Mistletoe and other parasitic plants are ecosystem engineers

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Photosynthesis – the ability to convert sunlight, water, and atmospheric carbon dioxide into sugar – is the defining characteristic of plants. However, some plant lines have moved away from this self-sufficient lifestyle and adopted a lifestyle in which they rely on other organisms to provide them with resources. What may seem like a bizarre lifestyle is pretty ubiquitous around the world: Parasitic plants make up one to two percent of all plant diversity, with 292 genera spread across various ecosystems.

Parasitic plants

Parasitic plants are incredibly diverse in appearance and distribution. They are united by the presence of a specialized structure, known as a haustorium, which allows them to attach themselves to the stem or root of a host plant. Thanks to this, they steal resources from the host, including carbon, water and minerals. Holoparasites (from the Greek holo– for “whole” or “whole”) are entirely parasitic, having abandoned photosynthesis and completely dependent on their host for their survival. Therefore, these plants often do not even look like plants; There is no point in keeping leaves green and filled with chlorophyll if they are not in use. The result is often striking and eye-catching.

Mistletoe and dodder, for example, are often seen as orange or dark purple “clumps” in the trees and shrubs they parasitize. Acting effectively as herbivores, these plants can harm or even kill their hosts. For example, genres Orobanche and Cuscuta are considered invasive species in America, with millions of dollars spent to prevent them from attaching themselves and devastating crops like corn and legumes.

Cuscuta. (Credit: Fritz Geller-Grimm / Wikipedia, CC BY-SA 2.5)

However, most types of parasitic plants try to have the best of both worlds. Hemiparasites are “half” parasites (from the Greek hemi– for half “). While they capture their own carbon dioxide, they attach themselves underground to the roots of other plants and parasitize them, undermining the host’s resources. With green leaves, fleshy tissue and sometimes showy flowers, hemiparasites appear “normal” above the ground. In fact, some of the most beloved and recognizable plant species in the United States are hemiparasites, most notably Castilleja, commonly referred to as the Indian brush, whose shiny, brightly colored, brush-like flower spikes are common in the mountain prairies, coastal plains, and interior prairies of North America.

Castilleja foliolosa. (Credit: Franz Xaver via Wikimedia Commons, CC BY-SA 3.0)

While much of the research on parasitic plants once focused on controlling invaders, a renewed ecological interest in hemiparasites has catalyzed an influx of studies highlighting the positive and often profound roles these plants can have. on the structure of the ecosystem. While one might suspect that a parasite only causes damage, this is far from the truth.

How parasitic plants benefit the ecosystem

Unlike most holoparasites, hemiparasites are generalists and parasitize a wide variety of host species. Therefore, plants that are very abundant in a natural community tend to be more parasitized than less common species, simply because they will be more often encountered by hemiparasites. By attaching disproportionately to dominant species and reducing their growth, hemiparasites release other plants from competitive exclusion, thus improving their chances of colonizing a new area or growing in population.

Perhaps counterintuitively, many published studies have illustrated that adding a hemiparasite to an area increases overall plant diversity. These communities are also more “balanced”; rather than having a single dominant host plant with sporadic instances of other species, plant species are present in relatively equal abundance.

Several studies have also shown that the impact of a hemiparasite goes well beyond the trophic level of the plant. In research published in 2018 by the journal Ecology, Dr Nate Haan and colleagues have shown that chemicals in leaf tissue of the hemiparasite Castilleja depended on the species of host plant it parasitized. In turn, the chemical composition of the leaves of the hemiparasite had an indirect impact on the fitness results of butterflies, which the caterpillars feed on. Castilleja tissue.

In addition to direct impacts on the composition of the plant community, parasitic plants have been shown to modify abiotic (i.e. physical rather than biological) conditions in their ecosystems. Since parasitic plants are constantly receiving material from their hosts, they often suffer from a wealth embarrassment, such as excess potassium. When they die, the litter of parasitic plants acts as a natural fertilizer, making the nutrients more available to neighboring plants and other organisms such as soil bacteria. Thus, due to their unique physiology, hemiparasites can have a disproportionate effect on their ecosystems and are considered not only to be key species but also to be eco-engineers.

Parasitic plants to the rescue

The potential to use parasitic plants in ecological restoration has long been discussed. In parts of central Europe, hemiparasites have been planted in areas infested with invasive grasses which are suitable host plants for the pests. Such efforts will soon reveal whether hemiparasites can be used as a natural biocontrol to help the global fight against invasive species.

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