The appearance of Common Buckthorn (Rhamnus cathartica) and Glossy Buckthorn (Frangula alnus) in North American woodlands represents a significant ecological challenge. These species, introduced from Eurasia, exhibit highly aggressive, invasive characteristics, quickly outcompeting native vegetation across forest understories. As a dominant herbivore across the continent, the white-tailed deer (Odocoileus virginianus) is often present in these invaded habitats, leading to the central question of whether deer consumption provides a natural check on buckthorn’s spread. The answer is complex, but generally, white-tailed deer avoid both species of buckthorn, classifying them as low-preference forage.
Consumption Patterns of Deer
White-tailed deer are highly selective feeders, often called “concentrate selectors,” meaning they seek out the most nutritious and easily digestible plant parts available. Buckthorn is largely avoided due to its unpalatable nature, making it one of the last plants a deer will choose to browse. Browsing on buckthorn stems and leaves, which are low in nutritional value, typically only occurs during periods of extreme food scarcity, such as harsh winters or in areas with extremely high deer populations that have depleted preferred native forage.
Deer do eat the dark berries produced by female buckthorn plants, often in the late fall or early winter. This consumption is not driven by the berries’ poor nutritional content, but rather by their availability when other soft mast has been depleted. The consumption of these berries has important implications for the plant’s ability to spread, as the seeds are often passed through the deer’s digestive tract intact.
Why Buckthorn is a Low-Preference Food Source
The primary reason deer avoid buckthorn lies in the plant’s chemical defense mechanisms and its low food quality. Both Common and Glossy Buckthorn contain high concentrations of secondary compounds known as anthraquinone glycosides. When ingested, these compounds act as a potent laxative, irritating the digestive tract and causing rapid bowel movements, which can lead to digestive distress.
This chemical barrier makes the consumption of buckthorn stems and leaves nutritionally inefficient and physically unpleasant. Furthermore, the leaves and woody tissues offer relatively low levels of protein and energy compared to many native browse species. The energy expenditure required to forage and process such low-quality, irritating food is not worthwhile for a concentrate selector unless the deer is facing near starvation.
Ecological Impact of Deer Browsing on Buckthorn Spread
Deer browsing does not control buckthorn; instead, it often accelerates the plant’s invasion through two distinct ecological mechanisms. The first is competitive release, where deer selectively consume palatable native species, which reduces competition for the unpalatable buckthorn. By repeatedly browsing native tree and shrub seedlings, deer remove the competition for sunlight, water, and soil nutrients, effectively “releasing” the buckthorn from competitive pressure.
The result is a shift in the forest understory composition, where native regeneration is suppressed while buckthorn thrives and forms dense, impenetrable thickets. Studies have shown that increased buckthorn cover correlates with reduced health and increased browsing damage on native tree species, demonstrating that the invasive plant indirectly intensifies deer herbivory on desirable species.
The second mechanism is endozoochory, or seed dispersal through animal droppings. When deer consume the buckthorn berries, the seeds pass through the gut and are deposited in scat, often far from the parent plant. This assists in the long-distance dispersal of viable seeds, helping the invasive species colonize new areas. A single fecal pellet group can contain viable seeds from dozens of plant species, highlighting the deer’s role as a vector for the rapid spread of invasive plants across the landscape.
Deer’s Preferred Seasonal Forage
In spring and summer, deer prefer high-protein, easily digestible foods like forbs (herbaceous broad-leaved plants), fresh agricultural crops, and the tender new growth of woody plants. As a concentrate selector, they prioritize plants like clover, alfalfa, and the new shoots of woody species such as dogwood, maple, and aspen.
During the fall, the diet shifts to include hard mast, with acorns being a highly sought-after food source, particularly from white oaks. Soft mast, such as grapes and berries from native plants, also provides a burst of energy. Only when these preferred food sources are exhausted in the depths of winter do deer rely heavily on woody browse, and even then, they choose the most palatable native twigs before resorting to species like buckthorn.