Non-Native Species (Invasive Species) — Overview & Impact

An island has humans, otters, dogs, crabs, algae, and tall grasses. The otters sometimes swim between this island and a nearby island. On occasion, humans also travel between the islands with their dogs. When one human returns to the island, his dog has the disease Brucellosis. The disease quickly spreads into the other dogs and can even make humans and otters sick. Which organism is an invasive species in this example?