91. Read the following passage, then answer the question below.

Of the leading food crops, about three quarters benefit at least partly from animal pollinators. Honeybees receive most of the attention, but they are only one of more than twenty thousand bee species, and wild bees, flies, beetles, butterflies, and even some bats carry pollen between flowers. Their work is easy to overlook, yet a farm that loses its pollinators may lose much of its harvest as well.

In recent decades, beekeepers in several countries have reported winter losses far above historical norms, and surveys of wild insects point to declines in both numbers and variety. No single cause explains the pattern. Pesticides called neonicotinoids, which attack the nervous system of insects, can weaken bees even at doses too low to kill them. A parasitic mite, Varroa destructor, spreads viruses through honeybee colonies. Meanwhile, the conversion of meadows into fields and housing has removed the flowers and nesting sites that wild species need. Scientists increasingly suspect that these pressures interact: a bee weakened by poor nutrition is more likely to succumb to a mite, and a colony infected with a virus is less able to tolerate a pesticide.

Because the causes are tangled, solutions are likely to be varied. Some farmers now leave strips of wildflowers along field edges, which supply food and shelter without sacrificing much cropland. Regulators in several regions have restricted certain pesticides, although manufacturers argue that the evidence of harm under real farm conditions is weaker than laboratory results suggest. Others propose that growers rely on a wider range of pollinators rather than renting honeybee hives, since a single managed species is vulnerable to a single disease. Whatever combination proves effective, pollination remains a service that ecosystems provide for free, and its value becomes obvious only when it begins to fail.

What is the main idea of the passage?