Exotic Snail Research Articles

Exotic snails dominate nitrogen and carbon cycling in a highly productive stream

Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.

Fertilization of Algal Resources By an Exotic Snail May Facilitate Invasion

We studied positive and negative effects of snail consumers on their resource to determine if positive consumer effects may be facilitating invasion. Consumer- resource interactions often focus on losses to the resource, even though the resource might benefit if consumers recycle nutrients. The New Zealand mudsnail, Potamopyrgus antipodarum, an exotic in western U.S. rivers, attains high densities and dominates macroinvertebrate communities. In one well-studied river, it consumes the majority of primary productivity, cycles most nitrogen and can grow faster at higher densities. In field experiments, we tested the hypothesis that this invasive grazer stimulates algal growth via nitrogen excretion, which might explain its self-facilitation and invasiveness. Using in-stream cages subdivided into "with snails" and "without snails" sections, we examined the response of periphytic algae to snail grazing and excretion and snail excretion alone at various levels of snail biomass. We found that chlorophyll a and GPP (gross primary production) decreased as the biomass of snails increased in the grazed sections. Snail excretion, in the absence of grazing, increased both chlorophyll a and GPP, demonstrating a positive effect of snails on the resource, consistent with the nutrient recycling and enrichment hypothesis. We found no evidence for increased algal growth at intermediate snail densities in grazed treatments, as predicted by the Herbivore Optimization Curve hypothesis. However, the difference in chlorophyll a between "with snails" and "without snails" treatments increased as snail biomass increased. This suggests that snail compensation of the resource, through excretion, decreases at extremely high levels of grazing pressure and the net effect of snail grazing becomes negative. Together, these results suggest that invasiveness in some rivers may be fostered by this self-facilitation and recycling of essential nutrients.

Influence of Bio-Pollution on Ecosystem Processes: The Impact of Introduced Lake Trout on Streams, Predators, and Forests in Yellowstone National Park

Yellowstone National Park (YNP) is a treasured national resource and an important element of tourism and the recreational economy in Wyoming. Because of its unique geological features and abundant wildlife and fisheries, YNP is a tourist destination for millions of people annually. Although this national symbol is cherished for its pristine condition and has been protected from most human influence for over 100 years, human mediated invasions of non­ indigenous species, such as several species of plants and animals, including an exotic snail (Potamopyrgus antipodarum), may alter this ecosystem. Recently an unauthorized introduction of lake trout (Salvelinus namaycush) to Yellowstone Lake was documented. Recent investigation at the University of Wyoming, indicated that in-lake predation by lake trout on juvenile and sub-adult native Yellowstone cutthroat trout (Oncorhyncus clarki bouvieri) could negatively influence recruitment of cutthroat trout (Stapp and Hayward 2002). This may lead to significant reductions in numbers of spawning adult cutthroat if current management actions are ineffective, or if they are not continuously pursued (Stapp and Hayward 2002). While lake trout invasion in Yellowstone Lake will likely have detrimental effects on in-lake communities and processes, reductions in populations of native cutthroat trout can potentially impact other aquatic and terrestrial ecosystems outside of Yellowstone Lake. Cutthroat trout in Yellowstone Lake annually migrate into tributary streams and rivers to spawn (Varley and Gresswell 1988), with runs up to 60,000 trout per season into small streams such as Clear Creek (Gresswell and Varley 1988). This spawning migration may significantly affect in­ stream communities (cf. Power 1990) and alter nutrient cycling within tributary streams (Peterson et al. 1993) and in the adjacent riparian forests (Ben­David et al. 1998; Hilderbrand et al. 1999). Therefore, spawning cutthroat trout not only have trophic effects on their ecosystem but also act as "ecosystem engineers" (i.e., species that influence structure and function of ecosystems through non­ trophic processes) because of their role in transporting large amounts of nutrients between ecosystems (Jones et al. 1994). Reductions in spawning adult cutthroat trout will likely alter in­stream processes. In addition, for piscivorous (fish­eating) predators, a significant decline in the number of adult spawning cutthroat trout may reduce recruitment and survival, and it could threaten viability of predator populations. In this project we are investigating the role of cutthroat trout in structuring stream ecosystems, their importance to a representative fish-predator - the river otter (Lontra canadensis), and possible effectson terrestrial plants through nutrient transport by otters to latrine sites (Ben-David et al. 1998 Hilderbrand et al. 1999). We hypothesize that the spawning migration of cutthroat trout will result in transport of nutrients from lake to streams, and from streams to terrestrial forests, through the activity of piscivorous predators. Because nitrogen (N) limits production in area streams (J. L. Tank and R 0. Hall unpublished data) and terrestrial ecosystems (Nadelhoffer et al. 1995) we focus our investigation of nutrient cycling on this element. These observations will enable us to predict how streams, trout predators, and the terrestrial landscape will be affected following cutthroat trout decline.

