While that is true that we shouldn't move organisms out of their native habitat, there are more organisms than cacti. Also, the cacti aren't really harmful. In fact, they provide food for some types of animals. While that is true that we should be aware of moving organisms out of their native habitat, we still are okay with regulating biodiversity.
Yes, the work scientists and regulatory bodies do in the name of biodiversity is sufficient. The fact that almost all species of cacti are native only to the Americas is a bit misleading, because there are many types of plants that are only native to one particular region or continent. There are many nature preserves across the Americas that help protect these plant populations by limiting the encroachment of human populations.
Mother nature is smart. The earth knows what can grow in certain places and what can't. Species also naturally migrate and change over time. Humans are not as smart as mother earth. We shouldn't presume that we can fix everything, and instead, unless it's hurting the earth, we should just let things happen.
Camels also originated in the Americas before spreading to Asia through the Bering land bridge, traveling the opposite direction from the Asian immigration to America. Horses too. Went from North America to Asia & eventually Europe thanks to landbridges, while they became extinct in the Americas. Then thousands of years later the Spanish brought them back to the Americas.
Many animals are in danger of becoming extinct. The habitat of animals is often being destroyed to make way for property development or by pollution.Whilst it is necessary for development on new land for national and local development, pollution of the environment is destroying much of the biodiversity. There needs to be more protection for biodiversity. If people complain about the loss of income, the local economy can benefit from tourist attraction.