Scale matters

Setophaga discolor - Credit: Julie Hart

Biodiversity is changing all around us and worldwide. Local species disappear and sometimes other species invade. Studying birds in the U.S. and worldwide, we show that patterns and implications of this ongoing change vary strongly with the scale.

A minor loss or gain of species richness or functional diversity at the local or county level can look like a major gain at the state or national level, and yet be a net loss when viewed at a global scale. Researchers at Yale University studied 50 years of data about nesting birds in North America and tracked biodiversity changes at different scales. They found significant differences in how much change had occurred, based upon how wide a geographic net they cast. In addition taxonomic diversity and functional diversity increased over all but the global scale. The larger the scale change in taxonomic diversity was higher than in functional diversity which suggests strong trait redundancy at those scales. Also, insectivorous birds (like the prairie warbler in the photo) showed the most drastic declines across all geographic scales, from local to continental.

Any reporting and interpretation of biodiversity change thus needs scale as a key qualifier. Better yet, researchers and practitioners of biodiversity science should adopt a multi-scale framework and consider all geographic scales simultaneously.