What are pollinators and why do we need them?
Pollinators are animals that move pollen from male structures (anthers) of flowers to the female structure (stigma) of the same plant species. Movement of pollen (analogous to sperm) to a flower’s stigma results in fertilization of the flower’s eggs. An adequately fertilized flower will produce seeds and the fruit surrounding seeds, ensuring that a new generation of plants can be grown.
Pollination is mutually beneficial to plants and to pollinators. Pollination results in the production of seeds and is necessary for many plants to reproduce. Meanwhile, pollinators receive nectar and/or pollen rewards from the flowers that they visit. Sugary nectar provides pollinators with carbohydrates while pollen offers proteins, fats, vitamins, minerals, and necessary phytochemicals. See  for a recent review of bee nutritional needs.
Examples of pollinators
Honey bees often come to mind first when people think of pollinators. However, many different animals, including other insects (other bee species, butterflies, beetles, flies), some birds and some bats are pollinators. Indeed, there are an estimated 300,000 species of flowering plants worldwide that require animal pollinators . This tremendous floral variety supports a corresponding diversity of pollinators, and the vast majority of these pollinators are insects. For example, while there are only about 1,000 vertebrate pollinator species, it’s estimated that there are at least 16,000 different species of bees world-wide .
Pollinators are necessary for three-quarters of our major food crops
Not every species of plant requires animal-mediated pollination services. For example, wheat is wind-pollinated. However, the majority of crops that we like most to eat and provide most of our nutrition (fruits, vegetables, and nuts) use animal-mediated pollination [4, 5]. Without pollinators, our diets would be severely limited, and it would be more difficult to acquire the variety of vitamins and minerals that we need to stay healthy.
Healthy pollinators and healthy ecosystems
Outside of agricultural systems, approximately 80-95% of the plant species found in natural habitats require animal-mediated pollination . Plants are the foundation of terrestrial food chains. The foliage and/or fruits and nuts that plants make are eaten by herbivores which in turn are hunted by predators. Furthermore, plants provide shelter and nesting habitat for many different animal species. Thus, in order to maintain the diversity of our natural ecosystems, we need healthy pollinator populations to ensure that the next generation of plants will be produced.
 Vaudo, A. D, Tooker, J.F., Grozinger, C.M. and H.M. Patch. “Bee nutrition and floral resource restoration.” Current Opinion in Insect Science 10:133-141 (2015).
 Ollerton J, Winfree R, Tarrant S: How many flowering plants are pollinated by animals? Oikos 2011, 120(3):321-326.
 Danforth BN, Sipes S, Fang J, Brady SG: The history of early bee diversification based on five genes plus morphology. Proceedings of the National Academy of Sciences of the United States of America 2006, 103(41):15118-15123.
 Klein, A-M, et al. “The importance of pollinators in changing landscapes for world crops“. Proceedings of the Royal Society B 274 (1608): 303-313.
 Eilers, E.J. et al. “Contribution of Pollinator-Mediated Crops to Nutrients in the Human Food Supply” PLoS One 6(6): e21363. doi:10.1371/journal.pone.0021363
Published by the Center for Pollinator Research at Penn State, College of Agricultural Sciences, Department of Entomology. https://ento.psu.edu/pollinators/resources-and-outreach/what-are-pollinators-and-why-do-we-need-them