Ecologia e modelagem da distribuição dos sítios de nidificação do jacaré-açu (Melanosuchus niger) em uma área de várzea da Amazônia Central, Brasil

Abstract

The black caiman, Melanosuchus niger, is primarily an Amazonian species, its largest populations are associated mainly with interior várzea areas, but its nesting ecology related with these hydrologic dynamic environments is still poorly understood. The objectives of this study were related to the generation of information on the nesting ecology of this species as to the size and number of eggs per clutch and its relation with the size of the nesting female, the influence of flooding regimes on nests occurrence; egg mortality and its causes; and the creation of a model to predict nesting areas. The present research was developed in the southeastern-most portion of the Mamirauá Sustainable Development Reserve (the so-called focal area), a várzea region influenced by the annual flooding of the Solimões and Japurá rivers. We analyzed a sample of 231 nests of the species distributed in 67% of all 66 surveyed water bodies during two nesting seasons (2007 and 2008). Approximately 40% of the nests found were opened to measure and weight the eggs. The mean size of the eggs was comparable to that reported in other localities for the same species, but the size of the nesting female showed significant relations only with the volume, but not with the number of eggs per clutch, as opposed to other species. Occurrence of black caiman nests was positively related to the seasonal stability of lakes water mirror areas. Water bodies where nests occurred are often reached by the annual flooding between November and April, as opposed to other water bodies that are often flooded already in October. This reduces the risk of flooding of nests and provides enough time to finish the incubation of the eggs. A low proportion of nests (13%) were flooded, yet the eggs of almost 70% of the nests were taken by predators mainly by humans and the tegu lizard (Tupinambis teguixin), resulting in production of alive hatchlings in only 14% of the nests, one of the lowest proportions reported for crocodilians in natural conditions. The occurrence of nests in water bodies with the most stable water mirror areas aided the identification of nesting sites in the study area using the algorithms of the maximum entropy (Maxent) species distribution model. The models showed a good predictive performance, with AUC (Area Under the ROC Curve) values higher than 0.99. Nevertheless, it showed a low transferability (i.e. the ability to predict distributions in not sampled areas). The water bodies identified as suitable for nesting are key elements for population maintenance and should be considered as high-conservation value areas within sustainable management programs for the species.