Once attracted, plants secured the services of these animal pollinators by providing rewards, most often nectar and pollen, but sometimes oils and perfumes. Some plants also provided nest sites or materials, food for the young, and protective chemicals. Some plants became specialists (e.g. orchids), targeting only a small number of animal species, perhaps only one, while others became generalists (e.g. many weeds) appealing to a much broader audience. The various floral shapes, organizations, colors, fragrances and structures we find today are sometimes classified in terms of the pollinators they attract.
Animal pollinators come in a variety of shapes and sizes, from tiny insects to bats, birds, honey possums and even a few lizards. Not all of them are equally efficient at transferring pollen. Among the insects, the honey bee, because of the large size of its colonies, is the most significant pollinator. The size range of the pollen it will gather is the largest of any known pollinator. Just because an animal visits a flower doesn't mean that it can or will act as a pollinator - some animals, even honey bees on certain kinds of plants, access floral rewards without contributing to the plant's reproduction.
In a natural setting, the composition of plant communities evolve to take advantage of the available pollinators. Since the flowering periods for plants pollinated by animals are usually shorter than the lifetimes of their pollinators, the community must supply a sequence of resources that supports the energy needs of any local pollinators. Floral resources may be spread out in time, perhaps over the course of the day, or the season (even this year in the desert, all the plant species did not flower at the same time). In addition this community must supply resources that support the various life cycles of local pollinators. If sufficient resources are not available locally at the right time pollinators may move on or cease to exist. Some pollinators migrate, following a "nectar corridor". Here timing is critical - if the pollinators are not available when the plants are flowering, no seeds may develop.
Plants and their animal pollinators have interacted with and thereby affected each other's physical and chemical evolution. In doing so they have also affected each other's life cycles and spatial distributions, essentially becoming co-dependent. The interactions found today have developed over time and continue to evolve. In some cases, based on its floral properties, a plant doesn't attract the pollinators we might expect. These unexpected pollinators may have stepped in because of some irreparable break in the relationship that had existed between the plant and its preferred pollinators - perhaps due to the local extinction of the pollinator.
Too often in planning and planting for the world around us we forget that pollinators are an integral part of the reproductive system of most plants. The seeds we plant in our gardens, the fruits and vegetables we consume daily, many of the products we use, and much of the beauty we see in the natural world around us would not be here if not for the work of pollinators. Project - to gain a true appreciation of these small work-horses, try hand-pollinating your garden.
Ron Lyons (volunteer 1990-1999)