Israeli Researchers Show Parasites Have Benefits

Tel Aviv University researchers have shown that parasites can actually be a benefit to many different forms of life in nature. This is in spite of their generally negative image as destructive. Parasites, say the scientists, can have a positive effect on biodiversity and a crucial role in maintaining it.

There is a story in the Bible about how Kind David’s life was saved when King Saul chased after him, wanting to kill David. David hid in a cave and a spider miraculously spun a big web really fast over its entrance. So, when Saul passed by that cave he thought no one could be in it and David’s life was spared.

You see, before that day King David once asked God why he created spiders, questioning their value in the world. And so, God sent a spider to save David’s life.

And spiders have an important function in the ecosystem. They prey on all manner of bugs and other pests that could be harmful. And then there is bacteria. People know how it can kill. But the human body hosts millions of bacteria at all times that help it function, such as with the digestive process.

So, this is the same when it comes to parasites.

The research was conducted under the leadership of Prof. Frida Ben-Ami and Dr. Sigal Orlansky from the School of Zoology and the Steinhardt Museum of Natural History, Tel Aviv University. The study was published in the journal Frontiers in Microbiology.
The researchers explain that in a healthy ecosystem, there is usually a wide variety of species living side by side. Related species are able to exist in the same habitat provided that they influence and are influenced differently by natural resources and predators. Without proper separation and balance between those species, they cannot coexist – one of the species will be driven to extinction by the other. This principle is termed the ‘competitive exclusion principle’, also known as Gause’s law.

Dr. Sigal Orlansky said, “Parasites and pathogens are an integral part of any ecosystem. Despite their bad reputation, 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 ecosystem.”

The research was conducted on tiny Daphnia water fleas, which in Israel can be found mainly in winter ponds. Daphnia fleas are about three millimeters in length, feed on single-celled algae and bacteria and serve as food for fish. Since winter pools are closed habitats, the competition between different species is particularly significant in its effect on the biological diversity in the pool. Aquatic species that live in the winter ponds cannot leave or migrate elsewhere independently, so the results of the competition are crucial to their survival. Like most animals, these species are also hosts or carriers of parasites and it is rare to find a species that is almost completely resistant to parasites.

According to Prof. Ben-Ami, “In the population of water fleas in Israel, we found one species called Daphnia similis, whose nickname in the laboratory is “Super Daphnia” due to its almost complete resistance to parasites. Nevertheless, this “Super Daphnia” does not become the dominant Daphnia species in ponds. The most common species is actually Daphnia magna, which is highly vulnerable to a wide variety of parasites.”

The scientists also said the results of their experiments emphasize the important role of parasites in shaping biodiversity, as the parasites can mediate competition between Daphnia species. This competition, they explained, enables the coexistence of a species that is indeed resistant to parasites, but its ability to compete is lacking and without parasites would probably become extinct when it shares the same habitat with the Daphnia species most common in Israel, the Daphnia magna, which while sensitive to parasites is otherwise strongly competitive.