Regional Effects of Hydrologic Alterations on Riverine Macrobiota in the New World: Tropical–Temperate Comparisons

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Abstract

Because 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 hydrologic alterations in the New World, emphasizing dams. This is followed by a summary of biotic patterns that have emerged in hydrologically altered rivers draining temperate 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. The scope and history of hydrologic modifications in the New World is integrally tied to what is known about regional biological effects of these hydrologic changes in tropical and temperate regions. Rivers draining North America have been modified extensively (see, e.g., Benke 1990, Dynesius and Nilsson 1994). The United States has more than 5500 large (defined as higher than 15 m) dams, more than twice the number of large dams in Canada, South America, Mexico, Central America, and the Caribbean combined (Figure 1a). The United States also has the most large dams per land area, followed by the Caribbean, Mexico, and Canada (Figure 1b). Seventeen percent of the approximately 5.6 million km of rivers in the United States have been dammed in the last century, and only 0.25% (approximately 8000 km) of them are protected (i.e., minimal or restricted regulation). Most of these dams were built over the last five decades, with rates of dam development peaking in the 1960s; in North America, more than 200 major dams were completed each year between 1962 and 1968. The rate of large dam development has since declined. For example, the rate of construction of US nonfederal dams decreased from 2000 per year in the 1960s to approximately 1240 per year in the 1970s (NRC 1992). Temperate drainages of Argentina (i.e., the arid Central Andean area and Patagonia) are also highly regulated (Figure 2; Bonetto et al. 1987). An inventory of the energy potential of the Plata Basin in Argentina (Figure 2) indicates that 278 dams could be constructed (Bonetto et al. 1987, 1989). Thirty-two dams have either been built or are under construction in the Magdalena Basin of Colombia. In contrast to the era of large dams in the temperate regions of North America, which began with the construction of the Hoover Dam in the 1930s, the development of large dams in tropical regions of South America is fairly recent. Their construction became widespread after 1970, when Latin American governments had relatively easy access to loans from the international banking system. Since then, the number of large dams has increased by an average of two dams every 3 years, a rate of increase that is projected to continue into the next century (Petts 1990), driven largely by the continued demand for hydropower. A study conducted by the World Bank (1984) indicated that only 7% of Latin America's hydropower potential had been developed. Moreover, Brazil's state-owned power monopoly, Electronorte, proposes to meet over half of Brazil's future electricity needs via hydroelectricity from the Amazon region (Best and daSilva 1989). Over 70 dams are planned for Brazil's Amazonian region alone (Figure 3). Although dam-building plans have been postponed because of financial constraints on the government, the overall scale of the plans remains unchanged (Fearnside 1995). The vast extent of hydrologic modification in the temperate New World allows identification of general landscape-scale effects on river fauna. Prominent examples of biological effects observed across regions are summarized in Table 1, based on information for fishes and molluscs. Regional effects include extirpation or imperilment of migratory fishes; faunal range fragmentation and population isolation; extinction or imperilment of geographically restricted taxa dependent on uniquely riverine habitats; reduction in abundances of flood-dependent taxa, as well as taxa dependent on freshwater inflows to estuarine habitats; and increases in lentic and exotic taxa. Most information comes from North America, but some is available from temperate regions of South America (e.g., Patagonia and the Central Andean area of Argentina). Long-range migrations in temperate New World rivers are best documented for relatively large-bodied fishes, especially diadromous (migrating between ocean and fresh water) taxa. On the Pacific Coast, the diversity contained within hundreds of anadromous (spawning in fresh water) salmonid stocks has been severely depleted, largely because of dams and hydrologic alteration (Nehlsen et al. 1991). Atlantic salmon and anadromous sturgeon species are similarly imperiled, in part by lost access to portions of their native ranges upstream of dams (Table 1). Commercial landings of American shad (Alosa sappadissima) on the Atlantic Coast have declined by over 90% in the last century, with blockage of spawning runs by dams a major contributing factor (Jenkins and Burkhead 1994). Some anadromous fishes (e.g., river herrings, Alosa spp.; striped bass, Morone saxatilis; and American shad) with natural distributions curtailed by dams have been established outside their native ranges and in landlocked populations. However, the upstream portions of many US river systems have lost once-common species, such as the American eel, whose migrations are impeded by dams. The net impact of these species extirpations on ecosystem function is largely unquantified, but it is likely to be substantial (Pringle 1997); for example—one among many—the postspawning mortality and subsequent decomposition of anadromous salmon and trout can represent an important nutrient addition to small streams (e.g., Bilby et al. 1996). Potamodromous (migratory within fresh water) fishes likewise include diverse taxa that have declined because dams block migratory paths. Effects on some