Invasive species are non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Invasive species can be plants, animals, and other organisms (e.g., microbes). Human actions are the primary means of invasive species introductions. Many invasive species are threatening the world's ecosystems and in Alaska over 100 invasive species have established populations with serious threats from Atlantic salmon, birch leafminer, Chinese mitten crab, gypsy moth, New Zealand mudsnail, signal crayfish, brown rat, roof rat, whirling disease parasite and many others. The columbid species most commonly seen around human habitation and in cities is usually the rock dove, also called the rock pigeon, domestic pigeon or feral pigeon. The introduction of the rock dove from Europe likely set the stage for extinction of the passenger pigeon. Learn about how passenger pigeon went extinct due to the introduction of rock dove and their diseases by selecting: Passenger Pigeon Extinction.