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Explorers Club Grant Recipient Nathan Lujan Continues Aquatic Inventory of the Rio Mamore
Posted on November 7, 2011

Working with The Explorers Club, Eddie Bauer is funding two significant grants for research and exploration. The grants are directed toward projects that contribute significantly to work regarding climate change, preservation or sustainability. Grant recipient Nathan Lujan is studying Aquatic Inventory of the Rio Mamore: the Last Untouched Corner of the Amazon.

By Nathan Lujan

Our fieldwork began with a three-hour drive north out of Santa Cruz, through the city of Montero, then on to the small town of Buena Vista, where we would spend two days collecting in the Rio Grande and nearby tributaries. The Rio Grande, like all the rivers we would be sampling this trip, ultimately flows into the Amazon via the Mamoré and Madeira rivers, and has it headwaters high in the Andes Mountains. At the elevations near Buena Vista, though, it is a broad meandering river with an expansive floodplain.

When we were there, at the peak of the dry season, the Grande is shallow enough to walk across, which we did repeatedly as we collected fishes with our seines and gillnets. Although the fish were sparse, we managed to collect some interesting, rare and possibly undescribed species—one of which continues to puzzle us and our ichthyological colleagues as to its true identity.

After sampling a few more sites near the Rio Grande, we moved on down the road toward Cochabamba and spent a night in the town of Villa Tunari. There, we started getting into more interesting (for me!), higher gradient piedmont streams. At the San Luis River, our first site near Villa Tunari, we decided to try out the electrofisher, and it worked splendidly! This site, because of its very clear water and many large rocks, would have been impossible to thoroughly sample with just our seines, but with the shocker, the fish could be stunned so that they drifted out from under the rocks and into our nets.

Over the subsequent days, we continued sampling at progressively higher elevations along the road from Villa Tunari to Cochabamba, spanning a vertical altitudinal range of about 1,500 meters. The higher one gets in the Andes, the steeper the river gradients get and the faster and colder the water is. In response to this change in elevation and stream habitats, the fish communities also change. There are a number of fish genera and species that live only in the cold, high gradient streams of the Andes and not at lower elevations in the Amazon Basin. I’ve recently become more and more interested in these specialized, high elevation fishes and, just the day before I left the United States for Bolivia, I was overjoyed to receive the news that the National Science Foundation had agreed to fund my research on the diversification of fishes specialized for life along the Andean flanks. I was therefore especially motivated to collect samples in these habitats!

The central question of my research concerns the effect that historical uplift of the Andes Mountains had on the diversification of fishes along its flanks. Although the Andes seem quite ancient, they are actually fairly young in terms of geologic time. They began uplifting in earnest some 30 million years ago, and really accelerated about 9-13 million years ago. This certainly seems like a long time ago, but in comparison with some other upland areas called ‘shields’ in southern Venezuela and northern Brazil, which began uplifting hundreds of millions of years ago, the Andes are still fairly young!

By studying the DNA of various organisms, we can determine with low but sufficient accuracy, approximately when they divided into different species, and how quickly they diversified, both geographically across the landscape, and also morphologically, into different colors and shapes. Recent studies of parrots, poison arrow frogs, and even plants in the coffee family have indicated that the uplift of the Andes had a strong and positive effect on their diversification. With the support of sponsors like the Explorer’s Club, Eddie Bauer, and the National Science Foundation, I hope to investigate this question in the organisms that have held my fascination since childhood, the fishes of rivers and streams!

All told, after sampling several more sites along the southern Andean flank during our return trip from Cochabamba to Santa Cruz, we collected a total of just over 50 fish species. Not a tremendously diverse collection, but this is not an especially diverse corner of the Amazon Basin. It is actually one of the most recent additions to the Amazon Basin, having been added some 9 to 11 million years ago, as a consequence of Amazonian headwater expansion driven by the increased amount of atmospheric moisture captured by the growing Andes Mountains. In South America and throughout the world, geological and climatic phenomena occurring over many eons have shaped the patterns of life that we interact with daily. Until recently, we have had little control or influence over these grand cycles, but it is increasingly apparent that the situation has changed, and that we are now in the driver seat. By studying historical interactions between geology, climate, and biological diversification, we may have some hope of predicting what our own influence might be into the future.

Note on the fate of specimens and equipment: As is typical of most of my expeditions, I identified and counted the fishes we collected before leaving Bolivia, divided them in half, and left half in Santa Cruz’s MNKP collection for Bolivian students to examine over the decades to come. The other half returned with me to the United States, where the specimens will be available to students and scientists from North America and around the world. The electroshocker and all the equipment purchased for the trip remained in Santa Cruz for scientists at the MNKP to use.

Author: - Monday, November 7th, 2011

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