Introduction

An ecosystem is best understood as a functional unit of nature — living organisms interacting with each other and with their physical surroundings. Ecosystems vary hugely in size, from a small pond to an entire forest or sea; some ecologists even treat the whole biosphere as one global ecosystem, made up of every local ecosystem on Earth. Because a system that large is too complex to study all at once, it's more practical to split ecosystems into two broad categories: terrestrial (forests, grasslands, deserts) and aquatic (ponds, lakes, wetlands, rivers, estuaries) — with crop fields and aquariums counting as human-made examples of each. This chapter starts with an ecosystem's structure, then looks at how energy and nutrients actually move through it — productivity, food chains and webs, nutrient cycling — and the losses and transformations that come with that flow.