Heather M. Malcom

Effects of zebra mussels (Dreissena polymorpha) on native bivalves: the beginning of the end or the end of the beginning?

Abstract The long-term effects of an alien species may differ from transient effects that occur shortly after its invasion of a new ecosystem. Conservationists fear that the invasion of North America by the zebra mussel since 1985 may lead to the extinction of many populations and species of native bivalves. The appearance of zebra mussels in the Hudson River estuary in 1991 was followed by steep declines (65–100%) in population size of all species of native bivalves between 1992 and 1999. The body condition of all unionids and growth and recruitment of young unionids also declined significantly. Initial declines in population size and body condition were correlated primarily with the filtration rate of the zebra mussel population but not with fouling of native bivalves by zebra mussels. However, samples taken since 2000 have shown that populations of all 4 common native bivalves have stabilized or even recovered, although the zebra mussel population has not declined. The mechanisms underlying this apparent reversal of fortune are not clear. Recruitment and growth of young mussels have shown limited recovery, but the body condition of adults has not. We found no evidence that spatial refuges contributed to this reversal of population declines. Simple statistical models project now that native bivalves may persist at population densities about an order of magnitude below their preinvasion densities. These results offer a slender hope that zebra mussels may coexist with unionids and sphaeriids in North America, as they do in Europe.

Submersed Vegetation as Habitat for Invertebrates in the Hudson River Estuary

We sampled epiphytic and benthic macriinvertebrates in 20 beds of submersed vegetation throughout the Hudson River estuary to assess the importance of plant beds in providing habitat for macroinvertebrates and to determine which characteristics of plant beds affected the density and composition of macroinvertebrates. Macroinvertebrate densities in plant beds were 4–5 times higher, on average, than densities in unvegetated sediments in the Hudson. The macroinvertebrate community in plant beds was dominated by chironomid midges, oligochaete worms, hydroids, gastropods, and amphipods. Many species of macroinvertebrates were found chiefly on submersed plants, showing that plant beds are important in supporting biodiversity in the Hudson. Macroinvertebrates were most numerous in beds with high plant biomass and in the interiors of beds, whereas neither bed size nor position along the length of the estuary affected macroinvertebrate density. Community composition varied strongly with position along the river (freshwater versus brackish), habitat (epiphytic versus benthic), and position within the bed (edge versus interior). Plant biomass also influenced macroinvertebrate community composition, but bed area had relatively little influence.

Causes of recruitment failure in freshwater mussel populations in southeastern New York

Populations of freshwater mussels (Unionoida) are declining or disappearing from many waters around the world. In many declining populations, recruitment fails before adult mortality occurs, resulting in relict populations that can persist for decades. We tested whether recruitment failure in populations of the freshwater mussel Elliptio complanata was associated with invasion of nonnative crayfish, loss of a primary fish host (American eel Anguilla rostrata), excessive inputs of fine sediments, or unfavorable interstitial water chemistry (too little dissolved oxygen or too much un-ionized ammonia). We sampled mussel populations, crayfish populations, and environmental conditions at 14 sites on wadeable streams in southeastern New York. Five of the mussel populations had little or no recent recruitment. We found no association between recruitment failure and crayfish, American eels, fine sediments, or interstitial dissolved oxygen. In contrast, recruitment failure was strongly associated with high concentrations (>0.2 microg N/L) of un-ionized ammonia. This threshold is much lower than thresholds for acute ammonia toxicity identified in laboratory studies. We suggest that excessive concentrations of interstitial un-ionized ammonia may be responsible for widespread declines of freshwater mussel populations, especially in agricultural areas.

Biodiversity in Hudson River shore zones: influence of shoreline type and physical structure

The shore zones of the Hudson River, like those of many developed waterways, are highly varied, containing a mix of seminatural and highly engineered shores. Our goal was to document the biodiversity supported by different kinds of shore zones in the Hudson. We chose six common types of shore zones, three of them “natural” (sand, unconsolidated rock, and bedrock), and three of them engineered (riprap, cribbing, and bulkheads). We measured selected physical characteristics (shore zone width, exposure, substrate roughness and grain size, shoreline complexity) of three examples of each of these shore types, and also sampled communities of terrestrial plants, fishes, and aquatic and terrestrial invertebrates. Community composition of most taxa differed across shore types, and frequently differed between wide, sheltered shores and narrow, exposed shores. Alien plant species were especially well represented along engineered shores. Nevertheless, a great deal of variation in biological communities was not explained by our six-class categorization of shore zones or the physical variables that we measured. No single shore type supported the highest values of all kinds of biodiversity, but engineered shore zones (especially cribbing and bulkheads) tended to have less desirable biodiversity characteristics than “natural” shore zones.

Long-term responses of native bivalves (Unionidae and Sphaeriidae) to a Dreissena invasion

A long-term (28-year) study documented complicated responses of populations of native bivalves (Unionidae and Sphaeriidae) to the dreissenid invasion of the freshwater tidal Hudson River. Early in the invasion (1993–2000), formerly large populations of all native species declined steeply, reaching 0 to 35% of preinvasion densities by the year 2000. These declines in population density were accompanied by large declines in body mass, body condition, and recruitment of natives. Since the year 2000, the population of sphaeriids has recovered to preinvasion densities, and body condition, recruitment, and juvenile growth of unionids have recovered substantially, even though the number of dreissenids in the river has not declined. However, population densities of unionids have not recovered, and 2 of 3 formerly common unionid species (Anodonta implicata and Leptodea ochracea) have not been collected in several years. The 3rd unionid species (Elliptio complanata) also appears to be on the verge of disappearing from the river. Statistical models suggest that the dynamics of native populations were more likely driven by exploitative competition than by fouling. They also suggest that changes in dreissenid body size may have modulated the strength of this exploitative competition over the course of the invasion. The lack of recovery of unionid populations despite substantial recovery of juvenile recruitment and growth, as well as adult body condition, appears to be a result of failure of juveniles to reach adulthood, probably because of high mortality from predators.