Origins

 by Elizabeth Pennisi

Why in tropical forests do tall broad-leaf trees tower over a layer of understory species? What dictates that shrubs and herbaceous plants pepper the ground below, creating an environment recognizable the world over as tropical forest.

Biologists have long wanted to know why forests and other ecological communities look the way they do. In the beginning of the 19th century, Prussian naturalist Alexander von Humboldt started to find out. He assembled the first comprehensive treatise on how vegetation varies with altitude, climate, soil, and other factors. The work was a groundbreaking exploration of the physical underpinnings of ecological structure: what determines the species that make up a community and their relative abundance.

More than a half-century later, Charles Darwin quietly conducted experiments in his garden at Down House that were even more seminal. Examining a patch of unkempt lawn as it went to seed, Darwin observed that the species changed through time and that competition led to the demise of less-vigorous ones.

Ever since, ecologists have wrestled with understanding what dictates the proportions of each plant in communities varying from meadows to montane forests. How these forces set up communities has "arguably been one of the most primary questions driving ecological science since its origins," says Brian Enquist of the University of Arizona, Tucson. Competition, predation, disturbance, and other factors have a heavy hand, and new research is showing the influential role of evolution, as well.

In this month's Origins essay, Erik Stokstad explores the thinking that has gone into understanding and interpreting community compositions. A combination of physical and biological forces organizes species into predictable communities such that a rainforest is recognizable no matter what part of the planet it grows in. He describes the role of these physical factors and the influence of different biological interactions. Some ecologists think competition is key; others contend the abundance and diversity of species in a community is determined mainly by random dispersal, speciation, and extinction. This latter idea, dubbed the "Unified Neutral Theory of Biodiversity," makes a radical assumption: It considers all organisms of the same trophic level (plants, say, or herbivores) as demographically identical; that is, each organism in a particular level has about the same chance of reproducing, dying, migrating, or giving rise to a new species. The neutral theory has had mixed support. Some researchers think that biological interactions and “neutral” factors work in concert. As challenging as sorting out the rules that govern ecological structure has been, the effort is worth it, say researchers, because of the potential conservation benefit.

Image: Katharine Sutliff