Less equals more: When does mortality lead to larger populations?

Featured Scientist: Zoey Neale (she/her/hers), M.S. 2015, Illinois State University. PhD anticipated Spring 2023, Department of Biosciences, Rice University.

Birthplace: Richmond, Virginia

My Research: My current research looks at how climate change influences the effects of predators on their prey.

Research Goals: I hope to continue to study how climate change and human activities change how species interact in nature.

Career Goals: I want to be a professor at a research university.

Hobbies: Guitar, hiking and backpacking, running, video games

Favorite Thing About Science: I love being on the front lines of expanding human knowledge about the world around us.

My Team: I conducted this research at Illinois State University in Dr. Steve Juliano’s lab. Dr. Juliano advised me in the design and implementation of this project. I wrote the publication with edits from Dr. Juliano. My lab mates in the Juliano lab included Geoff Ower, Kate Evans, and Kris McIntire. I also had assistance from several undergraduates: Keenan Longan, Amy Gensler, and Kaitlyn Frederick.

Organism of Study: Mosquitoes

Several mosquito larvae submerged in water. They have their heads down and their bottoms up so that the siphon can stick out of the top of the water.
Several mosquito larvae submerged in water. They have their heads down and their bottoms up so that the siphon can stick out of the top of the water. Mosquito larvae use their siphons to breathe. Image source: Dr. Steve Juliano Lab, Illinois State University.
A close-up of mosquito perched on a person’s skin. It has a red abdomen.
A close-up of an Aedes albopictus, also known as the “Asian tiger mosquito.” One of the four mosquito species we tested in this study. The mosquito’s abdomen is red because it is filled with blood. Image source: James Gathany, USCDCP on Pixnio.

Field of Study: Population Ecology

What is Population Ecology? A population is a group of organisms of the same species that live in the same area and mate with each other. Population ecology is the study of how the size of populations change over time. The size of a population can change when individuals of that population are born and when they die. Population size can also change when individuals leave or enter into the population.

Check Out My Original Paper: “Finding the sweet spot: What levels of larval mortality lead to compensation or overcompensation in adult production?”

A QR code that links to Joey's published paper.
QR code to the original publication

Citation: Neale, J. T., and Juliano, S. A. 2019. Finding the sweet spot: What levels of larval mortality lead to compensation or overcompensation in adult production? (doi: 10:9. 10.1002/ecs2.2855)

Research At A Glance: In nature, organisms are often killed by outside sources such as predators, diseases, extreme weather, and human activity. When outside sources kill organisms, it is called extrinsic mortality. While this type of death may at first lower the size of a population, killing individuals can sometimes lead to increases in the population size over time. This can occur when food is scarce, and many individuals are already at a risk of starvation. The extrinsic mortality kills individuals that would have starved to death anyway. When these individuals are removed from the population, there is now more food for the other members of the population. This can lead to an increase in the population size because these other members are now able to get the food that they need to survive. 

The number of individuals that are removed from a population should determine if the population will grow. Our goal was to find how many mosquitoes must be removed from a population for it to grow. We also wanted to know if the species of mosquito made a difference. To test this, we raised four different species of mosquito larvae in small containers. We then removed individuals from the population to imitate the extrinsic mortality that mosquitoes experience in nature. We either removed the larvae from their containers by hand or added predators that would eat individuals in the population. We removed between 0% and 70% of the larvae from each container to see what percentage of extrinsic mortality would increase the population size. Afterward, we counted the number of adult mosquitoes in each container to find the final population size. We found that how mosquitoes respond to extrinsic mortality depended on the species. We also found that species that can live on less food are less likely to experience population growth in response to extrinsic mortality. Even so, the population size grew for three of the four species that we tested. For two species, high and low mortality kept the population small, but intermediate mortality caused the population to grow. For one species, the population grew no matter how many individuals were removed.

Highlights: In our research, we studied four species of mosquito: the Asian tiger mosquito (Aedes albopictus), the yellow fever mosquito (Aedes aegypti), the common house mosquito (Culex pipiens), and the eastern tree hole mosquito (Aedes triseriatus). One of the most important results from this study came from the species, A. triseriatus.  A. triseriatus mosquitoes can be found in the southern part of the United States. When we removed 70% of the A. triseriatus larvae from the container, the population grew (Figure 1).

Each of the dots in Figure 1 represent one replicate from the experiment. The experiment was done several times in different containers to make sure that we consistently got the same results. This is called replication. Each container is a replicate. The lines show the relationship between the percent mortality and the number of survivors when you take all of the dots into account. When 0-40% of the population was removed by predators, the final population grew from 5-45 survivors (shown in orange). When 0-70% of the population was removed by hand, the final population grew from 0-45 survivors (shown in blue). Overall, the figure shows that removing individuals increased the population size.

