Answer:
A new organism in an ecosystem can either greatly downgrade the quality of the ecosystem, upgrade, or bring no significant changes. Although it does usually benefit the ecosystem, it's safe to say some organisms may bring a decline to vital animals already present in the ecosystem. For example, if a new and quite aggressive species are introduced to the ecosystem, it may not have natural predators that would control its amount. That would cause an overabundance of that species and a decline in other organisms that they feed on. It can overbreed and spread throughout the ecosystem eating all the species that can't compete with them (due to the fact that the ecosystem does not have a defense against them).