The “Mouse Trap Tree” is a curious plant, named for its surprising natural phenomenon. This name prompts questions about how a tree could act as a trap and what organisms might fall victim to it, revealing a unique botanical adaptation.
What is the Mouse Trap Tree?
The “Mouse Trap Tree” refers to species within the Pisonia genus, particularly Pisonia grandis and Pisonia brunoniana. Pisonia grandis is also known as Grand Devil’s-claws, Birdcatcher Tree, Birdlime Tree, or Cabbage Tree. Pisonia brunoniana is sometimes called Australasian Catchbird Tree or parapara. Both belong to the Nyctaginaceae family, which includes the common bougainvillea.
While the name suggests trapping rodents, these trees are known for entangling birds. Their common names reflect the plant’s distinctive sticky fruit.
The Sticky Mechanism Explained
The Pisonia tree’s trapping ability lies within its fruits. Female flowers produce elongate, cylindrical fruits covered in extremely sticky, hooked prickles. This adhesive substance, often described as a thick mucus or resin, is exceptionally strong.
When birds, particularly seabirds, come into contact with these fruits, the seeds readily adhere to their feathers. The tree’s open branching system and large size make it a favored nesting site for many bird species, such as Brown Noddy, White Tern, and Red-footed Booby. Birds can accumulate numerous seeds as they move through the canopy.
The stickiness is so potent that even a small number of seeds can weigh down a bird, hindering its ability to fly. In severe cases, birds become so encumbered with seeds that they are rendered flightless, falling to the ground. These immobilized birds then face starvation or become vulnerable to predators like crabs or rats.
Native Habitat and Characteristics
Pisonia grandis is a lowland tropical tree found across the Indo-Pacific region, from Madagascar to Polynesia. It thrives in wet habitats like coral cays and atolls, forming dense forests. These trees can grow large, reaching 20 to 25 meters (66 to 82 feet) tall, with some exceeding 30 meters (100 feet).
It has a cream-colored trunk, large spreading branches, and dense, light green leaves up to 30 centimeters (12 inches) long. The bark is smooth. Small, funnel-shaped, greenish-white flowers bloom in clusters from summer to fall. Pisonia brunoniana is a smaller tree or shrub, growing up to 6 meters (20 feet) tall, with soft wood and brittle branches. Its leaves are large, glossy, and dark green.
Ecological Role and Human Interaction
Pisonia trees play a complex role in their native ecosystems. They are important nesting sites for many seabird species, supporting nearly all tree-nesting birds in areas like Palmyra Atoll. The guano produced by these large bird colonies contributes to nutrient cycling within Pisonia forests.
While the sticky fruits can trap birds, this mechanism also aids long-distance seed dispersal. Birds that manage to fly away with fewer seeds can transport them to new islands, helping the species spread across vast ocean expanses. However, the high mortality rate of birds due to entanglement raises conservation concerns. Conservation efforts on islands like Cousin Island have involved replacing Pisonia trees with other native species and actively rescuing entangled birds.
In Hawaii, the sticky fruits of Pisonia brunoniana were traditionally used by bird catchers to trap birds for their feathers, which were then used to create capes and other ceremonial objects. The wood of Pisonia grandis is generally soft and weak, decaying rapidly, and is not typically used for timber.