fishes with obligatory migrations have been striking, such as the extinction or imperilment of all taxa of North American lakesuckers (Table 1)102. The extent of facultative movement by freshwater fishes before widespread dam construction is not well known. However, fragmentation and shortening of free-flowing rivers has most likely contributed to population declines in many wide-ranging fishes, freshwater and et al. Most North American fishes are riverine taxa with and have lost of natural riverine river rivers have been to of have been restricted to or driven to extinction (Table also case Mobile River in upstream areas by dams are to extirpation when or mortality (e.g., from be by from et al. 1991). riverine from dams, native and are limited by and by altered and hydrologic (e.g., et al. to the and of streams and rivers have declined because dams, and have and and and 1987, and 1987, et al. species that were in the River are et al. of migrations in rivers has contributed to the in North American fauna. all native on or more species to as for the and the diversity of by to has only recently to be dams have in of to extirpations et al. 1996). have altered the of in river systems across the United include of as in the et al. and and Rivers et al. the and of has been The effects of hydrologic alteration on are well effects on river are well not in the main of rivers use to some for and et al. et al. 1995). in of large systems in North America, with alteration in are by declines in et al. et al. landings in the River in are landings in the et al. The of and by has aquatic in the River by as as and of has also contributed to imperilment of fishes that that are by the and 1987, et al. 1995). dependent on freshwater inflows to estuarine are also because of with dams and hydrologic modifications. The provides a declines in of in have been tied to hydrologic modifications that of fresh et al. 1992). The of fresh and has of the of the northern of and into the of the (i.e., the and river for and estuarine species that in or the The as in under the US The of native species has also declined. them are the the which as in and the an important species of and 1992). of freshwater is the increased of and estuarine the New World temperate and dam have better to many or to a of the native than to native and southeastern US rivers for and anadromous fishes on spawning for and in fishes have been their native ranges to and from dams of fishes, fishes such as which were for The River Basin provides a Because of the basin more than twice the number of species than were 1995). fishes have also been into regulated rivers in temperate regions of South For example, in region have in the of species and in the (Bonetto et al. 1987). the in the approximately of of and the of the of the and the lack of fishes (Bonetto et al. 1987). riverine species and fishes to can dams and upstream into rivers et al. 1991). fishes or with native species et al. 1987, et al. and river by and altered The southeastern US is a of temperate freshwater and The Mobile River which approximately in and more than river km in major systems (Figure southeastern aquatic diversity and The contained more than native freshwater fishes, of which were species et al. species, with et al. et al. 1997); and species, of which are (e.g., most large river systems in the United the Mobile River Basin has been extensively for and hydropower and dams, of which are in the constructed (Figure in the and portions of the An 15 dams have been built for hydropower The dams approximately of river in the basin as well as portions of many there has been of free-flowing riverine For example, hydropower dams of the River of the riverine and with lentic of the riverine as from dams, of and (Figure Dam construction and hydrologic alteration have aquatic biota in the Mobile River as by effects on the of the (i.e., fishes and fishes eel, striped bass, shad) have been from portions of their ranges because dams between and freshwater fishes such as sturgeon sturgeon and southeastern have been or are severely in portions of their fishes also effects of and and species diversity are in river from dams (e.g., and et al. In fishes in the Mobile River basin are because of range reduction and fragmentation et al. 1989). have more in main (e.g., of species in of the et al. and in (e.g., extirpation in the Burkhead et al. 1997). species are the Mobile River basin of the native and species are on the of extinction et al. 1997). of the River as many as species, after construction of a of dams that the of the known extinction in the half of the 1997). The effects of hydrologic alteration to and to estuarine in the of the construction of hydropower dams with large were in and and in are because hydropower and which are to in the basin the and into the when are are by in the The extent of lost in the of the of altered not known. However, the of fishes such as US well the effects of In the Mobile altered and especially higher inflows in and have been with lost of estuarine fishes and that the with species in the Mobile River basin include approximately fishes, many of such as were to such as represent from in the basin in the of or more sturgeon of the by an exotic species is the of the in the system. A native to US the native species of species, and by the potential to the native fauna. information on river in South America is (Bonetto et al. 1987). For example, from South America contributed to either the on and or the and the landscape-scale effects of hydrologic modifications on tropical river are to are migratory runs and extirpation of fishes and between and fresh water) as well as the of and exotic taxa in in temperate zones, fishes have been into tropical of South America both from basins within the and from and 1991). and