Population structure and human pressure of two caiman species in Southern Brazilian Amazon

PEREIRA, A.; PORTELINHA, T.; SIMONCINI, M.; MALVASIO, A.

Skukuza

Congreso; 24th Working meeting Crocodile Specialist Group/UICN; 2016

Internationa Union Conservation of Nature/Crocodile Specialist Group

Studies in protected areas provide interesting data for population status of crocodilians. However, crocodilians populations in these areas also undergo negative effects of human presence in its abundance, structure,and animal behavior. In this sense, studiesrelating anthropic pressure and their effects on population of crocodilians areneed for a better management practice and conservation measures. Therefore, wedetermined and compared the population parameters of encounter rates, sizestructure, and sex ratio for Melanosuchusniger and Caiman crocodilus populationsamong three study sites (Coco River, Rico Lake, and Dentro Lake) in the CantãoState Park, Southern Brazilian Amazon, Tocantins State.Additionally, we tested two hypotheses of relationships between humans andcrocodilians abundance/size structure: (1) caiman will be less abundant closerto human community; and (2) human disturbance may have a positive effect onabundance in smaller caiman and a negative effect in larger individuals. Spotlightsurveys were carried out during dry season (August-November) between 2013 and2014 to collect data about: abundance (by encounter rate: individuals/km),species, sex ratio, injury frequencies, and animal size. The human disturbance analysis was conducted just in the Coco Riverwhere allowed a comparison between transects (about eight kilometer each) ofdifferent degrees (high, medium and low) of human disturbance (transit,presence of ports, boats, and houses on the banks). We counted 4,787 caiman (2,506 C. crocodilus, 201 M. niger, and 2,080 unidentified) and we captured 60 individuals of C. crocodilus and 24 of M.niger for sex ratio and injury studies. We found that M. niger and C. crocodiluspopulations was significantly most abundant in the Dentro Lake. Caiman crocodilus species waspredominant in all sites, where its pattern of encounterrates increased in sequence: Rico Lake (2.5 ind/km), Coco River (4.82 ind/km),Dentro Lake (8.13 ind/km); whereas the pattern was the reversed for M. niger, decreasing in sequence: RicoLake (1.59 ind/km), Coco River (0.33 ind/km), Dentro Lake (0.24 ind/km). Bothspecies was basically composed by juveniles (SVL < 70 cm for C. crocodilus- 91.5% of population; SVL < 100 cm for M. niger: 63.7% of population) and distributed in different sites:Dentro Lake for C. crocodilus andRico Lake for M. niger. The sex ratio was male-biased for both species in all study sites (C. crocodilus 2♂:1♀; M. niger 7♂:1♀). Males and juvenilespresented more injuries, mainly on the region of the tail. Additionally, the data suggestthat human presence maynegatively influence C. crocodilus andM. niger abundances, and it wasdecreasing as human disturbance level increases. Regarding the caiman size/humandisturbance relationship, we found smaller C.crocodilus individuals in high disturbance level areas, and larger individuals in low disturbance levelareas; for M. niger, we foundlarger individuals in areas with high human disturbance level. Proximity tocommunities and accessibility to the Cantão State Park is possibly the causesof human disturbance. Our results provide important implications highlighting howwild crocodilians population respond to human disturbance.