Thus, it can be important to know not merely which habitats a particular species of animal prefers, but how it moves about within that habitat to use to its best advantage. Does it tend to revisit particular patches of ground, and if so, how often? A herbivore, after all, wouldn't want to head back to a given place before whatever it had eaten there had had a chance to regrow.
Research on this topic has tended to focus on birds, especially those that feed on nectar, to which the rate at which flowers replenish their supply is obviously important. For the same reason, bees have not been neglected either, but there is rather less on mammals. Exceptions include herd animals, such as elk and impala, where we do often see a pattern in their wanderings, driven perhaps by herd-leaders' memory of where, and when, they had last found good food sources.
The koala (Phascolarctos cinereus) is an interesting animal for several reasons. It is the only living species in its family and, while the species itself only dates back to the late Ice Ages, it has been well over 20 million years since its ancestors diverged from those of its closest living relatives (wombats). It also has an unusually restricted diet, eating only eucalyptus leaves, with over 20% of its diet coming from just three species.
Eucalyptus leaves are not especially nutritious, so koalas are always on the look out for those with best nitrogen and sugar content, and the lowest amount of bitter chemicals that evolved to deter herbivores. Then, to conserve energy, they sleep for around 20 hours a day, picking trees that do not have edible leaves and that, so far as possible, keep them apart from other koalas. (No, they aren't very sociable).
While it isn't the first study of its kind, a paper published earlier this year used GPS radio collars to track the movements of 25 koalas living in the Liverpool Plains of New South Wales. This area is mostly agricultural, lying just south of the town of Gunnedah and dotted with smaller villages. Agricultural land is of no use to koalas, but there are many small patches of eucalypt woodland, with the largest about 10 km (6 miles) across. This is exactly the sort of fragmented landscape in which movements between patches of food should be easiest to detect.
It turned out that most of the koalas stuck to a routine, moving on after a set period of days in one place. This was often a 7-day pattern, although some had shorter or longer cycles of activity. In particular, males often moved about every other day, possibly because they were patrolling their territory as well as simply looking for food, making sure no rivals had snuck in while they were away feeding. Although this isn't going to be apparent from GPS positioning, they were probably bellowing at one another from strategic points, something that male koalas do to prove their bulk and masculinity.
In contrast, some females kept to a three-week pattern, perhaps reflecting a more intensive use of the local resources before they felt compelled to move on. Regardless of the length of their routine, however, koalas of boths sexes were more likely to stick to it when the weather was dry. Koalas drink very little, getting almost all of the moisture from the leaves that they eat, so it could be that were targeting the trees with the juiciest leaves at this time, similar to the way that elephants make more regular visits to water holes at the driest times of the year.
However, the patch of woodland they chose to move to seems to have been chosen essentially at random, unlike the more organised activity previously reported in deer and hummingbirds. This was less true of those koalas who only had a few nearby patches to pick from, but it's probably easier to slip into a routine when you don't have many choices and still need to make sure you aren't visiting the same place too often.
This last part may be significant for conservation. The koala is not generally considered an endangered species, with around 300,000 of the furry marsupials being found across much of eastern Australia. However, there is some evidence of population decline, with a 2012 estimate suggesting a fall of 28% over the prior 24 years. That may have changed for the better since, and there were uncertainties in the evidence even then, but climate change is unlikely to be helping, so we don't really know.
If koalas living in edge environments with limited choice of feeding grounds are being forced into stricter routines, the authors of the study conclude, we can make their lives easier by boosting connectivity. This could be done by maintaining the existing patches, restoring some corridors of tree growth along streams, and perhaps even by planting small groups of trees between the larger patches, so that the koalas can roam further and have more feeding and resting places to choose from. And, while this study was conducted outside of the breeding season, that would probably help them find mates, too.
Which has got to be good for them, and would likely benefit other species, too. The koala may be famous for only eating eucalyptus leaves, but the same is true of the southern greater glider (Petauroides volans)... and that really is endangered. If some koalas are getting stuck in a rut, rarer species may be finding it even harder.
[Photo by JJ Harrison, from Wikimedia Commons.]
No comments:
Post a Comment