Monday, October 25, 2010

Snakebite, Snake Activity and Climate

Climate has a far reaching impact on organisms, including snakes, like the Coastal Taipan to the left.. The Queensland news media are carrying numerous stories warning people about an increased risk of snakebite. Most recently Jodie Wilson, President of the Australian Veterinary Association of Queensland said yesterday (October 22, 2010) that veterinarians were expecting more envenomations in the coming weeks due to snakes being flushed out by torrential rain and dog owners walking dogs off-leash was apparently exacerbating the number of bites. But more importantly, the number of snakes encountered by pets and humans appears to have increased because of increased rainfall in a La Niña year. Northern and eastern Australia tends to have below normal temperatures during La Niña phase years and the cooling is most extreme from October to March. In contrast, temperatures are much warmer during El Niño years. El Niño occurs when sea surface temperatures in the central to eastern Pacific Ocean are significantly warmer than normal. This recurs every three to eight years and is associated with a strong negative phase in the Southern Oscillation pendulum. In most years (the normal pattern) the Humboldt Current brings  cold water northward along the west coast of South America, a pattern that is enhanced by deep cold water ‘upwelling’ along the Peruvian Coast. The trade winds push the colder water westward and as it flows westward along the equator it picks up heat. The result is the western Pacific is 8 to 10 C warmer than the eastern Pacific. In El Niño years, the normal cold water flow along the South American coast and in the eastern Pacific weakens or may even vanish completely, and the central and eastern Pacific may become almost as warm as the western Pacific. When the eastern Pacific Ocean is much cooler than normal the air rising over the normally warm seas cause moist air to rises higher in the atmosphere causing abundant rainfall over the over eastern and northern Australia, resulting in widespread rain and flooding to Australia – this phase is called La Niña. Thus, the Australia Broadcasting Company is now reporting the wettest September on record for Queensland. Snakes are flooded out of their retreats and move to higher ground.

But the El Nino-La Nina cycle does not only effect snakes and snakebite in Australia. Morrison and Bolger (2002)  monitored reproductive success of individual rufous-crowned sparrows (Aimophila ruficeps) in coastal sage scrub habitat of southern California from 1997 to 1999. They found annual reproductive output of this ground-nesting bird varied strongly with annual variation in rainfall. Birds fledged 3.0 young per breeding pair in 1997, when rainfall was near the long-term mean, 5.1 offspring per pair in 1998, a wet El Niño year. But in the drier 1999 La Niña year  only 0.8 fledglings were produced per pair. The reproductive output was consistent with the hypothesis that food availability was positively correlated with rainfall. But, the factor most responsible for the high reproductive output in 1998 was low early season nest predation which, combined with favorable nesting conditions, enabled more pairs to multiple-brood. Cool, rainy El Niño conditions in 1998 altered the activity of snakes, the main predator of these nests. The authors used video-surveillance of nests and found the California Kingsnake (Lampropeltis getula californiae) and the San Diego Gopher Snake (Pituophis melanoleucus annectens) were the principal nest predators and that they account for 90% of the predation events, where the predators could be identified. More recently, González-Andrade and  Chippaux (2010) evaluated the burden of snake bite in Ecuador in an attempt to identify the difficulties of snake bite management in the country's health facilities. They surveyed national health statistics to estimate the overall incidence and mortality due to snake bites. From1998–2007, the average annual incidence and mortality was respectively 11 and 0.5 per 100 000 inhabitants. The Ecuadorian snakebite incidence increased in the rainy season and in El Niño years. Snake activity changes with cllimate, a simple concept but one with far reaching consequences that are seldom appreciated.

Literature
Morrison, S. and  D. Bolger. 2002.  Variation in a sparrow's reproductive success with rainfall: food and predator-mediated processes. Oecologia 133: 315-324.

González-Andrade, F. and  J.-P. Chippaux. 2010. Snake bite envenomation in Ecuador. Transactions of the Royal Society of Tropical Medicine and Hygiene, 104:588-591.

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