Black Bears and the Uncertain Apex

The concept of the food pyramid is a central one in ecology. The idea is that since consumption cannot ever be 100% efficient, every type of animal must necessarily be less common than whatever it is that it feed on, at least in terms of its total biomass. Plants are more common than herbivores are more common than small carnivores are more common than larger carnivores.

The actual picture is more complicated than this. Many "carnivores" are at least partly omnivorous, and they often eat large herbivores more regularly than they eat small carnivores. Plus, we also need to consider the detritivores and parasites. But the general pattern holds, and at the top of the pyramid, we find the apex predators. 

It's possible to argue as to what exactly constitutes an apex predator. The general idea, however, is that they feed on other animals without being preyed upon themselves. At least among terrestrial mammals, an average body mass of more than about 15 kg (33 lbs) is generally about enough that predators need to manage their own population (through competition, territoriality, infanticide, etc.) rather than having to worry about something larger and scarier managing it for them. 

Even so, the non-human animals typically described as "apex predators" in most parts of the world are quite a bit larger than this. Such animals dominate the "landscape of fear", influencing what other animals choose to live and feed where, and, even if they aren't eating them directly, will have at least some competitive effect on the smaller predators nearby. Not always in the way you might think, either; one study found that jackals are more common where lions are also common, but only if there are many other predators around as well - if it's just lions, they will avoid them.

So what exactly counts, and what doesn't?

The American black bear (Ursus americanus) is one of the more common large mammalian predators. Of course, one could question the definitions in that statement, not least because black bears are omnivorous, and meat isn't even an especially large part of their diet. Nonetheless, they do have significant effects on the other animals around them. In Virginia, they were found to be responsible for around half of all deaths of white-tailed deer fawns, and, in Yellowstone, they have been reported to eat elk calves, on average, once every eight days. 

Understanding how black bears affect the other animals around them can be important for a couple of reasons. For one, they are relatively tolerant of human presence, often wandering into semi-rural residential areas and being more likely to come into conflict with humans. For another, their population has increased over the last thirty years or so, making them one of only two carnivoran mammals that are more common now than they were in the late 20th century. (The other is the giant panda, which was starting from a much lower point, and isn't a predator). 

There are currently thought to be just under a million American black bears in the world, more or less evenly split between the US and Canada, with a much smaller number in Mexico. Their population probably dipped to its lowest point around 1900 or so, and has risen erratically and unevenly since then. Certainly, they are no longer found in as many places as they were in the 18th and 19th centuries - they once inhabited the whole of the US except the southwestern deserts, Hawaii, and the colder parts of Alaska, and reached from Hudson Bay almost to Mexico City. But, with increasing numbers where they are found, understanding their ecology isn't important because they are endangered, but because of what could be wide-reaching effects.

A review published last year confirmed that, despite being omnivorous, American black bears do meet many of the criteria used to define apex predators in other studies. For example, cougar/puma/mountain lions have been described as apex predators in part because they are the primary predators of white-tailed and mule deer in areas where they live together. In eastern Canada, black bears are responsible for a full 94% of the deaths of young reindeer... and, yes, maybe they do eat a lot of honey, nuts, and berries, but that's probably not much consolation if you're a mother reindeer.

Having said which, the review was unable to find much evidence that black bear numbers affected the local deer population. But it did identify cases in which they influenced other mammalian predators, which could be argued as evidence for their "apex" role. For instance, in the western US, they scare away coyotes, just as cougars do. 

In fact, cougars, a fairly clear example of an apex predator, are themselves affected if black bears are in the neighbourhood. This is largely through kleptoparasitism, a fancy word for stealing somebody else's dinner. That is, the cougar kills something, and then the bear turns up, scares it off, and eats the carcass. No hunting (or, in the strict sense, predation) required, but the end effect is the same. And this does affect the cougars; when black bears are around, they are able to spend, on average, less time at each fresh kill, get less to eat and, in consequence, have to hunt again sooner than they would otherwise. So the cougars actually end up killing more deer than they would if there were no bears nearby.

Wolves seem to be less affected, possibly because, being pack hunters, they are more efficient at taking down deer, and perhaps defending carcasses when the need arises. While the presence of brown bears can, indeed, reduce the kill rate of wolf packs, in the case of the smaller and more peaceable black bears, it's probably they who have to change their habits when wolves are around.

Of the four main areas where most studies of American black bears have taken place, wolves are present in two - the Great Lakes and Yellowstone Park. Here, perhaps, they have less direct effect on the local ecosystem, not because nothing is eating them, but because other predators (including brown bears, in the case of Yellowstone) are more influential. But, in the other two main study sites - Newfoundland and the Great Smoky Mountains National Park on the Tennessee/North Carolina border - there is nothing else of quite the same size to compete with.

This underlines the fuzzy nature of the concept of "apex predator". It's reasonable to argue that black bears, being omnivorous and eating more plant matter than meat, don't really count. Because they are relatively harmless to humans who leave them alone, unless they have cubs to defend, they rarely fit our usual concept of the term. And, in the western and northern US, where there are also wolves, and in Alaska and limited parts of the northwest, where they share the landscape with grizzlies, that could well be fair enough.

But in the east, where there are no longer wolves and have never been brown bears or cougars, black bears may be as close to an apex predator as you're going to get. Certainly, it seems that they have a direct, top-down effect on the other animals around them, ungulates and smaller carnivores alike.

[Photo by "Cephas" from Wikimedia Commons.]