Showing posts with label food chain. Show all posts
Showing posts with label food chain. Show all posts

Monday, July 25, 2011

Why Are Some Snakes More Common Than Others?

Ringnecked Snakes, Diadophis punctatus 
reach some of the most dense populations 
found in snakes. JCM

There is a reason why more people study lizards than snakes, snakes are notorious for being difficult to find. Undoubtedly their cryptic nature and in some cases low population densities are likely contributors to this situation. A paper in Science this week by Hechinger et al. examined parasites (there are no known parasitic snakes), but the authors produced two general rules that they suggest can be applied to animal abundance – for any species. The following quotes were taken from a press release associated with this article. 


First they said that, "In addition to body size, the general rule for animal abundance must factor in the food chain and let both small and large animals be top consumers." The second rule was, "that the amount of biomass produced by a population does not depend on the body size of the animals in the population, or on what type of animal -- bird, fish, crab, or parasite. So, "If this rule is general, it means an aphid population can produce the same amount of biomass as a deer population," said Lafferty. "Furthermore, tapeworms that feed on the deer population produce less biomass than the deer, but can produce the same as a mountain lion population that also feeds on the deer. Predicting animal abundance is one of the most basic and useful things ecological science can provide for management and basic research," said Hechinger. "This simple rule helps with that because it may apply to all life forms and can easily be applied to complex ecosystems in the real world." So, does this apply to snakes? Snakes are all predators or facultative scavengers. Does it explain why many snakes are very difficult to find, while other species seem quiet abundant in their habitats?

Parker and Plummer (1987) compiled a table of population densities of snakes, undoubtedly many more have been published since - but this information was readily available for this post. I added column for size.

In looking at the table below, Hechinger et al seem to be correct, the snakes with the most dense populations are small species (Carphophis and Diadophis), species feeding on invertebrates that are close to their food source - a short food chain; or they are piscivorous species that are probably taking prey that are feeding on autotrophic protists, scavenging decomposing material, or have other abundant food resources. The rarer snakes tend to feed on mammals, have larger body sizes, or live in environments with low productivity (Lampropeltis, Pantherophis, Eryx).


Species
Density,
 no/ha
Size (m)
Acrochordus arafurae
19-97
1.5
Eryx tataricus
<1
0.7
Carphophis amoenus
373-729
0.28
Coronella austriaca
11-17
0.8
Diadophis punctatus
719-1849
0.45
Elaphe dione
<1
1.2
Pantherophis obsoleta
<1
2.0
Elaphe quadrivirgata
4-46
2.6
Pantherophis vulpina
1-3
1.0
Heterodon nasicus
2-10
0.6
Heterodon platirhinos
1-7
0.8
Lampropeltis calligaster
<1
1.1
Lampropeltis triangulum
<1
0.9
Lycodonomorphus bicolor
380
0.7
Protobothrops flavoviridis
2-3
1.5
Vipera berus
3-14
0.65
Vipera ursinii
7
0.5


Citations
Hechinger, R. F., K. D. Lafferty,. A. P. Dobson, J. H. Brown, A. M. Kuris. 2011. A Common Scaling Rule for Abundance, Energetics, and Production of Parasitic and Free-Living Species. Science, 2011; 333 (6041): 445-448 DOI: 10.1126/science.1204337.

Parker, W. S. and M. V. Plummer. 1987. Population Ecology. Pages 253-301 In: R. Seigel et al. (eds.) Snakes, Ecology and Evolutionary Biology. New York: McMillian Publishing.