Historical changes in the environment of Lake Titicaca: Evidence from ostracod ecology and evolution

Abstract

Lake Titicaca is a high-altitude, nearly closed, basin lake in the tropical belt of South America. The lake is subjected to the influence of the intertropical convergence zone (ITCZ) during the summer months, and is characterized by a flora and fauna that, while not as diverse as that of many other ancient lakes, nevertheless has groups such as molluscs and ostracods which show a fascinating ecological and morphological variability (polymorphism). For these and other groups. Lake Titicaca is an ideal natural laboratory for studies of evolution. This chapter demonstrates how, by applying transfer functions to quantitative information on the response of extant ostracod communities to habitat heterogeneity and environmental variability, past environments of Lake Titicaca can be reconstructed. Results show that recent history of Titicaca (since 8000 14 C years bp) to have been characterized by major lake-level fluctuations which separated the basin into three palaeolakes at various intervals of geological time, and which have possibly influenced population dynamics in the lake for groups such as fishes, molluscs and ostracods. The high-amplitude palaeoenvironmental variations indicated by the transfer function also seem to have had important effects on the dynamics of ostracod populations. Special attention is given to the formation of habitats and evolution of the lake ecosystem during the past 10 000 years, with particular emphasis on the ostracod crustaceans within the subfamily Limnocytherinae, a key ostracod group in the central Andes, and their highly variable carapace morphology. It is possible to distinguish among several morphs (or subspecies?) within this group, but the overlap is significant and individuals are sometimes difficult to classify. Accordingly, while taxonomic identifications remain based on external carapace morphology only, it is impossible to propose a definitive hypothesis for phylogenetic relations in the Andean limnocytherinid flock and its taxonomic structure. Even so, preliminary surveys of this group beyond the Lake Titicaca basin and adjacent areas suggest that the actual Titicaca fauna is not an isolated lineage, but part of extensive radiations which span the entire Andes. The observed differences between extant ostracod communities of Lake Titicaca and lakes Huaron and Junín can be explained by differential effects of the ITCZ, which produced distinct climatic regimes during the Holocene and resulted in different evolutionary pathways for the respective faunas of these lakes. This evolution appears to be more uniform in the case of Lake Junín, probably because of its closer proximity to the equator (and hence a lower exposure to the influence of the ITCZ) than Lake Titicaca.