Parasitic Plants Can (Sometimes) Help Competing Plants Coexist

Source Article: Hodzic, Jasna, Ian Pearse, Evelyn M. Beaury, Jeffrey D. Corbin, and Jonathan D. Bakker. 2022. “Root Hemiparasitic Plants are Associated with More Even Communities across North America.” Ecology e3837.

Featured Image Caption: Castilleja miniata is one of many hemiparasitic plant species native to western North and South America. The genus Castilleja made up 25% of the total hemiparasite abundance observed in the study. Image Source: Wikipedia Creative Commons by Dcrjsr

Meet Your Friendly Neighborhood Parasitic Plants

You might assume that parasites are harmful to biodiversity, but some parasitic plants may help maintain diverse plant communities! Parasitic plants attach their roots to other plants’ roots, then leach energy in the form of sugars from the host plant. Hemiparasitic plants also perform their own photosynthesis, capturing sunlight as energy stored in sugars.

These hemiparasitic plants are found all around the world, and experimental findings suggest that they can increase species richness and evenness. Species richness is the number of species in a community, and species evenness is related to the abundance of each species. If only a small number of species are very dominant in the community, the community has low evenness. By contrast, if many species are quite abundant, the community has high evenness. Hemiparasitic plants are often rare, but they may play an outsized role in structuring their communities.

How Do Competing Species Manage to Coexist?

Why might hemiparasites bolster biodiversity? The answer lies in ecological theory. Species cannot coexist when they are competing too intensely or directly for the same resources. Hemiparasites might help less competitive plant species to persist in plant communities by weakening the stronger competitors. Hemiparasites may tend to prey on the most prevalent, dominant plant species, giving the other species a chance to get enough nutrients, sunlight, or space to survive.

Rhinanthus is the most heavily experimentally studied hemiparasite taxa. Image Source: Wikipedia Creative Commons by J.F. Gaffard

However, our understanding of how hemiparasitic plants affect communities in nature is limited. While many experiments have found effects on plant species richness and evenness, most of the experiments relied on a single hemiparasite genus in grasslands. As a result, there is little understanding of the true impact of diverse hemiparasites in the wide range of plant communities around the world where they can be found. Are the effects of hemiparasites evident in diverse, natural communities?

Using Continent-Wide Data to Study Patterns of Biodiversity

To address this uncertainty, researchers investigated a huge set of plant communities across America to study the relationships of hemiparasites to community structure, that is, species richness and evenness. Thousands of plant inventories (12,498) were conducted in pieces of land managed by the U.S. National Park Service. These inventories recorded all the plant species present and the percent area that they covered, a measure of species abundance relative to other plant species. The researchers classified the plant species as either woody or herbaceous. They then calculated the species richness and evenness at each location. Out of all the locations studied, around one eighth had hemiparasitic plants present.

Locations of inventoried U.S. National Park Service lands with hemiparasitic plants present. Image Source: Open Access Article. Hodzic et al. 2022. Ecology.

Next, the researchers looked for relationships between hemiparasite presence and abundance and the species richness and evenness of communities. But first, the researchers asked an important question: do more biodiverse communities tend to have more rare species in general? If so, how can we tell if hemiparasitic plants, which are frequently rare, were related to biodiversity because they are rare or because they are hemiparasitic? To disentangle these factors, the researchers looked at the abundances of hemiparasitic plant species and identified many similarly rare, non-parasitic plants for comparison in their analyses. If these similarly rare plants had the same relationships to species richness and evenness as hemiparasitic plants, then hemiparasitism was not driving clear patterns of biodiversity.

The genus Krameria made up 20% of the total abundance of hemiparasites in the study. Image Source: Wikipedia Creative Commons by Stan Shebs

The researchers then used models to investigate whether communities with a) hemiparasitic plants or b) similarly rare non-parasitic plants present typically had higher species richness and evenness. They investigated species richness and evenness of all plants, only woody plants, and only herbaceous plants. Then, the researchers repeated this analysis but using hemiparasite and similarly rare plant abundance instead of presence.

Context Matters for Hemiparasite Effects

The models revealed that communities with hemiparasites present did not tend to be any more species rich or even overall compared to communities with similarly rare, non-parasitic species. Additionally, hemiparasite abundance was not more strongly associated with species richness of overall communities, herbaceous plants, or woody plants than similarly rare, non-parasitic species. Hemiparasite abundance was not more highly associated with evenness of woody plants or overall communities either; however, hemiparasite abundance was more strongly associated with greater evenness of specifically herbaceous plants than similarly rare, non-parasitic plants. Also, some hemiparasite species had much stronger relationships to species richness and evenness than others. The top 10 most abundant hemiparasites were the most strongly related to increases in herbaceous species evenness, while less common hemiparasites were less strongly related.

The genus Pedicularis made up 32% of the total hemiparasite abundance in the study. Image Source: Wikipedia Creative Commons by Walter Siegmund

Are you disappointed? Don’t be! These results actually make a lot of sense. Woody plants tend to have tougher roots than herbaceous plants, which makes them harder to parasitize. As a result, hemiparasites probably only affect the community structure of herbaceous plants. Hemiparasites likely increase herbaceous species evenness by weakening the dominant species, allowing inferior competitor species to grow to higher abundances. However, this effect of evenness does not need to come along with greater species richness, which would require helping new species to establish in the habitat. Hemiparasites are typically found in species-rich habitats with lots of resources, like other similarly rare, non-parasitic species. They do not consistently increase species richness; they just tend to survive best in these habitats.

Only one genus of hemiparasites was seemingly associated strongly with species richness, Castilleja. This suggests that different hemiparasitic plants may have different effects on their communities. One possible reason is that hemiparasitic plants can be either annuals, which produce seeds then die each winter, or perennials, which live and regrow for multiple years. If a plant is an annual, it tends to prioritize acquiring resources during its short life, while perennials prioritize conserving resources for the future. Therefore, annual species, like some species of Castilleja, might be more aggressive parasites to their hosts than perennial species. Castilleja may weaken the dominant plant species more than most hemiparasitic plants, opening opportunities for new species for move into the community.

The genus Comandra made up 13% of the total abundance of hemiparasitic plants in the study. Image Source: Wikipedia Creative Commons by Fritzflohrreynolds
Species Evenness and Biodiversity Conservation

For some herbaceous communities, hemiparasitic plants may play a real role in maintaining species evenness. Evenness is beneficial for biodiversity conservation; if many species are very rare (a low evenness community), they are at high risk of local extinction. Then, because this community would have only the few dominant species, it would have a decreased ability to adapt and persist when environmental conditions change. Therefore, a plant community with low evenness has less stable biodiversity than a more even plant community. Hemiparasitic plants, therefore, may be particularly valuable for the maintenance of biodiversity in some communities!

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Julia Bebout

Julia Bebout

I am a second year Master's student at the University of Calgary studying how competing species coexist. I graduated from Lehigh University in 2021 with degrees in Biological Sciences and Earth & Environmental Sciences. As an undergraduate, I studied paleoecology and the microbial ecology of alpine wetlands. I'm especially interested in community ecology, wetland and alpine ecosystems, and regenerative agriculture. I also love hiking, climbing, baking, and dancing! Twitter: @BeboutJulia

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