Earthshots: Satellite Images of Environmental Change

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» Knife River Delta, Canada

These images show devegetated shoreline along Hudson Bay in central Canada, eaten bare by growing numbers of snow geese. In the Landsat images, red signifies vegetation. Hudson Bay appears black in the upper-right of the images, and north of the Knife River Delta is a bright strip of land eaten bare by the geese.

These Mid-Continent snow geese live in coastal marshes along the Gulf of Mexico in winter and along Hudson Bay in summer. They spend almost all spring and fall migrating back and forth. Their traditional winter diet is marsh hay cordgrass, saltgrass, bulrush, and other marsh plants. Snow geese are not only grazers but also grubbers, digging up even the underground parts of plants with their strong bills.

In the spring, the snow geese fly north along well-defined paths through the prairies, stopping often to eat along the way. They linger on the prairies of the northern United States and southern Canada until about early May, then fly nonstop across the northern forests to coastal areas around Hudson Bay and farther north.

Once at the nesting ground, they start breeding. The northern summer is just long enough for goslings to hatch and learn to fly. Female snow geese have strong homing habits, returning to the spot where they were raised and even using their previous nest sites. Snow geese form large colonies of a few hundred to over 100,000 birds. Nearby La Perouse Bay (marked on this map) has the best-known colony in the world, studied by scientists since 1968.

Up in the tundra, summer is short and the soil is shallow and dry. Nesting snow geese seek out the lusher "oases" in this arctic "desert," narrow productive strips of a few hundred yards along the Bay and along some inland lakes. But even here food is sparse, since the geese leave just as the growing season is ending, with no time for the plants to recover before the next spring.

So even in a good summer, the females rely on stored energy in their bodies to produce and incubate their eggs. To produce a large clutch of eggs, they need to arrive at Hudson Bay fat and healthy. So traditionally, the amount of food available during winter and spring acted as one of the limits on the snow goose population.

European settlement brought good times for snow geese, as the Louisiana-Manitoba corridor became one long buffet line for them. By the mid-1900s degradation and draining of the Gulf Coast marshes pushed many snow geese inland, where they learned to feed on the stubble and seeds of wheat, corn, and especially rice fields. There were rice farmers in Texas who suddenly went from having no geese to flocks of thousands in the late 1940s. As Midwestern agriculture intensified, some birds stopped going to the coast, instead wintering as far north as Iowa.

This southern "gravy train" improved the snow geese's winter survival rates and springtime weight gain. Fatter, more numerous females then produced more goslings. The young flying south for the first time found more food along the way. Flocks inevitably grew; in 25 years the Mid-Continent population increased from about 2 million to 5 million birds. The colony at La Perouse Bay grew from 2,000 nesting pairs in 1968 to 22,500 in 1990.

Map of the featured area.

All these snow geese overgrazed their nesting grounds. Many of these shoreline "oases" were transformed to mudflats. Comparison of the 1973 and 1996 Landsat images shows the area of bright, bare shoreline spreading inland into the vegetation, although the tidal change makes it hard to tell how much.

To make the changes easier to see, we simplified the Landsat images into one-dimensional "vegetation index" images, showing how much plant life there was in 1973 and 1996. Photosynthesizing plants reflect more infrared energy than visible energy; these images show that difference on a scale of –100 to +100. Green means growing plants, yellow means bare ground, and red means water (though we blacked out Hudson Bay and its inlets). Then, to make the changes even easier to see, we combined the vegetation images into a single change-image, in which green shows where vegetation increased from 1973 to 1996 and red shows where it decreased. There is a definite red swath north of the delta, inland from the beach, which indicates a sharp decrease in vegetation. The beach itself, like most of the image, shows a light yellow-green color, suggesting a slight increase in vegetative growth. This small change could be from an actual increase in vegetation, or just from sensor "noise."

Scientists evaluating 1,200 miles of shoreline habitat along Hudson Bay said about a third was severely damaged and another third destroyed. Once the soil is bare the surface temperature increases, which increases evaporation, which leaves behind an accumulation of natural salts on the surface. This salty layer inhibits the recovery of plants. Erosion also damages the thin soil. At nearby La Perouse Bay, scientists built a pen around some bared ground to keep the birds out, and after 12 years there was only 5% regrowth.

Despite the loss of forage, the geese keep homing back to the same area, and they keep reproducing using energy gained during migration. The goslings are not so lucky. Many snow goose families at La Perouse Bay walk up to 30 miles to find food, and only 10% of the goslings survive. But adults commonly live and reproduce into their teens, so the population keeps growing.

Destroyed habitat could make the aged snow goose population collapse. Already, less-numerous species have been affected; at La Perouse Bay, the numbers of American wigeons, northern shovelers, yellow rails, stilt sandpipers, Hudsonian godwits, and short-billed dowitchers have fallen 90 percent since 1980. Diseases could also sweep through the crowded snow geese, and spread to other bird species. Even if the snow geese don't collapse, the population could sink to a low level of health and productivity.

This is a new situation for wildlife managers, who worked for many years to increase population numbers. Many managers want more hunting along the migration flyways. Controlling the food supply is impossible, and reducing habitat on public lands could harm other species. It is also hard to use the nesting geese and eggs for food, since geese and people there live far apart. Government programs would be expensive to set up, and hunters already kill about a half million snow geese annually, which accounts for about two-thirds of adult snow goose mortality.

Managers have proposed removing various restrictions on hunting and even subsidies or awards for hunting. Some believe that increased hunting is "too little, too late" to prevent destruction of the habitat and collapse of the population.


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