Minnesota moose experts generally agree that global warming is forcing the southern edge of the distribution of the moose northward into Canada, threatening this important US population of this ginormous deer species. Global warming denialists insist that this is the moose’s fault, and has nothing to do with global warming. This is the first of a two part look at this question.
Moose are one of five species of deer that are common in North America. (For those of you tuning in from Europe, this can get confusing because the common terms are all mixed up.) Most of these deer are members of the subfamily Capreolinae. The genus Odocoileus includes the mule deer and the white-tailed deer, and together they have a distribution that runs from southern Canada throughout the United States, across Mexico and into Central and South America. While Odecoileus tolerate a wide range of conditions, they are limited in the north by very cold weather, hampered by deep snow which causes them to work harder for forage and be susceptible to predators, and short warm seasons.
The genus Alces includes the American Moose (Alces americanus), the subject of this essay, and a very similar animal (Alces alces) in the Old World called Elk (which is really a moose that the Old World people have misnamed, apparently). Moose are really large, semi aquatic, and unlike the Odocoileus, it is warm conditions they can’t tolerate rather than cold. For this reason, they are mainly a Canadian species, though their range extends into the northern tier of US states to varying degrees, this range having expanded during the last 50 years following a period of contraction probably caused by over hunting and habitat loss.
New York State and Massachusetts, for instance, never had moose during the 1950s and 1960s. The first several moose to show up in those states were wandering moose, animals with a deadly brain parasite that causes them to wander aimlessly until they die. Such moose were shot on sight by authorities. But then autopsies of moose so killed started to show the animals to be healthy, and over time it was realized that the moose range was actually expanding. (So they mostly stopped killing them on sight.)
One of the most interesting things I ever saw was a moose walking casually across a swamp in central Connecticut in or about the year 1990. There were no moose in Connecticut in 1990. Except this one, I guess. (And no, I did not turn that moose in to the authorities.)
A third genus of deer within the subfamily Capreolinae that are numerous in North America is the caribou. These are all classified in one species, Rangifer tarandus despite the fact that like the Moose/European Elk dichotomy caribou are both Old World and New World, and despite the fact that there are two distinct North American populations, woodland and tundra caribou. These animals are restricted to even cooler climates than Moose, possibly because of forage, although historically woodland caribou probably extended much farther south than their present range despite warm weather limitations.
More distantly related than the rest, part of the subfamily Cervinae, is Cervus elaphus, or the American Elk (called red deer in Europe, and related to the sika, which are Asian but do have one transplanted population in the US). We are going to ignore elk in this discussion. The point of talking about several species of deer is in part to discuss geographic range and its link to climate change. The best available prehistoric and historic evidence suggests that elk were fairly cosmopolitan in their former distribution, but have been negatively affected by hunting and other human influences more than any other deer in North America. So, it is impossible to draw a species distribution map for elk, or for that matter, to make clear statements about their habitat preferences. If one were to do so based on their current distribution, they would seem to prefer national forests and parks in hard to get to places or that are heavily guarded by smoky-the-bear hat wearing rangers, but we find their teeth and bones on Native American sites across a huge area and linked to both open and closed habitats in the past. So forget about elk.
The following graphic is a lame effort on my part to draw the distributions of, from north to south, caribou, moose, and white-tailed deer.
These distributions roughly reflect natural ranges, but if Elk had not been hunted out, there might be areas where they are the predominant species. Moose eat a lot of aquatic vegetation and they browse, caribou eat lichen in the north and evergreen browse in the south, and white-tailed deer eat browse. By “browse” I simply mean leaves on trees or bushes. There are definite differences in the exact preferences between, say, moose and white-tailed deer to the extent that if you see certain plants being eaten and look for prints, the species represented by the prints is fairly predictable. Munching on willows? Moose. Munching on mountain maple? White-tailed deer. And so on. But I believe there is a great deal of overlap between what white-tailed deer and moose eat, or at least can eat. This means that there is some competition between them for food.
It is almost certain that white-tailed deer are overpopulated in most of their range owing to state and provincial game department practices, and the presence of vast reservoirs of unhunted habitat that lack large predators (such as the area around reservoirs, military reservations, and suburbs). white-tailed deer are virtually ubiquitous in the indicated range, and numerous enough that anyone living in this range can decide they want to see a white-tailed deer and succeed in that effort within a few hours if they know where to look. It does not matter if you live in Chicago, Little Rock, or Ely. This has not always been the case. Before highly regulated hunting, when the US was a more agrarian nation, and farmers harvested the deer on their land, white-tailed deer were absent from huge areas. A friend of mine tells the story of her grandfather (or grand uncle, or someone) who caught a white-tailed deer on his farm in the Midwest and was able to charge people to see it. If you go to that same farm area now, what do you see? Suburbs and deer everywhere.
