Why is the World Green?

Viewed from space by human eyes, the predominant colours of our planet are the blue of the oceans and the white of the clouds. The blue of the oceans forms the subject of another of our chapters. However, if one focuses on the land masses other colours dominate. On land the white colour still features prominently in the polar areas covered with snow and ice, but zoom in on lower latitudes and much of the land is a mix of the green of vegetation and the brown of more arid areas. Green dominates large areas of land, so unless you are reading this in a desert, during the high-latitude winter, or in a highly urban area, then green will probably feature prominently in your surrounding landscape. One answer to the question that heads this chapter is that the climate (often rainfall) allows some parts of the land to be green with plant life, while making other areas arid and brown. However, this green of extensive plant life is still a puzzle—plants are food for a wide range of animals, so why is so much food left unused? Swarms of locusts, destroying most plants in their path (be they biblical plagues or modern day outbreaks), are the exception not the rule. But why is this so? Why are so many parts of our world green in the face of this threat from herbivores? As we will see, if herbivores are the key to our question, then what starts as a question in plant ecology ends up being a question about factors that limit the size of herbivore populations. In effect, we need to understand why herbivore populations do not increase in density to such a level that they destroy all the available plants, giving a land that is brown rather than green. Until the middle of the twentieth century if you had put the green world question to biologists, many of them would probably have suggested that it was not in the interests of a species to consume all of its food reserves.