The bright side of parasitic plants: what are they good for?

Abstract

Parasitic plants are mostly viewed as pests. This is caused by several species causing serious damage to agriculture and forestry. There is however much more to parasitic plants than presumed weeds. Many parasitic plans exert even positive effects on natural ecosystems and human society, which we review in this paper. Plant parasitism generally reduces the growth and fitness of the hosts. The network created by a parasitic plant attached to multiple host plant individuals may however trigger transferring systemic signals among these. Parasitic plants have repeatedly been documented to play the role of keystone species in the ecosystems. Harmful effects on community dominants, including invasive species, may facilitate species coexistence and thus increase biodiversity. Many parasitic plants enhance nutrient cycling and provide resources to other organisms like herbivores or pollinators, which contributes to facilitation cascades in the ecosystems. There is also a long tradition of human use of parasitic plants for medicinal and cultural purposes worldwide. Few species provide edible fruits. Several parasitic plants are even cultivated by agriculture/forestry for efficient harvesting of their products. Horticultural use of some parasitic plant species has also been considered. While providing multiple benefits, parasitic plants should always be used with care. In particular, parasitic plant species should not be cultivated outside their native geographical range to avoid the risk of their uncontrolled spread and the resulting damage to ecosystems.

Figure 1

Illustration of functional roles of most significant parasitic plant groups (mistletoes and root hemiparasites) in ecosystems. A, Competitive alien invasive species and native invaders thriving in the absence of parasitic plants. B, Change in the plant community, increasing biodiversity. C, Diversified food supply for herbivores. D, Increased diversity and abundance of pollinators (e.g. Hymenoptera, Lepidoptera, Diptera). E, Root hemiparasite as ecosystem engineer and food opportunity for pollinators and herbivores. F, Sward gap for seedlings establishment. G, Increased soil organic matter and edafauna (Collembola, microorganisms). H, Increased food opportunity for seed herbivores and predators, seed dispersion by herbivores. I, Increased nutrient-rich litter, decompositors, and other animals living in litter. J, Mistletoe – opportunity for nesting, food, shelter.

Figure 2

Representative taxa, active compounds, and pharmacological activities of medicinal parasitic plants. A, representative parasitic plant taxa used in various continents. B, primary phytochemical ingredients reported in medicinal parasitic plants. C, medicinal activities reported for parasitic plants.

Figure 3

Cultivation of parasitic plants with high economic values promotes ecological services. A–B, A sandalwood (S. album) resort area in southern China, photos by Guohua Ma. C, Reforestation using Malania oleifera in karst regions of southwestern China. D, Grassland restoration using Rhinanthus alectorolophus in Czech Republic, photo by Stanislav Hejduk. E, Desert revegetation with root holoparasitic Cistanche tubulosa and its host Tamarix chinensis in northwestern China, photo by Pengfei Tu. F, Desert revegetation with root holoparasitic Cistanche deserticola and its host Haloxylon ammodendron in northeastern China, photo by Pengfei Tu. G, Santalum acuminatum in fruit in Southern Australia, photo by Richard McLellan

Figure 4

High aesthetic values of parasitic plants, which are used or are suitable for use in horticulture. A–C, Himalayan Pedicularis species (P. olivcriana, P. oxycarpa, and P. tricolor from left to right). D, Nuytsia floribunda, photo by Owen Roberts. E, Melampyrum arvense. Photo by Jakub Těšitel. F, Castilleja latifolia, photo by Huiting Zhang. G, Taxillus delavayi, photo by Yang Niu