Today, white-tailed deer populations are probably abnormally high and abnormally contiguous across the landscape because of the practice of shooting far more males than females in hunting season, a lack of predators that would normally eat them, and abundant food in certain areas. These fluctuations in white-tailed deer populations are important to note because it suggests that there are probably genetic bottlenecks followed by population spread and possibly relaxed selection over the last century. This is a situation that is good for parasites but not so good for the white-tailed deer or any other mammals that may share parasites with them. (Consider the rapidity and completeness of the spread of Lyme disease across the entire country over just a few decades.) When you look at the map above, realize that the distribution of moose within the indicated area is patchy, because of their semi-aquatic habitat preference and other factors. That range is a region within which you will find moose, but they are not ubiquitous. They are only located in some spots within this larger area of distribution. The white-tailed deer, on the other hand, are like mammalian air. They are everywhere, carrying and dispersing diseases, like an ether for parasites. Any moose population that might otherwise be isolated from other moose populations are connected to each other by this continuous sea of white tails.
Now, refer back to the map. The northern edge of the white-tailed deer population is limited by temperature. So is the southern edge of the moose distribution. One would expect that if the overall medium or long term pattern of temperatures changed, that these distributions would also change, up to a point. White tailed deer are forest animals, so their distribution would not move north too far because of the limitations of forest cover, at least until the forest also moved north, during a warming period. Moose may be somewhat restricted int heir move south because many of the marshy habitats they like are found in greater abundance in glaciated (young) terrain than in “driftless” or unglaciated (older, more eroded) terrain. So during a cooling period, moose range may move south in a patchy, irregular way.
But the issue at hand is warming and moose. The Minnesota moose population is actually an excellent place to look for changes in the southern boundary of the moose population owing to supposed global warming. There are several details of this region that allow more than just a vague correlation to be tested. Consider:
Recent warming trends are not uniform across space and come in fits and starts in time on a local level. Nationally, warming is more obvious in the Dakaotas and less so in the Wester Great Lakes, and Minnesota is right between these two zones. Thus, it would be predicted that if a warming related decline of moose in Minnesota occurs, it would start in the western part of the state and occur later in the eastern part of the state.
Locally, there are pretty good climate records. If the moose population can be divided into, say, eastern and western segments, the climate records can be checked to see if warming, possibly different in each area, corresponds to declines in moose populations depending on the exact temperature records. A warming – moose decline link would be strongly supported if different temperature change patterns in the two areas correlated with moose decline. You’ve heard the fallacy that “correlation does not imply causation” (that is exactly how I heard it used just today, by someone). Well, it does. Imply. “Correlation does not mean causation” is the correct phrase. But, two correlations can be thought of as a repeat experiment. That would be what such a finding meant.
In order for there to be a link presumed between temperature change and the viability of moose populations, there needs to be mechanisms. Notice that I used the plural. We already know that basic climate variables are at the base of many systems. A climate change that enhances or diminishes the viability of a given geographical region for a given species is expected to do so in many ways. Here is a short list of categories of effect:
- Winter or dry season parasite die-offs (e.g. shorter winters = more parasites).
- Critical season forage availability. Pre-winter feeding is important for some animals. For moose, longer or warmer summers, or more sunlight because of less cloud cover, can affect aquatic plant communities changing forage amounts.
- Snow related predator effects. Deep snow is good for hunting wolves no snow is bad for animals that turn white in the winter, etc. More snow cover is probably bad for white-tailed deer. Even more snow cover could then be bad for moose. (Global warming effects does not equal less snow, BTW). So as snow increases, white-tailed deer are expected to become less abundant and moose should be fairly happy, until a certain point is reached.
- Competition with other species. All these different effects will increase or decrease the abundance and ubiquity of white-tailed deer, which compete with moose for some food and spread parasites.
- Sifting metabolic demands. Moose are selected to thermoregulate in cold climates. They survive summers by laying low and holing up during the day. Many warm days in a row can be deadly for moose.
There are certainly more effects, and these effects can be broken down in a number of ways. The question is, is the balance of effects negative or positive for a given species. Given what we know about moose and the obvious fact that their southern range is limited by temperature, we would have to guess that the balance of increasing temperature is negative for moose. Is this what we see when we look at the die-offs of the Minnesota moose?
Continued in the next installment of “Minnesota Moose and Climate Change”