most exotic species are introduced into a new environment accidentally as a result of
Introduced species Unintentional introductions occur when species are transported by human vectors. Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they are not considered native. For example, three species of rat (the black, Norway and Polynesian) have spread to most of the world as hitchhikers on ships. There are also numerous examples of marine organisms being transported in ballast water, one being the zebra mussel. Over 200 species have been introduced to the San Francisco Bay in this manner making it the most heavily invaded estuary in the world.[17] There is also the accidental release of the Africanized honey bees (AHB), known colloquially as "killer bees" or Africanized bee to Brazil in 1957 and the Asian carps to the United States. The insect commonly known as the brown marmorated stink bug (Halyomorpha halys) was introduced accidentally in Pennsylvania. Another form of unintentional introductions is when an intentionally introduced plant carries a parasite or herbivore with it. Some become invasive, for example the oleander aphid, accidentally introduced with the ornamental plant, oleander.
Hawaiian crow The Hawaiian crow faces an ample number of threats in the wild, which are considered contributing factors to their extinction in the wild. Small population size makes the species more vulnerable to environmental fluctuations, leading to a higher likelihood of inbreeding, which could result in lethal deformities. Unlike most crows, Hawaiian crows did not adapt well to human presence. Persecution by humans is another threat to their survival. Farmers have shot ʻalalā because they were believed to disturb crops. Illegal hunting has continued even after legal protection was granted to the crows. Humans have also caused habitat degradation and deforestation through agriculture, ranching, logging, and non-native ungulates. Loss of canopy cover exposes the ʻalalā to dangerous predators. Chicks are vulnerable to tree-climbing rats, and after they leave their nests, to cats, dogs, and mongooses.[18] Deforestation also increases soil erosion and the spread of invasive plants and mosquitoes.[8] This directly relates to the primary cause of the Hawaiian crow's extinction, disease.