Aad van Meerkerk, Wikimedia Commons

The Emerald Isle has its share of invasive species that threaten biodiversity. Here we present Ireland’s 10 Most Unwanted.

Although its greenish-brown hue might match St. Patrick’s Day attire, the Asian clam -- native to China, Korea, southeastern Russia and the Ussuri Basin -- does not belong in Ireland. According to Invasive Species Ireland, it arrived in a boat's bilge water, as bait and as escapees from the aquarium trade. The clam competes with native species for food and space.

Alexey Krasavin, Flickr

Lambay Island, off the coast of Dublin, is home to an unwanted population of black rats. They arrived as stowaways on freight containers and ships. Non-native rodents such as these dramatically affect island ecosystems, which tend to be small and specialized. Rats have a hunger for eggs, and prey upon birds, reptiles and other small species.

S. Pothoven, Great Lakes Environmental Research Laboratory/NOAA

Like the Asian clam, bloody red shrimp probably came to Irish waters in bilge water, as bait, or due to the aquarium trade. Native to the Black and Caspian Seas, this bulbous-eyed species competes with native Irish marine life for resources. 

Noel Burkhead, USGS

The brown bullhead catfish was intentionally introduced for anglers, who still consider it to be a valuable game-fish. Unfortunately, the Northern Ireland Environment Agency and other groups suspect that the catfish likely threatens trout, eels, crayfish and other marine species. It also could spread parasites and disease.

Christian Fischer, Wikimedia Commons

The Chinese mitten crab suffers from the fact that it is tasty. It was probably brought into Irish waters due to the live food trade, smuggling, and as a result of other human activities.

"As well as damaging natural biodiversity, invasive species can cause serious problems for local communities,” Cathy Maguire of Queen’s School of Biological Sciences said. Chinese mitten crabs burrow into soft sediment banks, increasing the potential for erosion.

Noel Burkhead, USGS

Biologist Dave Tosh of Queens University discovered the white-toothed shrew while studying the diet of Ireland’s barn owl. Owl poop yielded the remains of the shrew.

“Having looked at hundreds of pellets from Ireland, already I knew that what I was looking at was very unusual as our native pygmy shrew is very small in comparison,” Tosh said. The white-toothed shrew likely is a threat to the tinier native shrew, along with other small mammals.

Mediterranean earthwormNoel Burkhead, USGS

The Mediterranean earthworm P. amplisetosus is the only species on this list that is not technically an invasive species, because it doesn't compete with resident species for resources. It probably came to Ireland via nursery plants or as live bait. What it can do is increase carbon dioxide, a greenhouse gas, in the atmosphere.

"If other soil decomposers like P. amplisetosus start to expand their habitat ranges we could see increasing amounts of CO2 being released from the soil where previously this carbon had been locked up because it was inaccessible to native earthworm species," said Olaf Schmidt, who studied the worm and is a researcher at University College Dublin.

Ryan Hagerty, USFWS

The feral ferret problem in Ireland is yet another case of human intentions gone wrong. People originally brought ferrets to the green isle to help control rabbit and rodent populations. The ferrets themselves have since bred and multiplied. They now threaten Ireland’s native bird species.

Robert Orr, Flickr

Ireland has a native red squirrel whose resources are threatened by the grey squirrel, which originated in North America. Humans intentionally introduced the squirrel to Ireland in the late 19th century during a misguided effort to improve biodiversity.

Muntjac deerJoe Pell, Flickr

Originally from China, muntjac deer were deliberately introduced into the U.K. in the early 20th century. In some instances, they escaped from safari parks and then multiplied. According to Invasive Species Ireland the deer overgraze, help the spread of disease to livestock, and cause non-native plants expand their footprint.