Look but Don't Eat Toxic Plants
Catalina Island Conservancy
Catalina is home to two of planet's most toxic plants, both imports from the Old World. There are also native plants here that contain powerful chemicals that cause harm when ingested. On Catalina Island, we might think about toxins as defense against mammals that munch. But the evolution of plant defenses is frequently a story driven by insect herbivores.
The Island’s two most toxic plants were introduced for their ornamental appeal. Oleander, (Nerium oleander), a drought-tolerant shrub native to the Mediterranean region, was brought to California for its ability to remain lush and green with minimal watering. All parts of the plant are toxic and, if ingested, will cause nausea, vomiting, dizziness, irregular heartbeat and, in extreme cases, death.
Castor bean (Ricinus communis) was introduced for its tropical appearance and the commercial production of its oil as a lubricant. It hails from the Southeast Mediterranean, East Africa and India and has been transported around the world. Beans contain ricin, one of the most toxic compounds known. As few as two or three seeds have been known to be fatal in humans. Castor bean is also considered invasive in Southern California. Its fast-growing, suckering habit allows it to spread in canyon bottoms in this part of the state.
Plants in the nightshade family (Solanaceae) include edible garden favorites like tomatoes, egglant and peppers. Other plants in this family contain toxic alkaloids that, when ingested in high quantities, can result in horribly unpleasant side-effects. Catalina is home to four notable members of this family: Douglas’s nightshade (Solanum douglasii), Wallace’s wild tomato (Solanum wallacei), jimson weed (Datura wrightii) and tree tobacco (Nicotiana glauca). All parts of these plants are poisonous and can result in anything from a scratchy throat, nausea, and vomiting to delirium, coma and death.
Although numerous members of the pea family are edible (this family includes all peas and beans), nearly 100 others are considered to be toxic. Poisonous members of this family found on Catalina include rattleweed/locoweed (Astragalus trichopodus), wild sweetpea (Lathyrus vestitus) and lupine (Lupinus albifrons albifrons). All are extremely harmful when consumed in large quantities. Their bitter tastes make them uncommon choices for human consumption and natural deer repellants. However, livestock elsewhere have been known to die from consuming these plants.
The poisons in these plants are all expensive to manufacture — they take energy and resources away from growth and reproduction. A simple evolutionary explanation is that plants with such defenses ultimately were more successful than those without.
An interesting twist to this story is that research from around the world has shown that plants on islands can lose defense mechanisms in the absence of pressure from herbivores. Research from Santa Cruz Island found island plants have lower levels of toxins than their mainland relatives.
Pepperdine University research found deer chose Catalina Island toyon (Heteromeles arbutifolia) leaves over toyon leaves from the mainland in feeding experiments. That experiment did not measure toxins but did find Malibu leaves had significantly more and longer spines. Island plants that lose their expensive defense mechanisms over the millennia are at a big disadvantage when suddenly faced with introduced herbivores like goats and deer.
We humans know to avoid plants that poke us but don’t always know when something is poisonous. Our advice is to avoid eating any plant unless you are 100% certain it’s edible! Get a second opinion. Get a third. And if you’re still in doubt, DON’T EAT IT!
By Alexa Johnson
Alexa Johnson is the Catalina Island Conservancy’s Outreach and Naturalist Training Specialist. For more information, please visit our website.