Declining snake populations in Costa Rica

Imantodes cenchoa

Many herpetologists who have done fieldwork over the last several decades have commented on declines in snake populations. Snakes, unlike frogs, don't vocalize, they usually don't congregate for reproduction, and they are quite secretive. Many roads that I use to drive in search of snakes at one time passed through grassland, forest, or desert but are now lined with agricultural fields, housing developments, and industrial parks. Field notes suggest snakes were common on some of those roads 30 or 40 years ago, but have mostly disappeared due to human activity.

In a forthcoming paper, Barquero-Gonzalex and colleagues (2019) investigate if reports of decreasing snake populations at Drake Bay, Costa Rica had a real basis and if environmental factors, particularly temperature and precipitation, have played a role in that decrease. The researchers worked at Drake Bay from 2012 through 2017 and made over 4000 h of transect counts of snakes. Using head flashlights and a schedule mostly from 1930–2200 hours, several times every month they surveyed one transect covered by lowland tropical rainforest at an altitude of 12–38 m above sea level, near the Agujas River. We counted all the snakes along the transect; identified species in situ and also photographed them. They found snake counts increase from August to September and then decline rapidly. The May snakes/rainfall peaks coincide, but the second snake peak occurs one month before the peak rainfall; they counted more snakes on dry nights, with the exception of Imantodes cenchoa which was equally common despite rain conditions. We saw less Leptodeira septentrionalis on bright nights, but all other species were unaffected. During the six years, the number of species with each diet type remained relatively constant, but the number of individuals declined sharply for those that feed on amphibians and reptiles. They conclude that night field counts of snakes at Drake Bay, Costa Rica, show a strong decline from 2012 through 2017.

Barquero-González, J.P., Stice, T.L., Gómez, G. and Monge-Nájera, J., 2019. Are tropical reptiles really declining? A six-year survey of snakes in Drake Bay, Costa Rica, and the role of temperature, rain and light. BioRxiv, p.731174.