Why is the world green? The interactions of top-down and bottom-up processes in terrestrial vegetation ecology

摘要

A classic question in plant ecology is "why is the world green?" That is, if plants are food for animals why do not animals eat all the available food-changing a 'green world' into a 'brown world'. We first reviewed this question in 2009 and now revisit our arguments in the light of new data and new thinking. Here we argue that (1) the top-down bottom-up dichotomy is probably too simple for understanding a complex system-such as vegetation-rich in feedback processes. (2) Nevertheless it appears that bottom-up processes are generally more important for maintaining the presence of some sort of vegetation while top-down control process are generally more important in determining the type of vegetation at a site. (3) Although this review mainly takes a qualitative and experimental approach to the question, we also argue that simple well-known mathematical models from population ecology can be very informative in thinking about the types of explanations for the green world phenomenon, and demonstrating that it is rarely a simple choice between one form of control or another.