A honeybee colony is a busy community. One queen lays all the eggs, thousands of female workers do the jobs, and a few male bees, called drones, mate with queens from other colonies. Young workers clean the hive and feed the young bees. Older workers leave the hive to collect nectar and pollen from flowers. When a worker finds a good source of food, she returns and performs a special movement known as the waggle dance. The direction and length of the dance tell other bees where the flowers are. Bees do not only help themselves. As they move from flower to flower, they carry pollen with them, and this helps many plants to produce seeds and fruit. For this reason, farmers often value bees highly, and many crops depend partly on them. Because bees are so useful, scientists worry when their numbers fall.

Question: In the sentence 'this helps many plants to produce seeds and fruit', what does 'this' refer to?