Ah, trees! Is there any living thing more representative of the environmentalist movement than an ancient oak or towering sequoia? Trees are bastions of life in a variety of ways: they create habitats for arboreal species, provide nutrients to herbivorous animals, stabilize soils to prevent erosion, and convert carbon dioxide into oxygen through photosynthesis. In a 2021 paper in Nature Climate Change, scientists calculated that forests worldwide absorb and sequester over seven billion metric tons of carbon each year, about 1.5 times the amount of carbon that the USA releases in that same period of time. As such, proper forest management is one of the most effective tools humans have to fight climate change (though, as I have discussed before, tree planting needs to be done strategically in order to have positive environmental effects).
But even trees can have dark sides. While many trees are content to provide habitat and food to animals in their ecosystems, others wage wars on those living things that try to consume them. Many trees produce tannins, for example, bitter compounds designed to deter animals from munching on their leaves or unripe fruit (humans use these compounds to flavor wines—go figure). Other trees, such as the acacias of Africa are covered in sharp spines designed to make dining on their leaves about as enjoyable as eating a barbed wire kabob. In the mangrove forests and brackish-water swamps across central America, though, there lives a tree that takes the life-threateningly poisonous cake. Every single part of the plant, from its leaves to its bark to its serendipitously apple-shaped fruit, is laced with a cocktail of multiple poisons that can cause swelling, severe pain, and even blindness in those who encounter it. It’s good to be a tree-hugger, but maybe don’t hug this particular tree. Consider: the Manchineel, Hippomane mancinella.
Manchineels are shrub-like trees, meaning that they have multiple woody stems as opposed to a singular trunk. As such, they tend to be quite wide, though they can also grow up to 15 meters (50 feet) tall. They have grayish-brown bark with a recurrent suture-like striping pattern, and football-shaped leaves with finely-serrated edges. Taxonomically, manchineels are in the Euphorbia family, a massive family of plants that can be found on all continents except for Antarctica. Notable examples of other euphorbias include cassava, castor plants (which incidentally are the source of the famous poison ricin), and poinsettias. Manchineels can be found throughout the American tropics, with populations in Mexico, the Caribbean, and Central America. They grow in coastal areas, such as beaches, mangrove forests, and brackish-water swamps. Most trees would die if planted in these saltwater environments, but manchineels have the ability to control the concentration of salt in their tissues, allowing them to thrive where other trees would fail.
Manchineels produce short stalks of yellow-green flowers which, once pollinated, ripen into fruits that bear an uncanny resemblance to small green apples about 2-5 cm (1-2 inches) in diameter. The fruits have two different names, and which name you call them says a lot about you as a person. If you are the kind of person who likes to admire vegetation from afar, perhaps in the form of a high-quality .JPEG on your favorite conservation blog, you might call them “Beach Apples”, for they are often found lying on warm, sandy beaches. If you are the kind of person who likes to pick random fruit off of the ground and bite it, however, you will probably call them the name they were given by Spanish colonizers: “la Manzanilla de la Muerte”—“the Little Apple of Death.”
Every single part of the manchineel tree is extremely poisonous, from its roots to its shoots. Its trunks and stems are filled with a milky sap which causes inflammation and blistering on contact with skin or mucous membranes, and its leaves contain oils with the same toxic effect. The fruit of the tree, the death apples, cause swelling and excruciating pain in the mouth, throat, and lymph nodes when ingested. The fruit’s poison can also cause bradycardia (a slowing down of the heartbeat) leading to hypotension (low blood pressure) which can be fatal. Should you manage to work your way through swallowing the fruit, its toxins then wreak havoc on the digestive system, damaging the intestinal lining and causing extreme discomfort. Most people foolish enough to eat the fruit die, though the radiologist Nicola Strickland famously took a bite of one of these fruits while on vacation and lived to tell the grisly tale.
As if deadly apples weren’t enough, manchineels have many more ways to ruin your day with poison. Manchineel toxins are water-soluble, meaning that when it rains, they dissolve into the raindrops which then fall down from the canopy. Standing under a manchineel tree in the rain can cause contact dermatitis (skin irritation) in any place where the raindrops touch bare skin, as well as ophthalmitis (eye irritation) and temporary blindness. Even the smoke from burning manchineel wood can cause these same painful and debilitating effects. So notorious is the tree for its toxicity that some indigenous people of the Caribbean use its sap to poison arrows. Some historians believe that the Spanish conquistador Ponce de Leon died after being shot with an arrow coated in this poisonous sap, which is precisely what he deserves for invading Florida (he’s lucky the Floridians didn’t kill him with a golf club-wielding alligator).
