David Bruce Lewis

TRADE‐OFFS BETWEEN GROWTH AND SURVIVAL: RESPONSES OF FRESHWATER SNAILS TO PREDACIOUS CRAYFISH

Foraging organisms must often balance the conflicting demands of feeding and avoiding predators. I investigated whether the freshwater snail Amnicola limosa trades off obtaining food for reducing the risk of mortality from predatory crayfish (Orconectes). Field surveys and experiments were conducted to determine, first, whether snails altered habitat use when presented with predation risk, and second, whether this behavior had consequences for growth. Surveys of 20 northern Wisconsin lakes revealed that snail occupancy of food-poor macrophytes was positively related (and occupancy of food-rich bottom sediments was negatively related) to crayfish abundance. Similarly, in a lake where crayfish (O. rusticus) are abundant, but differentially active among seasons, seasonal sampling indicated that snails occupied sand during the spring period of low crayfish activity but were absent from this habitat during summer. These distributional patterns derive from a combination of active habitat shifts and differential mortality among habitats, as indicated by laboratory experiments and field estimates of snail dispersal rates. Furthermore, aggregation of snails on macrophytes reduces snail growth. The average length of adult snails collected from bottom sediments was greater than the length of snails on macrophytes. Similarly, in the laboratory, snails that migrated up vertical surfaces in response to chemical alarm signals from feeding crayfish exhibited a growth depression. This trade-off of maximum growth rates for increased survival from crayfish by A. limosa likely poses constraints on snail fitness, which, in turn, can affect interactions with other predator species.

Landscape-scale Variation in Taxonomic Diversity in Four Groups of Aquatic Organisms: The Influence of Physical, Chemical, and Biological Properties.

We evaluated several factors influencing the taxonomic richness of macrophytes, benthic invertebrates, snails, and fish in a series of northern Wisconsin lakes. We chose the study lakes to decouple the potential effects of ionic strength of lake water and stream connection, two factors that are usually highly correlated and therefore have been confounded in previous studies. In addition, our study lakes covered a wide range in a variety of characteristics, including residential development, abundance of exotic species, nutrient concentrations, predator abundance, and lake size. Species richness within each of the four taxonomic groups was significantly positively related to ionic strength (as measured by specific conductance); we also found secondary associations with other variables, depending on the specific group of organisms. The relationship between richness and lake area was dependent on the specific conductance of the lake and the vagility of the organisms; less vagile groups of organisms showed stronger and steeper species–area relationships in low-conductivity lakes. Further, after variance owing to specific conductance was removed, the presence of stream connections was positively related to species richness for fish, snails, and macrophytes as well as familial richness in benthic invertebrates. Our results indicate that lakes with relatively more groundwater input have lower extinction rates for all four groups of taxa and that lakes with stream inlets and outlets have enhanced immigration rates for fish, snails, benthic invertebrate families, and macrophytes. These findings link processes of immigration and extinction of four groups of organisms of varying vagility to landscape-level hydrologic characteristics related to the glacial history of the region.