Probably the one who has no predators, no competitors, who is resistant for pathogens, who can shape its habitat according its own needs. Usually there is no such a species in the natural ecosystems. General rule in ecology: if there is something alive, there should be some organisms which are able to eat it. During the time food network becomes quite stable. As a result most of the species are well adapted for their role in it. However sometimes new species (the aliens from planet Earth) arrive and then things get complicated.
Alien VS Invasive species
Most of the alien species do not affect native ecosystems considerably. Especially if they do not find suitable nutritional resources or native species which are much better competitors than they are. Even changed climate, soil, humidity or other abiotic conditions can be very challenging for the aliens or disrupt their reproduction cycle. However sometimes these aliens can find a perfect set of conditions during their journey to other ecosystems. As a result they can start to act as a superior species. If their effect on native ecosystems is significant or even harmful these species are called invasive species.
Why invasive species have a benefit compared with native species?
No one in native ecosystem knows what they are. Natural potential predators do not identify them as a food source. They can be resistant for pathogens and repellents of the competitors due to different chemical composition which evolved in completely different ecosystem far away.
How invasive species spread around the world?
The same way as humans are traveling: airplanes, ships, trucks, railways etc. Do not be surprised if you find some alien travelers got stuck in your shoe sole. One more way for this species to spread is release of exotic pets, many aquatic species travels across the world in ballast waters.
Why should we be concerned about invasive species?
Some people might think that more species means increase of biodiversity. However, most of them spread across new ecosystems and out competes native species, spread the deceases native species are not resistant for, occupies space, light, food sources etc.
As a result, spread of non-native species should be strictly controlled because of negative consequence for native biodiversity and human interests. What invasive species you are familiar with? What species are spreading across your home, city, country? Please share this info in comments 😊