Exotic Snails Dominate Nitrogen and Carbon Cycling in a Highly Productive Stream

Individual animal species can impact ecosystem processes, but few exotic animal species have demonstrated ecosystem-scale impacts, in spite of large population sizes. We combined whole­stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated carbon and nitrogen fluxes in a highly productive stream. Exotic snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. These large fluxes were due to high snail biomass rather than high per-biomass rates of excretion or consumption. This exotic snail may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport. Individual animals species can alter ecosystem functioning, such as nutrient cycling and storage, by directly altering carbon or nitrogen flux through grazing or excretion of ammonium (Grimm 1988, Frank et al. 1997, Vanni 2002), or indirectly via predation (e.g. Schindler et al. 1997). Despite numerous examples linking animal species with ecosystem processes, e. g., nutrient fluxes, there have been only a few examples showing how exotic animals may affect ecosystem processes or overall functions (Strayer et al. 1999, Lovett et al. 2003). Exotic animals might provide a model system for examining single-species impacts on ecosystem processes for several reasons. There are potentially strong ecosystem-scale impacts from exotic animals because they sometimes can dominate invaded ecosystems in terms of biomass. They also might bring a new trait to the invaded ecosystem, e.g. a generalist predator. However, there are few generalizations about how an exotic animal will impact native ecosystems, in spite of many impact studies which are mostly at the population or community level (Parker et al. 1999, Byers et al. 2002). Parker et al. (1999) presented a framework for considering exotic species impact as: I = RxAxE, where R =range (unit area), A =biomass per unit area, and E = per-biomass impact. Local­scale impact can be determined by either high biomass (e.g. Strayer et al. 1990) or by high per­biomass impact relative to native species, which may include an exotic species that brings a novel trait (Vitousek 1990, Byers 2000). Despite this framework, we do not know the degree to which either high biomass or high per-biomass impact contributes to overall ecosystem-scale impact by invading animals. Separating these two will allow better prediction of impacts in that we can focus research and management on understanding either specific traits or potential maximum biomass of invaders. We studied the role of the exotic New Zealand mud snail, Potamopyrgus antipodarum, on carbon. (C) cmd nitrogen (N) fluxes in Polecat Creek, Wyoming (Fig. I). We scaled their per-biomass rates of orgcmic matter consumption and ammonium excretion by snails to whole-stream rates in an 800-m reach. We compared these scaled estimates with whole-stream measures of C-fixation and N cycling to estimate the snail's contribution to stream C and N cycling (Grimm 1988, Vanni 2002). Because we scaled the impact of snails by multiplying per-biomass rates by snail biomass, we were able to estimate the degree to which high biomass or high per-biomass rates contnouted to the dominance of N and C fluxes in this stream. We compared both the per biomass rates and dominance of ecosystem N fluxes by Potamopyrgus with literature values for other freshwater invertebrates.

Linking Modeled and Experimentally Measured Interaction Strengths Between an Exotic Snail and Algae in Rivers in Yellowstone National Park

Exotic species are one of the largest human­caused threats to ecosystems and are becoming a central focus of research for ecologists. We do not know what factors allow exotic species to invade, nor can we predict what effects the exotic species will have once it has invaded. Invasive species, including mollusks, have greatly affected processes in freshwater ecosystems. A recent mollusk invader to North America is the New Zealand mud snail, Potamopyrgus antipodarum. It is a small snail (ca. 4mm long) in the family Hydrobiidae. Potamopyrgus has recently invaded the Snake and Madison Rivers and the Great Lakes In the Madison River, it is numerically dominating the benthos in certain locations with densities over 300,000 individuals/m2 with a corresponding biomass of 30 g/m2 (R. Hall, unpublished data). Because mud snails have high biomass and potentially high secondary production, they may alter algal species composition, and lower algae biomass and production in rivers. Algae form the base of the food web in these rivers; hence, mud snails may be competing with other invertebrates by lowering algae biomass as has been shown for a caddisfly grazer (Kohler and Wiley 1997). It is relatively straightforward to measure grazer impact on algae population using small-scale experiments. However, it is harder to scale up these estimates to whole-river primary production because we cannot manipulate snail densities at this scale. From a management perspective, we cannot do experiments every time a prediction is needed, e.g. when this snail invades a different river. For this reason to predict impacts based on knowing snail and algae biomass using a modeling approach may become useful. Ecologists are just beginning to link energy flow with experimentally measured interaction strengths. Recently Wootton (1997) provided a theoretical means to link energy flow with experimentally measured interaction strength for bird foraging in the rocky intertidal zone. He estimated parameters of a Lotka-Volterra predator-prey model using consumption rate estimates. He then compared these estimated parameters with those measured (using the same units) using experiments. He found a positive correlation between the two suggesting that it is possible to predict interaction strength from energy flow from the consumer to the resource. The objective of this study is to estimate interaction strength between exotic mud snails and periphyton by using the modeling approach developed by Wootton (1997) and to test these estimates by using controlled field experiments.

