The positives of parasitic plants(!)

Parasitic plants – angiosperms that directly attach to another plant via a haustorium, a modified root that forms a morphological and physiological link between the parasite and host – tend to get a bad press. And it’s little surprise with the antics of such villains as Striga, the ‘violet vampire’, which greatly reduces the production of staple foods and commercial crops such as maize, sorghum, millet, rice, sugarcane and cowpea in many African countries, and can cause up to 100% crop loss. Slightly less devastating and livelihood-threatening is Rhinanthus minor – ‘yellow rattle’ – a hemiparasite on grasses, which is found in Europe, Russia, western Siberia, northern USA and Canada.

Whilst it is generally recognised that such plants have major negative impacts on plant community structure via influence on host productivity and competitive ability, James Fisher et al. show that nutrient-rich leaf litter from R. minor has a positive effect on plant community structure: ‘critically, in the case of grass and total community biomass, this partially negates biomass reductions caused directly by parasitism’. From sub-terranean to supra-terrestrial community impacts now, with another hemiparasite – mistletoes – and work by David Watson and Matthew Herring. Having already been established as ‘keystone resources’ – species providing important resources for a broad range of taxa and determining local diversities in these habitats – Watson and Herring experimentally investigated the role of Australian mistletoes such as Amyema miquelii (Loranthaceae, bog mistletoe) in eucalyptus woodland. After 3 years, sites from which mistletoe was removed lost, on average, a fifth of their total species’ richness, 26.5% of woodland-dependent bird species and more than one-third of their woodland-dependent residents.

The researchers, from The Institute for Land, Water and Society at Australia’s Charles Sturt University suggest that ‘nutrient enrichment via litter-fall is the main mechanism whereby the mistletoe promotes species’ richness, driving small-scale heterogeneity in productivity and food availability for woodland animals’. They further propose that this explanation applies to other parasitic plants with high turnover of enriched leaves, and that the community-scale influence of these plants is most apparent in low-productivity systems. I wonder if they had R. minor in mind? Prescience is, after all, a virtue…

[In the interests of fairness, it should be stated that Fisher et al. do cite Watson and Herring’s paper – Ed.]