All animals require food to live. The availability of food is a major factor in how many animals live in an ecosystem. Areas like rainforests with rich food supplies have more species of life than other areas like deserts and the Polar Regions where there is less food.
The arrival of new predators in an ecosystem can have a devastating effect. In balanced ecosystems, predators and prey have evolved together. Predators can catch enough prey to survive, but not so many that they kill all of their food.
The arrival of a new predator can upset this balance. An example of this is the introduction of the red fox to Australia, which has caused concern over their effect on native birds and small mammals. Introducing new predators can cause a rapid decline in the numbers of prey, which then reduces the food supply for existing predators.
When organisms inhabit new ecosystems they often bring new pathogens. As an example, Europeans first colonised North America, and introduced new pathogens, like the influenza virus. Many Native Americans had not developed immunity to new diseases such as this, and so many were killed by them. There are many examples of new pathogens being introduced to the UK. Ash dieback is a disease caused by a fungus which has killed many ash trees since it was first found in the UK in 2012.
These new pests might devastate farmer's produce.
Pathogens have also been introduced on purpose. Myxomatosis is a disease that affects rabbits. It is caused by a virus and infected rabbits develop skin tumours and may go blind. In the 1950s it was purposefully released into the wild in the UK to reduce the population of rabbits. It did exactly this and some people estimate that more than 99 per cent of rabbits in the UK died. However, our rabbits developed immunity to it and the population has now returned to previous levels.
The introduction of a new species into an ecosystem can result in it out-competing another native species. Several hundred years ago grey squirrels were brought over from North America by wealthy people and let free in their grounds. Our smaller native red squirrel couldn't compete with the newer, larger grey squirrel. Because grey squirrels are larger they can store more fat and survive harsher winters. So the numbers of red squirrels and the places they live has reduced dramatically.
Other examples of out-competition of native species by newly introduced species include the Canada goose in Europe and the cane toad in Australia, Himalayan balsam in Cambodia and harlequin ladybirds are also current concerns.