Maintaining and Increasing Animal Numbers:
Reproduction is a vital process for animals as it serves several important purposes. It allows animals to maintain their population size, increase their numbers, and maintain the balance of nature. Let's explore each of these reasons in detail:

Maintaining Population Size:
- Animals need to reproduce to maintain their population size. If animals do not reproduce, their numbers will gradually decrease over time due to natural factors such as predation, disease, and death. Reproduction ensures that new individuals replace the ones lost, thereby sustaining the population.

Increasing Population Size:
- Reproduction is also necessary for animals to increase their numbers. By giving birth to offspring, animals can expand their population and occupy new habitats. This is particularly important for species that have experienced a decline in numbers or are at risk of extinction. Reproduction helps to replenish and restore their population size.

Maintaining the Balance of Nature:
- Reproduction plays a crucial role in maintaining the balance of nature. Each animal species has a specific role or niche in the ecosystem, and their numbers need to be balanced for a healthy and functioning ecosystem.
- Predators and prey need to maintain a balance to avoid overgrazing or overpopulation of certain species. Reproduction ensures that there are enough individuals to fulfill these roles and that the ecosystem remains stable and sustainable.

Summary:
In conclusion, animals need to reproduce to maintain their population size, increase their numbers, and maintain the balance of nature. Reproduction is a natural process that ensures the survival and continuation of animal species. By reproducing, animals can sustain their population, expand their numbers, and fulfill their ecological roles in the ecosystem.

Without balance nature cannot sustain. Though number of animals increasing or decreasing, equilibrium should exsist. Hence reproduction is needed.