A whole cocktail of toxins can be found within manchineels, but their most dangerous compounds are phorbols. Many euphorbias, as well as other plant lineages, produce phorbols, but none with the variety and quantity that manchineels do. Phorbols work by mimicking a naturally-occurring chemical in the body called diacylglycerol, disrupting the systems that cells use to communicate, produce energy, and regulate their growth. These factors also make phorbols carcinogenic, so if you manage to survive manchineel poisoning, you can look forward to an increased risk of cancer.
But why the hell are manchineels so toxic in the first place? Is it really advantageous for a tree to produce enough toxins to make any living thing that touches it beg for death? After all, fruits generally exist to entice animals to eat them in order to spread the plant’s seeds—producing fruits this deadly seems counterintuitive. There are two theories that together can explain this bizarre adaptation. Firstly, manchineels don’t require animals to help spread their seeds. Manchineels live in coastal habitats and their fruits are buoyant, so they can use tides and ocean currents for seed dispersal. As such, the toxins may help defend the plant’s seeds until they are ready to drop off of the tree and into the sea.
A second theory for the manchineel’s extreme toxicity is an evolutionary arms race against herbivores. Plants typically evolve toxins in order to defend themselves from predation. Considering they can’t exactly run away, a plant’s best defense against being eaten is to make itself unappetizing to an herbivore. Herbivores, however, need to eat plants to survive, so over time, they develop resistances to plant toxins. The plants, in turn, develop stronger toxins, and the cycle repeats. Over millions of years, this can lead to ridiculously-poisonous plants and insanely-resistant herbivores. Going bananas with so many different toxins may have allowed manchineels to escape this arms race, as evolving resistances to so many different poisons at once wouldn’t be worth the effort for herbivores. Why spend so many resources developing the tools to digest manchineel leaves when you can just eat some other, more appetizing plant? This process of multiple organisms evolving in response to one another is called coevolution, and it has led to many of the bizarre relationships we see between different species across the world, from bees pollinating flowers to mosquitoes haunting your summer barbecue.
As coastal trees, manchineels have very deep and robust root systems to help anchor them into the ground. In addition to anchoring the trees during storms and tides, manchineel roots help to stabilize shorelines, preventing erosion from sweeping whole beaches out to sea. As climate change has warmed the world’s oceans, severe storms have become more common in Central America, and as such manchineels have become even more important to their ecosystems. Additionally, humans often plant manchineels as windbreaks, stands of trees designed to slow down ocean winds to prevent the erosion of inland soil. Manchineel wood is also quite strong, so carpenters will sometimes use it to make cabinets, though they first have to dry the wood out so that it becomes less horrendously dangerous to work with. Personally, I want to meet the madman who came up with the idea of turning the world's most dangerous tree into a wardrobe.
In most of its geographical range, manchineels are actually faring quite well, with an International Union for the Conservation of Nature (IUCN) rating of Least Concern, but the population in the US state of Florida is endangered. As of right now, there is not much information about what is causing manchineel populations in Florida to decline, and there are no publicly-available conservation initiatives to protect them. I would, however, like to take some time to talk more generally about the ecosystems of Florida and the numerous threats they face. Florida in general has a lot of endangered flora and fauna due to human activity, especially due to the introduction of non-native species. A ton of species, from emerald ash borers (Agrilus planipennis, a type of beetle which damages and kills trees) to Burmese pythons (Python bivittatus) have now established invasive populations in the state. Many foreign plant species have established invasive populations in Florida as well, such as Old World climbing ferns (Lygodium microphyllum) and the incredibly-destructive kudzu (Pueraria sp.) which has completely devoured much of the southern United States. All in all, over 100 foreign animal species and 80 foreign plant species have now developed invasive populations in Florida.
Why Florida? Do species all over the world really like Mickey Mouse and conservative Republicans enough to want to move to Florida? Two factors have combined to make this particular state a magnet for invasive wildlife. Firstly, Florida is a major point of exchange for both the reptile and the plant trade in the United States. More exotic reptiles are bought and sold in Florida than anywhere else in the world, and irresponsible pet owners will sometimes intentionally release animals into the wild when they no longer want to take care of them. Many pet owners do not recognize the challenges inherent to caring for exotic reptiles, especially large species like pythons and tegus; when Little Jimmy’s snake that was once the size of a shoestring is now the size of a fire hose, the solution for many people is to make that giant snake someone else’s problem. Additionally, ¾ of all non-native plants entering the US pass through Florida on their way elsewhere, allowing them to gain a foothold should any plants or seeds be left behind. Kudzu, for example, was intentionally brought to the US as a decorative plant, but was able to easily escape into the wild, where it has become the bane of many southerners.
