Long-Term Monitoring Reveals Population Decline of Spectacled Caimans (Caiman crocodilus) at a Black-Water Lake in Ecuadorian Amazon
Knowledge on long-term population trends in crocodilians is essential to assess the effectiveness of conservation areas and to guide sustainable management practices. We studied changes in population size of spectacled (Caiman crocodilus) and black caimans (Melanosuchus niger) over a period of 17 yr at Mateococha, a black-water lake located in Cuyabeno Wildlife Reserve, western Amazonia, Ecuador. Using standardized spotlight counts and two mark–recapture surveys, we estimated the population abundance, body size structure, and sex ratio in 1994, 2004, and 2011. The maximum number of C. crocodilus recorded in 1994 (147 individuals; 33 individuals/kilometer [ind/km] of lakeshore) declined by 2004 to 94 individuals (21.1 ind/km) and by 2011 to 63 individuals (14.2 ind/km). The number of M. niger recorded was low but constant (5–7 individuals; 1.1–1.6 ind/km) throughout the study, suggesting that factors causing the decline in C. crocodilus were not related to the M. niger population trend. Large C. crocodilus adults (total length 150–220 cm) were frequent in 1994 but became rare in 2004 and 2011. The sex ratio of captured C. crocodilus was male-biased during all periods. The causes of the population decline are unknown, but available evidence suggests that illegal hunting and habitat degradation are likely culprits despite the protected status of the Cuyabeno Reserve. Our findings question the effectiveness of protected areas to ensure long-term survival of caiman populations in Ecuador. El conocimiento sobre tendencias poblacionales a largo plazo en crocodilianos es esencial para evaluar la efectividad de áreas protegidas y guiar prácticas de manejo sustentable. Estudiamos cambios en el tamaño poblacional de caimanes de anteojos (Caiman crocodilus) y caimanes negros (Melanosuchus niger) durante un período de 17 años en Mateococha, un lago de aguas negras ubicado en la Reserva de Producción Faunística Cuyabeno, Amazonía occidental, Ecuador. Usando muestreos nocturnos estandarizados y dos experimentos de marcaje-recaptura, estimamos la abundancia, estructura de tamaño corporal, y proporción de sexos de las poblaciones en 1994, 2004, y 2011. El número máximo de C. crocodilus registrado en 1994 (147 individuos; 33 individuos/kilómetro [ind/km] de orilla de lago) disminuyó en 2004 a 94 individuos (21.1 ind/km) y en 2011 a 63 individuos (14.2 ind/km). El número de M. niger registrado fue bajo pero constante (5–7 individuos; 1.1–1.6 ind/km) durante todo el estudio, sugiriendo que los factores causantes de la disminución de C. crocodilus no estuvieron relacionados con la tendencia poblacional de M. niger. Adultos grandes de C. crocodilus (longitud total 150–220 cm) fueron frecuentes en 1994 pero se volvieron raros en 2004 y 2011. La proporción de sexos de C. crocodilus capturados fue sesgada hacia machos durante todos los periodos. Las causas del decline poblacional son desconocidas, pero la evidencia disponible sugiere que cacería ilegal y degradación de hábitat son probablemente los responsables a pesar del estado protegido de la Reserva Cuyabeno. Nuestros descubrimientos cuestionan la efectividad de áreas protegidas para garantizar la supervivencia a largo plazo de poblaciones de caimanes en Ecuador.Abstract
Resumen
Number of caimans observed in 1994 (n = 7 spotlight counts), 2004 (n = 1), and 2011 (n = 7) in Mateococha Lake, Amazonian Ecuador. (A) Total number (Caiman crocodilus + Melanosuchus niger + ‘eyes only'); (B) number of C. crocodilus; and (C) number of M. niger. The maximum number was observed during the first count in all periods, and a regression line of this number against study period is presented for A and B.
Body size structure for the Caiman crocodilus population estimated from three periods: (A) 1994 (n = 7 spotlight counts); (B) 2004 (n = 1); and (C) 2011 (n = 7) in Mateococha Lake, Amazonian Ecuador.
Linear regression analyses between estimated total length (ETL) and real total length (TL) for samples of Caiman crocodilus individuals that were observed and subsequently captured and measured. In each period, the regression model had a reasonable predictive value; (A) 1994: n = 85, r2 = 0.936, F1,83 = 1224, P = < 0.001; (B) 2004: n = 24, r2 = 0.819, F1,22 = 104.9, P = < 0.001; (C) 2011: n = 69, r2 = 0.915, F1,67 = 728.5, P = < 0.001. The regression equations were used to correct size estimates for not-captured individuals.
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