The role of parasites in food-web topology and dynamics of subarctic lakes

Food webs constitute an important biological concept, depicting ecological communities as detailed networks of trophic interactions. Parasites have rarely been included in food-web studies, despite their omnipresence and the fact that parasitism represents the most common consumer strategy among organisms. With a growing awareness of the potential importance of parasites in trophic networks, there has been a comprehensive call for their inclusion in food-web analyses. This project has addressed key issues related to the topology and functioning of food webs with special emphasis on the role of parasites, using subarctic lakes as model system. Through detailed studies of plants, animals and parasites and their trophic interactions, a complete trophic network has been established for Lake Takvatn, northern Norway. The resulting food web is highly resolved (comprising 175 free-living and parasitic species and 4316 trophic link) and also includes trophic information of different life stages (313 nodes and 6088 links in total). The established food web represents a state-of-the-art network in respect to details and resolution, and is the first detailed food web including parasites from the Arctic, and also one of a few worldwide. Our studies demonstrate that parasites are key food-web constituents that are involved in a majority of trophic links, increase network complexity and are thus important for the functioning and stability of the ecosystem. Trophically transmitted parasites are particularly embedded in the network as they use feeding links between their hosts to vie up through the food chains, and they thus permeate the food web with their complex life cycles. Besides being infective agents, parasites may also serve as prey for non-host predators, either through concomitant predation (ingestion of a parasite along with the host) or by predation on free-living life stages of parasites. Such predator-parasite interactions appear to be prevalent and important links in the studied food web. Our findings also reveal that important prey for fish like amphipods, snails and copepods, have central roles in the trophic network and are hosts for many parasite species of which several end up in fish. Our network analyses further show that introductions of new fish species may have unforeseen impacts through the arrival of their associated parasites. Brown trout used to be the single fish species present in Takvatn. Following introductions of Arctic charr and three-spined stickleback, large changes occurred in parasite infections and the pelagic network, partly because of concomitant introductions of several new parasite species. Additionally, four bird species at the lake rely on stickleback and charr as prey. Hence, the introduction of the two fish species facilitated the arrival of at least 11 new species, with major consequences for species interactions and parasite transmission in the trophic network. Charr and stickleback are also identified as two of the most central species in the present food web in Takvatn. Long-term studies in Takvatn have shown that parasite infections in fish may be reduced through management efforts, but pertinent insight to the trophic network and its parasite community is required. An experimental culling of Arctic charr in the 1980s resulted in large changes in the fish community of the lake. Data from more than three decades of studies have revealed a dramatic decline in the infection of charr by two species of Diphyllobothrium tapeworms that often constitute a problem in salmonid fish. The decline was related to a change in the trophic niche of charr following the population reduction, where copepods and sticklebacks that are important intermediate hosts for these parasites, largely have been omitted from the diet. For large trout, the Diphyllobothrium infection has increased, likely because of increased transmission through extensive predation on sticklebacks and small charr. Comparative studies of lakes with different fish communities have revealed that high Diphyllobothrium infection in trout is related to the presence of sticklebacks. This is further influenced by Schistocephalus solidus, a large-sized tapeworm larva that manipulate the behavior of sticklebacks and make them more vulnerable for predation by e.g. trout, thus demonstrating a large complexity in predator-prey-parasite relationships. By combining long-term and detailed studies of a model system with comparative studies of a number of other subarctic lakes, the project has provided important new insight to the role of parasites in northern ecosystems and food webs, and thereby improved the knowledge for nature management and sustainable resource use. Suggestively, parasites may also influence the important relationship between biodiversity and ecosystem functioning, thus underpinning the importance of including parasites in trophic network analyses.

Food webs constitute an important biological concept, depicting ecological communities as detailed networks of trophic interactions. Parasites have rarely been included in food-web studies, despite their omnipresence and the fact that parasitism represent s the most common consumer strategy among organisms. With a growing awareness of the potential importance of parasites in trophic networks, there has recently been a comprehensive call for their inclusion in food-web analyses. The proposed project address es key issues related to the topology and functioning of food webs with special emphasis on the role of parasites, using subarctic lakes as a model system. Two contrasting but complimentary approaches are employed; the first being related to the establish ment and analysis of the detailed structure of a whole-lake food web including parasites in a subarctic lake ecosystem, and the latter to the investigations of a key sub-web interaction module (fish predators, their prey and their trophically transmitted parasites) in time (long-term within-lake variations) and space (between-lake variations). The adopted framework facilitates in-depth exploration of the importance of parasites in food webs, including analyses of compartmentalisation and main energy pathw ays, assessment of the impacts of fish species additions and their potential hitchhiking parasites on food-web structure and dynamics, and an evaluation of the role of fish species composition for parasite transmission and the functioning of the trophic n etwork. The addressed topics represent fundamental objectives in ecology and conservation biology that presently are receiving increasing attention from the scientific community. The project will also provide important insight to the functioning and persi stence of vulnerable arctic ecosystems.