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Review
, 11 (6), 533-46

Parasites in Food Webs: The Ultimate Missing Links

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Review

Parasites in Food Webs: The Ultimate Missing Links

Kevin D Lafferty et al. Ecol Lett.

Abstract

Parasitism is the most common consumer strategy among organisms, yet only recently has there been a call for the inclusion of infectious disease agents in food webs. The value of this effort hinges on whether parasites affect food-web properties. Increasing evidence suggests that parasites have the potential to uniquely alter food-web topology in terms of chain length, connectance and robustness. In addition, parasites might affect food-web stability, interaction strength and energy flow. Food-web structure also affects infectious disease dynamics because parasites depend on the ecological networks in which they live. Empirically, incorporating parasites into food webs is straightforward. We may start with existing food webs and add parasites as nodes, or we may try to build food webs around systems for which we already have a good understanding of infectious processes. In the future, perhaps researchers will add parasites while they construct food webs. Less clear is how food-web theory can accommodate parasites. This is a deep and central problem in theoretical biology and applied mathematics. For instance, is representing parasites with complex life cycles as a single node equivalent to representing other species with ontogenetic niche shifts as a single node? Can parasitism fit into fundamental frameworks such as the niche model? Can we integrate infectious disease models into the emerging field of dynamic food-web modelling? Future progress will benefit from interdisciplinary collaborations between ecologists and infectious disease biologists.

Figures

Figure 1
Figure 1
Graphical depiction of a simple five-node food web before (a) and after (b) adding two parasites. Taxa represented are basal (B), grazer (G1, G2), predator (C1, C2), parasite (P) and hyperparasite (HP). The parasite (P) has an adult stage (A) using C1 as a host, a free-living larval stage (L1) and a parasitic larval stage (L2) in an intermediate host (G2). Transmission from intermediate host to final host occurs when a final host eats an infected intermediate host. The yellow L-shaped box contains the three life stages (yellow circles) of the parasite, P. Ellipsoids indicate parasites occurring within hosts. Arrows represent feeding links with the arrow pointing from the resource to the consumer (depicting energy flow). There are three types of predator–parasite links (dashed lines): feeding on the free-living stage of a parasite (L1–G1), ingestion of an infected intermediate host with the possibility of transmission of the parasite to the predator (L2–C1) and incidental ingestion of a parasite in an infected prey (L2–C2); the latter two we merge with predator prey links. Below the stick and ball figures are who eats whom matrices where consumers are rows and resources are columns. The matrix in (b) has four quadrants, clockwise from the top left, predator–prey, predator–parasite, parasite–parasite and parasite–host. Note that in the matrix of the free-living web, 20% of the possible links (directed connectance) are present while after adding parasites this increases to 24.5%– but only if predator–parasite and parasite–parasite links are included. This is also substantially higher than the 14% (7/49) predicted if the number of conventional links were to scale with the square root of possible interactions. Also, note that while the parasite P feeds on two hosts, it is not a generalist because it requires both to persist.
Figure 2
Figure 2
Three-dimensional visualization of the complexity of real food webs with parasites using data from the Carpinteria Salt Marsh Web (Lafferty et al. 2006b). Image produced with software available from the Pacific Ecoinformatics and Computational Ecology Lab, http://www.foodwebs.org. Balls are nodes that represent species. Parasites are the light-shaded balls and free-living species are the dark-shaded balls. Sticks are the links that connect balls through consumption. Basal trophic levels are on the bottom; upper trophic levels are on the top.

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