Abstract
Caiman yacare (lagarto) and Melanosuchus niger (black caiman), sympatric species in the Bolivian Amazon basin, have been severely overexploited in the past. We present the results of a standardized survey of C. yacare and M. niger populations in order to evaluate their actual population status in twelve oxbow lakes of the Ichilo River floodplain. Additionally we explored the effect of environmental and anthropogenic variables on caiman distribution patterns. The average density of C yacare and M niger in the shoreline of floodplain lakes was of 6 and 1 ind/km, respectively. For both species, the population was composed mainly of juvenile individuals. We used regression tree analysis (RTA) to assess patterns of M. niger and C. yacare densities with eight environmental and two anthropogenic variables. The RTA analysis showed that the variation in the densities of both C. yacare (52.4%) and M. niger (36.8 %) was related to water conductivity. For C yacare, higher densities occurred at higher values of water conductivity, while M. niger densities followed an opposite trend, resulting in relatively well spatially segregated populations of the two species. After excluding conductivity, Lake-River Distance (LRD) was shown to be the main splitting variable in the RTA analysis. The observed distribution patterns may be the result of the historical post-hunting situation, in combination with differences in habitat selection by the two species, and competitive exclusion processes between the two species. M. niger, a species reported to be recovering slowly from previous low population levels, appears relatively well protected in the Ichilo river floodplain.
Highlights
Before the 1950s, the crocodilian species Melanosuchus niger (Spix, 1825) and Caiman yacare (Daudin, 1802) were widely distributed in the upstream Amazon basin in Bolivia, Perú, Ecuador, Colombia and Venezuela (Medem 1983, Ergueta and Pacheco 1990)
We present the results of a standardized survey of C. yacare and M. niger populations in order to evaluate their actual population status in twelve oxbow lakes of the Ichilo River floodplain
Counts and population structure of C. yacare and M. niger: in 1999, 58 black caimans and 341 lagartos were counted in the 12 surveyed oxbow lakes, whereas in 2000 a total of 34 black caimans and 297 lagartos were encountered
Summary
Before the 1950s, the crocodilian species Melanosuchus niger (Spix, 1825) (black caiman) and Caiman yacare (Daudin, 1802) (locally called “lagarto”) were widely distributed in the upstream Amazon basin in Bolivia, Perú, Ecuador, Colombia and Venezuela (Medem 1983, Ergueta and Pacheco 1990). Though King and Videz-Roca (1989) and Llobet and Goitia (1997) failed to find black caiman (M. niger) in this area, local indigenous people confirmed in 1998 that both species were still present in the inundated floodplains, the oxbow lakes and occasionally in the main river channels The objectives of this study were 1) to determine the population status of C. yacare and M. niger in the oxbow lakes of the Ichilo river flooding area; and 2) to assess the environmental and anthropogenic factors that influence the distribution and abundance of both species in the study area. This study aims to contribute to a better understanding of the influence of actual and historical disturbance on distribution patterns of caiman species in Bolivia, which is a prerequisite for the design of effective conservation strategies
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