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Aquatic Biogeography

Geography is inseparable from studies of taxonomy, ecology, and evolution. Most of my research in these fields has focused on the three major South American highlands: the geologically ancient Guiana Shield and Brazilian Shield, and the much younger Andes Mountains. Highlands are intriguing areas for fish biogeography for several reasons. First, geologic uplift is often datable via multiple, independent geologic methods. Since highland uplift often functions as a vicariant event, separating once contiguous populations, the ability to date these uplifts provides often rare opportunities to calibrate absolute rates of evolution. Second, geologically ancient highlands like the Guiana Shield harbor many narrowly endemic, relictual lineages – the discovery of which can have far-reaching evolutionary ramifications. Third, elevational gradients are correlated with a host of physical habitat gradients that can create natural, replicated experiments for both ecological and evolutionary research.

Amazonian rivers can be broadly divided into lowlands, where broad flooplains interact with main river channels via lateral connections, and uplands, where high gradients and incised channels lead to longitudinal gradients in water physicochemistry, species richness and trophic ecology.   In 2010 I led an interdisciplinary investigation of the Arazá-Inambari River, a headwater of the Amazon that starts at 4300 m asl (meters above sea level) and descends to 200 m asl before joining the larger Madre de Dios River. We sampled water physicochemistry, epilithon and macroinvertebrate diversity and abundance, and fish diversity at 18 main-stem and 14 tributary sites, each separated by an approximate average elevation of 250 m. We found nonlinear relationships but strong coefficients of determination between elevation and temperature and dissolved oxygen, and trends toward increased diversity of epilithon and decreased diversities of macroinvertebrates and fishes at higher elevations. Fishes were only collected below 2600 m asl and displayed a steep increase in trophic ecological diversity (plot left), from invertivores at high elevations to a diverse mix of invertivores, piscivores, and herbivore/detritivores at low elevations. Macroinvertebrate abundance showed an immediate decline with the onset of fishes, but insect functional feeding groups showed strong nonlinear coefficients of determination when regressed against elevation (plots below). These patterns displayed clear differences with predictions of the temperate River Continuum Concept that deserve further study.

The Guiana Shield is an aggregation of highlands in northern South America that give rise to headwaters of the Amazon, Orinoco and Essequibo rivers (plus many other coastal drainages and tributaries of the lower Amazon). Since my first expedition to  South America – to the Ventuari River, a right-bank tributary of the upper Orinoco that is nested within the westernmost flank of the Guiana Shield – I have been especially interested in biogeographical patterns around the western lobe of the Guiana Shield (pictured). The western Guiana Shield is dominated by a dramatic landscape of tabletop mountains, sheer-sided cliffs, and a diverse fish fauna that continues to reveal incredible new discoveries. This book chapter summarizes the geologic and hydrologic history of the Guiana Shield, and what was known in 2011 about the aquatic biogeography of this region. It postulates null hypotheses for geographic patterns in the diversification of Guiana Shield fishes and taxonomic and phylogenetic support for these models. Patterns described in this chapter serve as the foundation for several published and ongoing studies of northern South American fish biogeography.

The extreme physical conditions of high elevation stream habitats affect not only the taxonomic richness of communities and their ecological interactions (see Example 1 above), but also the molecular evolution and biochemistry of proteins within stream-dwelling organisms. Since 2012 I have collaborated with PhD student Gianni Castiglione and Professor Belinda Chang at the University of Toronto on an investigation into evolutionary modifications of the vision protein rhodopsin in response to life at high elevations. This research has the potential to help predict how long a given species or lineage has occupied high elevations, and thus the patterns of biogeographic dispersal across high elevation headwaters of the Amazon and Orinoco rivers.

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