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Review
, 5, 239

Where Are the Parasites in Food Webs?

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Review

Where Are the Parasites in Food Webs?

Michael V K Sukhdeo. Parasit Vectors.

Abstract

This review explores some of the reasons why food webs seem to contain relatively few parasite species when compared to the full diversity of free living species in the system. At present, there are few coherent food web theories to guide scientific studies on parasites, and this review posits that the methods, directions and questions in the field of food web ecology are not always congruent with parasitological inquiry. For example, topological analysis (the primary tool in food web studies) focuses on only one of six important steps in trematode life cycles, each of which requires a stable community dynamic to evolve. In addition, these transmission strategies may also utilize pathways within the food web that are not considered in traditional food web investigations. It is asserted that more effort must be focused on parasite-centric models, and a central theme is that many different approaches will be required. One promising approach is the old energetic perspective, which considers energy as the critical resource for all organisms, and the currency of all food web interactions. From the parasitological point of view, energy can be used to characterize the roles of parasites at all levels in the food web, from individuals to populations to community. The literature on parasite energetics in food webs is very sparse, but the evidence suggests that parasite species richness is low in food webs because parasites are limited by the quantity of energy available to their unique lifestyles.

Figures

Figure 1
Figure 1
Illustrations of A - negative asymmetry, and B – positive asymmetry at predator–prey nodes. In these hypothetical examples, the perspective is that of a parasite whose larval stage is in the rabbit intermediate host, and adult stage in the fox definitive host. In this scenario, the other hosts are not parasitized. C. Summary of mean asymmetry values for all nodes (open circles) and parasitized nodes (closed circles) from 5 published webs; Pin- Pinelands NJ, Car – Carpenteria marsh CA; CaK- Carpenteria modified web CA; Yth -Ythan estuary, Scotland; Tuck –Tuckerton salt marsh, NJ by Rossiter and Sukhdeo 2011 [21].
Figure 2
Figure 2
Two analyses of the same topological data from a host food web in the Meadowlands salt marshes of New Jersey by Anderson and Sukhdeo 2011 [[24]]. (a) A traditional food web diagram showing linkages among participants. This is a parsimonious arrangement of species, so even though it seems as though there are 8 trophic levels, there are really only 4, with the graphing program spacing them out a little for the sake of visualization. (b) A network clustering algorithm partitioned the food web into 15 distinct modules of highly interacting species independent of trophic position, and suggested that parasites preferentially colonized highly connected modules of tightly interacting species which experience fewer fluctuations in abundance relative to those in the periphery.
Figure 3
Figure 3
The life-cycle of a typical trematode. There are six distinct free-living parasite strategies during transmission from definitive host to definitive host that could only have evolved if there was an evolutionarily stable configuration between the hosts involved. Only one step, trophic transfer, is considered in the topological approach.
Figure 4
Figure 4
Standing stock biomass patterns of autotrophs, consumers and predators in a food web recovered from a fairly pristine Pinelands stream by Hernandez and Sukhdeo 2008 [[2],[19]]; this illustrates the ‘rule of ten’ in standing stock biomass pyramids. Parasite biomass (in black) was recovered from two trophic levels in this system.
Figure 5
Figure 5
Direct energetic measurements of net production (kj/m2/yr) values for each host species in a Pinelands stream food web based on bomb calorimetry. Each compartment represents the total yearly production energy for each organism in the food web; the black compartments represent those hosts which are parasitized, Lettini and Sukhdeo, in prep.
Figure 6
Figure 6
Unreliability scoresV2/xfor each of 68 host species in a Raritan river food web. Total biomass for each species was measured for 8 consecutive seasons to determine biomass fluctuations used to calculate unreliability scores. Rossiter 2012 [22] found that parasitized hosts were among those hosts with the lowest unreliability scores. The parasitized host with the highest unreliability score (arrow) is a seasonal frog species.
Figure 7
Figure 7
A. Hernandez and Sukhdeo 2008 [[19]] measured the proportion (%) of isopod intermediate hosts in the diets of four different hosts as determined from stomach content analysis. B. Prevalence of infection (%) with the acanthocephalan parasite Acanthocephalus tahlequahensis whose larval stage is found in the isopod Ceacidotea communis. Bluegill sunfish are recent invaders in this system and are relatively resistant to the parasite; this is a good example of the dilution effect where adding non-competent hosts to the system compromises the transmission rate of the parasite to its normal host.
Figure 8
Figure 8
Typical pattern of standing stock biomass pyramids of a black water stream in New Jersey (black), and the estimates of actual production energy at each trophic level (grey). Parasites were included in the appropriate predator and consumer trophic levels. These data suggest that the real energetic costs at lower trophic levels (grey) can be significantly higher than estimates according to the rule of 10 (Lettini and Sukhdeo, in prep.).

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