Patterns of species and growth form diversity along altitudinal gradients in the south-western Ethiopian highlands.
P 2.6 in Biodiversität, Arten- und Naturschutz
Understanding how biodiversity is organized across space and time has long been one of the central concerns of ecologists and biogeographers. Latitudinal gradients and island patterns of richness/diversity were the most documented ones1. In contrast, altitudinal gradients of species richness were poorly understood2 and simply considered as a reflection of latitudinal gradients. However, climate change has revived the attention in recent times to investigate the patterns and potential mechanisms that structure richness patterns along altitudinal gradients2,3. In addition, different patterns of richness may appear if the vegetation components are disagregaed3
We investigated the richness patterns of total vascular plant richness and several growth forms along altitudinal gradients in the south-western Ethiopian highlands. Boosted regression trees were used to predict the patterns of richness distributions. The results indicate that total richness did not show any response to altitudinal effects (energy, productivity). In contrast, dis-aggregation of the components of diversity to several growth forms such as woody, herbs, graminoids, climbers and ferns revealed contrasting patterns. Woody, graminoids and climbers showed a uni-modal structure yet peaking at different altitudes with graminoids partitioning the lower elevation, then climbers and finally tree species at higher elevation, about 2000 m.a.s.l. This is in congruence with the intermediate stress hypothesis of Grime4 where both the opposing process of decreasing species pool and relaxed competition with increasing altitude induce higher diversity at mid-elevation ranges5. Ferns and herbs showed a monotonic increase towards higher altitude, but the pattern of herbs has a dented structure in the mid-altitude where there is possibly high intensity of disturbances (e.g. grazing). Soil sand fraction, silt, slope and organic matter were found to contribute large shares in predicted variance of richness for various growth forms while surface stone cover and bedrocks (nutrient) were important contributors for woody and herbaceous richness, respectively.
In conclusion, the pattern of richness observed in our study varied with the level of aggregation of vegetation components. Yet, generally a hump-shaped richness patterns observed for several categories. A mid-altitudinal richness peak could possibly be the effect of interaction of declining species pool (or mass effects) and decreasing level of competition along a stress gradient towards higher altitude. In addition, altitude is a proxy variable for temperature and rainfall and hence a simple gradient while a number of other environmental variables may assume primacy at landscape scale. Thus, considering multiple gradients would help reveal better pictures of richness patterns and potential mechanisms structuring the distributions of biodiversity in high mountain regions of the tropics.
1 Gaston, K., 2000. Global patterns in biodiversity. Nature, 405: 220—227.
2 Sanchez-Gonzalez, A. & Lopez-Malta, L. 2005. Plant species richness and diversity along altitudinal gradients in the Sierra Nevada, Mexico. Diversity and Distributions, 11:567—575.
3 Bhattarai K.R. & Vtaas, O.R. 2003. Variation in plant species richness of different life-forms along a subtropical elevation gradient in the Himalayas, east Nepal. Global ecology and biogeography, 12: 327—340.
4 Grime, P. 1979. Plant strategies and vegetation processes. John Wiley and Sons Ltd. Chichester.
5 Bruun, H. et al., 2006. Effects of altitude and topography on species richness of vascular plants, bryophytes and lichens in alpine communities. J. Veg. Science, 17:37—46.