January 28th, 2007

Ann Vole

Beaver population... then and now

I threw out some estimates that beavers are no way near the population density it was before the white-man came to harvest them. I suggested that a hundred times the current population was taken per year... well that might be a bit off but here is what I found: a study of harvest numbers of different trading posts was done to determine the catch for a sustainable population. These numbers were around 30000 pelts per trading post per year. There were several hundred trading posts. accounts by trappers were that only a small percentage of furs actually made it to the trading posts due to tipped canoes and damaged furs from insects or rot on the long journey to the trading posts. The sustainable catch seems to be about 10% without predation but back then a healthy population of wolves, bears and other large predators would have taken lots of the beavers so the percentage of sustainable catch would be much lower. I seem to remember reading somewhere that there were about 300 trading posts run by the Hudson Bay company so I will use that and ignore the competition. I will guess at 5% for a sustainable catch with all those predators (multiply by 20 for population from sustainable catch). 300 x 20 x 30000 equals 180 000 000. In the states that have beavers returned in the last 2 decades, they are seeing numbers around 30000 but one ecologist suggested (based on earlier beaver population studies) that the population generally peaks then drops to a steady 25% of the peak. Some states have found their numbers jump to even 70 000 lately but those ones did their survey by counting beaver huts and most had a healthy fur trade happening so their assumed 6 beavers per colony my be too high. I estimate there are 12 large states that would work for that 30000 beavers per state estimate and that would cover about a quarter of the beaver habitat based on a map I found of current beaver range. 4 x 12 x 30000 equals 1 420 000 or a very rough guess at current beaver population. This is less then one percent of the population of the heyday of the fur trade which, even though it is very rough and wildly inaccurate, still suggests that the current population density is definitely lower then before the white-man came. My whole point is that the land covered by water was much greater back then and if that water-covered land had continued to this day, we may not be seeing the rapid increase in carbon dioxide levels in the atmosphere.

Methane is a worse greenhouse gas then Carbon Dioxide and a recent estimate is that 1% of the methane released into the atmosphere is due to beaver flooding but it failed to point out that beavers are returning in record numbers to a large part of North America due to conservation of wetlands, decreased farm incomes causing farmers to leave some beavers be, and new methods of preventing problems from beavers by making permanent pond level stabilizing leaks so beavers do not flood more then they need (the alternative of removing the beavers does not work because more new beavers will always come back). This methane is only from recreation of wetland habitat. Another survey of geologic history of pond sizes suggests that the beaver ponds of today are about 15% the size they used to be 80 years ago. This is not unexpected being that if the pond gets too big, humans will be affected and break the dam (or put in the new water level controls). The other thing to note is that beavers use the same ponds for hundreds of generations (some huts are radiocarbon dated to be over 1000 years old and still in use) and they will continue to make the dam bigger as long as water flows somewhere due to instincts to deal with any water movement. In undisturbed areas, some beaver dams are hundreds of feet long and represent the work of thousands of beavers over hundreds of years. My point is the same: more land was under water before white-man came and wiped them out and that same level of water retention will never return while humans are here and that same level of carbon sequestering in wetlands and in small wetland trees that beavers love like aspen and willow as compared to evergreens that never lose their leaves (which returns sequestered carbon to the forest floor each year in trees that lose their leaves).