What are invasive species, and how do they destroy ecosystems

    20 Sep 2021

    Almost the entire history of mankind is somehow connected with displacement. Sometimes it occurred for quite considerable distances. Going on a sea voyage, pilgrimage or military campaign, we, intentionally or accidentally, carried away various species of animals and plants.

    Let’s get to know on Vsevolod Rudy, author of the Telegram channel Bar “Crazy Naturalist”, opinion about what invasive species are. Also we’ve found what is fraught with the entry of a stranger into the existing ecosystem, and how people struggle with it where it has already happened.


    Navigator Pausanius

    From the banks of the distant Nile

    Also brought deer skins to Rome,

    And Egyptian fabrics

    And a big crocodile.

    Nikolay Gumilev, “Navigator Pausanius”

    Species introduced by humans to new habitats, usually far beyond the limits of their natural range, are called introduced species. Introducent is not yet invasive; a creature caught in alien conditions can simply die without giving offspring, or form a small, not particularly noticeable population on the outskirts of a foreign ecosystem.

    But if a news species begins to spread in a new place, causing damage to the local fauna and flora, and sometimes to people, biological invasion begins. To be honest, it is impossible to predict the consequences of the introduction of a particular species into the ecosystem – there are too many variables for this to be taken into account. So usually they learn about the introduced species when the triumphant procession of the invader can no longer be stopped.

    Snakes, snakes around: how the brown tree snakes ate the island of Guam, and the pythons ate the Florida peninsula

    As we noted above, not every species is capable of becoming invasive. But what exactly helps the invaders to gain a foothold in a new place? Let’s try to answer by considering two examples of snake infestation.

    The tiny island of Guam is 544 sq. km is located in the Pacific Ocean, halfway between Australia and Japan. It is a US territory. Guam was captured by the Japanese during World War II and liberated by American forces in 1944. But along with American military equipment shipped from neighboring Papua New Guinea, new, much more dangerous invaders came to the island. Namely – brown tree snakes (Boiga irregularis), medium-sized weakly venomous tree snakes.

    It should be noted here that of all existing ecosystems, the island ones are the most sensitive to invasions. Often, there are few predators on remote islands, they simply cannot enter isolated territories, and conditions there do not change for years. Here evolution forms endemics – unique, unlike anything else, which are not found anywhere else – and disappear from any adverse external influence.

    This is the fate that befell almost all Guam birds. Having never seen snakes, they simply could not resist the tree snakes which occupied the free ecological niche of the bird hunter. Snakes, having fallen into suitable conditions – a warm climate, the absence of predators and an abundance of food, began to multiply uncontrollably, growing up to 3 meters instead of the usual size of 1-2.

    Reptiles literally ate 9 of the 13 native species that once lived on the island. Two endemics were miraculously saved and kept in captivity: the Guam shepherd boy (Hypotaenidia owstoni) and the Guam kingfisher (Todiramphus cinnamominus) disappeared from the islands, but several individuals survived in zoos. What to do with them is not entirely clear: the native island is still teeming with snakes, and an attempt to return to the wild means certain death for the birds.

    Two more species survived in the wild. These are the Guam salangan (Aerodramus bartschi), whose rocky nests are less accessible for snakes, and the Micronesian aplonis (Aplonis opaca), which has adapted to live next to humans. All other birds, as well as several species of local bats, were exterminated.

    The island’s forests have become the backdrop for horror movies. Absolutely silent, devoid of bird voices, full of snakes and hung with cobwebs – after all, having lost their natural enemies, local spiders also began to reproduce at an incredible speed.

    But the forests themselves are now having a hard time: before, birds and bats pollinated trees and spread seeds. Now the seeds have become much smaller, and those that have formed remain under the parent trees, where the lack of sunlight reduces the chances of survival.

