Sallie P. Sheldon

The Effects of Herbivorous Snails on Submerged Macrophyte Communities in Minnesota Lakes

The influence of herbivorous snails on freshwater macrophytes was examined both in the laboratory and the field. In the laboratory, 14 macrophyte species grown in the absence of herbivores had growth rates ranging from 1-10%/d. When grown with four densities of herbivorous snails, species that grew fastest in the absence of herbivores were, in general, most negatively influenced by grazing. In food choice tests, snails typically preferred the plant species that grew fastest in the absence of herbivores. Snail densities were manipulated in a small mesotrophic lake, Christmas Lake. In areas where high snail densities were maintained, macrophyte species richness decreased, and the plants remaining were the species least preferred in laboratory choice tests. In surveys of eight additional lakes having a range of snail densities, the relationship between snail density and macrophyte species composition and diversity was consistent with the laboratory and Christmas Lake results. The levels of herbivore damage in the lake were measured for one macrophyte species. For the leaves examined, from 6 to 13% of the total leaf area was lost to herbivores. These results demonstrate that herbivorous snails can strongly influence the distribution, abundance, and diversity of freshwater macrophytes, and suggest that the role of herbivores in freshwater macrophyte communities may be similar to their role in terrestrial and marine communities.

SUSCEPTIBILITY OF SALT MARSHES TO NUTRIENT ENRICHMENT AND PREDATOR REMOVAL

Salt marsh ecosystems have been considered not susceptible to nitrogen overloading because early studies suggested that salt marshes adsorbed excess nutrients in plant growth. However, the possible effect of nutrient loading on species composition, and the combined effects of nutrients and altered species composition on structure and function, was largely ignored. Failure to understand interactions between nutrient loading and species composition may lead to severe underestimates of the impacts of stresses. We altered whole salt marsh ecosystems (;60 000 m 2 /treatment) by addition of nutrients in flooding waters and by reduction of a key predatory fish, the mummichog. We added nutrients (N and P; 15-fold increase over ambient conditions) directly to the flooding tide to mimic the way anthropogenic nutrients are delivered to marsh ecosystems. Despite the high concentrations (70 mmol N/L) achieved in the water column, our annual N loadings (15-60 g Nm � 2 � yr � 1 ) were an order of magnitude less than most plot-level fertilization experiments, yet we detected responses at several trophic levels. Preliminary calculations suggest that 30-40% of the added N was removed by the marsh during each tidal cycle. Creek bank Spartina alterniflora and high marsh S. patens production increased, but not stunted high marsh S. alterniflora. Microbial production increased in the fertilized creek bank S. alterniflora habitat where benthic microalgae also increased. We found top-down control of benthic microalgae by killifish, but only under nutrient addition and in the opposite direction (increase) than that predicted by a fish-invertebrate-microalgae trophic cascade. Surprisingly, infauna declined in abundance during the first season of fertilization and with fish removal. Our results demonstrate ecological effects of both nutrient addition and mummichog reduction at the whole-system level, including evidence for synergistic interactions.

USE OF A NATIVE INSECT AS A BIOLOGICAL CONTROL FOR AN INTRODUCED WEED

We have evaluated the potential of a North American aquatic weevil, Euhr- ychiopsis lecontei, to serve as an agent of biological control for an exotic weed, Eurasian watermilfoil (Myriophyllum spicatum), which is currently found throughout much of the United States and in some southern provinces of Canada. We have found this weevil on M. spicatum in lakes where populations of the exotic weed have declined. We introduced weevils into enclosures in two lakes dominated by M. spicatum. In both lakes there was 50% less M. spicatum biomass in enclosures with weevils than in enclosures without weevils. Also, in control enclosures, M. spicatum formed a canopy on the water surface as it did outside the enclosures, while there was no plant canopy in any of the weevil enclosures. In laboratory feeding trials we quantified the effects of weevils on ten native aquatic plants including a native watermilfoil, Myriophyllum sibiricum. Weevils did not have a significant effect on the increase in plant length or final dry mass of any native plant, however the weevils did feed on the native watermilfoil. All native plants added new leaves, leaf whorls, or side branches under all weevil densities. The results from these studies suggest that a North American insect may be a suitable control agent for this introduced aquatic weed. Native biological control agents, when they can be found, offer potential advantages over classical biological control agents; they may have little impact on non-target native species that have coexisted with the control agent, and may save the time and expense of foreign research and quarantine procedures.

Weevils and Watermilfoil: Did a North American Herbivore Cause the Decline of an Exotic Plant?

