When humans transform wetland landscapes, the relative cost to crucial ecosystem services such as improved water quality can significantly exceed the amount of land lost, scientists have reported.
Examining aerial imagery and historical documents describing the California Delta, the researchers found that 77% of its wetland habitats have been lost since the 19th century. However, the region has lost a worrying 94% of its net primary production, a measure of the amount of energy produced by photosynthesis. Understanding these patterns could help guide conservation plans in the delta and other wetlands, the researchers reported April 16 in Science of The Total Environment.
"This is a major disturbance of an ecosystem, and it's one of the underlying reasons why we're having difficulty recovering and sustaining populations of wetland-dependent species," including some species of indigenous fish and migratory birds, said James Cloern, an emeritus senior scientist at the U.S. Geological Survey and first author of the study. "This is the first time that any group has tried to answer the question: What does this loss of wetlands mean in terms of the essential ecosystem function of primary production?"
Wetlands are rapidly diminishing around the world as they're developed and converted into agricultural lands. The extent of wetlands in Africa, Asia and Europe all shrank more than 30% between 1970 and 2013, Cloern and his colleagues noted in the study.
This is troubling because wetlands are home to nearly 20,000 plant and animal species, improve water quality and absorb excess water from storms. They're also among the most productive ecosystems on Earth, according to the U.S. Environmental Protection Agency.
Net primary production describes how photosynthetic organisms convert carbon dioxide into sugar molecules that are stored in the organisms' tissues. This energy source fuels the rest of the food web, from tiny microbes to top predators. While human activities have reduced global net primary production in terrestrial ecosystems by nearly 25%, the damage to wetlands is unknown.
"We don't have a clue what the loss of wetlands means in terms of lost primary production to support species that are dependent uniquely on wetlands," Cloern said. "We don't have a global-scale number of this manifestation of human filling and draining and diking of wetlands."
As a first step to answering this question, his team investigated how the Sacramento-San Joaquin Delta, or California Delta, has been altered over time. His colleagues at the San Francisco Estuary Institute-Aquatic Science Center had previously used hundreds of documents including explorers' notes, landscape surveys and photographs to piece together what the wetlands looked like back in the early 1800s.
Before European settlement, the California Delta was the largest estuarine wetland ecosystem on the West Coast. The 2,300-square-kilometer wetland formed a complex network of freshwater wetlands, tidal channels and river habitats. Today, nearly all the tidal marshes and floodplains have been drained and replaced by croplands, pastures and areas of open water.
"The process of putting levies around marshes, having the land naturally subsiding, and then levies being broken by storms or large floods has created these open-water sections of the Delta," Cloern said. "It's a habitat type that really wasn't present historically."
The California Delta now supplies fresh water to two-thirds of the state's 39.5 million residents. It's also a vital migration route for salmon and steelhead trout and is home to numerous threatened or endangered species, such as Delta smelt.
The researchers found that the Delta has lost 98% of its marshes since the early 19th century.
While sitting on an advisory panel for the project, Cloern said, "I was wondering, what does this kind of landscape change of this magnitude mean in terms of ecosystem functions?"
To find out, he and his colleagues examined five types of habitat in the Delta: open water, tidal marshes, non-tidal marshes, floodplains, and forests and scrub that grow along the channels and drop leaves and needles into them. In each habitat, the researchers identified photosynthetic organisms belonging to five categories, including marsh plants such as cattails, phytoplankton, woody plants, microalgae attached to plant stems and other surfaces and submerged or floating aquatic plants.
"At that point it was simple arithmetic," Cloern said. Based on the areas of the different habitats and the estimated productivity rates of their photosynthetic residents, he and his colleagues calculated net primary production across the Delta in the 19th century and present day.
Since the 1800s, his team observed, the Delta has lost a little over three-quarters of its traditional wetland habitats but more than 90% of its annual net primary productivity. The reason for this mismatch is that highly industrious marsh habitats were disproportionately eliminated, Cloern says, while less productive open-water habitats expanded.
This pattern, in which marshland is replaced by open water, is a common theme for other wetlands around the world.
In the Delta, the shift has created ideal conditions for nonnative nuisance aquatic plants such as water hyacinth, which in turn create prime habitat for invasive species such as largemouth bass that prey upon indigenous fish.
"We've completely transformed the plant communities in the waterways and are now managing them with contaminants," Cloern said.
Still, he and his team found that habitat restoration efforts could improve the situation. Should California's Delta Plan meet its proposed targets for the restoration of tidal marshes and other diminished habitats, 12% of the net primary production in the Delta that has been lost due to human activities could be replenished.
It's not clear how representative the California Delta is of wetland losses more generally, Cloern says. However, the findings could provide a starting point for broader estimates of the ecological cost of wetland destruction.
"If we're going to develop global-scale estimates of the loss of wetland primary production, it's important for us to know and understand the details of, what are the proportions of different habitats that were lost during these landscape transformations?" Cloern said.
The study, "On the human appropriation of wetland primary production," published April 16 in Science of The Total Environment, was authored by James E. Cloern, US Geological Survey; Samuel M. Safran, San Francisco Estuary Institute - Aquatic Science Center and University of Minnesota; Lydia Smith Vaughn, April Robinson, Alison A. Whipple, and J. Letitia Grenier, San Francisco Estuary Institute - Aquatic Science Center; Katharyn E. Boyer, San Francisco State University; Judith Z. Drexler, US Geological Survey, California Water Science Center; Robert J. Naiman, University of Washington; James L. Pinckney, University of South Carolina; Emily R. Howe, The Nature Conservancy; and Elizabeth A. Canuel, William & Mary.