Biology concepts – introduced species, invasive species,
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The United States is a melting pot, and it is one of our greatest strengths. The questions is, is it also a good idea for plants and animals? |
The United States is an amazing place to live; nearly everyone’s family is from some place else. But if you ask the people you meet on any given day, they will likely say they are from the USA. Most have had time to assimilate and find their niche in their family’s adopted homeland. Can you name a place on Earth where this situation applies to its animals and plants?
If a place like this existed, it would probably be some place young. It would probably also be someplace isolated, where the exchange of species would not have been easy. Sounds like an island to me; probably a volcanic island(s), something like the big island of Hawaii, in a chain that is only 300,000 years old….. O.K., it is the big island of Hawaii.
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Hawaii is the biggest and youngest of the Hawaiian Islands. Formed from five active volcanoes, it is growing larger everyday. |
Since they are islands, it makes sense that many of Hawaii's species came from somewhere else. A key question is, how did they get there? If seeds were brought by migratory birds or washed up on shore, or if an animal wasn’t quite dead when a predatory bird dropped it on the island, that’s one thing. But if humans brought plants or animals and released them by accident or deliberately, that is something else.
History shows the latter mechanism has been responsible for most of the diversity in Hawaii. Estimates are that the rate of species introduction in Hawaii has outpaced the natural rate of diversification by 2 million times. Over half of the island’s plants species are there because of people, not nature.
Many of the plants and animals that have been brought to Hawaii were introduced deliberately, but that is not the definition of an introduced species. Whether an organism is imported and released to serve some specific purpose, or whether it is a stowaway on a ship or otherwise unknowingly allowed to get loose, it is an introduced species (also called neozoon, alien, exotic, non-indigenous, or non-native species). Introduced species are those plants and animals living outside their native range due to some human intervention, whether intentional or not. For Hawaii, this began in the 19th century.
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The Hawaiian monk seal is known as Ilio‐holo‐i‐ka‐uaua in Hawaiian, meaning, “the dog that runs in rough waters.” They are critically endangered, with only about 1100 remaining individuals. |
Hawaii has only two native mammals, the Hawaiian monk seal (Monachus schauinslandi) and the Hawaiian hoary bat (Lasiurus cinereus semotus), and no native terrestrial (land) mammals. Many other mammals were introduced in the name of making money, like cattle and goats, while others were brought in specifically to make hunting more enjoyable.
In the last few posts, we have been discussing the activity patterns of different organisms, and we suggested that the organisms that interact most likely have the same or overlapping activity patterns. A tragic story illustrating this concept has played itself out in Hawaii since the late 1800’s.
Ladd & Company (from Maine) had established a lucrative sugar industry on the big island by 1834, and this increased the ship traffic in and out of Hawaii. The ships brought rats, an all too common introduced species.
In Jamaica, the problem of rats in the sugar cane fields was an old one. A successful sugar planter, W.B. Espeut, thought that introducing the Indian mongoose to the sugar cane fields could help with their rat problem. Apparently it did, and Espeut told everyone he could find. Subsequently, 72 mongooses were brought to the big island in 1883. There were some objections raised in the local papers, but as with most good ideas, they were roundly dismissed.
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It is sad that mongooses in Hawaii are causing damage, while at the same time, mongooses are endangered in their native India due to habitat loss. |
The problems became apparent not too long after mongoose introduction. In Jamaica there is a predator that eats mongooses, the fer-de-lance snake, but no such predator in Hawaii. What is the number one cause of death for mongooses in Hawaii? Old age!
No predator means unfettered reproduction, and female mongooses can have two litters each year. The result has been lots of mongooses, all looking for a meal and a mate.
Did all these Hawaiian mongooses do their job, did they get rid of the rats? Not really, and this is where a biology class could have helped. Mongooses are diurnal, they hunt during the day, but the rats on Hawaii are nocturnal. Oops. You would have thought someone might have noticed that beforehand.
I’m sure the odd mongoose runs into the odd rat as one goes to bed and the other begins its day, but that isn’t enough to force either species to adapt; everyone is happy keeping to his old schedule. Now Hawaii has too many rats and too many mongooses.
Mongooses (mongeese?) will eat almost anything (fruits, snails, mammals, insects, amphibians, lizards, spiders), but they really like bird eggs. Many species of bird in Hawaii, including the Nene (Hawaiian goose, the state bird) are on the brink of extinction because of the mongoose.
The story of the mongoose and the rat illustrates another point. Not only are there few mammals on Hawaii, there are even fewer native predators. As a result, many animals and plants that happen to be from Hawaii originally (or least for a long time) have adapted to this lack of predators by not developing defense mechanisms. This is an advantage, in that energy normally spent on defense can be saved for other metabolic or reproductive activities. Nature always wants to reroute energy if it is being used unnecessarily.
