The paper reviews the published literature on the studies of competition and allelopathy in aquatic plant communities. Taking a broader view of the community, the studies on interactions between macrophytes and microphytes, macrophytes and macro-invertebrates and microbial communities are also reviewed. The role of these interactions in the structure and dynamics of aquatic communities has been discussed in light of the current hypotheses concerning competition in terrestrial communities. The available information suggests that the aquatic plants of various growth forms differ greatly among themselves in their responses and adaptations to competition and allelopathy. The possible application of these interactions in biological control of plant pests and in agriculture is also summarized. We conclude that the observed differences in these interactions between the terrestrial and aquatic environment are due to the effects of water as a non-resource variable as well as due to special adaptive characteristics of aquatic plants. Further we hypothesize that the aquatic plants adopt both competitive and allelopathic strategies under different conditions and in interactions with different plants. The review highlights that our knowledge of both competition and allelopathy among aquatic plant communities is inadequate and fragmentary, and therefore, both extensive and intensive studies are required.

Competition and allelopathy in aquatic plant communities

The paper draws together a large and scattered body of empirical evidence concerning the prevalence of snail infection with schistosome parasites in field situations, the duration of the latent period of infection in snails (and its dependence on temperature), and the mortality rates of infected and uninfected snails in field and laboratory conditions. A review and synthesis of quantitative data on the population biology of schistosome infections within the molluscan host is attempted and observed patterns of infection are compared with predictions of a schistosomiasis model developed by May (1977) which incorporates differential snail mortality (between infected and uninfected snails) and latent periods of infection. It is suggested that the low levels of prevalence within snail populations in endemic areas of schistosomiasis are closely associated with high rates of infected snail mortality and the duration of the latent period of infection within the mollusc. In certain instances, the expected life-span of an infected snail may be less than the duration of the latent period of infection. Such patterns generate very low levels of parasite prevalence. A new age prevalence model for schistosome infections within snail populations is developed and its predictions compared with observed patterns.The implications of this study of observed and predicted patterns of snail infection within molluscan populations are discussed in relation to the overall transmission dynamics of schistosomiasis.

Prevalence of schistosome infections within molluscan populations: observed patterns and theoretical predictions

In 2002, the U.S. Department of Agriculture requested assistance from the American Malacological Society in the development of a list of non-native snails and slugs of top national quarantine signifi cance. From a review of the major pest snail and slug literature, together with our own experience, we developed a preliminary list of gastropod species displaying signifi cant potential to damage natural ecosystems or agriculture, or human health or commerce, and either entirely absent from the United States to our knowledge or restricted to narrow areas of introduction. Comments on the list from the worldwide malacological community were then solicited and led us to modify the original list. We then evaluated the taxa on this list by ranking them according to 12 attributes—seven biological variables and fi ve aspects of human interaction—based on thorough review of the detailed literature. The ranked list that emerged from this risk assessment process included 46 taxa (species or species-groups) in 18 families. The highest ranked taxa were in the Ampullariidae, Hygromiidae, Cochlicellidae, Helicidae, Veronicellidae, Succineidae, Achatinidae, and Planorbidae. We validated the risk assessment model by scoring a suite of non-native snail and slug species already present in the United States. The list is not defi nitive but rather is offered as a framework for additional research. There remain important gaps in biological knowledge of many of the taxa evaluated, and rigorous reporting of economic impacts is extremely limited. We expect the prioritizing and listing of taxa to be dynamic, not only as these knowledge gaps are fi lled but also as environmental, agricultural, international trade, and societal factors change.

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Alien Non-Marine Snails and Slugs of Priority Quarantine Importance in the United States: A Preliminary Risk Assessment

Although a disease of great antiquity, scientific studies of schistosomiasis began only 150 years ago. The complete life-cycle was not described until just before the First World War, making it possible at last to plan proper community control programmes. Inadequate tools prevented their effective implementation until well after the Second World War when new tools became available, thanks to the newly formed World Health Organization. Molluscicides spearheaded control programmes until the late 1970s but were then replaced by the newly developed, safe drugs still used today. Whatever the method used, the initial goal of eradication was, in the light of experience and cost, gradually replaced by less ambitious targets; first to stop transmission and then to reduce morbidity. The most successful programmes combined several methods to minimise reinfection after chemotherapy. Comparisons between different programmes are difficult without using appropriate, standardised diagnostic techniques and the correct epidemiological measurements. Some examples will be presented, mainly from our studies on Schistosoma mansoni in Kenya. Drug resistance on a scale comparable with malaria has not occurred in schistosomiasis but the likely withdrawal of all drugs except praziquantel leaves its control extremely vulnerable to this potential problem. An effective, affordable vaccine for use in endemic countries is unlikely to be ready for at least 5 years, and developing strategies for its use could take a further decade or more, judging from experience with drugs and molluscicides. In the interim, by analogy with malaria, the most cost-effective approach would the use of drugs combined with other methods to stop transmission, including molluscicides. The cost of molluscicides needs to be reduced and fears allayed about their supposedly adverse ecological effects.

