Lose one, alter the ecosystem

By Meghan O'Connell

A veil of sediment covered the bed of the Rio Las Marias within 24 hours of removing one type of fish from the species-rich river, marking an exception to the theory that if one species is lost from an ecosystem, then another will fill the niche.

A new study has documented that the carbon flow of this Venezuelan river is disrupted without the fish, called the flannelmouth characin.

Carbon flow is an indicator of the transfer of energy within an ecosystem and from one organism to another, such as when fish eat algae. This energy flow ensures all species receive nourishment through eating or photosynthesis.

The findings come at a time when humans are changing the environment more than ever in the past. The report of the Millennium Ecosystem Assessment, an international effort launched by the U.N. in 2001 and concluded in 2005, states that over the last half century, “humans have changed natural ecosystems more rapidly and extensively than in any comparable period in human history.” The consequence, the report states, is that diversity of species is being depleted at an accelerating rate and the environment is producing fewer resources.

The silver bodies of the flannelmouth characin shine in the lucent waters of the Rio Las Marias as they graze on the plant and animal matter called detritus that accumulates on the rocks. Lines etched in this covering of the riverbed are evidence of the flannelmouth characin’s housekeeping skills. These fish remove detritus that would shade algae growing in the stream.

“Over a long time there may be another species that comes in, but we haven’t seen that in the 20 years that we’ve been working here,” said Dr. Brad W. Taylor, a research associate in Dartmouth College’s biological sciences department. “And over longer time periods, the system will change so much that if another species comes in it’s not going to be the same ecosystem anyway.”

Taylor led a study to measure the impact of flannelmouth characin loss on a river in the Orinoco basin in Venezuela. Taylor and his colleagues erected a 680-foot barrier in the center of the stream and removed the species from one side.

The result was striking. During the six-year period of study, Taylor found that the river contained four and a half times more organic debris after the removal of the flannelmouth characin. The loss of this one fish had an effect on the ecosystem equal to changes caused by removing all fish, invertebrates, shrimps or predatory fish in other areas that have been the focus of other studies.

“As you lose species you lose ecosystem function,” said Dr. J. David Allan, a professor with the School of Natural Resources and Environment at the University of Michigan who studies stream ecology and human influence on inland water. “Is it a one-to-one relationship or are there enough substitute species that if you lose one, another will fill its place?”

At least 80 fish species can be found in a five-mile segment anywhere in the Rio Las Marias. And detritivores, or fish like the flannelmouth characin that consume detritus, account for 50-80 percent of the fish catch and the total mass of fish in the Orinoco and Amazon river basins, according to previous studies.

“The expectation is that diversity provides resilience against species loss, that more diverse systems are more resilient,” Allan said. “[The flannelmouth characin] is heavily fished and it’s unique. You take it out and there’s nothing that does what it does, so that’s a clear example of where a species appears to be irreplaceable.”

Detritus is a major pathway for the flow of energy among river organisms. The flannelmouth characin contributes to this aquatic energy distribution three ways. One is through its movement, causing matter to dislodge and float downstream. A second is by consuming the bacteria and algae in the detritus. A third is by ingesting matter and then excreting it, making it more susceptible to drifting downriver.

Other studies have found that the flannelmouth characins in the Orinoco and Amazon basins are currently threatened by dams, deforestation and pollution. That means Taylor’s research reflects the consequences of some imminent threats to this fish and the functions it provides.

Taylor recognized evidence of his own that the flannelmouth characin is vulnerable to human exploitation. Using data from field and museum specimens, Taylor concluded that the body mass of the fish had been decreasing for the past 25 years, with the current average maximum size being 20 percent smaller than that at which females typically reproduce. This reduction in body size is a hallmark of size-selective overfishing, he noted.

“Fishermen will come through and they’ll take a grain-sized sack of fish out of a one-mile length of the river in a day,” Taylor said. “And then there’s intensive fishing downstream where people put up gill nets and they collect all the large fish that way.”

Taylor also found that the fishermen had halved their mesh size of their nets in order to catch the smaller fish. A smaller body size for the flannelmouth characin means less consumption of detritus per fish, which could change the energy flow in the river.

Overfishing in marine waters is a globally recognized and well-documented problem. Inland overharvesting however, is performed on a smaller scale with less regulation, and therefore is not ably recorded, according to Allan. Inland waters are more part of regional and subsistence economies than a global structure, Allan said.

“I think overfishing is really important from a people point of view,” Allan said. “It is the employment of last resort for so many people.”

Photos by Brad Taylor and Jake Allgeier