The patches of red in these false-color Landsat images are the forests where monarch butterflies spend the winter. Starting in late summer and fall, monarchs in the United States and Canada migrate south to Mexico. Some travel up to 3,000 miles. The delicate insects are capable of flying 50–100 miles a day.
Cold weather drives the monarchs to head south to hibernate for the winter. They head for the only habitat suitable for their hibernation, oyamel fir (Abies religiosa) forests. These forests grow in only small areas of mountain tops in central Mexico, about 3,000 m above sea level. The monarchs usually cover whole trees as they keep each other warm. The short needles of the oyamel fir allow the butterflies to cluster together better than they could on flat-needled cedars or long-needled pines.
The oyamel forests provide a microclimate for the butterflies. Temperatures stay above freezing. If the temperatures were lower, the monarchs would have to use their fat reserves. The humidity provided by the forest also keeps them from drying out.
The monarchs stay in Mexico from about November to March. In the spring, they fly back north. On the way, they lay eggs on milkweed. These eggs hatch into caterpillars, who devour the milkweed leaves, then metamorphose into monarchs. These monarchs live about 5 weeks or so.
What’s especially amazing about the monarch migration is that not all monarchs take part in the journey. And unlike birds and whales, the monarchs that do complete the migration only make one round trip.
The great-grandchildren of the overwintering monarchs migrate south the next fall, somehow landing at the same trees as their ancestors. Without the use of maps or satellite imagery!
The monarch migration is an endangered phenomenon. Logging in the overwintering sites in Mexico reduces the area available for the monarchs. The Monarch Butterfly Biosphere Reserve, established in 1986, is intended to protect the overwintering areas. But studies have shown that deforestation has occurred within the reserve. Some of that deforestation can be tracked from space with Landsat imagery.
According to Monarch Watch, during the 2012–2013 season, monarch colonies occupied the lowest number of hectares of forest in the previous 20 years. But the 2013–2014 season was even lower. Recent seasons have seen a slight increase in area of forest occupied, but monarch populations this low are extremely vulnerable. Just one winter storm could severely decrease their numbers.
Landsat sensors use infrared reflectance. One of the wavelengths of light they use is called near-infrared. Actively growing vegetation reflects this wavelength, so when this wavelength is assigned the visible color red, vegetation is displayed as red in these false color images. As you take a closer look at the zoomed in areas in the other sections, watch for gray patches in areas that were once red. This indicates a degraded forest.
Not all of the reduction in the monarch population can be attributed to oyamel forest loss in Mexico. In the United States, expansion of agriculture and herbicide use are reducing the milkweed that the caterpillars need. Extreme weather conditions have also been harming the monarchs. Satellite imagery continues to be useful in tracking the monarch population in these overwintering sites and can help recover this remarkable migration.
Researchers from the University of Maryland and USGS have mapped changes in forests globally from 2000 to 2013. The maps displayed here are from that database and are paired with a corresponding Landsat image.
The study defined trees as all vegetation taller than 5 meters. In the forest change map, green indicates forest extent, and red is forest loss during the period 2000–2013. Any blue that appears is forest gained during that time period. Black is land that was non-forest during the time period.
The study used data from over 650,000 Landsat scenes and used 143 billion 30-meter Landsat pixels.
These close-up images show a portion of the reserve. The red spots in the global forest map indicate forest loss. The area in the center of the image is in a location called Lomas de Aparicio, which is within the core zone of the reserve. The forest loss here has been identified as large-scale logging activity by Vidal and others (2013) and by Google Earth imagery from 2004 and 2013.
All of these images clearly demonstrate that the forest was heavily impacted between 2004 and 2013. It is unlikely monarchs will form overwintering colonies at the Lomas de Aparicio site in future years. If they do, they will face much greater environmental risks during their overwintering stay.
Even though the monarchs use only a small portion of this oyamel forest habitat, they depend on the entire dense forest structure. The forest canopy acts as a blanket and keeps them warmer and captures moisture that they need for their survival. A fragmented forest will not do this.
Subtle changes in vegetation are sometimes difficult for us to see when just looking at the imagery. The Normalized Difference Vegetation Index (NDVI) uses the difference in Landsat’s red and near-infrared reflectivity to assess vegetation health.
The graph below shows how NDVI significantly decreases in two areas of disturbed vegetation (red symbols) relative to undisturbed sites (green symbols) within the oyamel forest. The site locations are shown on high-resolution imagery in the second figure. The NDVI data can help determine if a given area is suitable to monarch overwintering.
NDVI analysis done by Birgit Peterson, scientist with ASRC Federal InuTeq, contractor to the USGS EROS Center.
These close-ups show another core zone area of the reserve called Pelon. Forest disturbance is most visible in the 2000 image. Later Landsat images reveal some red filling in in this area. In the Landsat imagery, red indicates any actively growing vegetation, so it may not be the tall trees of oyamel forest recovering yet. The Global Forest Change data do reveal some blue pixels in the Pelon site, so there is potentially some recent recovery occurring.
The northern side of this site had been stripped of several historical colony sites by the time of the 2000 Landsat image. The Global Forest Change data don’t show some of this area as forest loss because that study uses Landsat data from 2000 to 2013, so the areas that are black were already non-forest by 2000.
The good news about the monarchs is that they are a beloved bug. Kids in classrooms nationwide capture the caterpillars and watch them transform into a chrysalis and eventually a monarch, learning all about life cycles along the way.
Increases in the area occupied at the overwintering sites are encouraging, but researchers continue to keep an eye on breeding habitats—milkweed restoration from Texas to Minnesota is key.
A recent study indicates that people would be willing to donate or buy nectar plants and milkweed to help preserve the monarchs’ habitat in their non-wintering locations. The willingness of Americans to buy nectar plants and milkweed could potentially be a multimillion dollar industry. Diffendorfer and others (2014) estimated that Americans are collectively willing to pay $933 million for nectar plants and $473 million for milkweed. That seems like a good start for the monarchs on their long journeys.
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