Define struggle for existence.

Explanation

**Step-by-Step Solution:**

1. **Definition of Struggle for Existence**: The struggle for existence refers to the competition among individuals for limited resources necessary for survival and reproduction. This competition can occur both within the same species and between different species.

2. **Types of Competition**: The struggle can manifest in various forms, including competition for food, space, mates, and other essential resources. Organisms must compete to secure these resources to survive and reproduce.

3. **Example of Intraspecific Competition**: An example of this struggle can be seen in the mating behavior of tigers. When two male tigers compete for the attention of a female tigress, they may fight each other. The winner of this competition is more likely to mate with the tigress, demonstrating the struggle for existence in terms of reproductive success.

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