If the loosestrife was to spread in an ecosystem, how will it affect the a animals

It's dangerous. It chokes wetlands and other areas. In wetlands, fish (and other creatures) use the area to hide, feed, mate, and produce the next generation. If it becomes a tangle of roots, the fish cannot exist there. Animals can no longer penetrate the area. On drier lands, it chokes out plants which starves the animals who would prefer to eat plants native to their area. Overall, the biodiversity is decreased. Think of it as a rapidly spreading killing machine. Where hundreds existed before it, but now there's only one.