The mystery of the mountains: Why does plant and animal diversity change with elevation?
Species diversity changes with elevation, but differently for each plant and animal taxon. New model combining environmental preferences of organisms, changing with the environment and with physical limitations of habitable areas, caused by the fact that the mountains tend to be spiky, can predict the actual distribution of species remarkably well.
New study in the September issue of a leading ecological journal Ecology Letters explains why the number of species simply keeps decreasing towards the mountaintop in some plant and animal taxa, while in others it peaks at mid-elevation somewhere on the mountain slopes. An international team of researchers led by R. Colwell (University of Connecticut) and including also V. Novotny’s lab at the Institute of Entomology (Czech Academy of Sciences) analysed the distribution of 4,500 species of organisms, from insects through vertebrates to plants, in 16 montane ranges from all over the world, including those from the highest peak of Papua-New Guinea, Mt. Wilhelm (see cover of the journal issue). The authors developed a “pencil model” of species distribution from the foothills to the montane summit. This elevational range can be envisaged as a box of pencils, with individual pencils representing species; some long, i.e. distributed across wide range of elevations, others short, with more restricted distribution (Fig. 1a). When we shake the box and thus randomly distribute the pencils, we get most pencils in the middle of the box. This corresponds to the maximum species diversity at mid-elevations. This classical model has been improved here by replacing random species distribution with a more realistic distribution that prefers certain elevation, with its environmental conditions. We can model this preference by inserting a steel ball into each pencil and placing a magnet at the optimum elevation (Fig. 1b). The strength of the magnet can be varied and thus its impact on the distribution of randomly shaken pencils in the box. A mathematical version of the “pencil model with balls” captures elevational distribution of various plant and animal taxa rather well and suggests that it could be successfully modelled by a combination of environmental factors and physical constraints on the species’ distribution given by the geography of a particular mountain range.
Colwell R.K., Gotelli N.J., Ashton L.A., Beck J., Brehm G., Fayle T.M., Fiedler K., Forister M.L., Kessler M., Kitching R.L., Klimes P., Kluge J., Longino J.T., Maunsell S.C., McCain C.M., Moses J., Noben S., Sam K., Sam L., Shapiro, A.M., Wang X., and Novotny V. (2016) Midpoint attractors and species richness: Modeling the interaction between environmental drivers and geometric constraints. Ecology Letters 19: 1009–1022.
