According to the second paragraph, by the middle of the 20th century ___.

are not used to the local environment

never get along well with other native species in the same environment

tend to do harm to exotic species

have been part of a given biological landscape for a long period

are not popular in Asian markets

are never in the trade of animal parts

are used for making traditional medicines

cannot be found in Asian markets

non-native species have caused badly damage to native ones

introducing new exotic species to local environments is necessary

exotic species have never been introduced on islands

very few native species have been damaged by exotic species

do no harm to native species and the local environment

may kill native species for food

always share the environment peacefully with native species

help to make the local environment more ideal to survive

are not used to the local environment

never get along well with other native species in the same environment

tend to do harm to exotic species

have been part of a given biological landscape for a long period

are not used to the local environment

never get along well with other native species in the same environment

tend to do harm to exotic species

have been part of a given biological landscape for a long period