David L. Strayer

Freshwater biodiversity conservation: recent progress and future challenges

Abstract Freshwater habitats occupy <1% of the Earths surface, yet are hotspots that support ∼10% of all known species, and ∼⅓ of vertebrate species. Fresh waters also are hotspots for human activities that have led to widespread habitat degradation, pollution, flow regulation and water extraction, fisheries overexploitation, and alien species introductions. These impacts have caused severe declines in the range and abundance of many freshwater species, so that they are now far more imperiled than their marine or terrestrial counterparts. Here, we review progress in conservation of freshwater biodiversity, with a focus on the period since 1986, and outline key challenges for the future. Driven by rising conservation concerns, freshwater ecologists have conducted a great deal of research over the past 25 y on the status, trends, autecology, and propagation of imperiled species, threats to these species, the consequences of biodiversity loss for ecosystem functioning, metapopulation dynamics, biodiversity hotspots, reserve design, habitat restoration, communication with stakeholders, and weaknesses of protective legislation. Nevertheless, existing efforts might be insufficient to stem the ongoing and coming multitude of freshwater extinctions. We briefly discuss 4 important challenges for freshwater conservation. First, climate change will imperil both freshwater species and human uses of fresh water, driving engineering responses that will further threaten the freshwater biota. We need to anticipate both ecological and human responses to climate change, and to encourage rational and deliberate planning of engineering responses to climate change before disasters strike. Second, because freshwater extinctions are already well underway, freshwater conservationists must be prepared to act now to prevent further losses, even if our knowledge is incomplete, and engage more effectively with other stakeholders. Third, we need to bridge the gap between freshwater ecology and conservation biology. Fourth, we suggest that scientific societies and scholarly journals concerned with limnology or freshwater sciences need to improve their historically poor record in publishing important papers and influencing practice in conservation ecology. Failure to meet these challenges will lead to the extinction or impoverishment of the very subjects of our research.

Effects of alien species on freshwater mollusks in North America

Alien species (those carried outside their original ranges by human activities) have strongly affected the distribution and abundance of mollusks in many North American fresh waters. The best known of these aliens in North America is the zebra mussel (Dreissena polymorpha), which has nearly extirpated native unionid clams from infested lakes and rivers by fouling their shells and outcompeting them for food. Zebra mussels also have reduced populations of native sphaeriid clams, and both increased and reduced populations of snails. The effects of the other well-known alien bivalve in North America, Corbicula fluminea, are surprisingly poorly known. Corbicula probably caused some populations of native bivalves to decline, but other native populations seem to coexist with Corbicula. Several plausible mechanisms of interaction between Corbicula and the native biota have been proposed, but not demonstrated. Other aliens, including the recently arrived snail Potamopyrgus antipodarum, probably compete strongly with native freshwater mollusks under some circumstances. Several alien species, such as round goby and some sunfishes and crayfishes introduced outside their native ranges in North America, are effective predators on native mollusks and have strong effects on their distribution and abundance. Other aliens (particularly aquatic plants) affect mollusks by altering the food base or the physicochemical environment. Alien species can affect water quality, cycling of contaminants, and performance of biological indices of water quality. Because of ineffective control of aliens in North America, they may be an increasingly important factor in molluscan distribution as new species arrive from other continents and established species spread throughout the continent.

Challenges for freshwater invertebrate conservation

Abstract Freshwater invertebrate conservation faces 5 important challenges. First, ˜10,000 species of freshwater invertebrates around the world may already be extinct or imperiled. Second, human pressures on freshwater resources are intense and will increase in the coming decades, putting yet more species at risk. Third, scientific knowledge about freshwater invertebrates, although substantial and useful for many groups, is far less than for the vertebrates for which much of contemporary conservation biology was designed. Even the best-known freshwater invertebrates that have achieved legal protection are perhaps 1% as well studied as the typical vertebrate. Fourth, because freshwater ecosystems are downhill from and embedded in their watersheds, freshwater conservation usually has to manage entire watersheds rather than small local sites where imperiled species occur. Fifth, society spends only modest amounts of money for freshwater invertebrate conservation. The median expenditure in Fiscal Year 2003 for freshwater invertebrate species on the US Endangered Species List was only US

Use of flow refuges by unionid mussels in rivers

24,000, and only a small minority of imperiled species is listed and receives even this modest attention. Considering these serious challenges, I believe that we need to think deliberately about the best approaches for conserving freshwater invertebrate biodiversity. The best solution may be to move away from a species-based approach that is largely derived from a terrestrial model towards broader, regional approaches that try to satisfy legitimate human needs for fresh water while preserving as much biodiversity as possible.

Microhabitat use by an assemblage of stream-dwelling unionaceans (Bivalvia), including two rare species of Alasmidonta

The considerable within-reach patchiness of unionid mussels in rivers has not been explicable by simple habitat features such as water depth, current speed, or sediment grain size. As an alternative, I hypothesized that mussels are found chiefly in stable areas of the river bed where hydraulic stresses during floods are low. I used movement of marked rocks to locate flow refuges during floods in 2 small rivers in southeastern New York. Mussel beds were spatially coincident with flow refuges, but not with other measured features of the habitat (water depth, current speed, sediment grain size) in both study sites. Use of flow refuges by unionids may partly explain the characteristic patchiness of unionid communities in rivers, and contribute to the success of these longlived, slow-moving animals in running waters.

