Ecological roles of parasites in food web structure, animal biomass and secondary production in pond ecosystems

Background/Question/Methods

Freshwater ecosystems are traditional model systems for understanding food webs and ecosystem energetics. Despite the central role of such studies in the advancement of ecology, there has been little effort to incorporate infectious agents into food webs and energy flow within pond ecosystems. This omission is particularly important considering the growing recognition of the roles of parasites in shaping community structure and ecosystem processes. Our aims for the present study were to: 1.) integrate infectious agents into a freshwater pond food web and evaluate their impacts on web structure and 2.) quantify the biomass and productivity of trematodes (hypothesized to make the largest energetic contribution of all parasitic groups), alongside the biomass of free-living aquatic organisms within three ponds. Our species list for the food web came from sampling conducted between 2009 and 2011 using stovepipe samplers, dip nets, seine nets and visual encounter surveys. We integrated parasites into the web by dissecting over 1,600 hosts and by using literature records to verify other host-parasite relationships. Biomass estimates came from direct measurements of parasites and length-to-mass regressions for most free-living taxa, and parasite productivity was estimated based on in situ measurements of the production of free-living trematode life stages.   

Results/Conclusions

The total pond food web included 1088 links, of which 55% involved traditional predator-prey interactions, 36% involved predator-parasite interactions, 8% involved parasite-host interactions and 1% involved parasite-parasite interactions. Parasites increased important web metrics including connectance (from 25.9 to 27.4), linkage density (12.4 to 17.3) and relative nestedness (1.3 to 2.1). Across three ponds, intermediate trematode hosts dominated the biomass of free-living organisms (snails = 3.2 g m^-2; larval amphibians = 3.1 g m^-2). The mid-summer aquatic trematode biomass averaged 0.09 g m^-2, which was comparable to or exceeded that of the most abundant insect orders (coleoptera = 0.10 g m^-2; odonata = 0.08 g m^-2; hemiptera = 0.07 g m^-2; ephemeroptera = 0.03 g m^-2). Most trematode biomass (85%) consisted of larval stages within snail hosts. Assuming infected snails released free-living parasite stages (cercariae) for three to four months a year, trematode communities produced an average of 153 mg m^-2 yr^-1 of dry cercarial biomass (range = 70 to 220 mg m^-2 yr^-1). Our results suggest that the contributions of parasites to pond food web structure and ecosystem energetics may exceed those of many free-living taxa that often play key roles in structuring freshwater communities.