The Eurasian watermilfoil (Myriophyllum spicatum) population in Brownington Pond, Vermont, declined between 1986 and 1989. Watermilfoil covered ≈10-11 ha of the littoral zone in 1986. Less than 0.5 ha remained in 1989. An herbivorous weevil (Euhrychiopsis lecontei), which is native to North America, was found associated with this watermilfoil population and we hypothesized that this weevil played a role in the decline. We monitored watermilfoil and E. lecontei populations in Brownington Pond from 1990 through 1992 by (1) mapping the location and extent of beds in the pond, and (2) determining watermilfoil biomass along permanent transects. The abundance of weevil eggs, larvae, pupae, and adults were monitored by sampling individual watermilfoil stems. Watermilfoil cover increased to ≈ 2.5 ha by 1991 and then declined again to ≈1 ha by 1992. The reduction in watermilfoil biomass from 1991 to 1992 ranged from 4- to 30-fold depending on location. Mean weevil abundance increased from 1990 (<1/stem) through early 1992 (3-4/stem) and then began to decline. The number of weevil eggs/stem also peaked in early 1992 and then declined. These survey results are consistent with the hypothesis that the weevil played an important role in producing both observed declines. The effect of weevils on watermilfoil was evaluated in two experiments. In an aquarium experiment, we found that the viability of stem fragments damaged by weevils was reduced compared to stem fragments without weevil damage. Watermilfoil commonly spreads by producing fragments so the spread of watermilfoil beds by fragmentation may be reduced when weevil abundances are high. In a pond enclosure experiment, weevils suppressed the production of new watermilfoil biomass. The biomass of lateral stems and roots of watermilfoil plants damaged by weevils was significantly less than those of control plants. There was no significant difference in the biomass of the stems originally planted in the enclosures. Weevil damage did have a negative effect on the buoyancy of these original stems, however. These experimental results also support the hypothesis that this native weevil played an important role in the two watermilfoil declines observed at Brownington Pond.

The effect of feeding by a North American weevil, Euhrychiopsis lecontei, on Eurasian watermilfoil (Myriophyllum spicatum)

Abstract The North American weevil, Euhrychiopsis lecontei (Dietz), has been found associated with a declining population of Eurasian watermilfoil, Myriophyllum spicatum L., in Brownington Pond, Vermont, USA. Feeding experiments revealed that adult weevils can reduce watermilfoil growth, and that they concentrate their feeding activity on the stem and leaves at the tops of plants. When presented with both M. spicatum and a native milfoil, Myriophyllum sibircum Komarov ( = Myriophyllum exalbescens Fernald), adult weevils fed primarily on M. spicatum . Surveys in Brownington Pond and other lakes have shown that all life stages of this weevil are associated with M. spicatum . These results suggest that E. lecontei has either expanded its diet to include M. spicatum or undergone a host shift from a native, North American macrophyte to M. spicatum .

Resource partitioning and competition for shells in a subtidal hermit crab species assemblage

SummaryShell and habitat utilization are quantified for 12 hermit crab species occurring subtidally in the San Juan Archipelago, Washington. The mechanism of competition for shells between these species is investigated using laboratory experiments to determine shell preferences, shell acquisition rates, and rates of exchange of shells via shell fighting. This information is used to estimate relative intensities of inter- and intra-specific competition for shells between the species in this assemblage. In contrast to earlier findings on intertidal hermit crab assemblages, a significant number (5) of the species in this assemblage appear to experience a greater reduction in their shell supply due to members of other species than due to other members of their own species. The relative amounts of inter- and intra-specific competition differ greatly for different species in the community. The high average figures for interspecific/intraspecific competition are largely a result of the presence of three abundant and very generalized species. In spite of the large number of species and relatively high ratios of interspecific to intraspecific competition, the species in this group are not close to a limiting similarity in resource use. There is suggestive evidence that greater selection pressures for divergence in habitat use may have resulted in the lower amounts of overlap observed in intertidal hermit crab assemblages in previous studies.

