单项选择题

TEXT C
Northern marshes are being turned into empty, desecrated mud flat wasteland. The culprit Snow geese.
These marshes are the breeding ground for snow geese. Once destroyed, some fear the species will take over the habitat of the Canada goose—a popular game bird in Minnesota. If this happens, Minnesota hunting and land conditions could be greatly affected.
The snow goose population has been on the rise in the last 25 years, but numbers are hitting an all-time high. This year there is an estimated 4.5 or 6 million birds, triple what the population was 25 years ago.
Although effects of the snow goose invasion aren’t apparent in Minneapolis, northern Minnesota and Canada can clearly see the signs. The population growth is due to the birds’ wintering habits. They fly south to Louisiana, Texas and Mississippi to nest. The conditions and food availability there have made it possible for more birds to survive the winter and make the trip back north. The period over which they’ve increased in number correlates to a change in agriculture practices in the region.
After World War II, there was an increase in man-made fertilizers, yielding an increase of corn, rice, wheat and other crops. There have also been other changes in agricultural practices causing an increase of production in cereal crops.
The geese find the agricultural areas better than the natural areas. The geese have escaped from any natural limits. They are not doing this on their own; it is in response to human practices.
Usually, about 70 to 75 percent of the birds make it back to Canada in late winter and early spring. But the surviving number of snow geese has steadily climbed each year to reach 95 percent in the last couple of years. Because so many survive, they strip the capacity of the breeding ground.
The snow geese are destroying salt marshes where they nest in the summer, about 30 percent of the salt marshes are completely destroyed, leaving them as inhabitable mud flats. Another 35 percent of salt marshes are significantly damaged.
There are three possible solutions: Let the problem take care of itself and wait for the population to crash, deal directly with the population by changing hunting limits and regulations or address the cause of the problem in the south.
The sharp rise of the snow geese population is mainly caused by ______.