I think you need to do a little more reading. Perhaps a lot more. Wildlife population fluctuations are almost never attributable to a exclusive single factor. In your passenger pigeon example, for instance, the popular teaching is that hunting wiped these out. I suggest that the wholesale fragmentation and deforestation of the hardwood forests (the feeding and nesting areas supporting the PP) in the eastern US is a major contributing cause. Remember that the PP population crashed at exactly to same time that rapid expansion of settlement and attendant large scale development of agriculture happened in major areas of PP habitat.
I have done my reading. Fluctuating widlife numbers may have several or many factors, but plummeting stocks not so many. And in every instance that I cited overhunting or overfishing were the leading cause. In almost every instance that I cited temporary hunting or "kill" fishing closures brought back stocks as well. When a hunting or "kill" fishing closure results in rapidly rebounding stocks it is very strong indication, even patently obvious, that the leading cause of the plummetting population was too much take.
Now for the passenger pigeon, please read this:
"The notable decrease of passenger pigeons started when professional hunters began netting and shooting the birds to sell in the city markets. Although the birds always had been used as food to some extent, even by the Indians, the real slaughter began in the 1800s.
There were no laws restricting the number of pigeons killed or the way they were taken. Because the birds were communal in habit, they were easily netted by using baited traps and decoys. The birds were shot at the nesting sites, young squabs were knocked out of nests with long sticks, and pots of burning sulphur were placed under the roosting trees so the fumes would daze the birds and they would fall to the ground. Hundreds of thousands of passenger pigeons were killed for private consumption and for sale on the market, where they often sold for as little as fifty cents a dozen.
By 1850 the destruction of the pigeons was in full force, and by 1860 it was noticed that the numbers of birds seemed to be decreasing, but still the slaughter continued.
One of the last large nestings of passenger pigeons occurred at Petoskey, Michigan, in.1878. Here 50,000 birds per day were killed and this rate continued for nearly five months. When the adult birds that survived this massacre attempted second nestings at new sites, they were soon located by the professional hunters and killed before they had a chance to raise any young.
The concerned voices of conservationists had little effect in stopping the slaughter. Finally a bill was passed in the Michigan legislature making it illegal to net pigeons within two miles of a nesting area, but the law was weakly enforced and few arrests were made for violations.
By the early 1890s the passenger pigeon had almost completely disappeared. It was now too late to protect them by passing laws. In 1897 a bill was introduced in the Michigan legislature asking for a ten-year closed season on passenger pigeons. This was a completely futile gesture as the birds still surviving, as lone individuals, were too few to reestablish the species."
The quote is from the Smithsoneon Museum. The article discusses changing land uses and deforestation, as it should, but goes on to point out that numbers of passenger pigeon didn't begin to plummet until market hunting began in earnest. Essentially, the article mirrors most.
http://www.si.edu/encyclopedia_Si/nmnh/passpig.htm
Or another:
"The real onslaught began with the onset of large-scale commercial hunting carried out by well-organised trappers and shippers in order to supply the developing cities on the east coast of the United States with a cheap source of meat. It began once railways linking the Great Lakes area with New York opened in the early 1850s. By 1855 300,000 pigeons a year were being sent to New York alone. The worst of the mass slaughter took place in the 1800s and 1870s. The scale of the operation can be judged by figures that seem almost incredible but which were carefully recorded as part of a perfectly legal and highly profitable commerce. On just one day in 1860 (23 July) 235,200 birds were sent east from Grand Rapids in Michigan. During 1874 Oceana County in Michigan sent over 1,000,000 birds to the markets in the east and two years later was sending 400,000 a week at the height of the season and a total of 1,600,000 in the year. In 1869, Van Buren County, also in Michigan, sent 7,500,000 birds to the east. Even in 1880, when numbers had already been severely reduced, 527,000 birds were shipped east from Michigan."
JPK
