Invasive Species

We all know that invasive species are nonnative species that take over a certain area. This invasion may lead to health, economic and ecosystem problems. They may even lead the native species to extinction through competitive exclusion, niche displacement, or hybridization. But when we think of evolution, what are the effects that these "alien species" have?
When considering invasive species through the lenses of evolution, we see it not as a harm, but as a proof of the capability of some plants to adapt. Invasive species are not good when they lead the native species to extinction, but that is not always the case. Sometimes, these invasive species make the native species adapt to the consequences, and if this evolution occurs fast enough, the change is not necessarily negative. Because of international trade, and tourism,  invasive species have started spreading. They already dominate 3% of all the ice-covered land of the world. If the species are not able to adapt, they will eventually be led to extinction. An example is the soapberry bug. Their beak lengths have varied during the last 50 years in order to adapt to the new invasive hosts. The reality is that international trade is ever-accelerating, which means, that the plants will have to adapt because the invasive species will only spread more and more. That is dangerous because some plants will not be able to adapt on time and will therefore become extinct, but the ones that will, will become stronger and fitter for their environment. So, to conclude, in terms of evolution, invasive species is neither good nor bad. It is just a cause that has evolution as a consequence. Because it happens, plants and animals evolve. Sometimes, evolve to something that they would not need to become if the invasive species was not there, but that is not necessarily bad. As long as they do not change their role in the environment affecting the food chain, the ecosystem will remain stable and the invasive species will have a neutral effect. 

Invasive species

Species -> Criteria |	Anoplolepis gracilipes (insect)	Acacia Mearnsii (shrub)	Acridotheres tristis (bird)	Aedes albopictus (insect)	Anopheles quadrimaculatus (insect)
Do they grow fast?	Ö	Ö		Ö	Ö
Do they harm other populations?	Ö	Ö	Ö
Are they a threat to human health?	Ö		Ö	Ö	Ö
Do they have a high dispersal ability?	Ö	Ö	Ö	Ö
Are they tolerant to a wide range of environmental conditions?		Ö	Ö

1) In order to determine the worst invasive species, I chose the five categories listed above: growth, harm to other populations, threat to humans, dispersal ability, and tolerance to environmental conditions. The reason why I chose these five categories is that I believe they are the main determinants of invasive species. Invasive species are commonly defined as being widespread and disruptive. All of the categories that I chose fit into this definition: the growth, the dispersal ability and the tolerance to environmental conditions fit into widespread and harm to other populations and threat to human health fit into disruptive. 
2) In order to choose the invasive species that I would study, I accessed the ISSG database and observed the 100 worst invasive species in South America. I studied the first invasive species and determined the ones that fit into my categories. However, during this process I made a mistake. I did not realize that they were in alphabetical order not from worst to least worst as I had thought. Because of that, my results were not precise. On the other hand, I was able to find species that fit into most of my categories, so my results were not entirely compromised. 
3) In order to eliminate the first three species from my list, I studied the five and determined the three that fit into the least amount of categories: one of them, the Anopheles quadrimaculatus, only fit into two, so determining that that was the least worst of the five was easy. Then, Aedes albopictus only fit into three of my categories so I determined that it was the second least worst. Finally, to determine the species that would get third place I had a problem. Of the five that I had selected, the three that I had left all fit into only four of the categories. Because of that, I had to rank my categories, determining which I thought was worst and seeing which ones met them. With that, I determined that Acridotheres tristis deserved third place because it did not grow fast. 
4) Lastly, to determine which invasive species I should take to the class debate, I followed the same step I used to determine which one should get third place: which ones have the categories that I believe make them be worst than the other. With that, I decided that Anoplolepis gracilipes won over Acacia Mearnsii. The first one, fit into all of the five except for tolerance to a wide range of environmental conditions. The second one fit into all five except for threat to human health. I determined that being a threat to human health made the Anoplolepis gracilipes be a worst invasive species that the Acacia Mearnsii.