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Schistosomiasis epidemiology and control: how did we get here and where should we go?

An account is given of an ecological investigation of freshwater habitats near Ibadan, Nigeria, which was designed to identify and evaluate key factors that might be manipulated to achieve control of the snail hosts of schistosomiasis and fascioliasis. Statistical analyses of data from 24 stations from both lentic and lotic habitats showed that there were statistically significant tendencies for the following snail species to coexist: at the macro-distribution level Lanistes libycus and Bulinus (P.) globosus, in lotic habitats, Lymnaea natalensis, Bulinus forskali, Biomphalaria pfeifferi and Bulinus rohlfsi in lentic habitats; at the micro-distributional level, B. rohlfsi, B. pfeifferi, B. forskali and Anisus coretus. Several species of aquatic macrophytes, including species that occur at water margins (Acroceras and Commelina spp.) and those that occur in more persistent aquatic habitats (Nymphaea and Ceratophyllum spp.) also coexist to a statistically significant extent both with each other and with snail species in the same microhabitats. However, Salvinia proved exceptional as it was negatively associated with four of the snail species. Persistent snail refugia, including areas at stream origins, backwaters and the deeper bays or inlets in lentic habitats, were characterized by the presence of particular macrophyte species. The reasons for the associations between snails and macrophytes are discussed and it is argued that they may be mutualistic. The snails proved tolerant to a wide range of chemical conditions with the exception of low oxygen concentration. However, the absence of aquatic macrophytes and lentic refugia in head waters and the presence of fast flowing water and high temperatures in shallow water are harmful. Although the indigenous snails, L. libycus and Aplexa waterloti, proved to be efficient predators on the eggs and juveniles of snail hosts, other laboratory and field evidence appear to preclude the possibility that they may be useful as biological control agents. In contrast, there is evidence that Melanoides tuberculata might exclude the snail hosts from eroding substrates in lentic habitats. The following integrated control measures in lotic habitats are discussed: (a) judicious drainage and canalization of marshy areas at stream origins, combined with replacement of key marginal macrophytes; (b) dredging and straightening of stream courses; (c) replacement of key macrophytes along the banks; (d) introduction of M. tuberculata if not present. In lentic habitats the following control measures are discussed: (a) prevention of immigration of both macrophytes and snails from feeder streams; (b) control of subaquatic emergent plant species by using grazing domestic animals or competitor species; (c) maximizing wind-generated surface currents to exclude floating plants; (d) eradication of submerged rooting plants by herbivorous fish or invertebrates; (e) the management of such water bodies as either plankton - herbivorous fish or plankton - herbivorous fish - carnivorous fish systems; (f) introduction of efficient, detritivorous non-host molluscs or fish to feed on bottom deposits.

Control of the snail hosts of schistosomiasis by environmental manipulation: a field and laboratory appraisal in the ibadan area, nigeria

The respective vectors of the two forms of bilharziasis in Egypt do not have the same ecological distribution. Bulinus truncatus is most abundant in large canals, and decreases in density as the water approaches and flows into drains. Biomphalaria alexandrina is most abundant in drains, and decreases in density upstream from these habitats. Both species are most abundant in the presence of aquatic vegetation, but they differ in their respective associations with the water hyacinth, Eichhornia crassipes. Biomph. alexandrina reaches maximum abundance in the presence of this plant, but Bul. truncatus is as uncommon in the absence of plants as in the presence of E. crassipes.Calculation of life-table parameters from field data shows that, under optimum field conditions, both species can double their populations in 14-16 days. The reproductive rates of both species are greatest in March and the death rates in midsummer. The observed peak densities in May and June give a false impression of optima because of undercollection of young snails, which are most abundant in March and April.Control operations should take advantage of the findings on population parameters. A single area-wide treatment with molluscicide in April is recommended. During the remainder of the year, search for isolated foci of snail breeding and individual treatment of these will effect large savings of chemical and will be effective in controlling the transmission of the parasites.

I. K. Dawood

Papers

The ecology of Bulinus truncatus and Biomphalaria alexandrina and its implications for the control of bilharziasis in the Egypt-49 project area.