Friday, March 30, 2012

Where Common Snakes are Rare and Rare Snakes are Common - On the Abundance of Tropical Snakes

Above: the arboreal cat snake, Boiga jaspidea; below the cryptozoic Gongylosoma baliodeirus. Photographed in the Danum Valley, Sabah, Malaysia.


Hans Breuer's post on Herp Nation's web site, Herping in the Tropics - Ecstasy or Nightmare?, brought back memories from another lifetime. In 1989 I spent a couple of months collecting data and specimens in Sabah's Danum Valley, an area of more than 400 sq km of lowland and hill dipterocarp forest that ranges in elevation from 150 to 1093 m asl. It was one of four trips to the Danum Valley run by the Field Museum to investigate the community ecology of the herpetofauna in Southeast Asia. From those four trips we put together a field guide and key to the snakes of the area (Murphy et al. 1994).

The article contained a bar graph of the 36 species found in Danum over 166 days of field work and how many of each species were collected. The graph is shown below, the photo insert is the species found most often, Pseudorabdion collaris, a small fossorial snake encountered while turning cover or raking leaf litter. 

Snakes are not easy to find in the tropics. The 166 days of field work produced 161 specimens (0.969 snakes per day). This was accomplished with three to four people working in the field a minimum of 6 hours per day (so at least 18 hours of effort per day to produce just less than one snake per day).

This experience is not unusual. William Beebe (1946) published the results of 36 months of field work at Kartabo, Guyana in one square mile of lowland tropical forest. He collected 425 snakes representing 52 species over 1080 days, or 0.39 snakes per day.

Dunn (1949) described a collection of snakes made by H.C. Clark in Panama. Dunn described 10,690 snakes representing collected over 13 years (4745 days)at four locations (the number of species varied between 40 and 60). The result was 2.25 snakes per day but this was a commercial venture involving many people.

Duellman (1978) reported on 1440 days of field work at Santa Cecilia, Ecuador and four nearby localities. His data show 564 specimens of 51 species, or about 0.38 snakes per day.

Today, the best answer as to why tropical snakes are difficult to find and when you do find them, rarely do you find a single species to be particularly abundant, seems to be exactly what Breuer concluded, the vast number number of hiding places in the tropics combined with the cryptic nature of snakes makes common snakes rare and rare snakes common. It also suggests, the diversity of snakes has been greatly under estimated and much remains to be discovered.

Literature
Beebe, W. 1946. Field notes on the snakes of Katabo, British Guiana, and Caripito, Venezuela. Zoologica 31, 11-51.

Duellman, WE. 1978. The biology of an equatorial herpetofauna in Amazonian Ecuador. University of Kansas, Miscellaneous Publications (65), 1-352.

Dunn, ER. 1949. Relative abundance of some Panamanian snakes. Ecology 30, 39-57.

Murphy JC, Voris HK, Karns, DK. 1994, A field guide and key to the snakes of the Danum Valley, a Bornean  tropical forest ecosystem. Bulletin of the Chicago Herpetological Society 29(7):133-151.

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