    The future of Guam is uncertain – although the US military is waging a chemical war with snakes, scattering poisonous baits stuffed with paracetamol through the forest, the lost ecosystem cannot be returned. Moreover, there are military bases and a large port on the island. Perhaps right now, in the hold of a ship, the boyies are going to other islands to wreak havoc there too.

    Let’s leave the plight of Guam and move on to another American territory – the Florida Peninsula. Paradise beaches, vast swamps with water cypresses… And a staggering number – more than 500 – invasive species of animals and plants. Asian rhesus macaques and Yemeni chameleons from the Arabian Peninsula roam the branches. Underwater, like vacuum cleaners, they suck in the local fauna

    poisonous lionfish that have penetrated the seas from Southeast Asia, and giant Burmese pythons (Python bivittatus) glide through the unique swamps of the Everglades in thousands. Let us dwell on the latter in more detail.

    Oddly enough, snakes reaching over five meters in length have long been one of the most popular pets in the United States. They were brought into the country literally in tens of thousands, and the release of a bored pet “free” by a negligent owner was only a matter of time. However, there is a version that hurricane Andrew, which struck Florida in 1992, was the key moment in the spread of snakes. Among the buildings he destroyed was a python farm designed to satisfy the needs of terrarium lovers.

    The first finds of scale giants in the Everglades National Park date back to 1990. After that, snakes went underground for ten years, and they started talking about a fixed population only in the 2000s when huge pythons began to be hit by trucks on Florida roads. Nowadays, the reptiles inhabiting the state are already number in the thousands. Due to the obstruction of vast tropical swamps, which happily warmed the snake on their chest, it is challenging to assess the population adequately. According to various estimates, there are now between 30,000 and 300,000 pythons living in Florida!

    As you know, one boa constrictor is 38 parrots and one parrot’s wing. But one Burmese python is dozens, if not hundreds, of mammals, birds and even reptiles that have been destroyed.

    Research in the Everglades from 2003 to 2011 showed that even populations of common species such as raccoons and possums have declined by 90%! And, for example, foxes and swamp rabbits disappeared altogether.

    Another study, which released rabbits with radio transmitters to track their movements, showed that 77% of all animals that died during the year met their end in the suffocating embrace of a snake.

    Moreover, pythons are not limited to mammals – scientists have found near-water birds like herons and even alligators in their stomachs. Alligators, by the way, are the only swamp animals that can periodically win in a battle with strangers. However, they are still not enough to keep the population of rapidly breeding snakes under control. Florida hunters, who annually kill hundreds of pythons wherever they see them, do not cope with this.

    The Coprolite Chronicle: What Fossilized Feces Tell About Dinosaur Life

    Summing up, we can say: invasion occurs when a species falls into conditions suitable for it and there are no natural enemies or competitors in the ecological niche for it. “Bonus points” also gives a quick reproduction – so, the number of eggs found in the body of one of the killed female Burmese python was a record 73! But this is not necessary, because the absence of threats means that almost all the offspring of the invaders survive, continuing the work of destruction of ecosystems started by their parents.

    Bottom, ballast and loud frog: the sad story of the Black Sea and not only

    We figured out what invasive species with their destructive effect on native flora and fauna are. However, they did not touch upon another important aspect: how does the appearance of “overseas guests” affect humanity? In particular, on the economies of those regions where the stranger did end up? And the Black Sea, familiar to many Russians, can tell us about this.

    The first swallow of a large-scale invasion of this region was the large rapana mollusk (Rapana venosa). Yes, yes, these large shells with the inscription “Anapa-2006”, which have been seen by everyone who has ever rested in the south of Russia. Rapana was first discovered in the Black Sea in the 1940s. It is believed that the tenacious shell arrived at the resort on the bottoms of ships that were transported along the Trans-Siberian Railway after the end of the Russo-Japanese War.

