David D. Hart

How dams vary and why it matters for the emerging science of dam removal

D are structures designed by humans to capture water and modify the magnitude and timing of its movement downstream. The damming of streams and rivers has been integral to human population growth and technological innovation. Among other things, dams have reduced flood hazard and allowed humans to settle and farm productive alluvial soils on river floodplains; they have harnessed the power of moving water for commerce and industry; and they have created reservoirs to augment the supply of water during periods of drought. In the 5000 or so years that humans have been building dams, millions have been constructed globally, especially in the last 100 years (Smith 1971, WCD 2000). If dams have successfully met so many human needs, why is there a growing call for their removal? The answers to this question require an appreciation of society’s changing needs for, and concerns about, dams, including the emerging recognition that dams can impair river ecosystems (Babbit 2002). But decisions about dam removal are complex, in no small part because great scientific uncertainty exists over the potential environmental benefits of dam removal. Certainly, the scarcity of empirical knowledge on environmental responses to dam removal contributes to this uncertainty (Hart et al. 2002). More fundamentally, however, a scientific framework is lacking for considering how the tremendous variation in dam and river attributes determines the ecological impacts of dams and the restoration potential following removal. Such an ecological classification of dams is ultimately needed to support the emerging science of dam removal. In this article, we develop a conceptual foundation for the emerging science of dam removal by (a) reviewing the ways that dams impair river ecosystems, (b) examining criteria used to classify dams and describing how these criteria are of limited value in evaluating the environmental effects of dams, (c) quantifying patterns of variation in some environmentally relevant dam characteristics using governmental databases, (d) specifying a framework that can guide the development of an ecological classification of dams, and (e) evaluating the ways that dam characteristics affect removal decisions and the future of dam removals. We restrict our analysis to the United States, where dam removals are currently hotly debated; however, the ecological framework we advocate could also be generalized to other parts of the world.

River flows and water wars: emerging science for environmental decision making

Real and apparent conflicts between ecosystem and human needs for fresh water are contributing to the emergence of an alternative model for conducting river science around the world. The core of this new paradigm emphasizes the need to forge new partnerships between scientists and other stakeholders where shared ecological goals and river visions are developed, and the need for new experimental approaches to advance scientific understanding at the scales relevant to whole-river management. We identify four key elements required to make this model succeed: existing and planned water projects represent opportunities to conduct ecosystem-scale experiments through controlled river flow manipulations; more cooperative interactions among scientists, managers, and other stakeholders are critical; experimental results must be synthesized across studies to allow broader generalization; and new, innovative funding partnerships are needed to engage scientists and to broadly involve the government, the private sector, and NGOs.

Community organization in streams: the importance of species interactions, physical factors, and chance

SummaryExperimental studies were used to examine the mechanisms governing the distribution and abundance of two major patch types in unshaded reaches of Augusta Creek, Michigan (USA). One patch type is dominated by Cladophora glomerata, a macroalga potentially able to monopolize space, whereas the other type is comprised of a low-growing, epilithic microalgal lawn inhabited by several species of sessile grazers (especially the caddisflies Leucotrichia pictipes and Psychomyia flavida). Cladophora patches are absent from mid-channel sites characterized by current velocities ≤ ca. 50 cm s−1; caging experiments indicate that their absence is due to grazing by crayfish (Orconectes propinquus). Cladophoras presence in sites with velocities >50 cm s−1 apparently results in part because crayfish foraging activity is impaired in high flow regimes. The presence of Cladophora strongly affects various other invertebrates due to its alteration of abiotic and biotic characteristics of the microhabitat. For example, the abundance of sessile grazers (e.g. Leucotrichia and Psychomyia) that inhabit microalgal patches is negatively correlated to the abundance of Cladophora, whereas the abundance of several other invertebrates (e.g. Stenonema mayflies and Taeniopteryx stoneflies) is positively correlated to Cladophoras abundance. Therefore, in some portions of this system, crayfish act as keystone predators because of their ability to regulate the abundance of Cladophora, which in turn has strong positive and negative effects on other components of the community. Cladophora does not always monopolize space at high velocities in the absence of crayfish, however. If sessile grazers arrive at such sites before Cladophora, they can prevent its establishment. Thus, where crayfish are absent, the likelihood that a site will be dominated by either Cladophora patches or sessile grazer — microalgal lawn patches depends on two sets of stochastic processes: (1) those that create bare space (e.g. disturbance and grazer emergence); and (2) those controlling the timing of recruitment by Cladophora or grazers at these bare sites. These priority effects (i.e. the ability of grazers and Cladophora to inhibit each others establishment) contribute to the marked spatial heterogeneity of these two patch types. Collectively, these results demonstrate how interactions between competition, predation, and physical factors can generate a complex mixture of community patterns.

