally thought of as highly resilient given their conditions of constantly moving sand and water, the study results show how sensitive these ecosystems are to human disturbance. This was particularly apparent for wrack-associated species — the small invertebrates that inhabit the upper intertidal zone and rely on stranded kelp wrack for food and shelter. This group typically represents around 40% of the biodiversity on Southern California beaches.

“We started out doing ecology for ecology’s sake, asking basic questions on the diversity and functioning of sandy beach ecosystems,” study co-author David Hubbard, said of this study. “The more we worked in Southern California, the more we realized how altered many of the beach ecosystems were.”