Abstract
As the data have poured in, and the number of published food webs containing parasites has increased, questions have been raised as to why free-living species consistently outnumber parasites, even though most general reviews on the subject of host:parasite species richness suggest the contrary. Here, I describe this pattern as it exists in the literature, posit both real and artifactual sources of these findings, and suggest ways that we might interpret existing parasite-inclusive food webs. In large part, the reporting of free-living species devoid of any associated parasites (termed here in the coding of food web matrices as "zeros") is a consequence of either sampling issues or the intent of the study. However, there are also several powerful explanatory features that validate real cases of this phenomenon. Some hosts appear to authentically lack parasitism in portions of their geographic ranges, and parasites are often lost from systems that are either in early phases of community re-colonization or are compromised by environmental perturbation. Additionally, multi-stage parasite life cycles and broad host spectra allow some parasite species to partially saturate systems without providing a corresponding increase in parasite species richness, leading to low parasite species richness values relative to the free-living community. On the whole, the existing published food webs are sufficient to, at least in principle, determine basic patterns and pathways associated with parasite establishment and persistence in free-living communities because (1) for the purpose of those features, species rarity is roughly analogous to absence and (2) the existing data seem to suggest that the addition of more parasite taxa would reinforce the patterns already observed. This is particularly true for helminth parasites, in which our understanding and the resolution of our work is most robust.
Highlights
At a recent meeting of the American Society of Parasitologists (Anchorage, AK, 2011), an important, though largely ignored, question was raised at the conclusion of my oral paper discussing the patterns of parasitism in a riverine food web; where are all the parasites? the discussion centered around the apparent paucity of parasites in published food webs relative to free-living taxa, and spawned a recent review offered by my mentor and coauthor on the paper (Sukhdeo, 2012)
For anyone working in the field of ecology, it would be hard to miss the fact that food webs have become a prominent way for biologists to study the structure and function of natural communities
If we are looking to describe the patterns of parasitism in natural communities, zeros may not be overly problematic to our interpretations
Summary
At a recent meeting of the American Society of Parasitologists (Anchorage, AK, 2011), an important, though largely ignored, question was raised at the conclusion of my oral paper discussing the patterns of parasitism in a riverine food web; where are all the parasites? the discussion centered around the apparent paucity of parasites in published food webs relative to free-living taxa, and spawned a recent review offered by my mentor and coauthor on the paper (Sukhdeo, 2012). It is estimated that there may be as many as 300,000 parasite species residing in just the 57,000 vertebrate taxa on the planet, and while the full culmination of the existing data suggest just 1.53 helminth taxa per host species (neglecting all protozoans, bacteria and viruses; Dobson et al, 2008), we still get the idea that there are many more parasites than free-living taxa in natural communities. Given their apparent ubiquity, it is odd that every published food web containing parasites contains many free-living species that do not act as a host for any parasites.
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