STRUCTURE AMONG FUNCTIONAL GROUPS OF FOOD WEB WITH PARASITES
Parasitism is an integral part of nature and parasites can play multiple roles in the process at multiple ecological hierarchies (Poulin 1999). In biological community, parasites often function as the top-down mediators engaging intra- and inter-species competition among their host species (Hatcher et al. 2006). Parasites may also manipulate host‟s
behaviour (Lefevre et al. 2009), allocate energetic budget via host‟s life history trait mediation (Lagrue and Poulin 2007) and result in profound energetic consequence in food webs (Mouritsen and Poulin 2003, Kuris et al. 2008). Moreover, parasites themselves can produce considerable
biomass and often serve as an important energy source in aquatic
ecosystem (Kuris et al. 2008, Thieltges et al. 2008, Johnson et al. 2010).
Food webs embraced parasites have progressively modify our views on the food web topologies and dynamics stemmed from the predation only webs (Poulin 2010). Although more studies now consider parasitism as an inimitable part of ecosystem (Lafferty et al. 2008), the functional role of parasites in food web remains inclusive. In particularly, how parasites interact with non-parasitic species through both direct and in-direct trophic interactions in the food web network is still an open question.
It has been emphasised that a completed food web with parasites
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consist four different subweb and each addresses interactions between predator-prey (HH), parasite-host (PH), predator-parasite (HP) and super-parasitism (PP) (Lafferty et al. 2006). By adding different subweb sequentially into the existing predator-prey subweb (HH), we can
demonstrate how the interaction structure been impacted when parasitism is considered into food web network.
In order to elucidate how parasite can affect interaction patterns, the signed digraph approach (Liu et al. 2010) was applied for the food web of Carpinteria salt marsh ecosystem (Lafferty et al. 2006). We quantify the impacting intensity of all paired species, including parasites, based on their topological positions in the food web network under five food web scenarios. They are (1) the predator-prey only food web (HH); (2) the predator-prey and parasite-host food web (HH+PH); (3) the predator-prey, parasite-host and super-parasitism food web (HH+PH+PP); (4) the
predator-prey, parasite-host and predator-parasite food web (HH+PH+HP) and (5) the completed food web which has all four subwebs (ALL). In addition, trophic species were assigned into basal, middle and top species based on the positions they occupied in the food web and into the
predefined groups of parasites. We therefore summarised the interaction pattern among these four major functional groups under five different food web scenarios.
Fig. S1 showed the interaction structures among major functional
groups under the five food web scenarios described. It demonstrates (1) all functional groups make negative impact to themselves; (2) the
resource groups positive impacts to all of their consumer groups; (3) although the consumer groups would generate negative impacts to their immediate preys, they compose none or slightly positive impact to their preys‟ resources; (4) parasite groups follow the general observation found in the food web scenario HH+PH and HH+PH+PP and represents an important top-down controller while receive no regulation from other functional groups in these food web scenarios; (5) when parasites were treated as the food source, i.e. the subweb HP was included, the parasite group became positive interactor to the top group and was negatively regulated by all other functional groups.
Apparently, the treatment of parasite as the food source for
non-parasitic species is not only to create the parasitic-bond bottom-up energy flow but also alter the signs and intensities of interactive structure among the major functional groups in the food web. Such substantial structural alternation by parasites presents a unique feature which is typically not observed in the predation only food web. Moreover, the creation of cyclic pathway involving parasites may further affect food web dynamic and stability by providing more feedback control loops in the complex trophic network.
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Fig. S1. The interactive structures based on the averaged signed
topological importance among four functional groups using five food web scenarios in Carpinteria ecosystem.
-0.03 0.00 0.03
-0.03 0.00 0.03
Basal->
Middle->
Top->
Parasite->
-0.03 0.00 0.03
-0.03 0.00 0.03
-0.03 0.00 0.03
HH
HH+PH
HH+PH+PP
HH+PH+HP
Basal Middle Top Parasite All