Ferns are an ancient group of non-flowering vascular plants (pteridophytes) that have thrived on Earth for millions of years. Despite their abundance across diverse ecosystems, they are surprisingly neglected by most generalist herbivores. This widespread avoidance suggests ferns possess highly effective defense strategies that deter the majority of animals. The few creatures that consume them are either highly specialized or graze on them out of seasonal necessity.
The Specialized Invertebrate Eaters
The insects that regularly feed on ferns are a small, highly adapted group that has evolved mechanisms to circumvent the plants’ defenses. One example is the Florida fern caterpillar (Callopistria floridensis), the larval stage of a moth that feeds heavily on varieties like the Boston fern. This specialized caterpillar appears unaffected by the phytoecdysteroid compounds that typically disrupt the growth of other insects.
Other invertebrates, such as the fern weevil (Syagrius fulvitarsis), have larvae that bore into the stipe (the fern stalk), causing the frond to die off. Generalist feeders, including slugs and snails, consume ferns, but they target young, tender growth, such as newly emerging fronds, which are less chemically defended than mature foliage. Wooly bear caterpillars also feed on fronds, especially later in the summer before they overwinter.
Occasional Mammalian Grazers
While insects have adapted to ferns, larger mammals generally consider them a last resort, consuming them only when preferred forage is scarce or out of seasonal urgency. Deer and rabbits browse on ferns occasionally, but consumption is sporadic and limited to the most palatable parts of the plant. Livestock, particularly goats and sheep, may graze on some species, though they are often poisoned when they ingest too much of the common bracken fern (Pteridium aquilinum).
Bracken fern is a globally prevalent species that causes significant issues for grazing animals, including cattle and horses. Animals typically avoid it due to its high toxicity, only turning to it during periods of drought or overgrazing when other food sources are unavailable. In some cultures, humans also consume the young, coiled fronds, known as fiddleheads, but they must be carefully prepared to reduce the concentration of toxins.
Chemical and Physical Defenses of Ferns
The primary reason ferns are avoided is their sophisticated and multi-layered defense system, which combines physical barriers with potent chemical toxins. On a physical level, mature fern fronds are often tough and leathery, making them difficult for many herbivores to chew and digest. Many species also possess trichomes, or fine hairs, on their surfaces that can deter smaller insects from reaching the edible leaf tissue.
The chemical defenses are the most effective deterrents, targeting both insect and mammalian physiology. For insects, ferns often produce phytoecdysteroids, which are hormone mimics that interfere with the molting process, causing developmental defects and often death.
Against mammals, many ferns, including bracken, contain thiaminase, an enzyme that inactivates Vitamin B1 (thiamine) in the digestive tract. Consumption of thiaminase-containing ferns can lead to a neurological disorder in animals like horses and pigs due to Vitamin B1 deficiency. Furthermore, bracken fern contains ptaquiloside, a compound that is carcinogenic and can cause bone marrow damage and tumors in grazing animals like cattle and sheep.