    Once in the warm Black Sea waters, the rapana began to eat all the bivalve molluscs that it could reach actively. It not only destroyed several species of endemic species, but also practically exterminated the commercial species – that is, the Black Sea oysters, mussels and scallops. Perhaps, in some other sea, the rapa would have been found, but not in the Black Sea, where sea stars, its main natural enemies, do not survive due to low salinity. But the settlement of the rapana was not the last page in the history of the Black Sea invaders. The oyster and mussel fishing that has sunk into oblivion was just flowers. Berries started in 1982. After all, it was then that the ctenophore Mnemiopsis (Mnemiopsis leidyi) entered the water area with the ships’ ballast waters.

    Ctenophores are unusual creatures, most of all similar to jellyfish but not directly related to them. They got their name for the rows of rowing plates stretching along with transparent jelly-like bodies. Most ctenophores feed on plankton – various, mostly microscopic animals and plants drifting in the water column. These small fry are the backbone of the food chain in almost any body of water. It forms the basis of the nutrition of many species of fish, which, in turn, are eaten by other organisms – predatory fish, dolphins, birds, seals… In addition, plankton plays an important role in water filtration: for example, a tiny crustacean – the Baikal Epischura (Epischurella baikalensis) – should be thanked for its crystal clear the water of Lake Baikal. That is why Mnemiopsis, which has been bred in the Black Sea, has become, without exaggeration, an ecological disaster.

    The comb jelly almost completely destroyed the stocks of plankton, and after it, herds of fish were on the verge of extinction. Already in 1989, the number of comb jelly reached 400 individuals per cubic meter of water!

    The lack of food hit the anchovy (Engraulis encrasicolus), almost the main Black Sea commercial fish, especially strongly.

    Like Mnemiopsis, a planktophagous fish, the fish could not stand the competition for food with gluttonous snot and practically died out. However, this was not only the case: the floating anchovy eggs developing in the water column were also mercilessly destroyed by invasive gluttons. Finally, due to the lack of natural filtration in the sea, the process of eutrophication began – pollution with excess organic matter. This led to a decrease in the amount of oxygen in the water, overgrowth of the reservoir and the flourishing of harmful microorganisms.

    The improvement came when another comb jelly, Beroe ovata, a natural enemy of Mnemiopsis, came to the sea with the same ballast waters. Having thinned its population, Beroe reduced the pressure on plankton, and fish returned to the Black Sea. However, it was not possible to return to the former level of reserves.

    Another interesting example is the coca frog (Eleutherodactylus coqui), introduced from Puerto Rico to Hawaii. The five-centimeter amphibian very quickly settled on the four main islands. It is physically impossible to fight it. The only way to destroy the thousands of tiny frogs that have occupied the entire archipelago – from parks and gardens to mountain forests at an altitude of 1,170 meters – is to burn Hawaii with napalm. But how can such a small creature affect the economy of an entire state? The answer lies in the very name of the frog. After all, “koki” is onomatopoeia.

    The amphibian’s calling card is a loud and prolonged males mating cry, “KO! KI!”. It is believed that the first part of the “phrase” drives away competitors from the territory, and the second attracts females. In the homeland of the frog, in Puerto Rico, there is even a legend explaining the origin of this sound. Allegedly, a certain goddess fell in love with Koki, the leader’s son, but did not have time to come to her beloved. An evil demon Yurakan flew into him, taking the guy with him. To perpetuate the memory of her beloved, the sad goddess created frogs that constantly repeat his name. But legends are legends, and the inhabitants of Hawaii had a hard time with these screams. The density of the frog population on the islands reaches 91,000 individuals per hectare – about five times higher than in its native Puerto Rico. The constant frog karaoke has seriously impacted the Hawaiian real estate market.

    People simply refuse to buy housing in areas infested with amphibians because it is impossible to sleep there! Moreover, there is a slight drop in the tourist flow – vacationers are also unhappy with screams of up to 73 decibels.

    But tourism is the most profitable branch of the state’s economy. Perhaps it’s time for the Hawaiians to pray for deliverance from the misfortune of that very Puerto Rican goddess. Something, but the name of her beloved, they definitely remembered for a long time.