RESOURCE LIMITATION IN A STREAM COMMUNITY: PHOSPHORUS ENRICHMENT EFFECTS ON PERIPHYTON AND GRAZERS'

A field experiment was used to test whether stream periphyton was nutrient limited, and if so, whether the grazing insects that consumed this resource were food limited. Eight replicate wooden flumes were placed in an unshaded riffle in Augusta Creek, a third- order, hardwater trout stream in southwestern Michigan (USA). Phosphorus, in the form of Ca(H2PO4)2, was added to four of these flumes for 105 d. The average concentration of soluble reactive phosphorus (SRP) maintained in P-enriched flumes (85 ,g/L) was >5 times as great as in control flumes (15 jug/L). After 77 d of nutrient enrichment, periphyton biomass per unit area on substrates with reduced grazer densities was 1.8 x as great in P-enriched flumes as in control flumes. The larvae of two species of grazing caddisflies (Leucotrichia pictipes and Psychomyia flavida) in P-enriched flumes also had higher individual mass, developmental rates, and population densities. These responses resulted in standing crops of final-instar larvae in P-enriched flumes that were 2.1 x and 1.5 x as great as those in control flumes for Psy- chomyia and Leucotrichia, respectively. This demonstration of food limitation supplements previous behavioral studies showing that both grazer species aggressively defend feeding territories, and contributes to a growing body of evidence suggesting that resource limitation is more important in some stream food webs than previously recognized. Resource limitation of these grazer populations may be due in part to the rarity of catastrophic flooding and drying in Augusta Creek. Such stable flows could permit consumer populations to reach higher densities than in frequently disturbed streams.

Density-dependent dispersal of black fly neonates is mediated by flow

To understand and predict the dynamics of a population it is necessary to determine whether processes such as dispersal, growth and mortality are density-dependent and how these processes may influence patterns of abundance and distribution. Newly hatched individuals (neonates) are a common dispersal stage in many terrestrial and marine invertebrates, and may affect where, and in what abundance, older stages are subsequently found, hence underscoring the potential importance of supply-side processes in governing the ecology of those systems. In streams, benthic invertebrates might be expected to experience strong density-dependent competition near oviposition sites due to the often clumped distribution of eggs, yet the ecology of the early life-history stages has been poorly studied. In laboratory experiments, we examined whether newly hatched black fly larvae (Simulium vittatum) disperse from egg masses, via water currents, in a density-dependent fashion, and the likelihood that the strength of density-dependence is modulated by current speed. To understand better the mechanisms controlling neonatal dispersal, we also determined the amount of time an average larva spent fighting, and the relationship between fights and dispersal events. The experimental results demonstrate that the dispersal rate of neonates from egg masses was strongly density-dependent. A second-order polynomial regression model reflecting this density effect explained 91% and 75% of the variation in dispersal rates for the fast and slow current speed treatment, respectively. Dispersal was lower at fast than at slow current speeds, indicating that these patterns of drift are not the result of passive dislodgment by water currents. Current speed also modified the effect of density on dispersal rate. The increase in dispersal with a unit change in density was lower at fast than at slow current speeds. Increasing larval density and low current speed increased the proportion of time a larva spent fighting, but most larvae did not disperse immediately after being attacked. The density effect suggests that dispersal by black fly neonates can be a voluntary response to reduced feeding rates stemming from competition with neighboring larvae. In general, it appears that the tendency of neonates to remain at the oviposition site depends on the suitability of the microhabitat for feeding. The high dispersal rates we documented (up to 4.5% of individuals min -1 ) occurred in response to levels of larval density, current speed, and food concentration that are probably typical of many field settings. This implies that many neonates may also disperse in a density-dependent manner via water currents in the field. The distances traveled by large numbers of dispersing neonates may decouple the number of larvae in an area from the number of adults that oviposited there, which suggests that supply-side phenomena may be important in streams. The development of a clearer understanding of the role of density-dependent dispersal as a potential regulatory factor in black fly populations depends upon the assessment of the fate of drifting individuals, coupled with measurement of other sources of mortality in these populations.

Restoring watersheds project by project: trends in Chesapeake Bay tributary restoration

Restoration of aquatic ecosystems is a high priority regionally and globally, yet only recently have such efforts adopted holistic approaches that include the restoration of streams and rivers flowing to coastal areas. As the largest estuary in the US, the Chesapeake Bay has been the focus of one of the most high-profile restoration programs ever undertaken in North America. While the primary emphasis has been on tidal waters, freshwater tributary clean-up strategies have recently been developed. We have compiled the first comprehensive database of over 4700 existing river and stream restoration projects in the Chesapeake Bay Watershed (CBW) to examine where dollars are being spent, what issues motivate restoration, and what approaches are used. By conservative estimates, in excess of

Experimental studies of exploitative competition in a grazing stream insect

400 million has been invested in restoration projects since 1990. The majority of projects were implemented to restore forest vegetation in riparian areas and improve water quality. Although the CBW has an extremely high density of restoration activities relative to other regions of the US, only 5.4% of the project records indicated that related monitoring of project performance has occurred. To provide cost-effective management solutions, we recommend that a centralized tracking system be developed that includes restoration projects associated with both tidal and non-tidal waterways, along with a substantial increase in investment in the comprehensive monitoring of individual projects following implementation.

