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.Infectious agents require a thresholdown health.Studies link stress to reduced immunehost density for transmission (McKendrick 1940).function and various associated maladies of theOutside stressors that reduce host vital rates willmodern age (Yang and Glaser 2002).Immune sys-depress host population density, thereby reducingtems are costly to maintain and stressed individualsthe chance of an epidemic process, or even the abil-ity of a parasite to persist at all in a declining or low1USGS Western Ecological Research Center.density population.2Marine Science Institute, University of California, SantaStressors may also induce a more negativeBarbara, California.3Department of Ecology, Evolution and Marine Biology.impact on parasites than on their hosts.This should113 114 PARASI TI SM AND ECOSYSTEMSincrease recovery rates of infected individuals and Bustnes et al.2000).Increases in trematodes are ofmitigate the population-level impacts of the disease.particular concern for those trematode species thatIn addition, infected hosts might experience differ- cause human disease.Deforestation reduces acidicentially high mortality when under stress.This leaf litter and increases algal growth in ponds andwould remove parasites more rapidly from the host streams, creating conditions suitable for snails thatpopulation than would occur without the stressor.serve as intermediate hosts for schistosomesWhile this increases the impact of disease on infected (Southgate 1997).The Aswan Dam that createdindividuals, it simultaneously decreases the spread Lake Nasser also created excellent habitat for theof an epidemic through the host population.Such a snails serving as the intermediate host for therelationship underscores the point that population trematodes that cause human schistosomiasiseffects of stress and infectious disease cannot neces- (Heyneman 1979).Construction of other largesarily be predicted from their effects on individuals.impoundments throughout Africa (e.g.PapernaIt is more likely that stress will have multiple 1969) has substantially increased schistosomeeffects on hosts and parasites such as increasing transmission, resulting in increased human morb-host susceptibility to disease while impairing host idity and mortality (Gryseels et al.1994).vital rates.This makes it unclear how a particular Due to concerns for human health, the literaturestressor should affect disease in a host population.tends to focus on the types of habitat changes thatAlthough stressed individuals should be more sus- increase disease.However, there are many ways thatceptible to infection if exposed, the stressor will habitat alteration, through its effects on biodiversitylikely also reduce the contact rate between infected loss, should decrease infectious disease (as discussedand uninfected individuals to the extent that the below).In particular, the wholesale draining andstressor reduces host density.Simulation models conversion of wetlands has dramatically reduced thehelp resolve the opposing predictions stemming transmission of various infectious diseases (Laffertyfrom these alternative effects.Stress is most likely and Kuris 1999; Reiter 2000).Management of waterto reduce the impact of closed system, host-specific sources for breeding mosquitoes, through drainageinfectious diseases, and increase the impact of other and controlled water levels, was instrumental in thetypes of disease (Lafferty and Holt 2003).successful malaria control campaigns in the southernUnited States and Israel/Palestine (Kitron 1987).7.2 Habitat alteration7.3 Biodiversity lossHumans have altered nature in ways that can affectdiseases (Lafferty and Kuris 1999) (see also Although authors disagree on the present rate ofChapter 10).Conversion of forest to agricultural extinction associated with human induced environ-land dramatically changes the environment for para- mental degradation, there is no denying that it issites and their hosts; and this has raised concerns orders of magnitude above background levelsfor human health (Patz et al.2000).In particular, (Regan 2001).None of these estimates considersdeforestation, damming, road construction, fish extinctions of parasites which, for some host groups,farming, and rice farming increase malaria trans- may exceed the extinction rate of host speciesmission by creating mosquito breeding habitat (Sprent 1992).Few will lose sleep over the notion of(Smith 1981; Desowitz 1991).In addition, domestic parasites going extinct but one only need imagineanimals may provide new food sources for mosqui- the diversity of now extinct parasites specializing ontoes, leading to increased malarial transmission in dinosaurs (Kuris 1996) to realize that parasite extinc-the associated human population (Giglioli 1963).tion has been a vast, but hidden, component of evol-Habitat alteration has also created conditions utionary history (see also Chapter 6).In addition,conducive for the transmission of trematodes.For given the possible role of parasites in stabilizinginstance, dumps and fish farms attract seagulls ecosystems (Freeland and Boulton 1992) conserva-which fuel trematode life cycles (Kristoffersen 1991; tion biologists may one day come to appreciate the ENVI RONMENTAL DI STURBANCES 115potential need to protect parasites (Combes 2001, change over the landscape would provide moresee also Chapter 8).Two caveats are: (1) many para- insight into this potentially large effect.sites are not strictly host specific and the fates of Parasites, particularly those with complex lifethese parasites are not tied to the extinction or per- cycles, should generally decline with a decrease insistence of single host species; and (2) parasites, due biodiversity (Robson and Williams 1970; Pohleyto the nature of density-dependent transmission 1976; Hughes and Answer 1982; Hudson et al.dynamics, are likely to go extinct well before their 1998).Digenetic trematodes are a good example.hosts (Lyles and Dobson 1993).For this reason, host Trematode communities can vary considerablyextinction may not be the key for understanding within a wetland (Lafferty et al.1994; Stevens 1996;parasite extinction.Reduced host species densities unpublished thesis) and among wetlands ( Laffertyand host species ranges are more likely to be good et al
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