We investigated the impact of regional climate change on the evolution of faunal communities within two different environments. On one hand, we analyzed the lacustrine fauna utilizing molluscs as proxy, and on the other hand the terrestrial fauna with bovids as model organisms.
Fig. 1: One of the most species rich areas of Lake Malawi is Cape Maclear in the South of the lake. In the past years, this region was investigated in the course of the Riftlink project.
We have investigated the impact of regional climate changes on the evolution of freshwater snail species in the African rift-lake Malawi. Although shell fossils from the Pliocene indicate that these species have lived in Lake Malawi already about two million years ago, our findings show that the modern species are much younger and evolved from a single surviving lineage no longer than 500,000 years ago (Schultheiß et al. 2009). This pattern was found to be almost identical for at least two different species flocks, indicating similar causes for the extinction of great parts of the Pliocene snail fauna on the one hand, and a relatively recent onset of speciation in the extant group on the other hand (Schultheiß et al., in prep.). These events were most likely caused by massive changes of Malawi's lake level in the Pleistocene. According to this scenario, lake level lowstands have ereased great parts of the Pliocene snail fauna whereas the re-flooding of the lake has triggered speciation processes in the modern species flock. In order to infer these events, we used DNA sequences of the snail species as well as a "molecular clock" approach, which enables time estimates of speciation events on the basis of mutations in the mitochondrial genome (Wilke et al. 2009).
Fig. 2: Lake Malawi is home to numerous endemic freshwater snail species, i.e. species which are only present in this lake. The image shows a male specimen of the species Bellamya capillata.
The causal link between climatic and faunal change has been hypothesized for faunal groups of different ecosystems (Bromage & Schrenk 1999). The climate of a certain region has an influence on the vegetation which in turn influences the composition of the faunal community. For our analysis we go back the way of this argumentation (Fig. 3). We analyzed the fossil bovid communities (antelopes and their relatives) of Lake Turkana (Kenya) and Lake Albert (Uganda) of the time span from around 8 to 0.5 Ma ago. Comparisons with modern bovid communities of Africa we got information on the vegetation and in turn on the possible climate present in the past. From modern national parks we know that some tribes of bovids are most abundant in certain environments, e.g. Alcelaphini (e.g. hartebeest) and Antilopini (e.g. Thomson's Gazelle) are found more often in open arid environments while Bovini (e.g. buffalo) and Reduncini (e.g. waterbuck) are more abundant in closed wet habitats (Vrba 1980). A comparison between the abundances of the bovid tribes of modern national parks with known vegetation and the so called members of the fossil record which represent a certain time span in the past reveals that most of the members of Lake Turkana show a high abundance of those bovids associated with closed wet environments. From the high proportion of Bovini and Reduncini at Koobi Fora in the eastern part of Lake Turkana (>45 %) we assume that this part of the Turkana basin exhibited closer conditions than the West Turkana area especially in the younger members from 1.8 Ma on (Bovini and Reduncini: <35 %) (Fig. 4). Another part of our analysis is the assignment of the bovids to diet categories such as browsers (feeding mainly on leaves) or grazers (feeding mostly on grass). The abundance of individuals/specimens in each diet category provides more information on the vegetation of a certain region than the taxonomic view on the bovid community. The oldest members under study exhibit a high abundance of mixed feeders while the younger members show high abundances of grazers (e.g. 2.68 - 1.88 Ma) or fresh grass grazers (e.g. 1.65 - 1.39 Ma). But we observed considerable variations in the abundance of the diet categories over time assuming several shifts in the environment from more open to closer and from less arid to wetter condition and vice versa. The abundances of the diet categories also differ between contemporaneous members of the localities studied in the Turkana basin. That stands for regional differences in the vegetation on a small scale. For example the amount of fresh grass grazers is in most cases higher in the eastern than at in the western part indicating a wetter climate in the eastern part. In general, the Lake Turkana region had a wetter climate in the past than today and a closer vegetation than the very open vegetation found today in the region.
Fig. 3: Steps of the analysis of bovid communities of certain time slides of localities at Lake Turkana (Kenya) and Lake Albert (Uganda).
Fig. 4: Amount of bovid tribes (relative values in percent) on the whole bovid community of certain time slides of four localities at Lake Turkana (Kenya).
Cited literature (Riftlink group members bold)
Bromage, T. and Schrenk, F. (Eds.) (1999): African Biogeography, Climate Change and Early Hominid Evolution. Pp. 1-498, Oxford University Press.
Schultheiß, R., Van Bocxlaer, B., Wilke, T. & Albrecht, C. 2009. Old fossils-young species: evolutionary history of an endemic gastropod assemblage in Lake Malawi. Proceedings of the Royal Society B-Biological Sciences 276: 2837-2846.
Vrba, E. (1980). The significance of bovid remains as indicators of environment and prediction patterns. In: Fossils in the making, Vertebrate Taphonomy and Paleoecology. A.K. Behrensmeyer, A. Hill (Eds). p. 247-271. University of Chicago Press, Chicago.
Wilke, T., Schultheiß, R. & Albrecht, C. 2009. As time goes by: A simple fool's guide to molecular clock approaches in invertebrates. American Malacological Bulletin 27: 25-4 5.