    Human friends: how species valuable to us destroy ecosystems

    Speaking about invasive species, it is worth mentioning separately the “approximate” of humanity. Cows and horses, bees and dogs, wheat and rainbow trout… To ensure his comfortable existence, people use dozens of species of animals every day. Nothing is surprising that most of them sooner and later move to new places after the owner. And, perhaps, one of the most illustrative examples is cats.

    The domestication of cats (Felis silvestris catus) began in the so-called Fertile Crescent, a region in the Middle East whose climate is best suited for farming. When people started to cultivate plantations and harvest crops, hangers-on rodents immediately reached out to their bins. It was at this moment that the need arose for purrrCats.

    Cats, ideally adapted to catching small prey, entered our life – and stayed close, even when the need to protect stocks disappeared. Ship cats caught rats in the holds of ships sailing to different parts of the world; someone took pets with them on a long journey… furry robbers.

    Their victims were a wide variety of creatures – from giant rodents hutia (Hutia) to the infamous dodo bird (Raphus cucullatus) and Stephens Island wren (Traversia lyalli). The last example is especially significant – it is believed that the first cats arrived on the island in 1894. Just a year later, in 1895, none of the attempts to find the bird were successful.

    Moreover, feral and “self-walking” cats continue their dirty work to this day. Scientists estimate that they kill between 2 and 5 million wild birds a year in the United States alone!

    And that’s just birds – no one counted reptiles, amphibians, small mammals, and invertebrates. By releasing pets for a walk, people increase their chances of dying from a car, disease, or predator and upset the natural balance of ecosystems.

    In conclusion, we will mention one more species. Having brought great benefits to man, he has brought tropical forests worldwide to the brink of extinction. This infamous plant is the oil palm (Elaeis guineensis). Long known to people as a food culture, in the 20th century, it began a triumphant march around the world from its native West Africa.

    Corporations became interested in a fast-growing crop that produced nine times more oil than, for example, soybeans, and it was only a matter of time before plantations in the tropics around the world were established. Whole hectares of forests were destroyed for the sake of palm trees, and endless rows of identical trees replaced the ecosystems full of unique fauna. The island of Sumatra stands out especially here, having lost 40% of its green cover due to the invading palm.

    Scientists estimate that between 1990 and 2008, palm clearance accounted for 8% of all global deforestation. At the same time, the restoration of the primary tropical forest – the type of forest with the highest biodiversity – takes, without exaggeration, hundreds of years.

    Its speed is ridiculous compared to how quickly the oil palm is taking over new territories.

    It is believed that every hour an area of ​​forest equivalent to 300 football pitches is cut down for the sake of new plantations. From food to shampoos, palm oil is used almost everywhere, and it’s no surprise that demand is growing exponentially. But how this will affect the fate of rainforest dwellers worldwide – from the famous orangutans (Pongo) to tiny insects – is anyone’s guess. And in most cases, the prognosis, alas, is disappointing.


    We can talk about invasive species for hours. People constantly encounter many of them in life – be it the Sosnovsky hogweed (Heracleum sosnowskyi), which occupied our fields and roadsides, the zebra mussel (Dreissena polymorpha), which invaded the American Great Lakes, or the box tree moth (Cydalima perspectalis ) that destroyed the boxwood forests of the Caucasus. Most likely, while I’m writing these lines, another uninvited passenger travels in the hold of a ship to the other end of the world.

    But the animals themselves are not to blame for the destruction of ecosystems. They do what they must: successfully adapt to new conditions, as required by evolution. The problem of biological invasions is a consequence of the irresponsible attitude of people to the surrounding nature. It is almost impossible to fight it, and its consequences are colossal. We can only hope that we are smart enough to prevent new environmental disasters.

    Read more about domestic cats – predators in the wild, fluffy at home, here.

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