How many animals are required per year by the average North American omnivorous diet to provide the required meat, dairy, and eggs? How much smaller is this number for vegetarian diets?
I am interested in per person numbers.
Getting the exact figures requires a lot of effort, so I am going to concentrate in getting the order of magnitude. Also, another factor that makes the effort grow considerably is the various types of meat that are being eaten: beef, pork, broilers, other Chicken, turkey, fish etc.
So, let's consider the beef. This article provides little insight about beef consumption within USA:
One personal act that can have a profound impact on these issues is reducing meat consumption. To produce 1 pound of feedlot beef requires about 2,400 gallons of water and 7 pounds of grain (42). Considering that the average American consumes 97 pounds of beef (and 273 pounds of meat in all) each year, even modest reductions in meat consumption in such a culture would substantially reduce the burden on our natural resources.
According to this source, USA had about 290M inhabitants in 2002 (the year above article seems to have been written). That is about 28B pounds of beef in the whole USA.
According to this discussion, there are about 500 pounds of editable meat per average cow. That means about 56M cows per year in the whole USA.
This article shows per capita consumption in USA over the last decades. For 2015, there were some 54 pounds of beef per capita (tends to lower over time).
This Quora answer tries to grasp exactly what you are asking and provides a rough estimation for USA in 2008:
- Cattle: 35,507,500
- Pigs: 116,558,900
- Chickens: 9,075,261,000
- Layer hens: 69,683,000
- Broiler chickens: 9,005,578,000
- Turkeys: 271,245,000
- Fish: 6,500,000,000 (not from the USDA)
- Shellfish: 64,000,000,000 (not from the USDA)
So we are talking about billions of farmed animals or about 28 animals per capita, most of which are chickens.
A rather poor graph grasps evolution over time for US.
Wow, big question. For now I'll only consider chickens in Canada because I know how to get stats for Canada, and chickens are the most numerous farmed animal (by far) so that will give an order-of-magnitude estimate for all farmed animals. This estimate excludes fish, because they are simply too numerous and too diverse to properly estimate.
There are a couple methods for estimating the number of animal lives involved in animal farming. The best numbers come from counting the number of animals bred into the system. If that is unavailable, you can compare per capita disappearance with typical weights at slaughter, but this will fail to account for early mortality. Both numbers fail to account for international trade import/export. Comparing the number of animals currently alive with the number of humans currently alive does not give a good estimate because it fails to account for the (potentially very short) lifespan of farmed animals.
The total number of chicken eggs placed into incubators in 2016 was 945 million.
Of these, 86.4 million were destined for egg production and 29.2 million female pullets were actually placed on farms. Since only females lay eggs, that means about 29.2 million male chicks were culled. Presumably the overall viability of incubated eggs is around 68%.
Another 859 million eggs were incubated for meat chickens, and 728 million were actually placed, meaning the viability of incubated eggs was around 85%.
On a per capita basis that works out to about 0.8 laying hens, 0.8 culled male chicks, and 21 chickens raised to slaughter age. Per person, per year.
Alternate calculation: The average Canadian purchased 32.51 kg (eviscerated weight) of chicken meat. With an average eviscerated weight of (6.16 lbs or 2.79 kg) that means 11.7 broiler chickens per person. This would be a low-end estimate because it doesn't account for culling and other losses.
Each Canadian following an average omnivorous diet requires the death of 23 farmed animals per year, or slightly more. Because chickens make up the majority of all animals slaughtered for meat and vegetarians don't eat chicken, a vegetarian would spare approximately the same number of lives as a vegan. These numbers are probably similar in the United States.