Who pollinates what
Pollination is the movement of pollen from one flower to another so that seeds can form. In forests this is done by wind, by water in a few cases, and by animals, most of them insects. The balance varies by forest type. Many conifers and a number of familiar temperate trees, such as oaks and birches, mainly use wind. Many other trees and nearly all showy woodland wildflowers rely on animals.
In tropical forests, animal pollination is thought to be especially common among trees, which is part of why those forests depend so heavily on insects, birds and bats. In cooler forests, wind plays a larger role among the dominant trees, while insects remain important for shrubs and for flowers on the forest floor.
The main insect groups
Bees. Bees are the best-known pollinators. They collect pollen and nectar for themselves and their young, and so they visit many flowers. Honey bees are only one of many bee types. Wild bees, including bumble bees, mining bees, sweat bees and mason bees, are numerous and diverse, and many are specialists that favor particular flowers. Early spring woodland flowers, which bloom before the canopy leafs out, often depend on queen bumble bees and small solitary bees emerging at the right time.
Flies. Hover flies, bee flies and many other flies visit flowers and carry pollen, and in cool, shady or wet places they can be among the most frequent visitors. Some flies are important for small flowers that are not suited to bees. Their larvae have varied roles, from feeding on aphids to breaking down decaying wood, so a fly that pollinates in summer may depend on rotting wood or damp soil as a larva.
Beetles. Beetles are among the oldest pollinators of flowering plants. Some magnolias and other primitive flowering trees are visited largely by beetles, which feed on pollen and floral tissue. Many beetle larvae live in dead wood, which links this group closely to the supply of rotting logs and snags.
Moths and butterflies. Moths often pollinate at night, visiting pale, fragrant flowers. Butterflies pollinate in daytime, particularly along sunny edges and in openings. Their caterpillars feed on specific leaves, so these insects depend on both flowers and host plants.
Wasps and others. Wasps, including small parasitic species, visit flowers for nectar, and some plants rely on wasps. Thrips and midges, tiny insects, pollinate certain tropical trees. In tropical forests, birds and bats also take on much of the work, but they fall outside the scope of this article.
What forest pollinators need
Pollinators need more than flowers. Different life stages and different species draw on different parts of the forest.
- A continuous supply of bloom. A forest that flowers in sequence, from early spring bulbs and tree blossoms through summer shrubs and into fall, supports more pollinators than one that blooms heavily for only a short time.
- Nesting places. Many wild bees nest in bare or sandy soil, in hollow stems or in old beetle holes in wood. Bumble bees often nest in abandoned rodent burrows or in thick ground cover. Leaving some bare ground, dead wood and leaf litter helps them.
- Light. Many flowers need sun. A closed canopy can shade out wildflowers, so openings created by canopy gaps, trails, clearings and forest edges often host more pollinators than deep shade. Natural disturbances and some forms of management create these sunny patches.
- Host plants and larval habitat. Moths, butterflies, flies and beetles often need particular leaves, rotting wood, damp soil or small pools for their young.
- Safety from pesticides. Insecticides applied nearby, including for forest insect pests, can harm non-target insects. Choices about timing and method matter, and the risks differ among products.
Why trees depend on them
Where trees rely on animal pollination, fewer pollinators can mean less seed set. Some trees can self-pollinate to a degree, but many do better with pollen from a different, distant tree, and a good cross-pollination often produces healthier offspring. This is one more reason fragmentation matters. When trees of one species are scattered at low density, as many tropical trees are, pollinators must travel farther between individuals. If forest patches are cut off from each other, pollen movement may drop and the trees left in small patches may produce fewer or less varied seeds. The timing of flowering also matters, and shifts in warming can potentially separate the dates when plants flower from the dates when their pollinators are active, although how widespread this is remains an area of ongoing study.
Pollinated flowers become fruits and seeds that feed birds, mammals and insects, and which grow into the next generation of trees. In that sense, pollination supports food webs and natural regeneration, as well as the forest’s long-term composition.
Threats
Pollinator declines have been documented for several groups, especially some bumble bees, but the picture varies widely by region and by species, and many forest insects are poorly studied. Common pressures include habitat loss, fragmentation, pesticide use, disease, invasive species that crowd out native flowers or compete with native insects, and climate shifts. Heavy browsing by deer can also strip the wildflowers that pollinators rely on.
What helps
Practical steps often recommended include keeping a mix of open and shaded areas, retaining dead wood and some bare soil, protecting native flowering shrubs and trees, avoiding pesticide use near flowering plants, and controlling aggressive non-native plants where they displace natives. Identification and counting of pollinators is a field in which volunteers can contribute through surveys. Because the details differ by region, a local extension office, native plant society or entomologist can give more specific advice.