that fishes and exotic have been into in However, from a study of in and studies have the ecological or ecosystem effects of such in tropical freshwater it is that the biota of rivers is to dams and the of the extent of migratory (e.g., and and the of of for and the or in tropical and The history of tropical riverine is and has to of and to temperate zones, tropical regions are by of Although the from freshwater in northern temperate regions and changes in the have contributed to in the by and are more than 2000 species of fishes in the Amazon with about 90% in contrast to species in the River of North America, with only 1992). and hydrologic modifications can in the of species, in some by faunal and Rivers and streams that tropical regions are to which have major for the of diverse faunal that were of these include for and the of among 1995). in the Amazon basin have been under for two and a to the with the Amazon and the Plata Basin (Figure 1995). The and of many tropical aquatic taxa them to fragmentation by dams. et al. that dams to be the most of for Amazonian in the of the large rivers in Latin America and their these large fishes migrations in rivers and their main for and and and 1997). an established in the and in migratory dynamics are more of and have been of the Dam the River in et al. 1995). is also potential blockage of of and and of upstream migrations that stocks river et al. 1991). and have all been upstream of which is the of the Magdalena and Rivers in 1995). Effects of dams on neotropical migratory (e.g., and (e.g., are summarized in Table and also can alter the biotic of streams draining tropical Caribbean because these streams are by migratory fishes and and are to hydrologic modifications (Pringle and This is of the extent of hydrologic modifications per land area (Figure in the Caribbean and it is not that faunal have been In and also in Caribbean dams for are that all native and from upstream et al. Table dams with have a as by of native species dams with than them or in dams also of fishes and when such dams are with For example, on an average half of the draining the Caribbean in of major is into which mortality of to the et al. The effects of mortality on upstream are not known. In tropical rivers with the most factor in year is the area of land which provides an of the and can of their The and of many tropical species are tied to of that in the as areas for in and and are important for fishes, which migratory species the and of the Amazon to et al. 1996). migrations of fishes into areas are by et al. and many fishes have for on and of Amazonian streams has and and of in tropical to in the temperate with effects on such as and For example, than of the by construction of the Dam the between and and only of the of by the of by in the on the between Argentina and is to have more than which were after the of the in Rivers 1994). the River the Dam in (Figure from the in of of the to the decomposition in of the and large of The of to the more than million conducted 3 after the dam indicated that of in the river began to only about km of the are to by of aquatic The rates in tropical and with hydropower For example, after the began to over half of the with The under by with but also many and 1989). and can have effects on biota in arid tropical in of of fresh to and the of fishes For example, of the in has native fishes that in fresh or with or highly some of these fishes have km upstream and 1994). The Plata River basin is the in South America and the in the than million in and Argentina in the The Plata River has main whose are in the the and the The in the is in the of the River and to or in the with the the River (Figure is by a that and more than (Bonetto et al. The extent of hydrologic modifications in the of the of the rivers in South than in the of the and Rivers (Figure in Basin more than of with the River development has been in the basin since the by a increase from the 1970s many of these were the River had an that in or with in of dams is based on power is in and basin dams have increased and the of has been in the basin of the River in to the of with and is in the Although the biotic effects of are to the regional within the Plata basin because studies are (Bonetto et al. documented effects include decomposition of and of in of increased and impeded native migrations and increases in spp.; Table Effects of dam construction have been to migratory fishes in the and Rivers in the a number of dams have been built in the (Figure are on most of these dams, and migratory species (e.g., have (Bonetto et al. 1989). effects could to areas as dams are A major regional biotic from the of the the of the which two (Bonetto and This of each more than is by the and The the dam allows of species, which not because the a has of the and the basin (Bonetto et al. 1987). In the Plata and taxa and have and the has from in the Plata and has also been a in the of taxa and which upstream from the in and the of and has decreased of most of the migratory species are severely in the and River The exotic has been as the within in the Plata and has been in the are to a of and used in and The (Figure for the and river increase and a major impact on the This is highly in part because of the potential and on the part of has and over the last (Pringle et al. The which approximately of is an important and highly diverse area that provides for species, and more than species of The for on the in the The modifications of the and and and The regional biotic effects of hydrologic modification could alter the and of the which in of the article research in the areas help inform decisions about future alterations in the New of the cumulative biotic effects of hydrologic alterations within river of of hydrologic effects addition to