Reductions in fitness due to an endoparasitic plant are comparable to the impacts of hemiparasites

Context Parasitic plants are widespread throughout the global flora and have diverse lifestyle strategies. In most cases these plants are detrimental to the host but may have some beneficial effects on the co-occurring plants in the sourrounding communities. Some have large macroscopic plant bodies and can photosynthesise, and are therefore able to produce some fixed carbon but do take water and nutrients from the host, especially if aerially attached. Very few species have vegetative parts completely enclosed in the host, having only reproductive structures externally displayed. Whether such internal parasites have as severe effects on the host as parasites with macroscopic plant bodies is unclear. Aims The endoparasite Pilostyles hamiltoniorum infests pea species (predominantly Daviesia species) in the south-west of Western Australia. We investigated the effect of this parasite on the vegetative growth and reproduction of Daviesia angulata in heathland vegetation. Methods Size, flowering and fruiting of parasitised and unparasitised host plants were recorded in three 6 × 30 m plots in a revegetated gravel pit in the Jurien Bay area of Western Australia. Key results A proportion of 21% of host plants was parasitised and these were significantly taller than unparasitised plants. These plants had 52% fewer flowers on average than unparasitised plants and subsequently far fewer fruits. Conclusions The reduction in reproductive output by this internal parasite was at least equal to or more severe than occurs in published examples of decreased productivity of other species parasitised by species with macroscopic plant morphology. Implications The reduced reproductive output of the host plants would be inimical to seed stores in the soil that this species relies on for regeneration after fires that commonly affect the vegetation in this region.