Quantitating Role of Parasites in Ecosystems Using Energy Flow
Donald A. Windsor
The work of Hatton et al. may have far broader implications than perhaps the authors and even their reviewer, Cebrian, realize. I suspect that their power law might reflect the important role that parasites play in ecosystems.
Hatton et al. report a predator-prey power law that applies across all ecosystems. Cebrian asks the critical question, “Where does this sublinear pattern stem from?”
I respond that it stems from the underlying actions of parasites. When predator-prey interactions depart too far from equilibrium, diseases take over, which is why our biosphere is so biodiverse. Monocultures are prevented by parasites. Parasites apparently maintain the predator-prey equilibrium at k = 0.75 by accounting for the other 0.25, which would make it linear.
Parasites have been considered as predators since at least 1927 (Elton). The difficulty is that parasite biomass is probably not comparable with free-living predator biomass. Besides, how to measure it would be another problem. It was unintentionally included in the biomasses measured by Hatton et al., which might also be an unrecognized problem. Effects of parasites per unit of their biomass would depend on their potency.
Parasites cannot be ignored in ecosystems, because they outnumber the free-living species that host them. Most of the species on Earth are parasites (Windsor). Why have parasites been so successful throughout evolution? Why have host species not been able to win the host-parasite arms race? My answer is because they are such an integral component of ecosystems that without them ecosystems would not have survived. It is as if hosts and ecosystems are addicted to parasites, at the species level.
The insidious, pervasive ubiquity of parasites renders it difficult to test my hypothesis because it is impractical to make field observations in parasite-free situations. Consequently, I am glad to see approaches, such as that of Hatton et al., which might be used indirectly.
Cebrian, Just. Energy flows in ecosystems. Relationships between predator and prey biomass are remarkably similar indifferent ecosystems. Science 2015 September 4; 349(6252): 1053-1054.
Elton, Charles. Parasites. In: Animal Ecology. New York, NY: Macmillan. 1927. Pages 71-82.
Hatton, Ian A. ; McCann, Kevin S. ; Fryxell, John M. ; Davies, T. Jonathan ; Smerlak, Matteo ; Sinclair, Anthony R.E. ; Loreau, Michel. Thepredator-prey power law: biomass scaling across terrestrial and aquatic biomes. Science 2015 September 4; 349(6252): 1070.
Windsor, Donald A. Most of the species on Earth are parasites. International Journal for Parasitology 1998 December; 28(12): 1939-1941.
This article was posted as a comment to the Cebrian article on 7 October 2015. Link is: http://comments.sciencemag.org/content/10.1126/science.aad0684