Recent work by Sures (2003, 2004) and colleagues has shown that parasitic helminths may play a substantial role in concentrating and ultimately removing heavy metals and other pollutants from their hosts. They can concentrate and withstand levels of cadmium, zinc, and other heavy metals that are up to 2,000% above background levels (and =1,000 times greater than the levels sustained by snails and other host species widely used as monitors of toxicants and pollutants). Parasites achieve this level of concentration through their preference for absorbing bile from the guts of their hosts. Most vertebrates attempt to minimize the impact of harmful substances in their gut by surrounding the offensive items with bile and passing them out in their feces. However, significant amounts of the substance are reabsorbed with bile in the lower intestine. This occurs to a much lesser extent in hosts parasitized by parasitic helminths; many of these parasites selectively absorb bile as a food source, thereby removing the pollutants from the host's gut and concentrating them in the worm (Sures, 2003).
Results from studies of salt-marsh ecosystems suggest that metazoan parasites constitute up to =3% of the biomass of major animal groups in the system (Kuris et al., 2008). If parasites are 3% of the animal biomass, then their ability to superconcentrate pollutants may mean that they contain 30-50% of the mass of pollutants in the system. This would amount to a formidable ecosystem service! We note, however, that this assumes that the many different groups of metazoan parasites studied in Kuris et al. (2008) are as efficient at absorbing pollutants as the adult stages of helminths in the guts of vertebrates studied by Sures (2003, 2004). Nevertheless, a relatively small biomass of adult worms in vertebrates may sequester a significant proportion of the pollutants that would otherwise disrupt the viability of host populations. This suggests that if parasites are lost via extinction of their hosts, or via replacement of intermediate hosts by nonviable invasive host species, then the free-living host species may experience enhanced levels of pollutants. Parasitic helminths of humans supply a similar ecosystem service when they selectively remove both pollutants and allergens from human guts. This provides a viable explanation for why allergies are much more common in human societies that have successfully reduced their parasite loads than in those that still bear a significant burden of parasitic helminths (Yazdanbakhsh, 2002).
In conclusion, we suggest that there is reason to join Sprent (1992) and Windsor (1995) in mourning the loss of the parasitic species that disappear when their hosts go extinct (Stork and Lyal, 1993; Koh et al., 2004). If significant increases in extinction rates now apply to birds, mammals, amphibians, and fish, then it is almost inevitable that extinction rates in host-specific parasite species are increasing at least concomitantly. As we
develop a deeper understanding of food-web structure and dynamics, it seems increasingly likely that parasitic helminths play a major role in ecosystem function and may even supply important economic services to humans. Understanding the structure of food webs remains among the deepest scientific challenges of the 21st century. Parasites will play a key role in developing this understanding, yet they are at least as threatened by mass extinction as are many other species—potentially even more so. A healthy functioning ecosystem will have a full complement of parasitic species (Hudson et al., 2006). Fully determining the role that parasites play in regulating natural systems remains a major challenge for ecolo-gists and evolutionary biologists. If the major job of conservation biologists is to maintain fully functional food webs, then it is crucial that we consider parasites as a vital and necessary component of biodiversity. It is then but a small step to acknowledge that these animals are well worth conserving.
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