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.Among the many fundamental effects ofoutlined above, demonstrate that increase in tem-human activity on ecosystems, the management ofperature significantly reduces infection levels.Forsocieties and agriculture has typically createdP.xenopodis (with continuous transmission), anunnaturally close confinement of some animaloverall increase in temperature operating propor-populations (including humans themselves) (seetionally throughout the year, or a longer period ofChapter 10).One effect has been reduction in theelevated summer temperatures each year, couldhazards of the external environment that regulatehave a critical negative effect on reproducing parasitetransmission of infection, producing conditions inpopulation size.For P.americanus (with once-per-yearwhich the hostile interactions between host andtransmission), if environmental temperatures wereparasite are most intense.In attempts to control dis-to remain high during winter, parasites would notease, humans have devised lethal factors to elimin-survive from one transmission season to the nextate specific target organisms with anti-parasitic(Tocque and Tinsley 1994; Tinsley 1995).Rainfallchemicals.However, these have become potentvariations interrupting parasite transmissionagents of natural selection, amplifying those para-represent an alternative environmental perturbationsite genotypes that have any capacity to blunt thethat could prejudice polystomatid survival.In thepopulation impact.The efficiency of the evolution-case study documented by Tinsley (1999b), threeary response by parasites is exemplified by thesuccessive years of interrupted transmission led torapid development of drug resistance.Otherextinction of P.americanus in local host populationsmedical interventions have also been important in(see above).manipulating the hostility of the host environment.Based on these lines of evidence, Tinsley (2003)Immunosuppressive therapy removes the capacityproposed that relatively simple environmentalto create a hostile environment and predisposes tochanges, such as those characteristic of pastuncontrolled infection.Vaccination creates hostileclimatic oscillations and operating over longer timeconditions before the experience (and the costs) ofscales, could have led to extinction of polystomatida primary infection (although it reflects the com-lineages.The critical factor in this process is theplexity of interactions with protozoan and helminthpowerful attrition of parasite burdens by hostinfections that effective vaccines to these parasitesimmunity evident in present-day host parasitehave proved difficult to develop).associations, resulting in a very narrow marginWhile it is ironic that human social organizationbetween survival and extinction.In natural hostmay have assisted the success of some parasitespecies populations, the majority of individuals areinfections (see Chapter 10), studies of truly naturalparasite-free by virtue of acquired immunity.Inhost parasite systems emphasise an alternativenatural anuran communities, a majority of speciesview: despite the superb adaptations of parasites,may be free of polystomatid exploitation because ofthe host constitutes a highly hostile environment.past interactions leading to extinction of host-These systems provide relatively rare quantitativespecific parasites.This scenario would reflect com-data on the extent of host-induced parasitebined operation of the most hostile conditions formortality.Even the favourable environments , inparasite survival: an interaction of host immunitywhich parasites can survive and reproduce, may bethat achieves very effective regulation of reproduc-responsible for major mortality within parasiteing parasite populations with critical climaticinfrapopulations and this contributes to the regula-changes that further reduce these infections andtion of host parasite interactions.The consistentprecipitate extinction. 110 PARASI TI SM AND ECOSYSTEMSestimates of >95% mortality between successful may continue over winter on Svalbard when airinvasion and first reproduction in polystomatid temperatures may be as low as 40 °C.However,monogenean life cycles (above) testifies to this the environment is not so extreme under protectivehostility.snow cover, where infective stages may occur.Hostility of the environment cannot be considered Fasciola hepatica must be assumed to exploit regulara characteristic property of the conditions that define short periods when temperatures are above thethat environment (with respect to high and low tem- minimum threshold permitting development in theperature, pressure, availability of water, oxygen etc.).high Andes.Parasites of ectotherms, that might beAdaptations of the organisms inhabiting the envi- expected to experience external temperature condi-ronment determine whether processes of growth tions directly, may also benefit from amelioration ofand reproduction can occur or whether survival is in environmental conditions resulting from hostjeopardy.This applies both to the external macroen- behaviour.The behavioural responses of S.couchiivironment and to the microenvironment provided in selecting optimum sites in the desert (includingby a host which, to a parasite specific to that host, excavation of hibernation burrows) coincidentallymay appear favourable whereas, to an unnatural provides its parasites with the most equableinfection, it may cause rapid destruction.micro-environmental regime.In many parasite life cycles, major external envi- An important requirement in strategies involvingronmental constraints with the power to block trans- a wait for the return of favourable conditions ismission and population growth can be identified, a trigger factor and its recognition by the waitingbut they sometimes do not actually represent condi- parasite stage.With appropriate life cycle timing,tions that could be described as hostile.In a series of the conditions experienced in hostile ecosystemsunrelated examples, the temperature threshold may not be hostile at all.In its brief excursion intobelow which parasite development is arrested is the desert environment, P
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