curb diversity. Maybe the harshness of the world prevents nature’s bus from ever becoming so full that species must fight over seats.
Another thing, besides environmental hazards, that opens up seats on the bus is species’ habit of eating one another. This could be a particularly effective way of maintaining biodiversity because, whereas storms strike all the trees in a forest more or less equally, predators and herbivores tend to focus disproportionately on the most common species in their environment. By reducing the numbers of species they feed on, these consumers prevent their prey from overwhelming their neighbors. If starfish are taken off a beach, the mussels they feed on take over, and the total number of species goes down. Likewise, herbivores tend to choose plants that are juicy and fast growing but not well defended. Without the browsers and grazers, these plants would take over. But with them other plants that are slower growing, because they have invested in poisons, thorns, or other defenses, can survive. One idea argues that every tree in the forest inadvertently attracts herbivores that specialize in eating it. Mature trees can cope with this, but it’s no place to raise a child—any young tree of the same species trying to establish itself in the same neighborhood is in trouble. These herbivores, however, would ignore nearby trees of other species, allowing a patchwork of diversity to build up. There is some evidence that tropical trees do indeed find it harder to germinate and survive close to members of their own species. The crucial common aspect of these predator effects is that they weigh heaviest on common species. This creates a benefit to being rare that maintains diversity by reducing strong competitors’ advantage.
These are just a few of the many ideas, none of them mutually exclusive, that different ecologists currently favor to explain why species do and don’t live together. There are variants of these, but they are all twists on the basic concepts of competition and how species avoid it, either by dividing up space, time, and resources, or because of forces beyond their control.
But maybe competition between species is not strong enough to control biodiversity. Maybe species are not adapted to slot into a niche,