W&L Biology Professors Explore Drought Impact on Amazon Fish Species

Washington and Lee biology professors Robert Humston, left, and Lawrence Hurd.

Robert Humston, left, and Lawrence Hurd.

While almost all of the literature on how climate change affects the Amazon has focused on the rain forest, two Washington and Lee University ecologists recently co-authored a paper that examines for the first time the effect of drought in terms of fish species' diversity.

Lawrence Hurd, the John T. Herwick, M. D. Professor of Biology at W&L, and colleague Robert Humston, associate professor of biology, co-authored the paper with Carlos Freitas and Flávia Siqueira-Souza of the Universade Federal do Amazonas. "An Initial Assessment of Drought Sensitivity in Amazonian Fish Communities" is published in Volume 705 of "Hydrobiologia: The International Journal of Aquatic Sciences."

The paper notes that freshwater biodiversity has experienced a greater decline than any other major ecosystem in modern times and that both global climate change models and recent experience show increasing intensity and frequency of droughts in the region.

According to Hurd, an ecologist, in order to do more conservation work it is necessary to learn much more about the basic biology of the region's aquatic system, why there are so many species and the interactions among those species.

One region of freshwater in central Amazon has more species of fish than anywhere else in the world, including coral reefs, with 2,000 species identified so far. Humston, a fish biologist and fisheries ecologist, called the region "one of the most bio-diverse aquatic systems in the world. The number of fish species is staggering."

A severe drought anomaly in 2005 in the Amazon, caused by the warming of surface waters in the Atlantic, provided the researchers with a unique opportunity to examine the drought's impact on the aquatic system. Freitas and his graduate students amassed a large data set of before and after fish samples between 2004 and 2007 in eight lakes of a floodplain in the region, although only samples from six lakes were used in the study. They collected 120 species of fish of which 70 species were used in the analysis.

The very dynamic hydrologic cycle between wet and dry seasons in the floodplain's lakes and rivers affects the biodiversity of the fish in the region. The water rises in the summer months and recedes in the winter months, with a possible difference in water depth of more than 50 feet. The lakes are isolated for a while and then become connected with the river due to flooding of the forest between the lakes and the river, thus allowing many of the fish to move into the river to complete their life cycles.

The 2005 drought interrupted this movement of fish by isolating the floodplain lakes from the river much more and for a longer period of time. The research shows that this caused the biodiversity of the fish to decline, although the responses to the drought were not uniform among species: some were transitory and others persisted through monitoring.

"The fish are not widely distributed and may be specialists to certain conditions, so if you cut them off from the river when they normally leave to reproduce, they become like landlocked salmon," said Hurd. "Either they are going to adapt by becoming lake residents or they are going to disappear. It's more likely to be the latter because this is happening much faster than we expect evolutionary adaptation to be able to keep up with."

The research found both winners and losers. Some fish that were in every lake before the drought were gone after the drought. Some fish that were not found anywhere were suddenly found in incredible abundance in some lakes after the drought.

"We thought originally that all the migratory species would suffer because they need to migrate between the lakes and the river," said Humston. "But while there was certainly a pattern to migratory species tending to respond one way, it wasn't uniform. Then we thought maybe the predators would really thrive because with the lakes becoming smaller due to the drought, the overlap between predator and prey would be incredible and the predator population would explode. But while some predator populations did explode, other predators almost disappeared completely."

Humston explained that maintaining biodiversity at the regional level is important because losing species in a particular area means losing genetic diversity in the overall population. This is a concern because diversity tends to engender overall stability (resistance to change) and resilience (returning more quickly to their original state).

The researchers pointed out that studying one species of fish at a time doesn’t work because all the species are interconnected and interact with each other to make the ecosystem. "We're looking at this from a community perspective because that will give us more information about how the ecosystem might respond to global warming," said Humston. "The nice part is that when you look at the community as a whole you're also tracking individual species at the same time."

Hurd and Humston were quick to give credit to the hard work of Freitas and his students in actually collecting the fish.

"They set gill nets in the water with a certain size opening in them so the fish swim into the opening and get caught," explained Humston. "Gill nets work best in low light conditions when the fish can't see the net, so they had to set the nets up in the evening and before dawn. It takes more than an hour to put the net out and two hours to bring it in.

"So they were out there in the Amazonian jungle in boats at night and there are crocodiles, and half the fish in these nets could probably take your hand off. It is an amazingly difficult field situation to work in. Then they had to identify all the fish and measure them in terms of the frequency with which they were caught. And they did this for eight lakes and for every part of the hydrologic cycle—low water, rising water, high water and receding water."

Hurd recalled that he had been on some of the collection trips. "If you're in the boat, then you have to work," he said. "You have to pull nets in and pull things out of the net that maybe you wouldn't want to put your hand on."

"In my opinion, this collaboration is creating opportunities for other people," said Humston." If we can demonstrate that the collaboration is productive, then the Brazilian government will continue to pay to send people here to work with us, such as the two [graduate] students from Brazil,” who worked with both Hurd and Humston last year.

Hurd has been collaborating with his Brazilian colleagues since he visited Brazil in 2007, and will continue to work with them under a fellowship he received from the Brazilian National Academy of Sciences.

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Sarah Tschiggfrie
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