CES - Pollination Tool Kit: What Is Pollination?

What is Pollination?

Pollination is central to successful reproduction in most plants. Simply stated, it is the transfer of pollen grains from the stamens of one flower to the stigmas of another flower. Some plants are self-pollinated or wind-pollinated, but most depend on insects, birds, bats, and other organisms, collectively referred to as pollinators, to transport the pollen for them. The co-evolution of pollinators and the pollination process is one of nature's unique solutions to the dilemma of sexual reproduction among stationary plant organisms.

The relationships between flowering plants and their pollinators have evolved since the early Cretaceous period, some 140 million years ago (13,22). These relationships are usually mutually beneficial to both parties. Pollinators assist in the reproduction of plants by transporting pollen. Flowers that are not pollinated are simply not able to produce fruits. In return, flowering plants produce nectar, a highly nutritious sugar-based substance and a critical source of food for pollinators. In some cases, however, this association is not mutually favorable. For example, many pollinators also ingest the pollen itself, as a useful source of protein, instead of transferring it to another flower. In addition, a few species of pollinators have become adept at 'stealing' nectar from flowering plants without transporting pollen (14,18).

In addition to being mutually beneficial, plant-pollinator interactions differ greatly in the degree of dependency that exists between the plant and pollinator species. The vast majority of flowering plants depend on multiple pollinators to fulfill their reproductive needs. Referred to as generalists, these pollinators make visits to many different flowering plants in search of forage. The honeybee (Apis mellifera), like many other bee species, is a good example of a generalist pollinator. Flowering plants benefit by attracting generalists because the number of pollinator visits is high, and one species can take the place of another should there be any decline in pollination activity. There are, however, drawbacks to this type of relationship. For example, some pollinators are more efficient than others; and by attracting generalist pollinators, flowering plants risk getting visits from less effective pollinators. In addition, plants that use generalist pollinators have to compete with one another for these pollinators (2,14).

A much smaller group of flowering plants are wholly dependent on a single species for pollination services. An example of this specialized (obligate) relationship exists between the yucca plant (Agavaceae) family and their yucca moth (Prodoxidae) pollinators. Highly specialized plant-pollinator relationships are ideal for both parties because pollination efficiency is high and competition for food resources among pollinators is low. Yet, this type of relationship makes both the plant and the pollinator vulnerable to fluctuations in their counterpart's populations (2).

Whether specialists or generalists, pollinators are essential to the survival of over ninety percent of the 250,000 flowering plants species on the planet today (14). The availability of pollinators is as important as moisture, sunlight, and soil fertility to the reproductive success of nearly half the world's flowering plants (16).

Pollination is also vital to the well-being of humans. The most obvious example of our link to pollination is through agriculture. Pollination services, by managed honeybees and wild pollinators, are a key component of the seed, fruit, and fiber yields of the crops that we eat (16). Almost all fruit and grain crops require successful pollination in order to produce the harvested crop. While it is true that some very important agricultural crops, such as canola, corn, and wheat, are self- or wind-pollinated, the majority requires the services of pollinators. In fact, pollinators are important for more than 150 food crops produced in the United States including apples, alfalfa, almonds, blueberries, cranberries, kiwis, melons, pears, plums, and squashes (6,14). (For a detailed list of cultivated crop plants serviced by pollinators, visit http://gears.tucson.ars.ag.gov/book/index.html)

Unfortunately, many people are simply not aware that pollination services are critically important for maintaining both healthy, diverse ecosystems and human sustenance. A recent survey showed that three quarters of the visitors to a pollination exhibit related pollen to allergies, but did not recognize its role in plant reproduction (4).