The impact of sterile populations on the perception of elevational richness patterns in ferns


  • Michael Kessler,

  • Sandra Hofmann,

  • Thorsten Krömer,

  • Daniele Cicuzza,

  • Jürgen Kluge

M. Kessler (, D. Cicuzza and J. Kluge, Systematic Botany, Univ. of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland. – S. Hofmann, Albrecht-von-Haller-Inst. of Plant Sciences, Univ. of Göttingen, Untere Karspüle 2, DE–37073 Göttingen, Germany. – T. Krömer, Centro de Investigaciones Tropicales, Univ. Veracruzana, Interior de la Exhacienda Lucas Martín, Privada de Araucarias s/n, Col. 21 de Marzo, C.P. 91019 Xalapa, Veracruz, México.


Dispersal may influence the spatial distribution of species richness through mass or source-sink effects, but the extent of sink populations at the community level remains largely unknown due to difficulties of identifying such populations. We compared the richness patterns of ferns in 333 plots along six tropical elevational gradients in America, the Mascarenes, and southeast Asia, using sterile populations as an indication of sink populations. First, we tested whether sterile fern records were more common towards the elevational range limits of the individual species, but found this pattern in only one out of ten cases. It is therefore uncertain if sterile records correspond to marginal sink populations. Second, we compared the elevational richness patterns of sterile and fertile species. In several cases, elevational trends for sterile and fertile records were quite similar, but in others they differed distinctly. The percentage of sterile records per plot decreased with elevation among epiphytic ferns along all six transects, whereas terrestrials showed mixed results (decrease, increase, and U-shaped patterns). The percentage of sterile species records per plot relative to the number of species per plot recovered four significant patterns among the twelve cases analysed: higher percentages at higher species numbers among terrestrial ferns on two transects and lower percentages among epiphytes on two others. Despite the problems with equating sterile records to sink populations, we thus found distinct elevational patterns of sterile records that clearly affected our perception of the overall richness patterns. Ignoring the impact of population dynamics on diversity patterns is thus liable to result in misinterpretations of the diversity patterns.