There are four levels of organisation in an ecosystem:
A species is a group of similar organisms that are able to breed together and produce fertile offspring. This includes every plant and animal species that can be found on Earth.
A population consists of individuals of the same species that live in a specific area at the same time.
A community is a group of populations belonging to two or more species that are living and interacting with each other in the same area. Each species in a community depends on another species for factors that are essential for their survival and reproduction (e.g. food, shelter etc). So, if you remove one species, it can affect the whole community.
An ecosystem is a biological community of organisms that interact with each other and their physical environment. Ecosystems include all the abiotic and biotic components that live and interact within the same environment.
An adaptation is a characteristic that helps an organism to survive and reproduce in its environment. Organisms can evolve over time to become better adapted to their environment.
Many organisms require the same resources. Due to a limited supply, they compete for these resources.
There can be competition between the same species or competition between different species.