She sits, her sleek, green body invisible in the tangle of leaves, and waits.

High up in the trees, in her favorite hunting spot, the green anole is patient, her bright, black eyes swiveling independently of each other, searching for the smallest flicker of movement. The green anole isn’t picky when it comes to food. Special sensors on her tongue will let her know if what she catches is edible, and most anything able to fit in her mouth will do just fine as a meal.

A slight rustle of branches dissolves the silence of her vigil. Her eyes pause their scanning, then snap in tandem to the roach that has just landed on a branch about 10 inches from her face. The bug is oblivious to the silent, scaled creature that could spell its demise. Now 3 inches away, 2 inches…

The anole strikes forward, breaching through her protective canopy of leaves towards the roach. She crunches her mouth down around the roach, its struggle to escape ceasing as she swallows it whole. Satisfied, she climbs up once again to her favorite spot, a cleverly camouflaged crusader among the leaves.

Green anole lizards, the only anole native to North America, stalk the treetops across the southeastern United States. Many Southerners witness these friendly creatures hunting for moths near the porch light. They might remember summertime stretches of childhood boredom spent catching green anoles and coaxing them to bite down on the soft flesh of their earlobes, wearing the live green earrings with pride, always a nice shock to the grown-ups.

Anoles display their own fanciful adornment. Males have bright red neck flaps known as dewlaps that puff out from under their chin in a crimson crest when calling for a mate or challenging another anole to a duel. They pump their bodies up and down in an impressive show of their pushup abilities, like middle-school boys in a gym-class contest. They “drop” their tails to distract a predator, or mischievous child, the detached appendage wiggling with a life of its own, allowing the crafty crawler to slip away.

But today, the green anole faces a more dire threat. Its new competitor is none other than its own cousin: the brown anole. Since the 1940s, the brown anole, otherwise known as the Cuban or Caribbean anole, found its way to Miami and has since spread rapidly across the South. As climate change expands its warm, habitable range northward, the prolific nonnative lizard is spreading further across the American South.

“It’s really good at traveling with humans,” said Yoel Stuart, an assistant professor and evolutionary ecologist at the University of Loyola Chicago who specializes in evolutionary biology through the study of the green anole.

The brown anole is not picky about its mode of transport. It travels in the wheel wells of cars, in the hidden crevices of boats and shipping containers, tucked away in bundles of firewood and, most frequently, in exotic plants, carted from lands afar and put up for sale at garden centers across the U.S.

The brown anoles are much more aggressive than the greens. They outcompete green anoles for food on the ground, fight even harder for territory and have even been known to prey on green anoles.

Plus, they’re everywhere. In Florida, brown anoles are now the most abundant vertebrate, according to Stuart. Researchers there put their numbers at 10,000 per hectare, or more than 5,000 lizards in a single acre.

Anoles lay eggs every four to six days during the spring and summer, and it takes only a little more than a month for baby anoles to emerge.

But the story of the anoles is more than just a lizard battle royale.

Their story of survival is also the story of human survival. Biodiversity supports the stability of food chains and ecosystems, which provide sustenance and income for people and protect the built environment from natural disasters. The introduction of species to an environment can have complex, unintended consequences that impact people, animals, plants and the landscape itself.

The case of the anoles is one small instance of how something as simple as a stowaway lizard can have a massive effect on ecosystems. The brown anoles were not necessarily spread with intention by humans like other invasive species such as kudzu, nutria or Burmese pythons. But negligence and willful ignorance can have just as powerful an effect.

Green anoles, previously left undisturbed on the entire continent, have never faced pressure like they do from brown anoles today. In a turn of events that intrigues evolutionary biologists, the presence of the brown anole on the ground has pushed the green anole from the lower parts of trees upwards, climbing skyward for a new chance at survival. There in the canopy, something extraordinary happened.

“As anoles moved up into the trees, they tended to have larger toe pads,” Stuart said.  “The exciting bit about that is, it happened quite fast,” he said.

Evolutionary change typically happens on a timescale of hundreds of years, if not thousands. This 5% increase in toepad size for green anoles happened over the course of only 20 generations. This means that, with an average life span in the wild of 5.5 years, this evolution is happening on a fast track within the last century.

That’s like seeing the average height of humans going from 5-foot-9 to the size of “NBA players,” according to Stuart. It’s a brown anole world in the modern-day South, and the greens are changing in order to survive.

These little green lizards show remarkable resilience. Faced with the options of total domination or change, they’ve adapted to the new world they find themselves in, those larger, grippier toes learning to climb on ever more delicate branches high up above the ground because they have to. Change is the only option for them.

But a new home higher up in the canopy isn’t necessarily what will save the green anole.

Martin Main, professor of wildlife ecology and conservation at the University of Florida, said the knight anole — another invasive, even more aggressive, predatory lizard from Cuba, also poses a serious threat to the green anole. They eat the same food as the green anole, prey on adult green anoles and live mostly an “arboreal” lifestyle, according to Main.

Knight anoles are also huge by comparison, at 13-20 inches in length and weighing in at close to 5 ounces at their largest — or about 21 times the size of the green anole.

The potential loss of the green anole comes with the potential for biodiversity losses, too.

Lots of species diversity means better chances of survival against diseases, severe heat and changes to the environment; all features associated with a warming global average temperature. A loss of biodiversity can lead to a “homogenization” of species in that niche of the food web, meaning only one species is occupying that space. This can have unintended consequences.

“If everything is exactly the same, there’s going to be very little of that variety that provides us resistant individuals that allow a particular species to continue,” Main said.

One study from The Royal Society Publishing found that brown anoles as more vulnerable to extreme temperatures caused by climate change than green anoles. The loss of brown anoles to extreme heat after after out-competing green anoles could result in the spread of diseases, with fewer creatures to eat pests like mosquitoes and attract their bite. This disruption of the food chain could lead to the spread of insect-borne diseases such as West Nile virus.

Lawrence Reeves, entomologist, assistant professor and researcher at the Florida Medical Entomology Laboratory, described how lizards are “dead-end hosts” for the West Nile virus, meaning that lizards cannot pass on the virus when bitten by an infected mosquito, unlike other animals that harbor West Nile.

“Every bite that goes toward a lizard is a bite that goes away from a bird or a mammal,” Reeves said.

This is just one instance of how life could be affected by the loss of a species. Ecosystems are so complex and delicately balanced that it can be hard to determine what impact an introduced species will have, experts say.

“By introducing new species and causing native species to disappear, we’re fooling with stuff that we don’t really understand,” Main said. “You never really know what’s going to happen when things change in an environment. It’s so complicated.”

Consider the green anole roach hunter who used the structures available to her: green leaves, brown stems and extra grippy toes. She took advantage of her natural surroundings rather than seeking to become dominant over them.

The idea that humans better connect with their environment when viewing their place on the planet as part of a system isn’t new. In Rachel Carson’s iconic environmental book ”Silent Spring,” she argues that “man is a part of nature, and his war against nature is inevitably a war against himself.”

Jennifer Skene, clinical lecturer at Yale Law School and the natural climate solutions policy manager with the Natural Resource Defense Council, describes how that shift in thinking can allow humans to adapt just as the green anoles have.

“I think conceptually, the way that conversations are moving gives me a lot of hope,” Skene said. “The rights of nature, conversations about rights of animals and animal welfare, and the way that we think about all of these is interconnected.”