The term, "Invasive species" is subbjective. Everything human beings have taken to new regions of the world could be considered invasive species. These include the rats that spread the Black Death through Europe, and the humble earthworms that till the soil in the United States. Many species are considered beneficial to human beings, such as the earthworms, or just desirable, such as parrots in Hawaii, or chameleons that eat other invasive insects.
To be fair, the archipelago of Hawaiian islands once had no mosquitoes, no ants, no feral pigs, cattle, goats, or sheep. Many of these species are a nuisance, but some have been of great benefit for human beings. Or at least they have been for the short term. What is certain is a few are completely catastrophic to native flora and fauna.
For biologists, the term invasive refers to those species of plants, animals, and marine life that once gaining ground in a completely new area wreak havoc upon the native species. This could include small pox decimating human populations, but let us examine the far more recent common threats of modern invasives. These are species that once introduced spread out of control, and are running rampant today.
Australia and Hawaii are two regions where introduced species have utterly changed entire regions. In Australia local fauna that had no build in fears of new predators and were eliminated in short order. Among the forever lost Australian species are flying fox, marsupials, giant thunder-birds and many, many more. Currently, down under, there are nearly 500 highly endangered due to replacement of habitat, introduced parasites, diseases and predators. But cascading crashes of natives is complex, too, usually involving a combination of over-population density, destroyed habitat, pollution, climate cooling or warming, and the aforementioned displacement that exacerbates all these things.
In Hawaii, once a true paradise due to its remote location, the story is more recent and no less tragic. Here there were few mega fauna to begin with, but the small birds, and few mammals indigenous to the islands existed no where else on earth. Cattle, pigs, sheep, goats and even dogs and cats brought by first settlers were quick to destroy not just the many birds, but their habitats as well. Most every bird, flower, tree, and animal now seen in Hawaii was brought here, and quickly displaced the native wild life. The loss of bio-diversity is staggering. Whole Sandalwood and Koa forests disappeared.
Yet, there is a unspoken truth about all species brought by humans, (including humans) and that is when they arrive native species plummet no matter what. This was never considered to matter until loss of biodiversity came to be understood as dangerous for all life on earth. The truth is that humans could be called the ultimate invasive species, yet the pets, plants and livestock they bring will not usually be considered "invasive" by them.
When invasive species become a major issue, everything about the region must be carefully assessed to determine what conservation measures are needed. Energy spent needs to concentrate on those species that destroy eco-systems and native Keystone species.
There are in Hawaii some wonderful aspects of invasives which are now ingrained into the culture. The pineapple, the orchid, macadamia and coffee trees, Parker ranch paniolos, (cowboys) and the colorful tropical birds are not looked upon as undesirable pests, but as part of the lifestyle of Hawaii.