Factors of the Extinction Crisis
Alien species: Spreading of non-native species by human activities
Planned introduction: settlers brought life stock, plants and seeds;new species are introduced to increase agricultural harvests
Involuntary introduction: organisms are transported unknowingly, e.g. ballast water of commercial ships; domestic animals escape
Predators are not used to new species and discriminate against them ? invasive species are not controlled by predation, so they can gain large population size and outcompete native species
Mitigation efforts: introduction of natural predators of invasive species from their origin ? mostly worsens the situation because natural predators might prefer other (native) prey, and now the introduced predators are without predation control themselves
Establishment of alien species often only recognized when it is too late (i.e. alien species have already established significant stocks)