Why does a species become a pest when introduced to a new land when it was not a pest in its country of origin?

Answer 1

Invasive SpeciesA pest is any living thing which has characteristics which people regard as injurious or unwanted. It is possible for something to be a pest in one setting but beneficial or domesticated in another.

When millions of years of evolution in one sub-climate can enable itself to repair and overcome an indigenous species does not mean that it can be introduced into another sub-climate.

Many species are introduced accidentally, but others are intentionally moved. An example of this is the cane toad, which was introduced into northern Australia in 1935 as an attempt at biological control. In tests where the toads were offered no food except the beetles that were destroying sugar cane crops, they ate them. When they found themselves in the cane fields with far more palatable food at hand - surprise, surprise. They ate that.

Toads bred wildly and swarmed out of control. There are millions of them in Australia now. They are pests because they compete with native species for food and places to live, their tadpoles are aggressive, and the toads themselves are toxic and kill carnivores that eat them.

Other animals were introduced unintentionally, such as feral horses, cattle, camels, sheep, goats and pigs (escaped from farms and multiplied). These animals are extremely useful in their farms, but outside of them, are pest animals because they destroy the environment in their search for food.

The main reason something becomes a pest when introduced to a new place is that it has no predators there. There's nothing that eats it, and so it reproduces exponentially (really, really fast). In its place of origin, there was something that preyed on it, keeping its population down.

A species when introduced into a new environment or community may or may not become a pest. The introduced species may become a pest if is better adapted to the community than other organisms occupying its niche. It may also become a pest if it has no biological enemies or predators in its new community to control it's numbers.

Answer 2

New species which survive are naturally invasive because they have needed to become so in order to survive.