On the other hand, many species are widespread with large populations and seem to be happy in a variety of different habitats. Often, these are animals with a broad diet, able to eat a range of different foods and still remain healthy - the red fox is a good example of this, especially once it started exploiting suburban habitats in the 20th century. Typically, they will not be as good at finding or processing these foods as those that specialise in one particular type but the fact that they can switch food sources easily makes up for this. Indeed, this can be a driver for evolution - an animal becomes really good at exploiting one narrow food source, out-competing the generalists, but the latter remain in the background and, when the world changes and the narrow food source is replaced by something different, become the basic stock from which the next round of specialists will arise.
Being able to put up with a wide range of climates is another reason for broad success, as we can see with animals such as wolves, which are found from Alaska to southern India. In addition to these purely biological factors, behaviour can also play a role. If the animal is capable of adapting its behaviour rapidly to changes in its environment or an increased number of predators, that will help it colonise wide areas. Wild boar are good at this, for instance, and it may even be that, as with some other species, they can switch to a more nocturnal mode of living as a response to human activity.
The brown hare (Lepus europaeus) is another example of a broadly successful, widespread mammal species. It's one of over thirty species of hare, and is by no means the only one that's wide-ranging - while most are restricted to smaller areas, there are two species that live across essentially the whole of Africa and others with similar ranges in Asia and North America. Those in the US, for example, may be familiar with the white-tailed and black-tailed jackrabbits - because, yes, "jackrabbits" are just American hares. In the case of the brown hare, it is native to most of Europe and extends into central Asia, as well as having been successfully introduced to the Great Lakes area of the US and Canada, southern South America and (albeit with subsequent regret) Australia.
Despite this, while still remaining under no particular threat worldwide, brown hare populations have been declining in Europe, perhaps partly due to the increasing amount of land taken up by single-crop agriculture. Hares are widely-studied animals so we have no shortage of research to look at when determining their habitat requirements and population densities and, as a recent review shows, their behavioural responses to the environment.
One thing we can look at here is the home range of the animals concerned. This is the area within which the animal lives and travels over a moderate period of time (say, two months). There is considerable variation between individual hares as to how much land they need to occupy but they are not particularly territorial, so the home range of one hare can overlap with those of several other adults, and any young they may happen to still be rearing.
One study from farmland in central Germany gave an average of 21 hectares (52 acres) but it's clear that individual hares may be more or less inclined to travel based on their individual circumstances and few will be exactly average. In particular, as is common with polygynous species, males generally have larger home ranges than females in order to maximise their mating opportunities.
Taking all of this into consideration it turns out that hares tends to have smaller home ranges in areas dominated by arable land. This indicates that while crop monocultures, such as wide fields of nothing but maize, may be bad for them, in general farmland provides them with plentiful food, and they don't need to travel as far to keep well fed.
This is especially true where fields are small, and there seem to be a couple of reasons for this. Firstly, the edges of fields tend to have the greatest variety of food types, whereas the centre will be dominated by whatever crop is being grown. Furthermore, hares, unlike rabbits, do not dig burrows, so hedgerows provide them with shelter meaning that hares will be more common where fields are small, increasing the population density and decreasing the home range size. Indeed, in this respect, plenty of fields may benefit hares even if the food being grown in them isn't one they want to eat, such as the "elephant grass" grown for biofuel.
Crop height may also be relevant - too short and the hares will be visible to predators, too high and it's difficult to travel through. It may be for this reason that, given the choice between farmland and forest, hares will actually opt for the former, even though they otherwise avoid human settlements; woodland provides less grass to eat and a patchier distribution of things to hide beneath.
Another way that animals may change their behaviour in response to their environment is by changing the time of day in which they are active. Hares are naturally nocturnal, so they do not need to switch to nighttime activity to avoid humans, but the precise details do vary through the year. This is because, in Europe nights are significantly longer in winter than they are in summer (Britain, for example, is roughly on the same latitude as southern Canada; jackrabbits in Texas would likely notice less of a difference). This means that during the summer, they are forced to come out in daylight if they want to get enough food to last through the day, although they mainly do so around dusk. In winter, they can stick to their usual nightly habits, and seek shelter before the sun comes up.
It's not just the length of the day that matters. Hares will also change their activity patterns with the phase of the moon, being less active on brightly moonlit nights, as is true for snowshoe hares (Lepus americanus) in the US and Canada. The likely reason for this is that moonlight makes them easier for predators to find and so safer for them to stay close to cover, and avoid going out altogether if possible. That this is not something inbuilt but a behaviour that the hares can adapt in response to changing circumstances is shown by the fact that, when kept in enclosures that foxes cannot enter and where aerial predators such as eagle owls do not naturally occur, the hares soon learn to ignore the moon and come out even on bright nights.
The ability to change behaviour in this way, in response to both predictable and unpredictable circumstances, may be a part of the reason why hares are successful as an invasive species, often to the detriment of native animals, such as the mountain hare (Lepus timidus) in Ireland. Not all hare species are so lucky, and two are officially listed as endangered, but, despite a declining population, the brown hare's adaptability and love of arable land should ensure its future and perhaps its expansion into new lands.
[Photo by "MOdmate" from Wikimedia Commons.]
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