But, if predators are then introduced (and they were by the bucketful after Western Europeans and Americans became involved) the native animals are particularly at risk. The lack of native predators and the introduction of alien predators is illustrated by the case of the western yellow jacket. Though often mistaken for a bee, the yellow jacket is actually a wasp, and a nasty one at that.
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Western yellow jackets have a smooth stinger, so they can sting multiple times. However, they rarely sting when away from their nest. |
The yellow jacket is an example of an accidentally introduced species, arriving in a shipment of Christmas trees. Law required that a percentage of the trees be shaken to knock off insects before shipment, but the required percentage of shaken trees was apparently too small, or the time shaken was too short, because it didn’t work.
In the continental U.S., the yellow jacket forms an annual nest and starts over building a colony each spring, but in the warmer climate of Hawaii, the species has become perennial, with nests as large as an SUV – more like a Lincoln Navigator, not the small Honda CR-V.
The yellow jacket is a carnivorous wasp only as a larva. The adults eat only nectar, but acquire meat to feed their young. This has decimated native Hawaii insect species, which in turn has reduced the amount of food available to bird species. In addition to insects, the wasps will devour dead birds and other large vertebrates, but will kill lizards and amphibians to feed their young.
In addition to accidentally introduced species, there are many feral (fera in Latin = wild beast) species on the big island. Feral species are animals that were domesticated, but have returned to the wild and propagated there. Their freedom could have come by accident or on purpose, as with cats and dogs in the cities. We call them strays, but they are correctly referred to as feral.
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The stories of sewer alligators inspired sculptor Tom Otterness to create a slightly scary piece in bronze. It is located in a14th Street subway station in NYC. |
All those alligators in the sewers of NYC aren’t feral, because they were never domesticated. And yes, alligators have been caught in the sewers of NYC; the latest Time and Post articles I could find were from August of 2010. They don’t live for years, grow to be monsters, or reproduce - but small ones, probably recently released, are found just about every year.
In most cases, feral animals cause problems and don’t offer many advantages. They can act as reservoirs of disease, compete with native species for resources, prey on indigenous species and eat native plants.
If there are benefits to feral animals, they will depend on your view of things. Some ranchers can make money by rounding up feral cattle or horses. Feral canines provide an income for the town dogcatcher. Stray cats can help keep the rodent population down; there seem to be many advocates for feral cats. There is even a website that advertises all the wonderful things about sterilized feral cats. Really? They want to catch them, sterilize them, and then release them again?
Literally, thousands of plants and animals of all kinds have been introduced to the big island, and now we get to the crux of the issue – introduced species that become invasive species, ie. those that do damage to the natural ecosystem by becoming dominant by killing or displacing native species. The question is – which is the exception, introduced species that become invasive and do damage, or introduced species that do little damage or even result in a benefit?
On the negative side, we have talked about the Hawaiian mongooses, rats, and yellow jackets. There are others; feral pigs and cattle graze on native grasses and other plants. The Formosan ground termite causes millions of dollars of damage to trees and structures each year. Alien plants, such as Florida prickly blackberry and molasses grass smother native vegetation and prevent their re-establishment. As a result of these and other invasives, Hawaii has more endangered species per square mile than any other place on earth. This is due, at least in part, to invasive species.
Other introductions have been moot. For instance, over 4,600 species of plants have been introduced into the Hawaiian Islands over the last 200 years. However, only 86, less than 2% of the total, have become serious problems for native ecosystems.
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Horses were introduced to the Americas by the Spanish in the 16th century, as were sheep and cattle. De Soto brought 13 pigs to Tampa Bay in 1539; these were the ancestors of the razorbacks of the Southeast… and the University of Arkansas. |
Some introductions have been wildly successful. Indeed, most of the cultivated crops (except for corn, turkeys, tomatoes, potatoes, and peanuts) and livestock animals in the continental U.S. are introduced species. Your pet cat, dog, bird, fish, or snake is probably an introduced species as well.
Of the approximately 5,000 alien animal and plant species in Hawaii, only about 300 to 500 have gone on to wreak significant damage and some have been beneficial. So the question remains, which are the exceptions- the failures and accidents that have resulted in destruction, or the successes and the accidental introductions that have had negligible effects?
Websites are full of lists of invasive species, the species they are displacing and the lost resources due to their introduction. We don’t see lists of introduced species that have worked out just fine. Perhaps this is as it should be; attention should be paid to those problems that need to be resolved. Attention should also be focused on the failures as a learning opportunity when future introductions are contemplated. But don't assume that a species is a problem just because it was introduced.
Lemoine NP, Burkepile DE, & Parker JD (2016). Quantifying Differences Between Native and Introduced Species. Trends in ecology & evolution, 31 (5), 372-81 PMID: 26965001
Nelson FB, Brown GP, Shilton C, & Shine R (2015). Helpful invaders: Can cane toads reduce the parasite burdens of native frogs? International journal for parasitology. Parasites and wildlife, 4 (3), 295-300 PMID: 26236630
For more information, classroom activities and laboratories on introduced and invasive species, see:
Introduced species –
Invasive species –