I encountered this last week as well, spurred on by the thought: where did the squirrels in NYC's Central Park come from? According to this article, they were introduced by humans in 1877: http://nymag.com/news/features/squirrels-2014-2/
But, I wonder if there's more natural migration going on now, or if Central Park is isolated enough from other green spaces that this population is cutoff from others. If that sounds absurd, remember: Manhattan is an island! I think squirrels are more free-roaming in the Bronx (with a bunch in the Botanical Gardens), but there's a river between them, and lots of paved concrete. Squirrels will swim, but there's quite a few blocks of mostly paved concrete between the river banks and Central Park. A squirrel could make the trip, but there may not be much cause to.
Although the squirrels in that park were introduced by humans, it should be noted (as mentioned in the article you cite) that those squirrels, Eastern Grays, are native to that area. They just were no longer in that particular place because we destroyed their habitat to make room for our cities there.
So in a sense introducing them to parks in those cities was just putting back what we had driven off.
It's interesting that the Eastern Gray squirrel was also introduced to parks in Seattle. The Eastern Gray is not native out here. The native squirrels out here are the Western Gray, the Douglas, the Red, and the Northern Flying squirrels.
That raises the question of why when they decided they wanted squirrels in the parks that they went to the trouble of importing Eastern Grays from the other side of the country, instead of just sending someone out to trap a few Western Grays and release them in the parks.
It turns out that the reason is that a lot of the parks feature non-native trees. Western Grays really want to live off of native trees, and so don't do well in those parks. Eastern Grays either are native to the same places the non-native trees are native to, or are happy with them anyway, so they do fine in Washington parks, regardless of whether or not those parks feature native plants or non-native plants.
That also means the Eastern Grays did fine when they left the parks, and they have pretty much completely replaced the Western Gray. There's now only three isolated populations in the state, where once they were widespread and common.
I once knew a girl who was visiting the US from a city in Brazil and she was shocked by squirrels and the fact that we just let them run around our cities and do whatever they do.
She was a little worried about attacks or food theft, but eventually she saw they were harmless. Even then she'd still excitedly point them out everywhere she went.
A potentially less labor-intensive way to estimate animal population is to capture a sample, tag them, release them back into the wild, and then later take a second sample and see how many of those animals are tagged: https://en.wikipedia.org/wiki/Mark_and_recapture
I was hoping this article would say something about improving the margin of error for this kind of statistical estimate, or at least provide a critique of mark-and-recapture statistics for counting squirrels (maybe they are just too... squirrely), but alas it sounds like they threw a bunch of volunteers at the problem:
> The trick is to divide and conquer. They drew a grid of 350 hectares—plots of land measuring 10,000 square meters—over Central Park. Think of them as something like Census tracts. Volunteers then fanned out and conducted two counts, one in the morning and another at night. The Squirrel Sighters, as they were called, spent 20 minutes per count searching for furry subjects, looking up in the trees and down in the bushes, and listening to the clawing and clucking sounds they make. Allen likens it to an Easter egg hunt; some volunteers found many squirrels, others saw none.
I was talking to a bird person who used to participate in various population studies with the national park service. There exist species for which tag-and-release doesn't work at all: jays and corvids in particular are verrrry good at avoiding the places and types of traps where they've been captured before.
300-ish man-hours from (probably) untrained volunteers, seems OK for an exhaustive census like this. I'd expect mark-and-recapture to be more expensive, just because it's tough to get people skilled in catching and tagging squirrels. Maybe you can get grad students to do it.
If they couldn't get enough volunteers, the natural next step would seem to be counting a sample of the 350 plots rather than all of them.
He said the methodology of dividing up into hectares had to do with squirrel behavior. They tend to mostly live within about a 100x100m area, so it was an easy way to be relatively accurate.
I do not mean this as a joke comment, but actually participating in a squirrel census would be a lot of fun. Sometimes the fun is in the journey, not the destination. And in this case, fun journey and the destination is reached.
I wouldn't trust an amateur citizen hobbyist to tag an animal population in a humane manner. There's no accounting for how the tagging is performed, or when the tags get removed and by whom.
If one person does it, others will justify their own efforts to apply tags to wildlife, and then we get into the legalities of animal cruelty, poaching on conservancy land, and on and on.
