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.1995), further reducing prevalence.For instance, As biomass increases due to increased productivity,cadmium kills amphipods infected with larval birds and mammals increasingly feed at enrichedacanthocephalans more readily than it kills un- sites.Hence, snails and fishes acquire increasinginfected amphipods (Brown and Pascoe 1989).In numbers of larval parasites that will be trophicallyaddition, pollution can negatively affect fish vital transmitted to the non-piscine top predators.Inrates.For example, oil pollution causes liver disease oligotrophic lakes, some of these same fishes are theand reduces reproduction and growth (Johnson top trophic level and harbour mostly adult para-2001).Such effects should reduce density and contact sites.Since larval parasites are more pathogenicrates, further reducing parasitism.than adult parasites there will be a further cascadeIn contrast to toxic pollution, eutrophication and of disease effects on a eutrophic ecosystem.thermal effluent often raise rates of parasitism in Acid precipitation associated with air pollutionaquatic systems because the associated increased can negatively effect parasites in waters with poorproductivity can increase the abundance of interme- buffering capacity.Marcogliese and Cone (1996)diate hosts.Parasites that increase under eutrophic found that yellow eels (Anguilla rostrata) from Novaconditions tend to be host generalists and have local Scotia have an average of 4 parasite species atrecruitment; cestodes with short life cycles and buffered sites, about 2.5 parasite species at moder-trematodes seem to be particularly favoured ately acidified sites, and 2 parasite species at acidi-(Marcogliese 2001).The most dramatic examples fied sites.This decline in parasite richness withinclude parasites whose intermediate hosts favour acidity is due to drops in the prevalence of mono-enriched habitats.These include some species of genes and digenes.The latter require molluscs astubificid oligochaetes and snails.Myxozoan para- intermediate hosts and these cannot survive insites of fishes, which require oligochaete hosts, are acidified conditions.Parasites that use freshwaterfrequently more prevalent at sites polluted by crustaceans as intermediate hosts may be similarlysewage (having high coliform counts) (Marcogliese impacted by reduced access to calcium ions.and Cone 2001).Beer and German (1993) describedhow eutrophication improved conditions for snails7.5 Climate changethat serve as first intermediate host for the digene,Trichobilharzia ocellata.Similarly, Valtonen et al.The most notable prediction of anthropogenic global(1997) found that eutrophication correlates posit- change is widespread increases in average tempera-ively with greater overall parasite species richness tures (Houghton et al.1996).This is particularlyin two fish species.An increase in frog deformities troubling to most parasitologists from temperatehas been linked to eutrophication of ponds which climes because many of the most deadly humanincreases the density of snails infected with Ribeiroia parasitic diseases we teach about, but are not atondatrae, a trematode known to cause abnormal direct personal risk to, are tropical (Rogers andgrowth in second intermediate hosts (Johnson et al.Randolph 2000).The fear is that if our world2002).The association between eutrophication and becomes more tropical, tropical diseases will gopollution is not likely to be linear.At high nutrient hand in hand with the more benign benefits ofinputs, toxic effects may occur and parasitism can pleasant weather.This is a bit simplistic; forecasts ofdecline (Overstreet and Howse 1977).The influence climate change do not predict that the weather inof pollutant stressors, must be analysed in the con- Milwaukee will necessarily resemble that intext of natural history.Some tubificids require clean Manaus.Still, there is a general expectation thatwater and will not be present at enriched sites temperatures will rise and precipitation patterns will(Kalavati and Anuradha 1992).change.The distributions of parasites, as for allEvaluating the changes in the fish parasitofauna species, are bounded by suitable climatic conditions.of oligotrophic and eutrophic lakes in Michigan, Thus, climate change should alter the future distri-Esch (1971) recognized that eutrophication opens bution of parasitic disease (Marcogliese 2001). ENVI RONMENTAL DI STURBANCES 119Some parasites should be more sensitive than out by observations of parasite communities in aothers to warming.Temperature would seem par- thermal effluent (Sankurathri and Holmes 1976).ticularly important when hosts are ectotherms that Nonetheless, it is hard to predict the effect of warm-do not actively regulate their temperature.In addi- ing on the parasite community as a whole.In onetion, parasites with free living stages should have case where this has been studied, parasite commun-more opportunity to interact with climatic condi- ities in turtles declined with increasing thermaltions (Overstreet 1993).For example, trematodes of pollution (Esch et al.1979).littorine snails that have free swimming cercarial Most fitness traits for hosts and their parasites willstages are not able to persist in arctic regions, pre- exhibit a peak performance at a thermal optimum.Ifsumably due to the effect of harsh weather the relationship between performance and(Galaktionov 1993).temperature differs between host and parasite, theModerate increases in temperature are likely to resulting gene by gene by environment interactionalter birth, death, and development rates in ways will either increase or decrease disease at a giventhat could conceivably favour parasites or interme- temperature, at least on the level of the individualdiate hosts.For example, if individuals are infectious host (Elliot et al.2002).For example, the optimal tem-for longer time periods under warmer conditions, perature of a fungal pathogen is higher than the opti-then disease will increase with temperature.The mal temperature of its sea fan host, placing the seaimpact of parasites on their hosts may increase with fan at risk to global warming (Alker et al.2001) [ Pobierz całość w formacie PDF ]