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 ecologists 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.
A.D.’s thinking about this whole topic was hugely shaped by many conversations with Robert M. May; we are very grateful to him for the insights provided by these discussions and to John Avise, Doug Erwin, Michael Donoghue, Nadia Talhouk, and Alejandra Jaramillo for comments on an earlier draft. The first draft of the chapter was written in Kilimanjaro, Nairobi, and Heathrow Airports; A.D. thanks British Airways and Precision Air for the patience, care, and attention of their ground staff. This work was supported by National Institutes of Health and National Science Foundation Ecology of Infectious Disease Program Grant DEB-0224565.