Introducing ecosystem engineers for grassland biodiversity conservation: a review of the effects of hemiparasitic Rhinanthus species on plant and animal communities at multiple trophic levels.
Parasitic plants have a strong negative effect on the growth of their hosts. On the community scale, parasitism can change competitive relationships among individuals, facilitate establishment of other species, and eventually increase diversity. Beyond the impacts on plant communities, parasites can directly and/or indirectly impact communities at other trophic levels. In the last two decades, scientists have started to use root-hemiparasitic plants - mainly of the genus Rhinanthus (Orobanchaceae) - as a tool in grassland restoration in Europe. As this may have effects on communities at various trophic levels, there is a need to summarise the existing knowledge on this topic and to identify the gaps in our knowledge. We review the literature dealing with the impacts of root hemiparasites on plant communities, herbivores, predators, pollinators, and soil biota. We particularly focus on the well-studied species of the genus Rhinanthus, as model root hemiparasites, and include species of other genera for comparison. In most cases, Rhinanthus and other hemiparasites had a negative effect on the abundance of grasses, while they rarely affected abundance of legumes and forbs. Hemiparasites had mostly either neutral or positive effects on plant species diversity. Several factors can limit the effects of hemiparasites on plant communities, such as a high level of nutrients in the soil, and a high initial diversity in the host community. For ecological restoration of degraded grasslands, a mixture of seeds of target plant species was usually sown in addition to the seeds of the hemiparasite. This can be particularly important at species-poor sites located in a landscape poor in propagule sources. Hemiparasite introduction should be carried out with caution in habitats prone to invasions by undesirable species. On the other hand, when a grassland habitat is dominated by an invasive species which is also a good host for a hemiparasite, root hemiparasites can be used as a tool against the invasive species, for their potential to increase the biotic resistance of communities. Relatively few studies have investigated the effects of hemiparasites on other trophic levels so far. In several laboratory studies, both positive and negative effects of hemiparasites on herbivores have been observed. In the field, a positive effect of Rhinanthus on the abundance, diversity, or species richness of several groups of herbivorous, predatory and detritivorous arthropods, and snails was observed, but the evidence is quite limited, and the underlying mechanisms poorly understood. Hemiparasites can influence microbial communities, and they can also have positive effects on pollinators, directly through their own flowers and indirectly by increasing the abundance of forbs relative to that of grasses, but more studies are needed to confirm these expectations. Because little is known about the effects of hemiparasites on animal communities, we call for additional research on these aspects.