Birds Help Spread Invasive Plant Species

Last Updated on August 27, 2020 by Kimberly Crawford

Birds disperse the seeds of invasive plants that often crowd or even kill native plants. Yet, these berries are often essential, providing birds with needed nutrients.

Hikers walking along the edges of woodlands in late fall and early winter are often amazed by large masses of red berries on shrubs and trees. These berries serve to nourish many birds through the winter months and are depleted at different rates by our winter residents and visitors.

To the birds, locating these berries is essential to their survival: The berries provide sugars for instant energy; protein and fat for longer energy release; and trace minerals and phytonutrients essential for functional metabolic enzymes and healthy immune systems.

The Two-edged Sword of Berries

Environmentalists have mixed feelings about these early winter berries. Because they fulfill acute needs for widespread food sources, these berries are essential for the birds. Because they are often the propagules for invasive plant species, they represent hordes of new plants that often damage native species and are difficult to eradicate.

Native winterberry hollies (Ilex verticillata) growing near ponds and streams bear large numbers of berries as do American holly (Ilex opaca), spicebush (Lindera benzoin), high bush cranberry (Viburnum trilobum), and the hawthorns (Crataegus species) that grow in drier areas.

But human alteration of the landscape has eliminated much of the habitat for these American beauties. In their places alien ornamentals such as burning bush (Euonymus alatus), Asiatic bittersweet (Celastrus orbiculatus), multiflora rose (Rosa multiflora), and barberry (Berberis vulgaris) have been planted – all of which produce multitudes of fruit that attract the attention of hungry birds.

Native plants generally exist in habitats that also contain the natural parasites, predators, and diseases that keep their numbers in bounds. Introduced alien plants are usually selected for hardiness and ease of propagation, then released into habitats without their natural controls.

Although their populations may be kept under control by consumers and diseases in their original homes, their introduction into areas free of predators and parasites combined with their proclivity for rapid propagation – often through both seed and asexual reproduction, allow these aliens to become invasive.

Becoming invasive, they outcompete native plants – often eradicating closely related species, and even killing off unrelated varieties.

Enter the Birds

Birds flock to feed on these aliens when their fruits become ripe – sometimes stripping an entire plant or field in a day or two. The skin and fruit are digested off the seeds, but their hard coats prevent the seeds from also being digested.

Instead, the grit (that birds eat to help them grind their food) scratches the seeds’ outer coats (allowing them to absorb water readily), the seeds are deposited in fecal packets of fertilizer – usually in a habitat similar to, but at a distance from, the habitat of the parent plant.

As a result, when the seeds germinate after the heavy rains of spring, a new generation rapidly becomes established. Without their natural control organisms, these new plants form new feeding grounds for birds which help to spread the invasive species even further.

Concerned citizens must balance the benefit to birds as well as the dangers to other plants when deciding what to do with these plants. Many gardeners and foresters prefer to eliminate the exotics and replace them with similar sources of fruit such as the native plants listed above.