Parasites in ecosystems: a call for appreciation of their non-trivial role in ecosystem biomass and energetics

Lake Hayes, New Zealand, South Island. One of the four lakes sampled during our study. Picture inserted show an amphipod host, Paracalliope fluviatilis, infected by the trematode parasite, Coitocaecum parvum. Note the proportionally large size of the parasite compared to its host.

EDITOR'S CHOICE JUNE 2015

by Clément Lagrue and Robert Poulin

The standing crop biomass of different populations or trophic levels reflects patterns of energy flow through an ecosystem. In particular, the ratio of total biomass between two consecutive trophic levels has been used to estimate the efficiency of energy transfer from resources to consumers. Parasites are deeply intertwined in food webs since they feed on many different hosts and occupy different trophic levels. However, the contribution of parasites to total ecosystem biomass is widely considered to be negligible, because of their small sizes and cryptic existence. Therefore, despite parasites having effects on hosts and species interactions that are disproportionate to their size, their contribution to ecosystem energetics is often assumed to be minimal although recent studies are starting to show otherwise.

Here, we quantified parasite biomass and contrasted it to the biomass of free-living animals to test its dependence on host biomass. We were thus able to assess how parasite biomass varies with host population biomass; data critical to determine what constrains parasite populations. We use an extensive dataset on all free-living and parasitic metazoan species from multiple sites in New Zealand lakes to measure parasite biomass and test how it covaries with host biomass.

Parasite biomass ranged from about 0.01 to 0.25 g m-2, surpassing that of minor free-living taxa. Parasites had local biomasses that correlated positively with those of their hosts. For assemblages of parasite species sharing the same host, we also found strong relationships between local host population biomass and the total biomass of parasites supported. Furthermore, the host­–biomass–versus–parasite–biomass relationships had slopes that were slightly lower from those predicted for the maximum carrying-capacity. This suggests that host populations are underexploited by parasites, i.e. that a slightly greater parasite biomass could theoretically be supported by the hosts. In our systems, the size (and therefore the biomass) of parasite populations may be constrained more by transmission dynamics and other epidemiological processes than by limited host resources.

Overall, as for free living taxa, parasite biomass correlates with host biomass in a predator-prey relationship. Interestingly however, data suggest that parasites are not making full use of available host resources. Host populations should be capable of supporting a little more parasite biomass, and may be open to expansion of existing parasites or invasion by new ones. Our findings add to the still very small, but hopefully growing, number of studies of parasite biomass in the context of whole-ecosystem energetics.

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