information on specific taxa or of the and migratory of aquatic biota within river and studies of aquatic biota in from to in regulated rivers of Latin of the of hydropower for temperate regions to tropical regions. Most on a emphasis has to et al. 1997). However, these to be landscape of dams within basins dam or of rivers in a basin be to the biotic of the effects of hydrologic alterations are based on such as modification or the or of specific taxa. be For example, to what extent has the example, and rivers been by hydrologic is also important to upstream as well as effects of dams and (Pringle 1997). are the upstream biological of what extent has of biota in upstream and how can of taxa be and is a to the effects of dams and on dynamics of aquatic fauna. what extent fishes and within river before fragmentation have altered their migratory what extent has fragmentation caused in of migratory and flood-dependent The lack of information these but and for be et al. 1994). The to general from studies of tropical rivers to temperate but tropical and temperate rivers to dams For example, temperate rivers have been regulated for that not have a understanding of the biological of main tropical rivers also an to examine the extent to which and is dependent on are for the of how hydrologic modifications fishes with migratory river basins be because the not the in river also that of dams in the tropics can help gaps in which could to better of and more about the effects of of the overall effects of dams on in South America has been by the lack of as is the case in North for the five major dams constructed in the Amazon the and not include of migrations before the were et al. 1996). ecological studies of fishes were conducted before the dam on the completed in were general of species before and after the et al. 1996). with the of Amazonian in to before of the most recently constructed dams in the A of before and after construction of the a reduction in diversity the dam and a reduction The in diversity is to of the migratory of many species et al. 1996). for temperate regions be in tropical regions (Pringle tropical streams by fishes be to temperate streams by diadromous Although salmon are by the best known of migratory fishes, hundreds of species have patterns (e.g., especially on large rivers in the Because these fishes not the anadromous migratory are are not as Dam have that are have been are based on the salmon and are for many native species 1996). For example, the Dam on South America's River with a of more than The were to based on and with anadromous migrations on the River in North America World Bank 1995). Little or to the that many species in the are and river their Little is known regarding or for important species in the et al. and and 1997). can to the of fishes in Latin America when with in North America, whose migratory patterns relatively A recent by the US indicated that of the regulated by the in the United only have some of in for the of upstream and only have are the of is not This article how hydrologic modifications have riverine on regional scales in the New World. In temperate regions of North and South America, rivers have hydrologic alterations over the We have extensively documented how these alterations have in biotic from population and to range fragmentation and increases in exotic and taxa. In tropical regions of Latin America, there are of dams and limited data to temperate which to generalizations about biotic but it is that as in these regions to rivers rivers are to hydrologic the of faunal the extent of faunal migratory the of of to combined with the and in tropical and the of hydrologic modifications in the New World ecosystem in both the and the in riverine the of aquatic biota to to changes in (e.g., which is across many of North and South On an many stocks of fishes that temperate-zone rivers after are and summarized in article the nature and extent of hydrologic how riverine to future on both and biotic on landscape and regional scales is an important in the and with hydrologic modifications. We to for in We for many and and for and in the on effects of We also and two for their on an of Table Regional effects of hydrologic alteration on riverine in the temperate New World Table Regional effects of hydrologic alteration on riverine in the temperate New World of dam development in the New World. of dams and number of dams per land from and The extent of river in regions of the Plata the arid or Central Andean and the dams and with a of more than are from Bonetto et al. planned and large dams and in Brazil's Amazonian region from 1995). The large than dams are in the of in in in in in and in in dams in the state-owned power are planned by governments or are not The Mobile River basin in the southeastern United the location of dams and and dams to the River the southern is of Mobile River basin as or under the A of riverine remains of Dam on the can migrations to spawning by river fishes such as the southeastern This species to be from the River of the Mobile River but in the River the and dam is by planned for South America that from 1995). The proposes to km of the and river systems into a that from to the Atlantic in Argentina The of tropical freshwater such as are by large dams. Rivers of the Plata River basin from Bonetto et al. with the location of completed or large dams in the River basin and are to in the

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The record

Venue

BioScience

Topic

Fish Ecology and Management Studies

Field

Environmental Science

Canadian institutions

—

Funders

National Science Foundation

Keywords

Temperate climateTropicsGeographyEnvironmental sciencePhysical geographyClimatologyEcologyBiologyGeology

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