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.
Traditional snow-goose life
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.1
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 U.S. and southern Canada until about early May, then fly nonstop across the northern forests to coastal areas around Hudson Bay and farther north.2
Once at the nesting ground, they start breeding as soon as possible. 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. Unlike other geese which nest far apart, 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 1968.3
Once the young are hatched, dried, and walking, the family leaves the nest area to find food. At this time of year the adults are growing new feathers, so the whole family walks. This is the only time the geese can be easily caught. When fall comes, the snow geese fly south to their winter grounds again.
Two facts are important for how many goslings hatch and survive:
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.5
Landscape change in the south
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. One study of spring migration noted that "[n]atural foods were largely unused by geese until they reached the marshes of Hudson Bay, when consumption of green vegetation resumed." This same agricultural development left in place the larger lakes where migrating flocks like to roost overnight, as well as the snow geese breeding ground up north.6
Landscape effects in the north
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, and is currently growing at about 5-8% annually. The colony at La Perouse Bay grew from 2,000 nesting pairs in 1968 to 22,500 in 1990.7
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 they do 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".8
What is happening? The geese have now eaten out much of the "oasis" land around Hudson Bay, and they are expanding into the less productive inland tundra. 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.9
But 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. This is true not only for the Mid-Continent population of snow geese, but for four of the five populations worldwide.10
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 dodwits and short-billed dowitchers have already fallen 90% 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.11
Proposed solutions
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.12
Several factors currently limit the hunt. Old geese are harder to shoot; they avoid humans and decoys, they keep to refuges, and they even learn to react to changes in hunting regulations. Hunter numbers have also been dropping, so the annual harvest is falling even though the season and daily bag limits have been increased to the maximum allowed by the Migratory Bird Treaty of 1916. And in some hunters' eyes, the overabundant snow geese have become "sky carp" or "white crows," less desirable than other goose species.13
Managers have proposed removing various restrictions on hunting. They suggest more spring hunting, currently-outlawed practices such as baiting and electronic calls, higher daily limits, hunting on refuges and private land, simplified hunting across state and national borders, more subsistence hunting and egging along Hudson Bay, and even subsidies or awards for hunting. As the managers expected, the situation has bred controversy. An executive of the Humane Society stated that the "effects that the snow geese have on the Canadian arctic do not warrant this indiscriminate destruction." Other critics think the reputation of hunting will be damaged, and that technology such as electronic callers will be a "law enforcement nightmare". Some believe that increased hunting is "too little, too late" to prevent destruction of the habitat and collapse of the population.14
Footnotes
Thanks to Andrew P. Jano, Ontario Ministry of Natural Resources, for his assistance with materials for this article.
1. Chris Madson, "Snow Drifts: The Mid-Continent's Lesser Snow Geese": Wildfowl Magazine, April/May 1997, p. 48-49.
2. Mike Johnson, "The Snow Goose Population Problem, Part I": North Dakota Outdoors, v. LIX, no. 2, August 1996, p. 17.
3. Johnson, "Population Problem, Part I", p. 16-176. Bruce Batt, "A Perilous Abundance": Ducks Unlimited, Jan/Feb 1998, p. 56.
4. Madson, p. 48. Lisa Drew and Chris Madson, "Out of Control!": National Wildlife, April 1998, p. 28. Johnson, "Population Problem, Part I", p. 16.
5. Johnson, "Population Problem, Part I", p. 17. Madson, p. 49.
6. Drew, p. 30, Ray T. Alisauska, "Spring Habitat Use and Diets of Midcontinent Adult Lesser Snow Geese": Journal of Wildlife Management, v. 56, 1992, p. 43. Long-term weather patterns may also be one of the many factors which might be boosting geese productivity. (Johnson, "Population Problem, Part I", p. 15.)
7. Johnson, "Population Problem, Part I", p. 16. Drew, p. 28. Batt, p. 56.
8. These NDVI images were created directly from the digital number (DN) values of registered Landsat data (MSS bands 4 and 2, TM bands 4 and 3), without converting them to reflectance or correcting for atmospheric conditions.
9. Drew, p. 30. Batt, p. 58.
10. Johnson, "Population Problem, Part I", p. 16-18. Drew, p. 31.
11. Drew, p. 28.
12. Johnson, "Population Problem, Part I", p. 14-15. Mike Johnson, "The Snow Goose Population Problem, Part III: Arctic Ecosystems in Peril": North Dakota Outdoors, v. LIX, no. 8, March 1997, p. 3. Mike Johnson, "The Snow Goose Population Problem, Part II: Working Toward a Solution": North Dakota Outdoors, v. LIX, no. 3, Sept/Oct 1996, p. 22.
13. Batt, p. 59. "U.S., Canada May Declare War on Geese": USA Today, 28 Apr 1998, Science Section. Johnson, "The Snow Goose Population Problem, Part II", p. 20, 22. Shannon Tompkins, "Snow Geese Debate Heating Up, But There's No Solution in Sight": Houston Chronicle, 4 Feb 1999, Outdoors section, p. 11 (online archive). Craig Bihrle, "Greater Goose Harvest: Higher Limits, Periodic All-Day Hunting Create Possibilities": North Dakota Outdoors, Sept/Oct 1998.
14. Johnson, "The Snow Goose Population Problem, Part III", p. 3-4. USA Today, "War on Geese". Tompkins, "Snow Geese Debate Heating Up".
Fred Cooke and others, The Snow Geese of La Perouse Bay (Oxford U. Press, 1995), 297 pages.
Andrew P. Jano, "Habitat Loss Assessment by Multispectral Analysis of Landsat Data in the Seal-Knife River Region", North American Arctic Goose Conference 1998, Poster Presentation.
Andrew P. Jano and others, "The Detection of Vegetational Change by Multitemporal Analysis of LANDSAT Data: The Effects of Goose Foraging": Journal of Ecology, 1998, v. 86, p. 93-99.
LM1035019007322690 (Landsat 1 MSS, 14 August 1973)
18 July 1996 Landsat 5 MSS, track/frame 32/19, from Radarsat International, Richmond, British Columbia, Canada.
Canada Centre for Mapping, 1989, Churchill Manitoba, Northwest Territories: scale 1:250,000
The geese regions and paths on the map of North America are after the map in Mike Johnson, "The Snow Goose Population Problem, Part II: Working Toward a Solution": North Dakota Outdoors, v. LIX, no. 3, Sept/Oct 1996, downloaded in January 1999 from ndwild.psych.und.nodak.edu/HTMLPages/sg2map.html, where it is credited to NebraskaLand Magazine, Nebraska Game and Parks Commission.
Campbell, Robert Wellman, ed. 1999. "Knife River Delta, Canada: 1973, 1996." Earthshots: Satellite Images of Environmental Change. U.S. Geological Survey. http://earthshots.usgs.gov. This article was released 14 February 1999.
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