Beavers in the Wetlands

The North American beaver (Castor canadensis) is one of two living species of beaver, along with its Eurasian counterpart. They are, of course, rodents - they are related to gophers - and relatively large ones at that. They are found across all but the most treeless parts of the US and Canada, as well as the border regions of northern Mexico; they have even been introduced to Finland, Belgium, and Argentina. They are a relatively common species across much of this range, something that has been helped in recent decades by restrictions on hunting.

Nonetheless, while not endangered themselves, they can be key to maintaining ecosystems, not least because they are one of the few nonhuman species that substantially modifies the land around them. Their ability to alter wetland habitats by dam-building has been identified as a key factor in maintaining other species at greater risk, such as amphibians in the Rocky Mountains. On top of which, their habit of cutting down trees affects the composition of the forests in which they live.

For these reasons, it can be useful to know what factors affect beaver population density and how the relevant habitat will be changed as a result. Which turns out to be a more complex question than it might appear at first glance.

Part of the reason that so many different species of animal (and plant) exist is that each adapts to a particular environment, or exploits the same environment in a slightly different way. What's good for one animal isn't necessarily good for another, even if they are otherwise similar. The species evolves to be good at exploiting whatever its particular favoured resource may be, and if some become particularly good at exploiting some other resource, then, even absent any physical separation, they adapt to that and may become a different species.

The particular resources an animal species may require can vary considerably. Food supply is obviously a key one, but there may be physical requirements as well, such as suitable places to sleep or hide from predators. But, whatever they are, the animal wants to seek them out, and we'd therefore expect that the more of them are in a given area, the more individuals of that particular species will live in it. 

This has been formalised scientifically as the "ideal free distribution" model, which states that, assuming no physical barriers are preventing free movement, population density increases as habitat quality increases. This is, however, a bit like those economic theories that assume markets always act in a perfectly rational manner, largely because that "no barriers" part is quite significant, and we can't always rely on a direct correlation between resource availability and population density.

Furthermore, if the animal lives in communal groups, as beavers do, an increase in population density could happen for either of two reasons (or both). A better habitat could support more groups of the animal, each group requiring a smaller area to support itself. Or the groups could become larger, with more individual animals occupying the same area. A key limitation here, if the animal is territorial, is how much effort it has to put into defending its borders against rivals and, therefore, how much it helps to have those borders be shorter.

This is one of the questions that remains open for North American beavers, with conflicting results from studies looking into it. In general, population density of beavers seems to be related to the availability of aquatic vegetation such as cattails and water lilies, since this forms over half of their diet, but likely also to the types of trees that are available, since they rely on those for food, too, as well as constructing dams from them. 

Poplars, aspen, and willows seem to be the most popular trees, but they often use ash trees where poplars are less common. The latest study to look at how habitat affects beaver population density was conducted in an area where this is the case - Plaisance National Park in Quebec. This is a relatively small park at 28 km² (11 square miles), occupying the north bank and the islands within a stretch of river between Ottawa and Montreal. 

Other than the river itself, it consists of woodland with numerous patches of swampy ground. The trees are about a quarter ash and a quarter maple, with a random mix of oak, elm, linden, and other species making up the remainder. While poplars are present, they are not numerous, so the beavers rely on the ash, being spread broadly across the flat countryside, where they have prospered since the Park was established in 2002. (The beavers, in contrast, largely ignore the maple trees).

Beavers were fitted with GPS collars so that they could be monitored as they moved across the Park. This should have given a reasonable estimate of the area each family occupied, but the researchers confirmed this by the simple practice of walking across the area, examining the scent-marked mounds that beavers leave to mark out the edges of their territories. 

Determining the size of each family group is, however, a little more complicated, since only a small proportion of the beavers will get the GPS collars. Here, the researchers used drones equipped with cameras to watch the beaver lodges and see how many animals came in and out around dawn and dusk. This, it should be noted, takes quite a bit of time, since you need numerous trial flights before the study proper to make sure that the beavers realise the drones are harmless and don't try to hide from them.

The study showed that most beaver families occupied an area of between 10 and 20 hectares (25 to 50 acres). There was considerable variation, with some families occupying areas smaller or larger than typical, but statistical analysis was unable to find any clear pattern relating to the number of ash trees or the availability of tasty water plants. However, the better the quality of the habitat, the more beavers there were in each family group, with the smallest just having three, and the largest, seventeen.

This tells us that, at least in this specific part of the world, beavers take advantage of high-quality land by raising larger families but occupying the same general area that they would have anyway. They could defend smaller territories, sharing the landscape around them with more neighbouring families, and raising the overall population that way, but they don't.

This could be because there are only so many places one can build a good dam, so that the ability to occupy the landscape is limited by the locations you have to start out from. A similar study on Eurasian beavers, for instance, seemed to show a "first-come, first-served" effect, where the first beavers to arrive in a given area get the best bit of terrain and spread out around it, while late comers end up with smaller territories dotted around it.

In badgers, on the other hand, we don't see the same effect. Here, the larger the badger group, the larger the area it will occupy, even though they wouldn't have to if the habitat quality (availability of earthworms, in their case) really were better. This may be due to differing behaviour patterns, with badgers being less cooperative than beavers; it's in the interest of male badgers to have as big a territory as they can get away with, so that other males who might sneak in to mate with the local females are kept as far away as possible.

Coyotes show a different pattern again: the better the habitat (e.g. the more deer there are to eat in it) the smaller the territory of any given group. This, again, is probably due to their social structure. Coyotes have to defend their territory from rivals, so, if they can keep it small and still find enough to eat in it, they're going to be better off. 

Beavers, in comparison, are more tolerant, a phenomenon known as the "dear enemy" effect. What this means is that they tend to leave their known neighbours to their own devices and only get defensive when a stranger turns up. It could also be the case that beaver society is structured such that the dominant male can let his younger relatives help look after infants, he can devote more time to protecting his territory - so a bigger family is more of a help, freeing up his time.

Since about 2002, emerald ash borers - invasive beetles from northeastern Asia - have been detected across most of the eastern US and neighbouring parts of Canada, where they cause significant damage to the native ash trees. Since, in the case of the area in the study, these trees were key to the beaver's population density, this could alter that, leading to smaller families occupying the same areas. Having said which, that obviously won't apply in beaver colonies where ash trees aren't common in the first place, and it wouldn't if they just get replaced by poplars, which the beavers prefer.

Especially in biology, things can often be more complex than they first appear.

[Photo by "finchlake2000", from Wikimedia Commons.]