Regional Effects of Hydrologic Alterations on Riverine Macrobiota in the New World: Tropical-Temperate Comparisons The massive scope of large dams and other hydrologic modifications in the temperate New World has resulted in distinct regional trends of bi

Abstract

B ecause there are many long-established dams in temperate zones, paradigms and theories of how hydrologic modifications caused by dams alter the ecological dynamics of rivers are based largely on studies of temperate basins (e.g., Poff et al. 1997). Little is known about biotic responses to hydrologic modifications in tropical streams; generalizations about the effects of dams in the tropics are constrained by limited data on recently constructed, and relatively few, dams. Moreover, general ecological understanding of the effects of dams in both tropical and temperate zones is constrained by a lack of baseline information on the distribution and ecology of aquatic biota before dam construction, as well as by an overemphasis on economically important species. This article has two main objectives: to examine what is known about regional effects of hydrologic modifications in temperate and tropical areas of the New World (i.e., North and South America and the Caribbean), with an emphasis on fishes and molluscs; and to discuss research needs regarding regional effects of hydrologic alterations in temperate and tropical regions. A better understanding of regional effects of cumulative hydrologic alterations could help inform decisions on the nature and location of future hydrologic modifications. We begin with a brief description of the scope of hydro-logic alterations in the New World, emphasizing dams. This is followed by a summary of biotic patterns that have emerged in hydrologically altered rivers draining temper-ate regions. We use the highly regulated Mobile River basin in southeastern North America as a temperate-zone case study to discuss specific biological effects. We then focus on the vulnerability of the biota of neotropical rivers and discuss biotic patterns that are emerging in response to relatively recent hydrologic modifications. The Plata River Basin of South America provides a tropical case study. (The term tropical is used to refer to the equatorial area [approximately 30° north and south of the equator] between the northern and southern subtropical dryland zones.) We end by examining research needs and gaps in our understanding of the ecological effects of hydrologic modifications on landscape and regional scales in the New World. Scope of hydrologic alterations in temperate and tropical regions of the New World