We received comments on our Facebook page discussing about the digging behaviour of wild boars and how it could affect trees in Hong Kong. We interviewed experts from HKU and would like to share their views with you all.
Q: Does the digging behaviour of Wild Boars cause more trees to fall during typhoons?
©️ WildCreatures Hong Kong
In terms of Wild Boars
Expert：Applied Behavioural Ecology and Conservation Lab, School of Biological Sciences, HKU
(from left) Dr. Hannah Mumby, Assistant Professor, Dr.Yifu Wang, Postdoctoral Researcher, Mr. Calvin Ma- Research Assistant
1. Do boars dig and eat tree roots? Does that damage the trees?
Yes, this is part of the natural behaviour of boars. Since they are a native species, this impact on trees is also a part of ecosystem functioning. In fact, wild boars can be seen as ecosystem engineers. They can open up space for new plants to grow and allow turnover in the system. If populations of boar are dense, the impact on vegetation is stronger and can cause issues with soil disturbance and vegetation cover.
They mainly feed on plant matter, which include some of the roots. We don’t think that the woody structural roots are what they are looking for, they prefer the smaller, juicier ones, but since boars can be quite strong, major roots could also be destroyed during the process.
2. Do they revisit the same tree?
No concrete data for that. From observation, they tend to use specific areas, e.g. if they forage at a specific rubbish bin or someone feeding them at a certain spot, their activities can be very predictable.
So far we have observed that they seem to like a lot of common plants, such as Turn-in-the-wind, Wild Coffee, Luofushan Joint-fir and Taiwan Acacia and more...
(from left) Luofushan Joint-fir, Taiwan Acacia
3. Are there any areas in Hong Kong where the boar population is so dense that there is soil disturbance and vegetation cover loss?
We observed some signs, at areas such as Siu Sai Wan and Shatin. However, since we haven’t conducted an ecological impact survey, it is hard to draw a conclusion for now.
Vegetation loss in some of the Hong Kong countryside areas, probably caused by Wild Boars. (©️Applied Behavioural Ecology and Conservation Lab）
4. Is the number of wild boars in Hong Kong out of control now? If so, what caused that?
It does seem to be increasing. Control is a big question mark, because it suggests that people should be intervening in the population. We don't know exactly what caused the increase. Top predators such as tigers that used to be present in Hong Kong are no longer able to exist here, and haven’t been part of the ecosystem since the 1940s. This means lager terrestrial mammals like wild boars have few natural predators. There have been some changes to policy of wild boar, for example hunting does not now take place and although there is a contraception programme, it only covers a small proportion of the population. We have to remember that wild boar can have piglets twice a year and that they have around five piglets each time, so the population can increase much more quickly than monkeys, for example.
5. Some people feed wild boars in Hong Kong. But even if no one is feeding them, would their strong adaptability eventually lead them to forage in the city?
Feeding Wild Boar is a topic we are interested in understanding. As researchers, we question everything and focus on understanding the situation before drawing conclusions. It's possible that the boar might forage in rubbish bins and other sources of food even if people didn't directly feed them. But feeding is not just about getting energy and nutrients. We have to consider the impact of all human action and how they can affect their natural behaviour of animals and their response to people.
(from left) White bread and plastic bag found in the countryside. Wild boar feeding on road in the city. (©️Applied Behavioural Ecology and Conservation Lab）
(fromt left) Food inside pastic bag put on the street. A Wild Boar eating a plastic bag. (©️Applied Behavioural Ecology and Conservation Lab）
Wild Boars chasing a taxi with possibly their regular feeder. (©️Applied Behavioural Ecology and Conservation Lab）
6. Is there enough food in the wild for them? What do they feed on naturally?
There is enough food. Wild populations do fluctuate around their "carrying capacity" or ability to feed and support animal populations, and that's completely normal. Wild boar are generalist feeders and eat lots of things including roots and shoots, green plants, fruits, small insects, bird eggs and worms.
7. Are wild boars 'dangerous'? What are the common conflicts between HK people and boars?
They're wild animals, so it's best to give them space to be wild animals. Their size and strength means that they have the potential to injure, but this is unusual, and happens if the boar is afraid. For example if people get too close or the boars are injured or protecting their piglets. Most of the time, even complaints about boars are not about physical injuries. Issues can arise when boars forage in crops, in rubbish or lose their way and get stuck in a residential or commercial area.
8. What should people do if they encounter a boar in the city or in the wild? Do wild boars have any 'warning signals' that we should be aware of?