The effect of a native biological control agent for Eurasian watermilfoil on six North American watermilfoils

Abstract There is increasing concern that introduced, classical biological control agents can have significant negative effects on non-target species. One alternative to classical biological control is the use of native species to control exotic pests. A North American weevil, Euhrychiopsis lecontei , is being used as a biological control agent for the introduced aquatic plant Eurasian watermilfoil ( Myriophyllum spicatum ) in the United States. Previously, we determined that E. lecontei did not have a significant effect on several unrelated species of native North American aquatic plants. Here, we examine the effects of this weevil on six native North American watermilfoils. In six separate experiments, individual shoots of each native watermilfoil species were exposed to zero, two or four weevil adults. Changes in length and final dry mass were determined for each shoot at the end of the experiments. We also recorded the number of weevil eggs and larvae on these native watermilfoil species at the end of the experiment. In treatments with two weevils per plant there were no significant impacts of weevils on the native watermilfoils. However, in treatments with four weevils per plant, final length of M. verticillatum after 11 days was 13% shorter than controls, and with four weevils final dry mass of M. alterniflorum was 65% less than controls and M. humile 43% less. Weevils laid fewer eggs on all native watermilfoil species than on M. spicatum controls. Few of the eggs laid on the native watermilfoils hatched. Our results suggest that when its density is high, E. lecontei can have impacts on some native watermilfoil species. However, due to reduced fecundity on native watermilfoils, E. lecontei will probably have little impact on the native species. E. lecontei appears to be an example of a native biological control agent that can reduce the abundance of an exotic species without a significant negative impact on closely related, non-target species.

Response of macroinvertebrates to blooms of iron-depositing bacteria

In field measurements and laboratory experiments we assessed the influence of high levels of iron, manganese, and concurrent blooms of iron-depositing bacteria, Leptothrix ochracea, on macroinvertebrates. Macroinvertebrate communities in five of six streams were depauerate inside blooms. Reasons for the decreased abundance vary among taxa, with our experiments demonstrating the importance, for one or more species, of (1) direct toxic effects, and,/or smothering, (2) behavioral avoidance of bacterial-coated substrates, and (3) an inability to use bacteria as food. Three mayfly species showed increased mortality when caged inside the blooms, but five trichopterans and one plecopteran did not. Five invertebrates avoided Leptothrix-coated substrate in choice trials, while three did not. Stenonema fuscum could not ingest Leptothrix, and Neophylax nacatus had reduced growth feeding on it, but Heptagenia umbratica grew equally well on diets of Leptothrix or diatoms. This study demonstrates the important role epilithic organisms play in modifying substrates, and how these changes may act to influence benthic abundance and distribution in streams.

Hybridization in Hydrophiles: Natural Interspecific Hybrids in Najas (Hydrocharitaceae)

Abstract Pollination by water (hydrophily) is a highly specialized mechanism that occurs rarely among aquatic angiosperms, which mainly retain the reproductive systems reminiscent of their terrestrial ancestors. Hydrophily is abiotic and typically associated with unisexual flowers, factors that predictably would promote xenogamy and outcrossing. Yet, there have been few reports of hybridization involving waterpollinated plants (hydrophiles), with no firm evidence of natural interspecific hybridization. The genus Najas comprises about 40 species of submersed aquatic plants, all characterized by subsurface hydrophily. Hybridization in this genus has been suspected, but verified previously only among infraspecific taxa. In this study we document the first instance of interspecific hybridization in Najas using genetic evidence from three populations that were identifiable as N. guadalupensis but yielded polymorphic DNA sequence profiles. To facilitate our analysis we first conducted a phylogenetic survey of New World Najas taxa using nuclear and chloroplast markers. Alleles cloned from a biparentally-inherited locus (ITS) in these aberrant populations associated with two distinct but phylogenetically sister species (N. guadalupensis subsp. olivacea and N. flexilis) thus confirming their hybrid origin. In all cases the chloroplast markers associated with N. guadalupensis subsp. olivacea, implicating it as the maternal parent. The hybrid Najas plants occur at the edge of the sympatric range of the parental species. They possess no readily distinctive morphological features and require genetic analysis for confident detection. One population grows aggressively, raising concerns that at least some hybrid Najas plants represent a potential conservation threat. The possible hybrid ancestry of the endemic N. guadalupensis subsp. muenscheri also was assessed, but could not be confirmed or refuted by the data evaluated.

Differential Colonization and Growth of Algae and Ferromanganese-Depositing Bacteria in a Mountain Stream

ABSTRACT Over a 1 km course of a small mountain stream, the dominant epilithon shifted from diatoms to a ferromanganese-depositing bacterium, Leptothrix ochracea, then back to diatoms. The rates of colonization and growth of the algae and Leptothrix, along with concentrations of phosphorus, nitrate-N, nitrite-N, ammonia-N, silica, iron and manganese were measured at eight sites along the stream. Leptothrix abundance was significantly correlated with iron and manganese but not with other nutrients. Algal species diversity was high above and below the bacteria-dominated section but was extremely low where Leptothrix was common. It is likely that the extensive sheath and ferromanganese deposits on the substrate inhibited algal colonization and growth.