This idea of repulsion \u2014 formally known as conspecific negative density dependence, or CNDD \u2014 goes back to the 1970s when the ecologists Daniel Janzen and Joseph Connell independently suggested that insects, herbivores and pathogens that prey selectively on one species could make the area around an adult tree dangerous for its seeds. Other species would not be prevented as effectively from growing in the area, though they would still be limited by nonspecific problems like a lack of sunlight under an adult tree\u2019s canopy. The result would be that adult trees of a species would tend to maintain a kind of minimum \u201csocial distance\u201d from each other.<\/p>\n