A. Modes of Pollination

The transfer of pollen is a vital process for reproduction in the majority of plant species—flowers that are not pollinated are simply not able to produce fruits—but moving pollen is not a simple task for plants, which, quite literally, are rooted to the spot. Movement of pollen between flowers on separate plants is called cross-pollination. Movement of pollen within a flower or between flowers on the same plant is called self-pollination. Self-pollination will produce seeds and fruit but the mixing of genes that result from cross-pollination is necessary to maintain a healthy plant community. In some cases self-pollination can lead to a condition called inbreeding depression in which breeding between close relatives may result in a reduction in genetic diversity, and the possible expression of negative traits in the population and a loss of ability to evolve in response to a changing environment.

A significant number of plants, particularly grasses, conifers, and oaks, rely upon the wind to spread pollen. Some of the world’s most important food crops, such as corn, wheat, and rice, are wind pollinated. A few even use water, such as horned pondweed, hornwort, and some species of water starwort. However, the great majority of plants, more than 70 percent of species, depend on insects, birds, bats, and other animals to transport the pollen for them.

The coevolution of plants and their pollinators and the resulting pollination process ensures precise transfer of vital genetic material between flowers of the same species. In contrast, wind pollinated plants release vast quantities of dust-like pollen grains of which only a few reach their target.

The relationships between flowering plants and their pollinators have evolved since the early Cretaceous period, some 140 million years ago.^{3 4} These relationships are usually mutually beneficial to both parties: flowering plants produce nectar, a highly nutritious sugar-based substance that has no direct benefit for the plant but is a critical source of energy for pollinators, and in return, pollinators assist in the reproduction of plants by transporting pollen. In some cases, however, this association is not mutually favorable. For example, many beetles eat the pollen instead of transferring it to another flower. In addition, some pollinators have become adept at “nectar robbing,” taking nectar from flowers without passing the anthers of the flower where pollen is located.^{5 6}

There are also differences in how effective pollinators are at transferring pollen. Bees, for example, are generally recognized as the most important single group of pollinators. There are three reasons for this. First, bees are the primary flower-visiting insects that actively collect and transport pollen (the exception is a small group of wasps, the family Masaridae, that also collects pollen). Other insects forage on flowers for nectar or pollen or eat parts of the flower and make contact with pollen only by chance. Second, bees are central-place foragers, consistently foraging in the same area around their nest, so plants within that area get many visits. Finally, bees visit a single species of flower during a foraging trip (a habit known as flower constancy), ensuring that pollen is efficiently and accurately moved.

By contrast, all but a few butterflies gather only nectar as they forage across the landscape, visiting any suitable flower. An interesting exception are some tropical Heliconine butterflies that collect balls of pollen from which they extract amino acids by regurgitating nectar onto them.⁷ Flies and beetles generally only take nectar, although some will also eat the pollen (a good source of protein and nutrients) or even parts of the flowers themselves. They also visit flowers to search for mates or to sunbathe for warmth and in doing so may make contact with pollen. The abundance of flies and beetles and their high levels of flower activity mean that although pollen transport is by chance and may only be a few grains at a time, they are valuable pollinators. This is particularly true for flies in colder (alpine or arctic) regions, where they may be more important than bees.⁸

Although 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. This is often reflected in the shape of the bloom or the accessibility of the nectar.⁹ Plants in the families Asteraceae (aster) and Apiaceae (carrot) are examples of flowers that have accessible nectar and thus many pollinators. Flowering plants benefit by attracting many different species of pollinators because the number of pollinator visits is high, and one pollinator species can take the place of another should populations fluctuate or decline. 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.^{10 11}

Some flowers, such as lupines (genus Lupinus) and larkspur (genus Delphinium) rely on a few, specialized pollinators. These plants have complex blooms with the nectar or pollen hidden away and accessible only to, for example, a hummingbird or bumble bee.¹² African water lilies (family Nymphaeaceae) are mainly pollinated by rhinoceros beetles (Scarabaeidae).¹³

Bat pollinated flowers tend to have several distinctive features. They are usually large, firm, and wide mouthed, shaped for a bat to push its head past the pollen bearing anthers and reach its long tongue down to the nectaries. Bats are thought to be color blind, and bat pollinated flowers are generally dull colored and nocturnal blooming, with strong fruity odors.Saguaro (Carnegiea gigantea) and organpipe cactus (Stenocereus thurberi) flowers are good examples of these features. 

A much smaller group of flowering plants is wholly dependent on a single species for pollination services. An example of this specialized (obligate) relationship exists between the yucca plant (family Agavaceae) and their yucca moth (family 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.¹⁴ Figs are another group of plants for which the wasp pollinators are typically highly specific.

C. Generalist and Specialist Pollinators

The pollinators themselves can also be categorized as generalist or specialist according to their foraging habits. Most pollinators are generalists, happy to take nectar or pollen from wherever they can reach it. Most bees are generalists, as are the majority of flies, beetles, and butterflies.

Examples of specialist pollinators can be found in both flies and bees. Specialized flies tend to be adapted to drink nectar from particular flowers. The tangle-veined fly Moegistorhynchus longirostris, found in southern Africa, has a proboscis three times the length of its body - a world record for fly tongue length. This extreme adaptation allows it to drink from Lapeirousia anceps, a plant in the lily family that has a long thin corolla.¹⁵ Specialist bees, in contrast, are specific about the flowers from which they collect pollen, and not where they drink nectar. An example is the aptly named squash bees (genus Peponapis), which collect pollen from flowers in the squash family. More choosy again are those bees that collect pollen from only a single species of plant. In the Virgin Basin of southwestern Utah, the Mojave poppy bee (Perdita meconis) gathers pollen only from the bearclaw poppy (Arctomecon humilis).¹⁶

Whether specialists or generalists, pollinators are essential to the survival of over 70 percent of the 250,000 flowering plants species on the planet today.¹⁷ 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.¹⁸