A Special Section on Dam Removal and River Restoration

SummaryField and laboratory experiments were conducted to determine whether intraspecific competition for food occurs during the larval stage of the periphyton-grazing caddisfly Glossosoma nigrior (Trichoptera: Glossosomatidae). Larvae were placed in field enclosures at densities less than, equal to, or greater than their natural densities. Most of these individuals began to pupate after ∼3 weeks, whereupon the mass of each individual was determined. Final mass declined significantly as larval densities increased, whereas neither developmental rate nor mortality/emigration rate was significantly affected by density manipulations. a supplemental experiment comparing the final mass of individuals grown at reduced densities in a laboratory stream with individuals from a natural stream bottom confirmed the results of the more extensive field experiment: reductions in density resulted in significant increases in final mass. Periphyton availability in field enclosures declined according to a negative exponential function as larval densities increased. Over the ∼25-fold range of larval densities used in these experiments, the final mass of individuals increased linearly with periphyton standing crops. This result suggests that Glossosoma larvae may compete for food even at densities below those employed in this study. Path analysis was used to explore the importance of indirect (i.e., exploitative) and direct (i.e, interference) mechanisms for the observed competitive effects. The analysis indicates that a model based solely on exploitation explains nearly as much of the variance in mass as a model incorporating both interference and exploitation.

Flow mediates predator-prey interactions between triclad flatworms and larval black flies

H activities have degraded many of the world’s ecosystems, which has created an urgent need for strategies that can restore their ecological integrity. This need is accompanied by many scientific challenges, however. In particular, ecosystems are among the most complex entities in the hierarchy of life, and the successful repair of damaged systems will require a deep understanding of the processes that determine their structure and function. Biologists have a critical role to play in creating this knowledge because of their expertise in such varied phenomena as the role of microbes in detoxifying anthropogenic contaminants, the effects of disturbance on population persistence, and the factors influencing competitive interactions between native and exotic species. By itself, however, biological knowledge is not sufficient for restoring degraded ecosystems. Two other types of expertise are also needed for developing integrated restoration solutions. First, because ecosystems are composed of many interacting abiotic and biotic components, biologists must collaborate with their colleagues in the physical sciences to learn how these systems work. Second, because humans are such strong interactors in these complex systems, we need to work with experts who can help us understand how human attitudes, institutions, and technologies influence the condition and management of ecosystems. Such enhanced interdisciplinary dialogue and unified approaches are essential for creating public policies that can sustain the planet’s life support systems (Lubchenco 1998, Covich 2000, Ludwig et al. 2001). Proposals to restore rivers via dam removal raise many issues that require broad discussion and teamwork. This approach to river restoration derives from the growing recognition that dams often disrupt the structure and function of river ecosystems by modifying flow regimes, disrupting sediment transport, altering water quality, and severing their biological continuity (Ward and Stanford 1979, Petts 1984, Collier et al. 1996). Future dam removal decisions can be enhanced by developing a more complete scientific understanding of the processes that determine how rivers are affected by different types of dams and how they respond to dam removal. There is an equally important need to understand the social, economic, engineering, and legal factors that influence dam removal decisions. Assembling a diverse array of experts to explore these different facets of dam removal was an exciting challenge for us. Listening to and participating in the dialogue that took place when those experts gathered at the annual meeting of the Ecological Society of America in August 2001 was even more rewarding. This special section of BioScience brings together those diverse authorities, and a few others, to examine the potential utility of dam removal as a method of river restoration. Our goal is not just to explore the many different scientific and social aspects of this topic but also to consider how these components can and should be connected. Bruce Babbitt, former secretary of the US Department of the Interior during the Clinton administration, is intimately familiar with the subject matter, having been present—sledgehammer in hand— at many dam removals across the United States. His passionate essay (Babbitt 2002) clearly frames both the scientific and human dimensions of the subject. In particular, he emphasizes the critical need for strong science, not just to predict what will happen when dams are removed but also to monitor dam removal outcomes so that we learn how to maximize the effectiveness of this restoration method.

Predator Prey Interactions In A Benthic Stream Community: A Field Test Of Flow-Mediated Refuges

The velocity preferences of suspension-feeding larval black flies (Simulium vittatum) and predatory triclads (Dugesia dorotocephala), both alone and together, were determined in a laboratory flow chamber in which a gradient of current velocities was maintained and manipulated (...) These results clearly demonstrate the potential importance of flow in mediating predator-prey interactions in benthic stream communities