What constrains life in the rainforest? Here we see plants growing upon plants growing upon plants – nearly every part of this tree trunk is covered with another organism. Ecosystems appear in miniature the closer one looks. So what prevents a baroque elaboration of this arrangement to infinite numbers of infinitely small plants? Resource limitation….

Broadway Boulevard – a paved multi-lane road, cutting a straight path across fifteen miles of desert, bringing the city uncomfortably close to the Rincon and Tucson mountains. This is progress, of a sort. Sixty years ago, the city would have been scarcely visible, with none of these highways or towers yet constructed. There would have…