Secondly, Florida has a climate that remains fairly warm year-round, allowing tropical plants and animals to thrive. Southern Florida winters very rarely drop below freezing, and as such reptiles that wouldn’t be able to survive winters further north find Florida to be a nice place to live (incidentally, this is also why Florida has so many professional golfers, who are also cold-blooded reptiles. Heard it here first, folks). In conjunction with irresponsible pet ownership, this leads to animals that were once captive developing native populations in a comfortable climate where they have no natural predators.
A rogues gallery of some of the most-destructive invasive species in Florida. 1) Red Lionfish, Pterois volitans. 2) Burmese python, Python bivittatus. 3) Island Apple Snail, Pomacea maculata. 4) Argentine Black and White Tegu, Salvator merianae. 5) Kudzu, Pueraria genus. 6) Emerald Ash-Borer, Agrilus planipennis.
Image sources: 1) Jens Petersen under CC BY-SA 2.5, 2) Mark J Andrews, under CC BY-SA 3.0. 3) RealGatba under CC BY-SA 4,0. 4) Jud McCranie under CC BY-SA 3.0. 5) Bernard Dupont under CC BY-SA 2.0. 6) UDSA APHIS, Public Domain
Invasive species can have a variety of negative effects on local wildlife. Most directly, they can prey on native species, which often don’t possess the instincts or adaptations to defend themselves against foreign predators. Burmese pythons have nearly cleared out the Everglades’ population of native small mammals, and large lizards, such as Argentine tegus and Nile monitors, often eat the eggs of native bird species. Emerald ash borers drill into ash trees, eating their inner bark and disrupting their ability to transport nutrients around their bodies. In the warm seas off the Floridian coast, voracious lionfish vacuum up young reef fish, devouring them before they can grow old enough to reproduce.
Less directly, invasive species can outcompete natives for resources. Island apple snails, for example, are larger and hardier than native snail species in Florida. They can live without food or water for weeks, scarf down vegetation incredibly quickly, and lay way more eggs than native snail species, making it difficult for the native species to find the resources they need to survive. Cuban tree frogs, which can now be found as far north as Maryland and Massachusetts, both compete with and eat native frog species, driving down their numbers. Plants like kudzu and Old World climbing ferns, meanwhile, can grow to blanket entire forests across the American South, crushing trees beneath their weight and depriving the rest of the forest of sunlight. While none of these issues are known to directly affect the manchineel, they all contribute to a loss of the unique wildlife found in Florida and make the place less habitable to the humans who live there.
So what can we do to help protect the ecosystems of Florida, the manchineel among its many bizarre species? Luckily, many conservation initiatives closely follow the state of Florida’s invasive species, monitoring for new populations, removing individuals from the wild, and educating the public to prevent further destruction. Plant quarantines and campaigns like Don’t Move Firewood have helped to slow the spread of Island Apple Snails and Emerald Ash Borers, respectively. Programs like the Florida Fish and Wildlife Conservation Commission’s Exotic Pet Amnesty provide a method for pet owners to responsibly get rid of exotic reptiles and amphibians that they can’t take care of. Education campaigns by the Nature Conservancy and the University of Florida have helped to teach native Floridians to identify non-native species and report them to authorities who can remove them from the wild. If you want to learn more about these initiatives and more, as well as donate to help protect ecosystems in Florida and beyond, consider checking out The Nature Conservancy by clicking here.
Manchineels are some of the strangest and deadliest trees on Earth. Their fruit can choke you, their wood can blind you, and their leaves and sap can burn your skin. Yet, they also protect the land in which they live, anchoring coastlines against hurricanes and shielding economically-important agricultural lands from ocean winds. Manchineels are one of many examples of the bizarre paths living things can take in an attempt to survive long enough to reproduce, and losing them in Florida would not only expose coastal areas to damaging erosion, but make one of the most ecologically-unique places in America a little less magical.
I hope you enjoyed this article. I tried something a little different this time, focusing on a plant as opposed to an animal, and I hope it was a nice change of pace. If you want to learn more about Consider Nature or receive updates when a new episode goes live, check out the links to our Discord and email list on the homepage. If you have any questions, suggestions, or you just want to say hi, you can always reach me at Considernatureblog@gmail.com
Sources and More Information:
Florida Fish and Wildlife Conservation Commission: Florida's Non-Native Fish and Wildlife
The Nature Conservancy: Stories in Florida—Slowing the Spread of Invasive Species
Sciencealert: This Tree Is So Toxic, You Can't Stand Under It When It Rains
University of Florida IFAS Extension: Manchineel
University of Florida IFAS Extension: THE INVASION OF EXOTIC REPTILES AND AMPHIBIANS IN FLORIDA
World Resources Institute: Forests Absorb Twice As Much Carbon As They Emit Each Year
Video Sources:
Bizarre Beasts: How An Invasive Snail May Save An Endangered Bird
Scishow: The Little Apple of Death (the inspiration for this article)