General considerations

The lesser scaup, Aythya affinis, is a common duck of the western North American prairies. Even the etymology of its genus name, Aythya (=water bird) suggests it is an unremarkable species of duck. Yet, for parasite ecologists it is far from unremarkable. In a classic study of parasite biodiversity, Albert O. Bush counted almost 1 000 000 parasites in a sample of 45 scaup, representing an astonishing 52 species of gut helminths alone (Bush & Holmes, 1986a, b). Although parasite biodiversity of this magnitude was foreshadowed by earlier studies on bird–helminth interactions by the fathers of parasite ecology in the former Soviet Union (Dogiel, 1964) and Poland (Wisniewski, 1958), Bush’s work was the first to rigorously quantify the occurrence and intensity of individual species within individual hosts. His study was also the first to place parasite communities into the conceptual framework of mainstream community ecology.

We highlight the scaup–helminth example to emphasize a point that lies at the heart of parasite community ecology, i.e., individual hosts almost always contain more than one species of parasite. This basic tenet prompts several intriguing questions. Do the species compete for limiting resources and then assort themselves in a non-random manner to reduce interspecific overlap? Does the presence of one species impose fitness costs on co-occurring species? Do species assort themselves to increase the probability of intraspecific mating or to avoid interspecific mating? Are biodiverse parasite communities less likely to be invaded than depauperate communities? Similar questions can be asked at different scales. Why, for example, is parasite biodiversity higher in scaup than in other species of bird, even sympatric ones with which they share bodies of water? What is the role of host phylogeny versus host ecology in determining interspecific variation in parasite biodiversity? It is questions such as these that have inspired parasite ecologists since the earlier studies by Dogiel and Wisniewski. Such questions provide the framework for this chapter.

General considerations

We now shift our attention to the geographical distributions of parasites. Although spatial aspects of parasite distributions were considered in the previous chapter, our focus here is on parasite distributions at broader scales, typically on the order of continents or regions. This perspective is the realm of parasite biogeography. In general, biogeographers aim to characterize patterns in the geographical distribution of organisms and to understand the role of historical events in determining present-day distributions of populations, species, and entire biotas (Brown & Lomolino, 1998). This fundamental aim seems straightforward, but the subject matter of biogeography is notoriously complex. Modern studies in biogeography combine aspects of phylogenetics, ecology, geographical information sciences, paleontology, and geology to understand present-day distributions, and to reconstruct the sequence of events leading to the assembly of faunas in space and time. For parasite biogeographers, the task is especially complex, and interesting, because understanding parasite distributions requires an intimate knowledge of the ecological and evolutionary factors that determine the distributions of all of their hosts.

For convenience, biogeographers recognize two broad research traditions. Historical biogeographers seek to understand the origin, dispersal, and extinction of species relative to geological events such as continental drift, glaciation, and the emergence and submergence of landmasses. A key paradigm that underlies historical biogeography is the notion that the geographical range of a species, or an entire biota, can be split into isolated parts by physical barriers to dispersal or gene flow (e.g., the isthmus of Panama, the Beringia land bridge) that results in the formation of sister species or sister faunas. This research arm is known as vicariant biogeography. In contrast, ecological biogeography tends to focus on the extent to which the distributions of species result from current ecological processes.

General considerations

The Nematoda (Greek, nemathread), or roundworms, are perhaps the most abundant and diverse group of multicellular animals on earth. Free-living nematode densities can exceed over 1 million individuals per square meter in some shallow-water marine sediments. Furthermore, free-living nematodes exploit a greater array of ecological habitats than any other metazoan. Many are significant detritivores or decomposers that play a disproportionately large role in recycling chemicals and organic nutrients in aquatic and terrestrial ecosystems. Others feed on bacteria and other microorganisms and are important in food web relationships. Approximately 20 000 nematode species have been described. However, it is certain that this is an underestimate of total biodiversity, with perhaps as many as one million species. The difficulty in describing nematodes, in part, is related to their small sizes and their notorious uniformity in internal and external morphology. However, it is clear that their small sizes and cylindrical, tapered shapes have contributed to their extraordinary adaptive radiation within diverse ecosystems, as well as their exploitation of both plants and animals as hosts.

