Metabolic theory of ecology is a way to help us understand why creatures and plants are the size and shape that they are. It explains why some creatures and plants are small and some are bigger. It also explains why some creatures and plants use a lot of energy and some use very little.
Metabolic theory of ecology is based on the idea that all living things use energy to stay alive and the amount of energy used is related to how big something is. A small creature or plant uses much less energy than a bigger one. If something needs to use a lot of energy, it will tend to be bigger so that it can store more energy and use it over a longer period of time. Similarly, if something doesn't need to use very much energy, it will tend to be smaller and will use less energy.
This means that the size and shape of something can tell us a lot about how much energy it needs and how it gets it. This can help us to understand why some creatures and plants are big and others are small and why some eat a lot and others don't.