After reading yesterday's Gizmodo article titled "The Fascinating Story of why U.S. Parks are Full of Squirrels" by Adam Clark Estes, we had our Curator of Urban Ecology and resident squirrel expert (he runs projectsquirrel.org, a citizen science project) Steve Sullivan, write a response. The result is a historic and eye-opening look into the population of squirrels (and other game animals) from a biologists point of vew.
This is a fun article as far as it goes. It neglects two important points though, one having to do with extirpation and the other with invasion.
Eastern grey squirrel
Sure, we encouraged squirrels to live in parks. Wildlife of all kinds has potential to bring joy as we watch and interact with it. In fact, there is a claim (I’m still looking for the primary source) that Oak Park reintroduced fox squirrels by trapping some in Oklahoma. So why did we have to add squirrels to our parks and why did Oak Park have to import them from so far away? Over hunting and habitat destruction.
As settlers spread they relied on wild game to supply much of their food. Since we need to eat all year, people were hunting and trapping all year. Bison, deer, and turkey are the ones we often think of in this context but in fact beaver, bear, bobolink, lark, curlew, duck, prairie chicken, and even squirrel were also on the menu. In fact, small animals like squirrel and many birds were likely on the menu more often than the larger species. Remember, prior to the present era, people ate far less meat than we do today and two or three bobolink were more than enough meat for a meal; a squirrel could feed four people. Nevertheless, relentless hunting reduced populations of these species significantly. Many that were once common disappeared from many states, some became extinct. Only a few decades ago, it was uncommon to see a deer and unheard of to see a turkey. Bison are found almost exclusively in preserves and elk are rare outside of them. Bobolink, duck and lark populations are tiny compared to pre-settlement times. Eskimo curlew are extinct (we ate them all) as are passenger pigeons, a species that was once one in four birds on our continent. During this time, squirrels diminished greatly, too.
Bull Elk, photo by Flickr user Amada44
Thankfully, uncontrolled hunting (in the US) has been largely solved. Hunting seasons, animal censuses, and hunter success reports ensure that our favorite game animals are almost all doing very well. So well, in fact, that many species have become pests in places where hunting is limited.
While we may have a good handle on over hunting, habitat loss is another issue altogether. One game species that is not doing so well is the bobwhite quail. This once seemingly ubiquitous species is seldom seen in many places where it once was about the only thing worth hunting. It is declining for a number of factors, most notably habitat loss. As farms are consolidated, fencerows are eliminated. These fencerows once provided shelter from the wind and rain for a wide variety of species. Without them, the landscape becomes little more than a biological desert of corn and soy fields. Almost nothing lives in these places, especially not game animals.
Bobwhite, photo by Steve Maslowski/USFWS
Thankfully for squirrels, the parks we create are often hospitable environments. When we make parks, we typically eliminate most of the natural biodiversity. Notably, we eradicate predators to the best of our ability, and we plant as many trees as we can. Both of these cases greatly benefit squirrels. As the article correctly points out, it is sometimes necessary to install nestboxes because we also remove large hollow trees just as they become naturally good homes for squirrels. Not all species respond well to such simple manipulations though. Think how hard it is to get a bat box populated.
And so, these days, after killing off most of the squirrels near us and destroying their habitats, we have created parks where they can live and, in many cases (but certainly not all), we have intentionally reintroduced them as a mobile part of the natural beauty we maintain in our cities.
Invasion is another story. The grey and fox squirrels that many of us are so familiar with are native to the eastern half of our continent. There are other tree squirrel species that evolved in the more diverse western ecosystems. However, as we settled the west, we brought our squirrels with us. The native species were sometimes shy, occurred in low numbers, or were too greasy; they were hard to hunt and not much good for food. So, we introduced the greys and foxes into these new ecosystems and, much as they took to the artificial habitats of parks, they also prospered in western habitats.
Eastern grey squirrel
Today, Eastern grey squirrels (Sciururs carolinensis) are invading the habitat of Western greys (S. griseus) and eliminating them. Project Squirrel participants may also be documenting a new invasion of fox squirrels in Colorado where they compete with Abert’s squirrel. Fox squirrels are also well-established in the Los Angeles region and are a major pest in some nut farms. Both grey and fox squirrels can also cause major damage to natural forests as they eat and scatter nuts and remove bark from trees. Over time, we expect them to change the look and feel (and thus the resident animal populations too) of some western forests. These same problems are being experienced in England and Italy where our grey squirrel has been introduced.
So, while it is enjoyable to watch squirrels in eastern parks, those that you see in western parks are often an indication of significant ecological problems brought about by people moving squirrels around.