Global patterns in plant environmental breadths

Abstract

The latitudinal gradient in plant diversity is one of the most famous patterns in ecology. It is hypothesised that narrow niche breadths and restricted geographic ranges in the tropics allow more species to coexist with minimal overlap relative to high-latitude regions. Although a wealth of studies have investigated these questions across different regions and taxonomic groups, these have consistently yielded contradictory results, leading to the continued persistence of numerous ecological explanations. Here, using a global occurrence database containing over 100 000 plant species, we provide the first globally standardised investigation into the geographic relationships among latitudinal range, environmental breadth, and latitudinal median. We find limited evidence for a global latitudinal gradient in species' ranges and environmental breadths, with results varying between hemispheres and along latitude within each hemisphere. In agreement with previous observations, we show consistent support for a latitudinal gradient in environmental breadth and latitudinal range, but only for trees in the Northern Hemisphere and for tropical species. In the Southern Hemisphere, conversely, these trends are inverted for non-tropical species, with latitudinal range and environmental breadth decreasing with distance from the equator. Moreover, these relationships are even weaker with environmental breadth, even though there is a strong relationship between environmental breadth and latitudinal range. By applying standardised methods at the global scale, these results suggest that variation in species' ranges is largely a by-product of biogeographic patterns rather than niche processes. Collectively, this work illustrates that existing ecological ‘rules' linking niche breadth to latitude predominantly reflect regional sampling biases and a historical focus on the Northern Hemisphere and certain taxonomic groups.