Why do living things need the other living things to survive?

Answer 1

This is not actually the case. There are a large number of living things called autotrophs, meaning that they produce their own food and are therefore not dependent on other living creatures to produce their own food. Plants, some varieties of protist (like seaweed), and cyanobacteria are all autotrophs that use photosynthesis. There are also bacteria that perform chemosynthesis, meaning that they use inorganic matter like hydrogen sulfide to make food for themselves and do not require other lifeforms. They are also considered autotrophs.

However, all animals and fungi are heterotrophs because their bodies cannot use the energy from the sun or from chemicals to perform carbon-fixing (the processes that autotrophs use to make food like photosynthesis or chemosynthesis). As a result, heterotrophs must consume organic material with "fixed carbon" to meet their energy requirements. This means that all heterotrophs minimally require an autotroph to feed them. Given the diversity of animals and fungi, there is a greater corresponding diversity of autotrophs to match the demand for resources.