Photo: Monika Feiling “By analysing ecological interactions in small-scale experimental systems, I realized that large-scale processes such as speciation and dispersal play an important role in regulating community structure and ecological interactions.”

The main reason for the broad interest is probably that the paper deals with a well-known and intriguing pattern of life—the high diversity of organisms at tropical versus temperate and boreal latitudes—which is of high interest to a broad range of scientists. By applying a new statistical method (quantitative meta-analysis) it became possible to directly compare these patterns for a wide variety of organisms.

The paper is a synthesis of the research done so far, but for the first time makes use of quantitative meta-analysis. Therefore, it places previous findings into a broader context and allows for more general conclusions.

Already in the 18th and 19th centuries naturalists described the high diversity of life in tropical regions. This increase of species richness from the poles to the equator has been termed the latitudinal gradient of diversity. Examples for this latitudinal gradient are highly abundant in the literature, but especially focus on certain organism groups—birds, mammals, higher plants. Whereas a large number of potential hypotheses have been proposed to explain this pattern, there is yet no consensus on ultimate causes. In this study, I used the information available in the literature to conduct a quantitative meta-analysis on latitudinal gradient. This method allows the derivation of general conclusions by synthesizing published information. The first main result of the study is that latitudinal gradients are highly general and occur in almost all organism groups investigated. The second main result is that the gradients are far from being uniform. There are substantial differences in how latitude and species richness are related. These differences occur between groups of organisms and between habitats as well as between different scales of observations. Latitudinal gradients are stronger for regional than for local estimates of richness, and are stronger for larger organisms. Another important factor related to an increasing strength of the latitudinal gradient is the trophic level of the organisms. The gradients are weaker in freshwater than in any other major ecosystem on earth and differ between different regions, but not between hemispheres. From the organism and habitat characteristics affecting the latitudinal gradient, conclusions can be drawn on the major models proposed to explain the latitudinal diversity gradient.

By analyzing ecological interactions in small-scale experimental systems, I realized that large-scale processes such as speciation and dispersal play an important role in regulating community structure and ecological interactions. This led me to the field of macro-ecology, where large scale patterns are investigated. Latitudinal gradients of diversity are perhaps the most widely known macro-ecological pattern in the literature.