Do you ever wonder why the deer population in your town varies by year? Have you ever noticed that the number of rodents scurrying around spikes the year following a high yield of acorns? Seems confusing doesn’t it, so why do these numbers change?
The answer is simple. The natural world is maintained by a series of checks and balances. Predation, parasitoids, and parasites regulate populations and help maintain an ecosystem’s balance. To better explain how populations are regulated, I will clarify the terms predator, parasitoid and parasite. Too often, these terms are used interchangeably, yet they have entirely different meanings.
The world’s food web is influenced by numerous interactions between organisms, plants and dead organic material. Predation is when one organism attacks or preys on another. Carnivores play a big role in predation, because they eat the meat of other animals . There are even carnivorous plants like the venus flytrap that eat insects . Other interactions that affect the food web include herbivory, detritivory and omnivory. Herbivores, such as deer and goats consume plant material. In order to digest plant material, they have very long intestines and specific bacteria to break down the plant cell wall, also known as cellulose. Detritivores consume dead organic material and omnivores consume both plant and animal material. All of these different feeding strategies influence the community structure and food web of an ecosystem.
Despite other feeding strategies mentioned, our focus today is on predation. Predators eat other living things to acquire the energy they need to survive and reproduce. As a result, they influence their prey’s’ population size. For example, if mosquitoes didn’t have predators like birds, insects and small fish, their population would grow exponentially making them an even bigger nuisance to humans. Deer or elk populations can be controlled by wolves or coyotes in wooded areas and as a result, the amount of vegetation lost to the herbivores is controlled. If coyotes or wolves were removed from the system, the deer population would rise and the loss of vegetation would increase. Predation is an integral part of the food web and regulates ecosystem dynamics.
Fig. 1 An example of a top-down food chain in Yellowstone National Park. Can you see how the different organisms interact? What would you expect to happen if there were no more elk? (Source: Lessons From Natures Tangled Web)
Parasitoids, also known as ‘the little things that kill other little things’, reside in or near a host, and eventually kill and consume it. Common parasitoids are wasps, flies and beetles. They first interact with their host after the female oviposits her eggs near, on or inside the host larvae .
There are two different forms of parasitoid development. Idiobiont parasitoids usually develop outside of the host by sterilizing it and preventing further development. Koinobiont parasitoids wait longer to kill the host and develop alongside it. They will feed on the outside of the host during development. Koinobionts are endoparasitoids and develop inside the host via female egg oviposition. They also can be ectoparasitoids and develop outside of the host’s body either attached or detached to the host tissue .
The development of a parasitoid within another organism is a fascinating phenomenon. My research focuses on parasitoid diversity and parasitoid behavior, specifically parasitoids of gall wasps. The gall wasp I study, Zapatella davisae, creates cavities or small holes under the bark of oak tree twigs. Its parasitoids cannot create their own galleries, so females oviposit their eggs on the inside or outside of the gall wasp (the host) and the parasitoid larvae kills the gall wasp host and develops in the gallery instead. It has not been determined whether these parasitoids are idiobionts or koinobionts. To determine this, researchers must document their lifecycle through twig dissections under the microscope. These wasps are about 1-2 mm in length. In this system, these parasitoids are beneficial because they lower the population size of the gall wasp, which is killing the trees. The parasitoid eats the gall wasp, and leaves the tree alone. Therefore, the more parasitoids present, the less tree feeding and mortality occurs. This is an example of how parasitoids, no matter how big or small, can have an impact on population dynamics.
Fig. 2 A gall wasp, Zapatella davisae (host) (left) and two of its parasitoids (right). (Source: Matt Buffington, Smithsonian; Monica Davis, University of Massachusetts Amherst)
Parasitism occurs when one organism benefits at the others expense. Parasites cannot survive if they do not have their host to live on. Unlike parasitoids, parasites do not always kill their host. There are three common types of parasites: protozoa, ectoparasites, and helminths .
Protozoa are microscopic one celled organisms that have the capacity to multiply in humans.
Their high level of multiplication increases the spread of serious infections. Parasites can be spread through fecal matter and oral ingestion via water contamination or human contact. There are four groups of protozoa based on the way they move and features of the cell: amoebae, flagellates, ciliates and sporozoa . A common example of a protozoan parasite in humans is Blantidium coli, which causes an infection of the gastrointestinal tract. The protozoan flagellate Giardia intestinalis causes giardiasis and absorbs nutrients from the intestine in humans, birds and animals.
Ectoparasites are blood-sucking arthropods, such as mosquitoes, ticks, fleas, lice and mites that burrow into tissue. Ectoparasites are good vectors for different diseases. For example, mosquitos are a vector for malaria by transmitting the parasite Plasmodium to humans through their salivary glands when they suck human blood.
Helminths are large multicellular organisms that cannot multiply in humans, but they are oftentimes human parasites. Some common examples are tapeworm, thorny headed worm and roundworm. Certain helminths will remain in the gastrointestinal tract, while other are capable of spreading disease throughout the body.
Fig. 5 Cycle of transmission of helminths to humans. (Source: Center for Disease Control and Prevention )
So there you have it: predators, parasitoids, and parasites. I hope that you have learned that life isn’t always what it seems on the surface - there may be something lurking underneath and it could be good, bad or just plain ugly. But no matter what it is, it has a place in the food web and the natural cycle of life.
 Hillis, David, David Sadava, Craig Heller, Mary Price. The Principles of Life. Sinauer Associates: Sunderland, 2010.
 Botanical Society of America. “Carnivorous Plants/Insectivorous Plants.” Last modified May 2016. http://botany.org/Carnivorous_Plants/
 Hoffmeister, T. Factors determining the structure and diversity of parasitoid complexes in tephritid fruit flies. Oecologia 89(1992): 288-297.
 Hochberg, M.E. and A.R. Ives. Parasitoid Population Biology (New Jersey: Princeton University Press, 2000), 139-149.
 Center for Disease Control and Prevention. “About Parasites”. Last modified March 5, 2014. http://www.cdc.gov/parasites/about.html
 Australian Society of Parasitology Inc. “Protozoan Parasites.” Last modified July 2010. http://parasite.org.au/para-site/contents/protozoa-intoduction.html
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