Scientists have been struggling for years to get the public to accept that predators are important to ecosystems. The public has seen initiatives such as the reintroduction to wolves into Yellowstone park working, and have slowly grown to accept that predators are important to maintain a healthy ecosystem.
Our next hurdle may be harder to surmount; Parasites are just as important to ecosystems, and we should love them. Okay… maybe not the second part. A parasite is loosely defined as an organism which lives in or on another organism, and causes harm to that organism.
We understand that if no predators exist in an environment, the prey population will grow out of control and eat almost everything. Eventually the prey will starve until balance is restored. In the opposite scenario, if the predator population rises, they will deplete their prey’s population, and eventually starve until balance is restored. Sometimes an organism is so dominant that they out-compete other organisms within their ecosystem.
Is there anything that can help prevent these situations? Parasites.
How parasites mediate competition between organisms
When the number of any individual organism increases, their parasites will also increase in prevalence. This is due to the increased contact that these organisms will have with each other, allowing transfer of parasites from one host to another, or even the introduction of parasites to a new area. These parasites may affect the organisms’ fitness (ability to mate and produce offspring), slowing their population explosion of either predators or prey.
Cordyceps is a genus of fungus that primarily infects insects. These fungi often live in high humidity areas. Spores from these fungi are released from dead hosts, infecting new insect hosts. These fungi are highly specific, each subspecies of fungus specialises in infecting one species of insect. This is because of how complex the mind-manipulation this fungus exerts on its host. In the case of Ophiocordyceps unilateralis, which infects ants. The fungus forces the ant to climb a plant high, bite into the plant, die, and produces a fruiting body from the host cadaver which will infect many other ants. In the case of Ophiocordyceps sinensis, which affects caterpillars at high altitudes. This parasite eventually drives the caterpillar up to the surface of the soil before killing its host. The fungus then develops its fruiting body, which infects other caterpillars. These 2 life cycles are so specific and specialised, O. unilateralis would be unable to effectively infect caterpillars, and O. sinensis so wouldn’t be able to effectively infect ants.
There are over 400 of these fungal parasites, most of which parasitise insects. Because of the specificity of these fungi to their host insects, the increased prevalence of any one insect, would inevitably increase the abundance of its associated fungal parasite, which would then drastically reduce the population of the host. In this way, no one species of insect can gain dominance over any other. This helps maintain proper ecosystem functioning and biodiversity.
Parasites can also prevent a species from dominating a certain niche, allowing for more biodiversity. In the case of Anolis lizards living in small Caribbean islands, a malarial parasite mediates the competition between two subspecies.
A. gingivinus is the dominant species, and outcompetes A. wattsi (Schall et al., 1992). The malarial parasite, Plasmodium azurophilum infects A. gingivinus but rarely infects A. wattsi. In areas where this parasite is absent, A. gingivinus is the only lizard present. Where the parasite is present, both A. gingivinus and A. wattsi can be found. The parasite reduces the fitness of A. gingivinus, enabling the less competitive A. wattsi to gain a foothold, and therefore maintaining biodiversity.
The importance of parasites is also vital for with organic farming. Without pesticides, we rely on nature to kill insect pests which may otherwise eat our crops. The aphid is a devastating pest, which can eat tonnes of our food every year, and spread plant diseases like mosquitoes spread them to humans. Even without considering the impact of spreading viruses like BYDV (Barley Yellow Dwarf Virus), aphids can reduce crop yields of cereal plants by ~10%. Parasitoid wasps, and fungal parasites are some of the most effective natural enemies of these pests.
The importance of parasites in controlling pests was highlighted in 2010, when there was an outbreak of mealy bugs (Phenacoccus manihoti) which, similar to aphids, attack and harm crops. These pests are native to South America. When they were accidentally introduced to Thailand, they multiplied out of control without their native parasites and predators to keep their population in check.
In a last ditch effort, farmers imported over 3 million parasitoid wasps (Anagyrus lopezi), to help control the mealy bug population. These wasps use their stingers to lay eggs inside the body of an insect host. These eggs will hatch, and eat the insect from the inside out, killing the insect. These wasps are highly specific for their mealy bug hosts, and therefore are unlikely to harm native insects.
Okay yes, they are gross, and they can be terrifying, but you have to admit that parasites have an important role in the environment. Parasites can prevent one species from dominating an ecosystem, can maintain biodiversity, and can help rebalance an ecosystem once an invading species gets out of control.
Peglar, T. (2019) 1995 Reintroduction of Wolves in Yellowstone. Yellowstone Park website. Available at: https://www.yellowstonepark.com/park/yellowstone-wolves-reintroduction
Coutts, B., Micic, S. (2018) Aphid feeding damage to cereal crops. Government of Western Australia website. Available at: https://www.agric.wa.gov.au/barley/aphid-feeding-damage-cereal-crops
JJ, Schall, J. J. (1992) Parasite-mediated competition in Anolis lizards. Oecologia, 92(1), 58-64.
Than, K. (2010) Parasitic Wasp Swarm Unleashed to Fight Pests. National Geographic. Available at: https://www.nationalgeographic.com/news/2010/7/100719-parasites-wasps-bugs-cassava-thailand-science-environment/