When an organism enters an established ecosystem, it can indeed compete with native organisms for food. This is a common phenomenon known as interspecific competition. Here's why it happens:

* Limited Resources: Ecosystems have a finite amount of resources, including food, water, shelter, and space. When a new organism enters, it adds another competitor for these limited resources.

* Ecological Niches: Every organism has a specific ecological niche – its role in the ecosystem, including its food sources, habitat, and interactions with other species. When a new organism enters, it may overlap with the niches of native species, leading to competition for those resources.

Consequences of Competition:

* Population Decline: The native species may experience a decline in their population size due to the competition for food.

* Extinction: In severe cases, competition can lead to the extinction of native species, especially if the new organism is better adapted to the environment.

* Ecosystem Changes: Competition can alter the balance of the ecosystem, potentially causing cascading effects on other species.

Examples:

* Introduced Predators: The introduction of the brown tree snake to Guam led to the decline and extinction of many native bird species, as the snake competed with them for food (birds and their eggs).

* Invasive Plants: Invasive plants, like kudzu, can outcompete native plants for sunlight, water, and nutrients, disrupting the plant community and impacting the entire ecosystem.

It's important to note that the success of a new organism in an established ecosystem depends on various factors like its adaptability, reproductive rate, and the presence of natural predators or diseases.