It's one thing to crowd source passive eyes spotting for squirrels. It's entirely another to invite people to try and handle them, and even interfere with instruments applied to their body.
Are they territorial? That might skew the sampling. Maybe tag one per hectare, then later capture one per hectare, the fraction that are tagged yields some kind of estimate of the total.
I bought the report from the website, which includes two maps, a booklet, postcards and (I'm not kidding) a vinyl record with the audio version of the report.
The whole package is BEAUTIFUL. It's some of the most delightful information design I've seen in a very long time. Totally worth the $75.
> The city used LIDAR technology, in which a drone or airplane shoots a light at the ground and the length of light is measured to determine elevation. The “phantom hills” are recorded when the drone or airplane mistakes, say, a building’s shadow as a hill.
Uhhh... No? Lidar definitely wouldn't have that problem.
Lidar captures a depth map and as such does have "shadowing" if you transpose an image taken from an angle to a vertical elevation map. Maybe that's what they meant, or that meaning got lost somewhere.
LIDAR doesn't really depend on any other illumination, so there are no shadows as such. If there is too much other illumination in the same wavelengths (or too little returned reflection) it might not register a return at all and this would appear as a hole in the data.
If not shining straight down, it might have an error for the horizontal positioning of a building's roof, but it will still measure a discontinuity like a cliff edge where one measurement hits the roof elevation and the next hits the ground adjacent to the building.
If measuring through a tree canopy, there will be multiple levels of returns as beams penetrate to different elevations before reflecting. These levels may include some measurement of the true ground level, unless the canopy is so dense that all beams hit a tree. So, a dense mass of trees or brush might obscure the true ground level beneath them. Perhaps the original article author conflated this with the idea of shadows.
Did a census bureau employee knock on their trees, driving them utterly nuts with redundant and intrusive questions? Questions like, "Did you gather nuts full time last week?", "Do you believe you've been given ample opportunity to gather nuts?" and "How many squirrels do you live with? Do they also gather nuts?".
Before reading the article, you can have fun doing a rough quantitative estimate. A Fermi question.
How many squirrels live in NYC's Central Park?
Is it more like 1, 10, 100, 1000, 10k, 100k, 1M, 10M, or 100M? What hard "no way is it more/less than" lower and upper bounds are you comfortable with and why? What about soft "I'd be surprised if it was more/less than" bounds and why? Doing iterative bounding and discussion, is a far more fruitful way to use the common estimation problems of education, than the traditional point estimates.
Everyone counts their "back yards" and they compile all the data. About the same difficulty as the squirrel count, but you are trying to identify what kind of bird.
On the other hand, I don't think that number is right. Based on my trips to NYC, and the amount of squirrels I've seen I think it's way low. But I may be skewed by the number of pigeons that are also in the park.
The census question not asked: are these squirrels native or invasive?
In most of the world the answer would be "invasive", particularly in the UK where they are wiping out the natives, but they appear to be native to New York.
They are native to New York yes, the eastern grey squirrel is a north american species.¹
However, the squirrels in Central Park, were deliberately introduced.
It's unclear whether they were absent from Manhattan naturally, or whether they were extirpated due to urbanization, and the re-introduced later.
And about the invasive status of the grey squirrel in Britain, it's sad that they were deliberately released in Britain, because it's not as if they swam across the Atlantic or hopped a freighter.
Now you guys are fighting an extermination campaign with all the regrettable suffering it causes to both squirrels and humans.
1: The Urbanization of the Eastern Gray Squirrel in the United States
Etienne Benson
J Am Hist (2013) 100 (3): 691-710.
So there was an episode on 99% invisible podcast about this topic, not sure if related to this census or not but it's an interesting listen for those who are curious: https://99percentinvisible.org/episode/uptown-squirrel/
But, I wonder if there's more natural migration going on now, or if Central Park is isolated enough from other green spaces that this population is cutoff from others. If that sounds absurd, remember: Manhattan is an island! I think squirrels are more free-roaming in the Bronx (with a bunch in the Botanical Gardens), but there's a river between them, and lots of paved concrete. Squirrels will swim, but there's quite a few blocks of mostly paved concrete between the river banks and Central Park. A squirrel could make the trip, but there may not be much cause to.