Thomas G. Horvath

CRAYFISH FEEDING PREFERENCES FOR FRESHWATER MACROPHYTES: THE INFLUENCE OF PLANT STRUCTURE AND CHEMISTRY

Abstract The omnivorous crayfish Procambarus clarkii fed selectively on several species of macrophytes, preferring delicate fresh plants that had filamentous or finely-branched architectures. When the macrophytes were dried, powdered, and reconstituted into an alginate gel (thus eliminating among-species differences in physical characteristics), crayfish preferences were altered; previously tough plants that were high in nitrogen and protein were preferred over previously delicate plants that were low in nitrogen and protein. Even though plant structure influences feeding decision of crayfish, the structurally identical macrophyte gels were fed upon differently, demonstrating that nonstructural traits are important feeding determinants. However, plant tissue constituents such as nitrogen, protein, phenolics, lignin, cellulose, or ash were not significantly correlated with feeding preferences. Two high-nitrogen plants that were avoided by crayfish as fresh and as reconstituted tissue (Nuphar luteum macrophyllum and Alternanthera philoxeroides) possessed extracts that reduced crayfish feeding in laboratory assays, demonstrating that macrophyte metabolites can deter some herbivores. As is often observed with large generalist herbivores and omnivores in terrestrial and marine systems, the freshwater crayfish made feeding decisions based upon multiple plant cues (structure, nutrition, chemical defenses).

Effect of Zebra Mussels, Dreissena polymorpha, on Macroinvertebrates in a Lake-outlet Stream

Abstract Zebra mussels, Dreissena polymorpha, increase macroinvertebrate abundance on hard substrata in lakes and large rivers because they enhance surface area, substratum heterogeneity and the accumulation of benthic organic matter (BOM). We tested the hypothesis that zebra mussel colonization of hard substrata in a lake-outlet stream would similarly increase macroinvertebrate abundance. Zebra mussels were attached to artificial rocks (0.05 m2) in a crossed factorial design with two states of mussels (live or dead) and three densities [low (200·m−2), medium (500·m−2) or high (1000·m−2)] plus a noncrossed control (no attached mussels). Rocks were placed in an erosional zone of a lake-outlet stream in southwestern Michigan that contained low densities of zebra mussels. After 28 d total macroinvertebrate abundance was significantly higher on rocks with high densities of zebra mussels, but live or dead treatment had no effect on macroinvertebrate abundance. Family richness differed significantly among density treatments (high = medium > low = control), but Simpsons diversity index did not differ among treatments. We conclude that changes in macroinvertebrate community structure probably were related to the increased substrate complexity provided by zebra mussels.

Abundances of Naked Amoebae and Macroflagellates in Central New York Lakes: Possible Effects by Zebra Mussels

Summary. Zebra mussels (Dreissena polymorpha) are bivalve mollusks that have invaded and altered the ecology of many North American lakes and rivers. To identify possible ecological effects of zebra mussels on naked amoebae, this study compared abundances of sediment and water column naked amoebae in shallow water zones of four lakes with and four lakes without zebra mussels. Additional data was collected on the density of macroflagellates. Although no statistically significant difference in naked amoebae density was found, higher ratios of sediment to water column naked amoebae abundances in zebra mussel lakes were observed due to increased naked amoebae abundances in the sediments. However, we did not observe a concomitant decrease in water column naked amoebae abundances. Flagellate abundances revealed no significant differences between the two lake types. Taken together, the data show that naked amoebae and flagellates thrive in shallow water zones of zebra mussel lakes and that the filter feeding activities of zebra mussels and reported reduced water column protists abundances may be offset by the flocculation of protists from the rich zebra mussel colonies.

Human Movements: Consequences to Global Biogeography

The examples discussed throughout this book show that globalization influences modern society’s social, political, and economic structures. However, globalization also impacts the biological and scientific environments. Lewellen’s definition that “contemporary globalization is the increasing flow of trade, finance, culture, ideas and people brought about by the sophisticated technology of communications and travel and by the worldwide spread of neoliberal capitalism and it is the local and regional adaptations to and resistances against these flows” (2002, 8–9) misses the accidental movements of nonhuman species. Although many of the trends in increased globalization have been going on since the beginning of trade itself, the scale has grown substantially in the past fifty years. The latter half of the twentieth century has seen an exponential increase in the number of species being transported between distinct geographically based ecosystems as a result of advances in technology, especially transportation. These technological advances are the direct result of globalization.1 As the number of multinational corporations has increased in the latter half of the twentieth century (Gabel and Bruner 2003, 3), so has the total tonnage of materials being moved between continents by these corporations. Additionally, Gabel and Bruner observe, “The transportation revolution led to shrinking the time it takes to get people and materials from here to there” (2003, 9).