Many ecologists believe that productivity is one of the most pervasive influences on diversity. But, as I completed this book, I realized that I hardly understood that influence. Yes, diversity exhibits spatial patterns that seem related to productivity. And, as you will see, we understand the pattern that occurs at small spatial scales. We even think we understand a part of the larger-scale pattern. Yet, unlike the other spatial patterns, the productivity pattern remains somewhat mysterious. Not that people have ignored it. They have caused a river of ink to flow in its name. We just aren't sure where the river is headed.

We have two patterns to account for. First, when experimental ecologists increase productivity to small patches of plants, plant diversity declines. Second, as productivity rises on a regional scale (c. 106 km2), animal diversity first increases, then it declines.

Experimental increase of productivity

Correlations help us search for patterns. However, patterns may not reflect causes. They may instead reflect phantom correlations, correlations of one of our variables with another variable, a variable we may not even have thought about, let alone measured. That caution applies to no variable more strongly than to productivity since productivity correlates with many other important ecological variables.