There is an intense competition, for the resources within a single species than between two different species. This is because different species have different niches. A niche refers to the place, eating habit, and resources utilized by an organism. No two species will share the same niche in the same place. Hence, eating habits and resources utilized will be different, thus avoiding the competition.
In addition, this is not the same case with the organism of the same species. Such organisms have to share the same niche to obtain their resources. They have to obtain their resources from the same place and sometimes even at the same time.
Due to this sharing, there arises a competition between the same species, which result in the survival of strongest competitors.