Guest Blog Post: Researchers split the birdcatcher trees (genus Pisonia) into three

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Guest blog post by Marcos Caraballo

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The birdcatcher trees – genus Pisonia – are infamous for trapping birds with their super-sticky seed pods that would frequently entangle the body of the ‘victim’. Left flightless, the poor feathered creatures eventually die either from starvation or fatigue, or predators. Similarly notorious are the birdcatcher trees for botanists, who have been baffled by their complicated classification for the last three centuries. 

Here’s why myself and graduate student Elson Felipe Rossetto of the Universidade Estadual de Londrina (Brazil) decided to take up the untangling of this issue with our recent taxonomic studies. You can find our research paper published in the open-access scholarly journal PhytoKeys.

We reestablished two genera: Ceodes and Rockia, where both had been previously merged under the name of Pisonia. Now, as a result, there are three distinct lineages of birdcatcher trees from the islands of the Pacific and Indian Oceans: Ceodes, Pisonia, and Rockia.

“Previous molecular studies on Pisonia species from around the world showed that species were clustered into three major groups, and here we assign names for each of them. With this new classification, a large number of the species known as Pisonia will be henceforth named Ceodes. This includes the Parapara (Ceodes brunoniana) and the Birdlime (Ceodes umbellifera) trees, both native to many islands, including Hawaii and New Zealand. They are commonly planted in gardens for their lush and sometimes variegated foliage, as well as their fragrant white flowers. However, the Cabbage tree (Pisonia grandis) will still be technically known as Pisonia.”

adds the study’s lead author Felipe Rossetto.

Birdcatcher trees have generated much controversy in the popular media because of their seed pods (technically called “anthocarps”) secreting a sticky substance that glues them to the feathers of seabirds or other animals for dispersal. Sometimes, though, too many seed pods can harm or kill birds, especially small ones, by weighing them down and rendering them flightless. This macabre practice has led to many controversies and local campaigns aiming to remove the trees, even illegally.

In spite of their forbidding reputation, however, we would like to stress that birdcatcher trees have positive effects on ecosystems and are important components of vegetation, especially for small islands. Sadly, there are many endemic and already endangered species of birdcatcher trees that only exist on a few small islands, where they are effectively placed at the mercy of local people.

Many species of birdcatcher trees are large and, thereby, tolerate harsh environments like seafronts and rocky cliffs, making them prime nesting spots for seabirds. Birdcatcher trees are also ecologically curious and could be regarded as keystone species in small islands, because their soft branches can sustain many types of invertebrates; their flowers are an important food source for bees and ants; their dense leaf litter nourishes the soil; and their roots have intimate interaction with native underground fungi (mycorrhiza).

All in all, clarifying the taxonomy of the birdcatcher trees is the first step to understanding how many species exist and how they relate to each other. 

Although most people relate birdcatcher trees with beaches and coastal habitats, there are species that are only found in mountains or rainforests. For example, the species now allocated to the genus Rockia is endemic to the Hawaiian archipelago. These are small trees able to grow in dry to mesic mountain forests. Using our new classification, future studies can explore in detail the hidden diversity of these enigmatic plants, and find out how trees with high dispersal capabilities evolve into species endemic to small island ecosystems.

About the author:

Marcos A. Caraballo-Ortiz is a research associate at the Smithsonian Institution (Washington, D.C., United States). His research interests include plant systematics and ecology, with a focus on flora of the Caribbean Islands. Dr. Caraballo-Ortiz has experience studying the taxonomy of several groups of tropical plants, with a particular interest in neotropical Mistletoes (Loranthaceae, Santalaceae, Viscaceae) and the Four O’Clock family (Nyctaginaceae). 

For more information about his projects, visit marcoscaraballo.com.

Research paper:

Rossetto EFS, Caraballo-Ortiz MA (2020) Splitting the Pisonia birdcatcher trees: re-establishment of Ceodes and Rockia (Nyctaginaceae, Pisonieae). PhytoKeys 152: 121-136. https://doi.org/10.3897/phytokeys.152.50611