Herbivores and carnivores:
Understandably if the prey species in an environment, say herbivores on the African veldt, increase in number, then the carnivores that hunt and feast on them will increase as well. But it turns out (Predator Biomass No Match for Prey, Nature vol. 525 no. 7568 September 10, 2015 page 161 reviewing work by Ian Hatton and colleagues at McDill University in Montreal, Canada (Science http://doi.org/713 (2015)) that the relationship is not simple. If 200 herbivores go to 400, then 20 carnivores don’t go to 40; they go to 30, or numbers something like that. The article infers. “An unappreciated degree of ecosystem organization.” I’ll say.
At least I think I’ll say.
I mean I’m not so sure, but follow my logic and keep me honest here. Carnivores range farther than herbivores. That’s as you would expect. The herbivore only needs to find the next blade of grass; the carnivore needs to find the next herbivore.
But, and I’m no expert, nor have I an expert to cite, the range difference is a lot greater. A male Florida panther will range over 200 square miles while the female ranges over 70 to 80. (https://www.google.com/#q=what+is+the+range+of+the+florida+panther) Since the female often has cubs to feed, at the very least the male range is redundant for eating purposes by a factor of 2.5. Now assuming a roughly circular domain, the male ranges over a distance of 8 miles from the center. But since perfectly circular domains are unlikely, in fact impossible if one is filling available space, let’s say the male ranges up to 10 miles from home.
By contrast our anthropology teacher showed us a classic film of some Kalahari bushmen chasing a giraffe. They pursued it for days, and when they had killed it and were processing the meat, they sent a runner back to camp. He knew the way and was back shortly with people to help with the work. It cannot have been more than an hour’s walk, or 3 miles. That camelopard sure did a lot of doubling back.
Of course I’m always looking for a fertility explanation when there is a puzzle, and one does indeed present itself. The higher density of the herbivores will perforce reduce their fertility because they will have occasion to mate with less kin individuals. This was worked out by Sibly, and if you’ve forgotten, go back to the New Year’s summary and redo your homework. But the carnivores are ranging far beyond their actual needs already. Boost their numbers and they are unlikely to contract the area of their social circle. They pay the higher price in fertility.
At least that’s what I think today. But that is the thinking of a believer; I wouldn’t try to use it to persuade a skeptic.
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