A data figure that shows the number of A. triseriatus mosquitos that survived the experiment. In the experiment, more mosquitos were removed by hand than when predators were added into the container. Regardless, this figure shows that removing individuals increased the population size.
Figure 1. The number of adult A. triseriatus mosquitoes that survived our extrinsic mortality experiment. The y-axis shows the number of adult mosquitoes that survived the experiment, or the final population size. The x-axis shows the percentage of mosquitoes removed from the container. Larvae were removed by hand (blue) or were eaten by predators (orange). Adapted from Neale et al. 2019.

These results are surprising because we would expect that removing a large number of individuals would cause the population size to decrease. The fact that we can remove 70% of the population and still see an increase is very surprising. When it comes to pest control, this means you could kill up to 70% of a pest population and end up with an even larger population than if you had done nothing at all!

What My Science Looks Like: In this experiment, we mimicked extrinsic mortality by introducing predators called Copepods to our containers. Copepods are tiny freshwater animals related to shrimps and crabs (see the image below).

A picture of a copepod on a white background. It has an oval-shaped, translucent body and what appears to be very large antennae. Its tail is bifurcated and about half the length of its body.
A microscopic view of a copepod predator. Copepods are approximately 1 millimeter long, and only hunt very small mosquito larvae. Image source: Don Loari on iNaturalist.

The Big Picture: Mosquitoes have been linked to several human and animal diseases. Mosquito-borne diseases such as the Zika virus, West Nile virus, and malaria are spread when someone is bitten by an infected mosquito. As a result, government agencies often invest in mosquito control techniques to lower the size of mosquito populations. Many types of mosquito control techniques target mosquito larvae.  People will apply insecticides to standing water to kill mosquito larvae, before they can become adults. Our results show why this might be a problem. Our study suggests that removing individuals from the population can increase the population size, depending on the species of mosquito and the number of larvae removed. Our study can be used to better inform mosquito control practices and avoid those that can increase in mosquito populations.

Decoding the Language:

Copepods: Copepods are small animals that live in water and are related to shrimp and crabs. They are predators of other small animals, such as young mosquito larvae.

Extrinsic mortality: Extrinsic mortality is an outside source of death that affects a population. The source of death does not come from the population itself. Deaths that are caused by predators, diseases, fires, and human activities, are all examples of extrinsic mortality. If an organism starves because it is competing for food with other members of the same species, then that is not a death caused by extrinsic mortality.

Insecticides: An insecticide is a substance that can kill insects and is often used in mosquito control. Examples include pyrethroids to kill adult mosquitoes, and BTi and pyriproxyfen to kill larvae.

Larvae: Larvae is plural for the word larva. A larva is the immature form of an insect. For example, a caterpillar is the larva form of a butterfly. Mosquito larvae live in water.

Population: A group of organisms of the same species that are close enough to one another to mate.

Predator: A predator is an animal that lives by killing and eating other animals.

Prey: An animal that is killed and consumed by a predator

Replicate: In experiments, replicates are used to make sure that the results of the study are consistent. The researcher will repeat the experiment several times on a small scale to make sure that they are consistently getting the same results. In the context of this research, the replicate was the individual container. For example, five containers may have had 20% of the mosquitoes removed to consistently measure how the population size changed in response to 20% mortality.

Siphons: A siphon is a respiratory organ that helps insects breathe underwater. In mosquito larvae, it is like a snorkel attached to their rear ends and serves as a breathing tube. The larvae will rest near the surface of water with their snorkels sticking out to breathe in air.

Learn More:

Juliano lab at Illinois State University, School of Biological Sciences, where I conducted this research for my master’s degree

Rudolf lab at Rice University, where I am currently working on my PhD

Centers for Disease Control (CDC) information on mosquito-borne diseases

The American Mosquito Control Association (AMCA) information on mosquito control

Other scientific papers on this subject:

Ower, G.D., S.A. Juliano. 2019. Effects of larval density on a natural population of Culex restuans (Diptera: Culicidae):  No evidence of compensatory mortality.  Ecological Entomology 44:197-205. (doi: 10.1111/een.12689)

McIntire, KM, SA Juliano.  2018. How can mortality increase population size? A test of two mechanistic hypotheses. Ecology 99:1660–1670. (doi: 10.1002/ecy.2375)

Synopsis edited by Kate Evans, PhD (Anticipated May 2024), Illinois State University, School of Biological Sciences and Rosario Marroquin-Flores, PhD (Anticipated August 2022), Illinois State University, School of Biological Sciences.

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