Red Crabs in Rain Forest, Christmas Island: Biotic Resistance to Invasion by an Exotic Snail

A widely held view is that biotic resistance to invasion on oceanic islands is weak. On Christmas Island, Indian Ocean, the endemic red crab (Gecarcoidea natalis) and the introduced giant African snail (Achatina fulica) showed inverse patterns of abundance along gradients from disturbed habitat to intact rain forest: empty, but intact snail shells were abundant in disturbed habitat and rare in adjacent rain forest, while red crabs were abundant in rain forest and uncommon in disturbed habitat

A survey of fascioliasis in beef cattle killed at abattoirs in southern Queensland

The overall prevalence of fascioliasis in beef cattle found in an abattoir survey completed in May 1983 in southern Queensland, was 1.1%. Prevalence was 36% in the Stanthorpe Shire, varied from greater than 1 to 10% in 12 shires and was 1% or less in 24 shires. Fascioliasis was not recorded from the other 46 shires in the area of the survey. Prevalence was less than 1% in female cattle of all ages. In males, prevalence increased with age from 0.6% in 17-month-old cattle to 4.7% in 4-year-old cattle. The differences in sex-specific prevalences may be due to differences in susceptibility to the parasite between sexes, differences in susceptibility to the parasite between sexes, differences in management between sexes, or artefacts associated with sampling for a disease of very low prevalence. The presence of the exotic snail species, Lymnaea columella and L. viridis was confirmed in southern Queensland in 1977. These snails are suitable intermediate hosts for Fasciola hepatica and may widen the endemic area for liver fluke because of their more aquatic character and greater tolerance to high temperatures than L. tomentosa, the traditional snail host in Queensland. We suggest that introduced snails have not yet had a substantial impact on the importance of fascioliasis to the beef industry of southern Queensland. The survey provides baseline data for the future monitoring of this potentially important parasitic disease.

The potential impact of exotic Lymnaea spp. on fascioliasis in Australasia

The presence of the following exotic Lymnaea spp. has been recently confirmed in Australia: L. columella (North and South America, South Africa, New Zealand,) L. viridis (China, Japan, Philippines, New Guinea, Pacific Area), L. auricularia rubiginosa (Indochina, Malaysia, Indonesia) and L. peregra (Europe). In the present studies the relative susceptibility of the exotic snails noted above to Fasciola hepatica was investigated. Two of the species, L. columella and L. viridis, were found to be highly susceptible to the Australian strain of F. hepatica and a substantial number of viable metacercariae were produced. However a strong disparity in the host— parasite relationship was evident when the parthenogenetic development of the fluke in the two exotic snails was compared with that in the native L. tomentosa. It was concluded that L. tomentosa is a more suitable intermediate host for F. hepatica than any of the introduced exotic species, but it has to be considered that the host—parasite relationships between new combinations of Fasciola and Lymnaea spp. may improve by adaptation. In view of the results of the present studies the potential danger of fascioliasis spreading to areas currently not endemic in Australasia through introduced exotic snails is discussed with particular reference to Queensland, Western Australia and the Northern Territory. There is strong circumstantial evidence that the exotic snails were introduced with aquatic plants through the relatively free overseas and Australian trade. Similarly, through the introduction of infected snails, dangerous parasites such as F. gigantica of cattle and sheep or Schistosoma ssp. which cause serious diseases of man and animals could enter the subtropical or tropical regions of Australia. Appropriate State and Federal quarantine regulations are essential to prevent further introduction and spread of the snails, and monitoring of the distribution of established exotic snails in Australasia must be continuous.

The epidemiology of Fasciola hepatica infections in sheep on a Lymnaea columella habitat in the Manawatu.

Abstract Extract Fasciola hepatica has become increasingly widespread in New Zealand in recent years following the colonization of large areas of the country by the exotic snail Lymnaea columella (Pullan, 1969 Pullan, N. B. 1969. The first report in New Zealand of Lymnaea columella Say (Mollusca: Gastropoda) an intermediate host of the liver fluke Fasciola hepatica. N.Z. vet. J., 17: 255–256. [Taylor & Francis Online] , [Google Scholar]; Pullan and Whitten, 1972 Pullan, N. B. and Whitten, L. K. 1972. Liver fluke Fasciala hepatica, in New Zealand. Part 1. A spreading parasite in sheep and cattle. N.Z. vet. J., 20: 69–72. [Taylor & Francis Online] , [Google Scholar]; Pullan et al., 1972 Pullan, N. B. and Mansfield, Caroline B. 1972. Fasciola hepatica in sheep: a preliminary epidemiological study. N.Z. vet. J., 20: 39–40. [Taylor & Francis Online] , [Google Scholar]). In October 1969, F. hepatica was found to be established in sheep and cattle on a farm 40 km from Palmerston North in an area previously regarded as free of fluke infection.