The Editor’s Choice for this issue of Journal of Ecology, the paper ‘Modelling the growth of parasitic plants’ by Yann Hautier, Andy Hector, Eva Vojtech, Drew Purves and Lindsay Turnbull, provides a nice example of how modelling simple ideas, when combined with analysis of experimental data, can lead to new insights into phenomena which were previously thought to be well understood.
Who determines productivity in the host–parasite system?
Hemiparasitic plants are photosynthetically active but rely on their hosts for water and mineral nutrients. They can reach extraordinary densities in European grasslands and play a key role in maintaining diversity, reducing the dominance of grasses and promoting forbs (Cameron et al. 2009). It is commonly observed that the combined mass of hosts plus parasites is less than the mass of the host plants when grown alone, leading traditional farmers to curse the parasite. The reduced yield has been attributed to some decrease in the efficiency of photosynthesis in the parasitized host or to reduced nitrogen-use efficiency by the parasite. This argument seems sound: if the parasite decreases the efficiency with which resources are gathered and/or used, then the total system productivity will be reduced. In contrast to this view, Hautier and colleagues simply assume that the system productivity is determined only by the host’s growth (they ignore parasite photosynthesis), and that this, in turn, depends on host size. In their model, the parasite growth rate therefore depends solely on the host growth rate; the parasite simply extracts mass from the host and stores it inactively. This mass no longer contributes to growth; hence it inevitably leads to decreased total productivity as long as host growth is size-dependent.
So does it work?
Well, the first question that Hautier and his coworkers ask is: “Is parasite mass determined by host growth rate?” and they show convincingly that this is indeed the case: faster-growing hosts support larger parasites. So, the basic premise of their model appears to be correct. This has several important corollaries: first, when the parasite is rare (so that hosts are typically either uninfected or infected by a single parasite), parasites should be prudent. This is because removing too much mass from their host will reduce their own future growth. Second, when hosts are infected by multiple parasites, a Tragedy of the Commons situation will occur, leading to over-exploitation of the hosts. This occurs because the prudent parasite will be outcompeted by a more aggressive parasite that takes a greater fraction of host mass—leaving resources for your competitors is rarely a good idea.
Finally, how should host resistance be measured?
The obvious metric is parasite mass: hosts that support lower parasite mass are obviously more resistant, aren’t they? Well, if all hosts grew at the same rate, this would be reasonable, but as hosts grow at different rates and so potentially provide the parasite with different amounts of resources, this no longer seems sensible. Looking at deviations from the expected relationship between host growth rate and parasite mass provides an alternative way of measuring host resistance. Using this method, Hautier et al. identify potentially resistant hosts, although most of the grass species they examine are similarly susceptible. Defining resistance then is perhaps not as simple as expected.
So, everything is clear then?
Well, perhaps not, because a slow host growth rate might itself be a consequence of being resistant (say having highly lignified roots) and so it becomes difficult to disentangle cause and consequence. Building models and exploring deviations from optimal behaviour provides a possible way forward in interactive systems such as these. An interesting discussion of these and related issues can be found in de Mazancourt et al. (2005).
Editor, Journal of Ecology
- Cameron, D.D., White, A. & Antonovics, J. (2009) Parasite-grass-forb interactions and rock-paper- scissor dynamics: predicting the effects of the parasitic plant Rhinanthus minor on host plant communities. Journal of Ecology, 97, 1311-1319.
- de Mazancourt, C., Loreau, M. & Dieckmann, U. (2005) Understanding mutualism when there is adaptation to the partner. Journal of Ecology, 93, 305-314.
- Hautier, Y., Hector, A., Vojtech, E., Purves, D. & Turnbull, L. A. (2010) Modelling the growth of parasitic plants. Journal of Ecology, 98, 857–866.
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