Ferns thrive across diverse global environments. Gardeners often wonder if animals consume them. This relationship is complex, depending on fern species, animal species, and local conditions.
General Consumption of Ferns
Fern consumption by animals is not universal. Many fern species are not preferred food sources for most herbivores. This selective feeding behavior is influenced by physical characteristics and chemical composition. Whether a fern is eaten depends on the specific species and the availability of other food sources.
Animals That Consume Ferns
Some animals consume ferns, though not a primary food source for many. White-tailed deer may nibble on ferns when other food is scarce. Feral pigs in Hawaii eat tree-fern trunks. The mountain beaver, a rodent native to the Pacific Northwest, is a notable fern consumer; ferns make up over 75% of its diet, including bracken and sword fern. Smaller vertebrates like the European wood mouse, short-tailed bat, Indonesian parrot, and Azores bullfinch consume fern spores or leaves.
Insects also feed on ferns. Sap-sucking insects such as aphids, whiteflies, and scale insects can infest them. Chewing insects like grasshoppers, crickets, beetles, weevils, and caterpillars, including the Florida fern caterpillar, feed on fern fronds. Beavers have been observed digging up and eating fern rhizomes.
Why Some Ferns Are Avoided or Toxic
Many ferns deter animal consumption with natural defenses. These include physical deterrents like a leathery texture, fibrous nature, or bitter taste, making some ferns unpalatable.
Ferns also produce unappetizing or harmful chemical compounds. Some contain thiaminases, which break down thiamine (vitamin B1), leading to a deficiency. Others contain ptaquiloside, a carcinogen, or cyanogenic glycosides that release hydrogen cyanide. Alkaloids, tannins, and terpenoids also contribute to their defensive chemistry.
Common Toxic Ferns and Their Effects
Several fern species are toxic if ingested by animals. Bracken fern (Pteridium aquilinum) is toxic due to thiaminase and ptaquiloside. Thiaminase can cause thiamine (vitamin B1) deficiency in horses, leading to neurological symptoms like incoordination and weight loss, often called “bracken staggers.” Sheep consuming bracken fern can develop retinal degeneration and blindness.
Ptaquiloside, a carcinogen in bracken fern, causes hemorrhagic syndrome and bone marrow suppression in cattle, potentially leading to urinary bladder tumors. Young fronds and rhizomes typically contain the highest concentrations. Prolonged consumption is often necessary for these toxic effects to manifest. Horsetail (Equisetum ramosissimum) also contains thiaminase, causing similar thiamine deficiency symptoms in horses, especially when present in hay.
Asparagus fern (Asparagus densiflorus) is not a true fern but a lily family member, toxic to pets. It contains sapogenin, which causes gastrointestinal upset if ingested by cats or dogs. Repeated skin contact with the plant’s sap can also lead to allergic dermatitis. Its berries are particularly toxic.
Implications for Gardens, Pets, and Livestock
Understanding fern consumption and toxicity informs decisions for gardeners, pet owners, and livestock managers. For gardeners, ferns are generally less appealing to deer due to their texture and taste, but deer may still browse them during food scarcity. To deter deer, gardeners can choose deer-resistant varieties, like some Maidenhair fern species, or use physical barriers and repellents.
Pet owners should know that many true ferns, such as Boston, Maidenhair, Bird’s Nest, Rabbit’s Foot, and Crocodile ferns, are safe for pets, but others like asparagus fern are toxic. Identify plants in and around the home to prevent accidental ingestion. Livestock managers should monitor pastures for toxic ferns like bracken and horsetail, as prolonged consumption can lead to severe health issues and economic losses. Awareness of these plant-animal interactions helps ensure the well-being of garden plants and animals.