Invasive species are present in most ecosystems, and are named as one of the 4 largest drivers of extinctions world-wide. It is therefore unsurprising that many conservationists strongly dislike them, and their eradication is a major focus for conservation. As would be expected, species which cause the most catastrophic damage have received the most attention, yet this gives the impression that all invaders are sprawling weeds, ravenous herbivores, or vicious predators. In reality, many introduced species are harmless or simply do not survive at all when introduced, and a minority may even be beneficial to native wildlife. In general the world’s ecosystems would be in a better state without them, but it is still fascinating to think about the question of whether all invaders should in fact be considered unwelcome.
Although it is commonly thought that all introduced species are ecologically damaging, in fact the majority are not. It has been suggested that invasive species follow the “10s rule”: only 10% of introduced species escape, 10% of those establish, and 10% of those become pests. For example, in France in the 19th century, many invasive plants were introduced accidentally through wool imports. 458 species were present in the wild in 1859, yet by 1950 the number had fallen to just 6, and today only 1 has become a pest. However, in some cases invaders are much more successful, for example if the climate in the introduced range is similar to the native range. Climate is not the only factor which determines whether a species can survive: which species are already present is also important. There are 2 possible scenarios: biotic resistance (which I will come to later) and invasional meltdown. The UK’s freshwater ecosystems are unfortunately spiraling into invasional meltdown caused by invaders from south-eastern Europe. Introduced zebra mussels, which out compete native mussels, provide a food source for invasive round gobies and a habitat for killer shrimps, which indiscriminately kill and maim native animals. The killer shrimps are themselves prey for the round gobies, boosting their population. The whole ecosystem is therefore coming to resemble something from south-eastern Europe rather than a native ecosystem. However, not all invaders are as monstrous as the killer shrimp. Little owls and brown hares were both introduced, yet seem to cause no problems, and have become a cherished part of our fauna. Some invaders may even have slight positive effects. In north America, feral horses grazing on salt marshes have increased crab populations and the diversity of foraging birds. Although not native, the feral horses may be filling the niche of other horse species which were abundant in north America until they were hunted to extinction around 10,000 years ago.
The proportion of invasive species which are pests is often lower if more native species are present, particularly apex predators. In Sweden, where White tailed eagles have recovered, they have eaten invasive American mink. In Ireland, booming populations of pine martens have killed grey squirrels, enabling red squirrels (which are better at evading capture) to thrive. If a species is introduced into a system with many species of predators and competitors, there is a high chance that something will either eat it or out-compete it, whereas if the ecosystem is more empty, the invaders have a field day. This idea is known as biotic resistance, and may also explain why introduced rats, cats, and goats are so devastating to small oceanic islands: small islands tend to naturally lack predators which would control the invaders. Ecosystems which retain keystone species and high native diversity are less likely to become dominated by a single species, either invasive or native. For example, by digging things up and destroying plants, wild boar increase the heterogeneity of forest floor plant communities, preventing a single species from taking over.
Attempting to eradicate invasive species may not always be the best option. It is very costly and is unlikely to succeed except on small islands. Reintroducing native species which could predate the invaders and simultaneously benefit other native species could be a better strategy. For example, reintroducing lynx would keep our populations of muntjac and sika deer in check, reducing damage to trees and shrubs. Even if the appropriate native species have gone extinct, ecological interactions can still be restored by using close relatives or ecological proxies. In the Indian ocean, giant tortoises were exterminated on Mauritius a few centuries ago, so conservationists recently introduced another species of tortoise from Aldabra, a nearby island. They have so far been successful at limiting the spread of invasive plants, as well as increasing vegetation heterogeneity by grazing, browsing, fertilizing the soil, and dispersing the trees of ebony seeds. A more outlandish proposal is to introduce giant tortoises to Hawaii (where no giant tortoise species are native) to recreate the browsing and grazing patterns of extinct moa nalos (giant flightless geese). The spiny leaves of some endemic Hawaiian plants are similar to plants adapted to tortoise herbivory on islands in the Indian ocean, suggesting that tortoises and giant flightless geese may have fed in a similar way. These examples suggest that some carefully chosen introductions could in fact be used to benefit native wildlife. Taken to the extreme, the idea of using non native species as ecological proxies for extinct native wildlife has caused some people to suggest ideas as crazy as introducing rhinos to Australia, cheetahs to the USA, and elephants to most of the world! (This is very interesting and controversial but I will not discuss it here. See http://biology.unm.edu/fasmith/Web_Page_PDFs/Donlan%20et%20al.%202006%20Am%20Nat.pdf for more info).
Although only a small proportion of introduced species actually become pests, those that do can cause irreparable damage to native ecosystems, and are a major threat to biodiversity and economies worldwide. But, whether we like it or not, in many places invasive species are here to stay, so we should consider whether focusing solely on eradicating them is the best strategy. Instead, reintroducing native species or carefully selected substitutions for extinct species may be a better way to restore biodiversity and improve ecosystem services. However, where this is not possible or not ecologically appropriate (for example in the case of invasive rats in seabird colonies), we should still definitely have a strong dislike for invaders.
Claim in intro: http://journals.sagepub.com/doi/abs/10.1177/2053019615591020
Lynx / sika deer: https://www.jstor.org/stable/pdf/3807969.pdf