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The effects of parasites on the kelp-forest food web


Parasites often track food web linkages through their complex life cycles, but most food webs do not systematically include parasites. Where studied, parasites have strong effects on food web structure. Kelp forests are famous for strong trophic interactions, and their dynamic and open nature make them very different relative to the systems where parasites have been thoroughly studied (salt marsh, sand flat, and lake ecosystems). The objective of this dissertation was to build a high-resolution topological kelp-forest food web that includes parasites. I used this food web to address the research question: How do parasites affect food-web structure? Chapter 1 provides background and motivation for this work by reviewing key areas of research in kelp forest ecology and the effects of parasites in food webs. Chapter 2 describes the study system and a free-living food web with 490 species across 23 Phyla, with 546 distinct life stages and 8,759 trophic interactions. Chapter 3 describes the parasites in the food web, which adds 422 species across 10 Phyla (521 life stages) and 2,745 trophic interactions between parasites and hosts to the network for a total of 11,504 links. Adding trophic interactions between predator and parasites (concomitant predation) adds a further 9,536 links to the network. Chapter 4 examines the effects parasite addition on food-web structure. The kelp-forest food web was greatly enriched through resolution of free-living species and parasites, and parasites made up a larger proportion of the kelp-forest food web than any other published food web with parasites. Some of the effects of parasite addition were related to increasing network size and contrasted patterns in other systems (e.g. decreased connectance). On the other hand, other effects (e.g. longest chain length) were consistent with predictions based on other systems and were not due to increased network size alone. Specialist parasites and concomitant links altered the degree distribution independent of network size. Parasite life cycles are embedded throughout diverse patterns of free-living species interactions and must navigate a dense network of predators to infect appropriate hosts. The kelp forest ecosystem provides a diverse source of food and a diverse set of predators for both free-living and parasitic species, and our understanding of kelp forest ecosystems is enriched by including them.

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