Nematodes are typically dioecious, exhibit sexual dimorphism, and vary in size from 1 mm to well over 1 m in length. The world’s largest nematode species is the massive Placentonema gigantissima in the placenta of sperm whales; female worms are approximately 8 m long! Nematodes have a fluid-filled pseudocoelom and are bilaterally symmetrical. All possess a complex multilayered cuticle that is molted four times before reaching sexual maturity. Like the acanthocephalans, nematodes also exhibit eutely whereby most adult tissues are composed of a constant number of cells and growth is due to increased cell size rather than an increase in cell number. Adults possess a complete, but simple, digestive system. Their reproductive systems are also relatively simple, but their life cycles are exceptionally varied.

The ability of parasites to cause disease has always been an important reason to study them, and the teaching of parasitology has almost always been stimulated by conditions conducive to disease, such as war or climate change. Currently, zoonotic diseases emerging from altered ecosystems, or carried by arthropod vectors spreading their ranges due to climate changes, supply that stimulation. However, most of us who teach, or have taught, parasitology have chosen that topic because of the fascinating life cycles of many parasites and their complex interactions with their hosts. Much of that fascination stemmed from learning how parasites can affect the population dynamics of their hosts, or the behavior of the hosts, or even the evolution of their hosts. In addition, that fascination was based on how much parasites could tell us about the life of their hosts, such as their diet, travels, or evolution. Or even of the earth itself – some of the earliest evidence for continental drift was the similarity in parasites of amphibians in Africa and South America. Examples of all of these influences are provided in this book.

Many of the systems that parasitologists have used to show these fascinating features have become relatively easy to study due to new techniques, such as those in genomics and proteomics, which have provided new and more powerful ways to study systematics, evolution, and host–parasite relationships. This has attracted the attention of biologists with a wide variety of backgrounds, so that much of the very interesting work done on host–parasite systems recently has been done by those trained in other specialties, such as ecology, behavior, neurophysiology, and evolutionary biology. Very few of the students in senior-level parasitology courses will go on for further study in parasitology, but many more will go on for further study in other biological specialties. Our courses, books, readings, and other materials used in our classes should be chosen to expose those students to the usefulness of parasites in investigations in their chosen fields.

General considerations

Our previous chapters focused on the functional morphology, life cycles and ecology of a wide range of animal parasites. One of our aims in these chapters was to highlight the complexity and fascination of the parasitic life style. Another was to introduce the idea that the seemingly infinite diversity of parasite life cycles and adaptations could be interpreted under an ecological umbrella. Armed with this background knowledge, we now consider unifying principles of ecology and evolution that can be applied to the phenomenon of parasitism.

We begin our transition with a consideration of the complex nature of parasite populations and the general ecological characteristics of the individuals that comprise them. We highlight the nature of enquiry at this level by first considering two examples. In a field survey, Cornwell & Cowan (1963) monitored the transmission of gut helminths into 180 canvasback ducks, Aythya valisineria, sampled from a small wetland in western Canada. The authors controlled for sampling heterogeneity by restricting their collections to ducklings within individual clutches. Their results showed that even within a single clutch, individual siblings harbored between 90 and 6000 worms! In another field survey, Valtonen et al. (2004) censused adult acanthocephalan populations in individual ringed seals, Phoca hispida, collected from the Baltic Sea. Although the collections spanned a 22-year period when the seal and intermediate host populations varied extensively, the prevalence of acanthocephalans was always 100% and the mean number of worms fluctuated within a single order of magnitude. Thus, on the one hand, mean parasite infrapopulation sizes can vary tremendously, even within very narrow spatial and temporal scales. On the other hand, population sizes can be remarkably stable, barely fluctuating around an equilibrium value. Characterizing this extreme variation at both narrow and broad temporal and spatial scales and understanding the underlying mechanisms that determine it are the central objectives of parasite population ecologists and ecological epidemiologists.