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Despite the negative connotation: parasites have a positive effect on biodiversity and play a crucial role in maintaining it

New research shines a positive light on biological stigma

Daphnias or not to be... a sick Daphnia (the red one) and a healthy Daphnia. Photo: Einav Marcus
Daphnias or not to be... a sick Daphnia (the red one) and a healthy Daphnia. Photo: Einav Marcus

When we think of parasites we usually have negative connotations towards them: how do they harm their host or how harmful are they to those who are destined to carry them? A new study by Tel Aviv University reveals that the presence of parasites in nature is not necessarily negative and sometimes they even help other animals to survive. 

The research was conducted under the leadership of Prof. Frieda Ben-Ami and Dr. Siegel Orlansky from the School of Zoology and the Steinhardt Museum of Nature at Tel Aviv University, and published in the journal Frontiers in Microbiology. "In the research we conducted, we show that parasites actually have a positive effect on the structure of society, and they play a key role in shaping the habitat and enhancing biodiversity," say the researchers.

A matter of balance

The researchers explain that in a healthy ecological society there is usually a wide variety of species living side by side. Related species will be able to exist in the same society provided that they influence and are influenced differently, by natural resources and enemies. Without separation and balance between those species, they will not have the possibility to coexist, that is, one of the species will become extinct. This principle is called competitive exclusion principle or Gause's law.

Dr. Siegel Orlansky: "Parasites and pathogens are an integral part of any ecological society. Despite their bad name, parasites play a key role in shaping population dynamics, community structure and biodiversity, thanks to their influence on the balance between the species in that society."

The research was conducted on tiny Daphnia type crabs, which in Israel can be found mainly in winter ponds. The size of the daphnia is about three millimeters and it feeds on single-celled algae and bacteria, which themselves serve as food for fish. Winter ponds are a closed habitat where the competition between the species is very significant in its effect on the biological diversity in the pond. Aquatic species that live in the winter pools cannot leave or migrate to another place independently, so the results of the competition are very critical to their survival. These species are also hosts or carriers of parasites and it is rare to find a species that is almost completely resistant to parasites.

Prof. Ben-Ami adds: "In the pond crayfish population in Israel we found one species named Daphnia similis whose nickname in the laboratory was "super Daphnia" due to its almost complete resistance to parasites. Despite all this, our "Super Daphnia" does not manage to be the most common Daphnia species in ponds. The common species is actually Daphnia magna, which is very vulnerable to a wide variety of parasites."


What would allow the coexistence of species?

To understand why immunological immunity is not a springboard for widespread distribution in the pond, the researchers set up a biological microcosm in the laboratory, where the two species shared the same habitat in the presence or absence of parasites. The results showed quite quickly that in a parasite-free habitat, the parasite-sensitive species, which is the most common Daphnia species in nature, is the one that won the competition and even caused the disappearance of the parasite-resistant "Super Daphnia". However, in habitats with parasites, the survival of the sensitive Daphnia decreased dramatically, and the "super Daphnia" population became established, which would allow coexistence between the two species.

Dr. Siegel Orlansky: "The results of these experiments emphasize the important role of parasites in shaping biodiversity, as those that can mediate competition between Daphnia species. This enables the coexistence of a species that is indeed resistant to parasites, but its ability to compete is lacking and without parasites it would probably become extinct when it shares the same habitat with a species sensitive to parasites, which is the most common Daphnia species in Israel, i.e., has a high competitive ability. Our research shows that the coexistence of these two Daphnia species is only possible through the mediation of a parasite."

Prof. Ben Ami summarizes and emphasizes the significant implications of the results of these experiments for a better understanding of systems in which species sensitive to parasites and less sensitive species co-exist. These consequences may affect dealing with biological invasions and even help reduce the threat to endangered species.

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