Macrohabitats of Freshwater Mussels (Bivalvia:Unionacea) in Streams of the Northern Atlantic Slope

We studied the microhabitats of six species of freshwater mussels, including two rare species of Alasmidonta, in the Neversink River, New York. In each of 270, 1-m2 quadrats, we measured water depth, current speed, bottom roughness, spatial variation in current speed, distance to shore, presence or absence of macrophytes, presence or absence of an overhead canopy, the extent of patches of fine sediment, and sediment granulometry, as well as recording the mussels present. Mussel populations in the Neversink are dense (mean=3.2/m2) and highly clumped. Stepwise discriminant analyses showed that current speed and spatial variation in current speed were the most useful predictors of the occurrence of mussels in quadrats. Alasmidonta heterodon was found most frequently at moderate current speeds and in quadrats that contained many patches of fine sediments. Alasmidonta varicosa occurred most frequently at moderate current speeds and in sediments with a high proportion of medium sands (0.25-1 mm). Nevertheless, the predictive power of discriminant models based on microhabitat variables is so low that we question the adequacy of a traditional microhabitat approach to unionacean ecology. We suggest that including geomorphological descriptors of the streambed or working at spatial scales of hundreds of metres might be more useful than a traditional microhabitat approach for predicting the distribution of freshwater mussels in streams.

Perspectives on the Size Structure of Lacustrine Zoobenthos, Its Causes, and Its Consequences

The goal of this study was to predict the broad-scale (1-10 km) distributions of freshwater mussels from readily available macrohabitat descriptors. All six of the descriptors used (stream size, stream gradient, hydrologic variability, calcium concentration, physiographic province, and the presence or absence of a tide) had some predictive power, but stream size and tidal influence were the most effective predictors of mussel distributions. Unexpectedly, several mussel species typically occurred in calcium-poor waters, which I tentatively interpret as evidence that these species might not tolerate eutrophication. In general, the macrohabitat distributions of mussel species identified in this study correspond only moderately well to previously published, subjective assessments of mussel habitat use.

Effects of low dissolved oxygen on juvenile Elliptio complanata (Bivalvia:Unionidae)

Benthic ecologists have recently begun to use animal body size as an alternative or supplement to traditional taxonomic descriptions of benthic communities, probably because of the successful use of a body-size based approach by planktonic ecologists. Relatively little information is available on the size structure of benthic animal communities, factors that affect the size structure of zoobenthic communities, or consequences of the size structure of a zoobenthic community. Marine and lacustrine zoobenthic communities have different size structures, and there are conspicuous differences among lakes in zoobenthic size structure. Factors likely to influence the size structure of lacustrine zoobenthos include water chemistry, lake productivity, and vertebrate predation. I conclude, however, that the effects of vertebrate predation on zoobenthic size structure probably are weaker than on zooplankton owing to the availability of physical refuges to the zoobenthos and to the widespread occurrence of morphological and behavioral defenses against vertebrate predation among zoobenthos. In turn, the size structure of zoobenthos may affect growth rates and productivity of fish, the fate of zoobenthic production, the turnover rate (and similar ecophysiological properties) of zoobenthos, food web efficiencies, and rates of sediment mixing. There are thus many opportunities for zoobenthic ecologists to use a size-based perspective in their research. Nevertheless, benthic ecologists should not apply planktonic paradigms about body size relationships uncritically to benthic communities, for body size has different implications for zoobenthos than for zooplankton.

Diacyclops dimorphus, a new species of copepod from Florida, with comments on morphology of interstitial cyclopine cyclopoids

Through use of a closed, circulating system, we examined the effects of low oxygen on the behavior and survival of juvenile Elliptio complanata, a common freshwater clam. Clams exposed to low oxygen exhibited increased stress behavior, extending their siphons, gaping, and surfacing more often than clams exposed to higher concentrations of oxygen. Stress levels rose with exposure time and led to increased mortality in a long-term study. Low oxygen in sediments may limit juvenile survival through direct mortality and indirectly through behaviors that can lead to increased mortality. Knowledge of the limitations may prove useful in clam reestablishment programs and for regulation of management practices near existing clam beds.

Changes in the Distribution of Freshwater Mussels (Unionidae) in the Upper Susquehanna River Basin, 1955–1965 to 1996–1997

Diacyclops dimorphus, new species, was collected from clean sand sediments of the Blackwater River, State of Florida, USA. The new species differs from congeners in that the female has all rami of the swimming legs biarticulate, but the male has most rami biarticulate and the leg 4 exopodite triarticulate. Only two other species of Diacyclops show comparable extreme reduction of the swimming legs. These are Diacyclops virginianus and Diacyclops trajani (a new name proposed for Speocyclops minutissimus sensu Petkovski 1954). The swimming leg articulation patterns of all species of Diacyclops are reviewed. This is the first report of consistent sexual dimorphism in swimming leg articulation in Diacyclops. Sexual dimorphism in the swimming legs rarely occurs in the subfamily Cyclopinae. We suggest that preferential reduction in endopodites of posterior legs of smaller cyclopines allows the female genital double somite to remain relatively large. Many interstitial cyclopine copepods possess distinctive morphological features. They are small, have a relatively large genital double somite, and produce relatively large eggs. Typically, their body appendages (antennule, antenna, mouthparts, and swimming legs) are short with relatively few segments, spines, and setae.