How to Study Invasive Species from Space

September 7, 2020September 7, 2020 Christina M. Marvin 0 Comments invasive species, satellite data

Original research article: Detecting Invasive Amur Honeysuckle in Urban Green Spaces of Cincinnati, Ohio Using Landsat-8 NDVI Difference Images.http://dx.doi.org/10.3368/er.38.3.139

Satellites transmit information to Earth, allowing researchers to study ecological variations over wide areas.

One way to study the Earth is from the ground. Another way is to study it from space. Bridget Taylor and colleagues found that they could monitor an invasive species using satellites, allowing them new means to better study these species and protect native environments. It could take months or years to monitor a large area for invasive species. Satellites would greatly speed up these processes. Early detection of invasive species can help stop their spread and further damage to native species.

Invasive Species

An invasive species is any plant, animal, or other organism that is not native to an ecosystem. Invasive species most often harm new environments by out competing native species for habitats and resources. According to the National Wildlife Federation, approximately 42 percent of threatened or endangered species are at risk because of invasive species. Invasive species also threaten human communities that rely on natural ecosystems for their health and economy.

Invasive species are usually not inherently “bad” in their native environment but become a problem for new areas. For example, take lake trout. Lake trout are native to the Great Lakes in the United States and necessary for the local ecosystem and human economy. However, these same fish are invasive in Yellowstone Lake in Wyoming because they compete with native cutthroat trout for habitat. The cutthroat throat that the Yellowstone Lake community relies on is becoming threatened by this new species.

A map of the United States showing Lake Trout in their native Great Lake habitat. When this species moves to Yellowstone Lake in Wyoming, they start competing with the native cutthroat trout in the area. — Lake Trout are beneficial in their native Great Lakes habitats. However, when introduced to Yellowstone Lake in Wyoming they become invasive intruders that compete with native trout for valuable resources. Image source: Wikipedia (Lake and Cuttthroat Trout) and Pixabay.com.

Invasive species are most often spread by humans, often unintentionally. Some people grow exotic plants, which then get carried to natural habitats. Seeds, insects, and small organisms are all carried by ships, pets, and even on the bottom of your shoes! To help prevent invasive species from coming with you, there are a few steps you can take. Plant native plants and remove any invasive plants in your garden. Also, clean boots and outdoor equipment often. And when camping, purchase firewood close (30 miles) from your campsites.

When invasive species gain hold, they are often hard to stop. Take for example, the Amur Honeysuckle in Cincinnati, OH.

The Problem with Honeysuckle

Published in the journal Ecological Restoration, researchers led by Bridget Taylor identified invasive Amur honeysuckle, an ornamental shrub introduced from Asia that has spread in forests across much of the United States. Amur honeysuckle takes land away from native plants in the hardwood forests of the Eatern and Midwestern United States. Its vigorous growth often outcompetes native plants and can reduce the growth rate of native trees by up to 50 percent.

An image of the Amur Honeysuckle plant. — Amur Honeysuckle. Image source: Wikipedia.

The scientists hoped to use Amur honeysuckle as a model species to show that satellite imagery could accurately identify invasive species from native ones. They focused on identification in urban green spaces in Cincinnati, OH. For this study, the researchers partnered with NASA and the U.S. Geological Survey to conduct the necessary satellite missions. Although NASA has partnered with the National Invasive Species Council (NISC) since 2005 to combat invasive species across the United States, the focus on an urban area is unique.

Contrary to what some might think, urban green spaces support wildlife and provide important ecological services such as air filtration. Urban landscapes tend to have a lot of noise that interferes with satellites, so the process used by these researchers is one that can better open the field to more studies in areas where many people live.

Viewing Cincinnati

The study areas consisted of five urban green spaces in Cincinnati. These locations were chosen, in part, for their variability in expected Amur Honeysuckle growth. Thanks to volunteers, some areas were expected to have little to no growth left:

Bender Mountain Nature Preserve (mostly eradicated)
Buttercup Valley Nature Preserve (mostly eradicated)
Parkers Woods (No eradication)
Avon Woods Nature Preserve (some eradication)
An undeveloped wooded section of Spring Grove Cemetery (some eradication)

Measuring “Greenness”

Researchers used a satellite called Landsat-8 to measure the “greenness” of the area. Green is a primary color that absorbs red light and slightly reflects near infrared light (NIR). The satellites used the ratio of red light to NIR to identify the prevalence of the invasive plant, the ratio of which will differ from native species.

A green leaf absorbing red light and reflecting near IR light. — Green leaves absorb red light and reflect NIR light. By measuring these effects from space, satellites can measure the “greenness” of an area by calculating red light/NIR light. Image source: Pixy.com

The researchers used two time points: November and January 2015. In November, the invasive species was green while native woody vegetation was dormant. In January, both plants were dormant. Previous research had suggested that an anomaly image (subtraction of an image from a reference point) more accurately predicts Amur honeysuckle than a single data point.

Therefore, they subtracted the January 2015 image from the November 2015 image to produce a difference image showing where vegetation greenness had changed the most between November and January.

Researchers then went out with GPS coordinates to see if there were correlations between the “greenness” values and plant species. They found that image differences could distinguish between invasive and native species. Specifically, they classified images with difference values greater than 0.035 as invasive species present and images with values less than 0.035 as invasive species absent.

The researchers are quick to point out that the 0.035 threshold is not perfect. More research with larger and more varied datasets is needed to determine how well this calculation can hold in a general sense. Other variables affecting the data include native vegetation that remains green year round, grass, and the presence of clouds on any given day. This approach works best in heavily invaded areas. Better resolution will be needed to detect smaller invasions.

The Future of Space View

Researchers hope this study will be of use to ecological restoration land managers as they plan restoration projects in urban areas. Satellites offer a large-scale alternative to reporting on the presence of invasive species from the ground, which can take a lot of time and work hours depending on the size of the area. However, it is usually difficult to use satellites in urban areas due to interference noise. This research group was able to develop a method using color ratios to accurately describe the presence of a heavily invasive shrub. They hope that with further study, this tool can be applied to less dense species as well. The tools are also accessible to other scientists, allowing more research groups to investigate other potentially pesky species early, before they become an ecological burden.

Volunteers taking care of natural resources in Michigan. Image source: Govdeliver.com.

Reviewed by:

Connor Rosenblatt