The Science of Conservation and the Removal of Invasive Mice on Midway Atoll

Researchers on Midway Atoll will give us more insight into the cascading ecosystem benefits of invasive mice removal through the study of mice poop.


Islands constitute only 5.3% of the Earth’s landmass, yet they are home to a disproportionate amount of biodiversity and endemic species. One such diverse group of animals is that of seabirds, which are found almost exclusively on islands. However, islands are also epicenters of extinction; nearly 2/3 of recent extinctions were island species. But what’s driving many of these extinctions?

Invasive species, including rats, mice, mongoose, and feral cats, just to name a few.

Invasive rodents are likely responsible for the greatest number of island bird extinctions as well as damaged ecosystems. Out on Midway, invasive house mice (Mus musculus) have started to attack and kill adult seabirds—consequently, Island Conservation and partners plan to remove invasive mice in 2020.

Invasive house mouse (Mus musculus) preying on Laysan Albatross (Phoebastria immutabilis) sitting on nest. Photo credit: USFWS.

The project sounds quite straightforward: mice attack and kill albatross; remove invasive mice; albatross flourish. So, why is there a research collaborative (including Island Conservation, the U.S. Fish and Wildlife Service, and Northern Illinois University) studying Midway’s mice? Once the mice are gone, there’s nothing left to worry about, right?

Mice are relatively small and cryptic—and because we wouldn’t expect such a small mammal to attack such enormous prey—they are a very understudied rodent. Predation of seabirds and other at-risk wildlife is a direct and alarming impact of mice, but there is more than meets the eye with these widespread rodents. Mice, directly and indirectly, affect islands by what they eat—seeds, plants, invertebrates. As opportunistic omnivores (a fancy way of saying not a picky eater), mice can substantially change island wildlife dynamics which can, in turn, can disrupt entire ecosystem processes. Without taking these impacts into consideration, the presence of mice on islands can hamper or even prevent successful restoration efforts.

On Midway, removing mice could greatly benefit restoration efforts much more efficiently than current labor-intensive restoration techniques (such as spraying weeds or hand-planting native plants). Combining research with eradication techniques on Midway presents an important case study for effective island restoration; the lessons we learn on Midway can help to restore islands all over the world. If we only focus on the physical, visceral impacts of mice (visible attacks on albatross), we’re missing the larger picture and broader implications of invasive mice impacts on Midway’s ecosystem.

As part of this project, we (Dr. Holly Jones Evidence-based Restoration Lab) are helping to coordinate ecological monitoring on Midway with Island Conservation and the U.S. Fish and Wildlife Service to understand the hidden impacts of mice on islands.

So, how do we figure out what mice are eating and affecting in Midway’s system? Poop and hair.

You’d be surprised by what scientists can learn from analyzing fecal matter and hair. Let’s start with poop. Fecal matter contains something called eDNA (short for environmental DNA), basically bits and pieces of DNA from food (or prey). eDNA can be found all over, in soil, seawater, snow, and even air. Using cutting-edge technology (specifically next-generation sequencing and eDNA metabarcoding), we can actually figure out all the different things that mice eat by finding and identifying the DNA of these diet components in their poop. First, scientists start with barcoding, whereby distinctive DNA sequences (called “barcodes”) are used like fingerprints to identify individual species. Next, we use metabarcoding, which is a complex technique to identify lots of different barcodes from lots of different organisms in a sample. In our case, our environmental DNA sample is mouse poop!

Processes involved in eDNA metabarcoding, including barcoding and metabarcoding. Photo credit: Woods Hole Oceanographic Institute

But, it’s one thing to know what mice eat—it’s another thing to know how much mice eat. Here’s where the hair comes into play, along with isotopes. Here’s a quick review on isotopes (don’t worry, this shouldn’t give you any bad flashbacks to high school chemistry labs): isotopes are atoms of the same element that have an equal number of protons and unequal number of neutrons, giving them slightly different weights. Isotopes can be divided into two categories—radioactive and stable. Isotopes are present everywhere in the world in which we live and breathe but the balance (or ratio) in which different isotopes of the same elements occur, vary between different substances (such as different types of food—plants, meat, and so forth) and ecosystems (between land, sea, or different climate zones). As organisms grow and develop, their tissues renew themselves—and the isotopes that are in the food that they eat are incorporated into all bodily tissues. By measuring the ratios of different isotopes in mouse hair, we can actually reconstruct the diets of mice. Using eDNA, we can figure out what mice eat; using stable isotopes, we can figure out how much of these diet items they eat.

Trophic levels are a defined by the position of organisms in a food chain or food web. Different trophic levels also have different ratios of stable isotopes, so diet “groups” like plants or insects can be easily differentiated from one another through stable isotope analysis. Photo credit: Fishionary

Still, you might wonder—why would we worry about what mice eat? Beyond mice, Midway is home to numerous invasive plant and insect pests, such as the once widespread golden crownbeard (Verbesina encelioides) and the Midway emerald beetle (Protaetia pryeri). Mice are known to eat a wide variety of plants, seeds, and insects; in turn, mice can suppress the populations of certain flora and fauna. Once mice are gone, this predation pressure disappears as well—and we could have another pest on our hands. So, by understanding mouse diet, we can predict what could happen to Midway after the mouse eradication, prepare, and ensure a robust and resilient island ecosystem into the future.

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But, we need your help to make Midway’s restoration a reality. Learn more at

Featured photo: Juvenile Bonin Petrel, Midway Atoll. Credit: Wes Jolley/Island Conservation

About Wieteke Holthuijzen

Wieteke Holthuijzen is a National Science Foundation graduate fellow at Northern Illinois University, where she studies the ecological impacts of introduced house mice on Midway through a collaborative research effort with Island Conservation and the U.S. Fish and Wildlife Service. Previously, Wieteke served as the Invasive Plant Control Specialist on Midway, helping to restore the atoll to a bustling seabird colony. She is intrigued by the nexus of nature and human presence and seeks to study and contribute to the conservation of imperiled species. In her spare time, she enjoys playing the cello, ukulele, banjo, and electric bass.

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