Give them plenty of space and let them move away if they want to. Be patient. If you accidentally get too close, you can move away and avoid making sudden movements or touching the animal. We are better at interpreting animal behaviour than we think, if the boar looks backed against a wall, is giving you eye contact, running or has piglets, the best thing is to give it as much space as you can. If you are concerned about a boar you can call the AFCD, if you are concerned about litter that boar feed on, you can call the FEHD.
9. Do other countries have similar wild boar situations like Hong Kong? Anything HK can learn from?
Boar are a really widespread species across their natural range in Eurasia and they've also been introduced to other places. Lots of regions have boar in cities, including Berlin and Barcelona in Europe. Many places do have regulated hunting of boar. Like Hong Kong, many areas have teams from wildlife departments working to ensure the health and safety of wild populations. Good waste management and knowledge of the public are known to be important in coexistence with wild boar.
Many cities which do not use any lethal means could have difficulties controlling the boar population, the management on other aspects has to be right on point to maintain the equilibrium.
For example, waste management in Berlin is a lot straighter than Hong Kong, but they still have an issue with boar. There is a very funny video, a man swimming nude chasing after a boar who grabbed his bag which has food and his laptop.
In terms of trees
Expert: Mr. Gavin S Coates, Senior Lecturer, Division of Landscape Architecture, HKU
1. What is the major cause of tree failure during typhoons in Hong Kong? Is that normal?
The major issue causing tree failures during typhoons in the urban areas of Hong Kong is the limited soil volume for tree root growth. Normally most tree roots spread out horizontally forming a ‘root plate’ often wider than the crown spread, but which does not delve deep. Usually 90% of the roots of a tree will be in the top 500mm or less of soil. The roots also intertwine with those of nearby trees, forming a mesh which helps all the trees stabilise themselves.
When we plant trees in tree pits and limited planting areas where the roots are confined, this often leads to problems. The roots are unable to spread out sideways because of solid foundations of built structures or very compacted ground. Now the roots can't go deep and they can't go sideways. Sometimes there's not enough root development at the base of the tree to support the crown of the tree. If the crown grows extensively without any selective pruning, it may become top heavy. In a typhoon the wind loading will act like a massive sail, then the tree may fail.
(left) The roots of a Chinese Banyan at Sai Ying Pun trying to reach ground, hit the concrete and couldn't grow properly.
(right) A banyan tree at Nathan Road, near Kowloon Park, a small stonewall planter extension has recently been built allowing the tree to send out new aerial roots into the planting area. (©️ Gavin S Coates)
2. Does the government do regular pruning to the trees in the city? Is that enough to compensate for the small size of the soil pots?
In the case of registered Old and Valuable Trees in Hong Kong, they are looked after very carefully with regular pruning, removal of dead branches and installation of cables to stabilise some of the vulnerable branches.
For example some of the banyan trees along Nathan Road next to Kowloon Park have been provided with planting area extensions making space for the aerial routes to extend down and grow into the soil, thereby lowering the centre of gravity of those trees and providing them with a bit more rooting space.
In some cases it makes sense to selectively and regularly prune the crowns of confined trees to maintain a balance between the roots and crown.
3. Are there any tree species in Hong Kong that are prone to tree failure? Is it because of the wood properties etc.?
We can‘t really conclude whether a particular species of tree is prone to typhoons or not just by looking at the hardness of the wood. Both softwood and hardwood trees have their ways to survive typhoons, it’s dead wood that will fall, because dead branches dry out quickly and become brittle.
Take for example, the Common Red-stem Fig (Ficus variegata) in the centre’s garden. If you rap your hand against the buttress roots, they resonate a hollow sound. Actually, it’s very, very soft wood! But it's flexible and the ability to bend in the wind is more important than having very hard wood. In addition, in the garden its roots can develop naturally and the buttress roots can extend freely. This is why your Fig tree has survived more than 30 years of typhoons, including Mangkut in 2018.
So the hardness of the wood is just one of many factors, but the major issue is lack of soil volume.
However, Bauhinia × blakeana is particularly prone to failure due to the properties of the wood as well as the disorganized way of branching – many of them just fall to pieces during typhoons!
The Common Red-stem Fig at our centre's garden, planted by ex-resident Stephanie Crockett around 30 years ago. Its figs are very popular among animals viisting our centre.
Flower emblem of Hong Kong, Hong Kong Orchid Tree (Bauhinia x blakeana) is a hybrid species, which cannot reproduce naturally. More about Hong Kong Orchid Tree, please click here.
4. Many trees are exotic, like Brisbane Box on Lung Fu Shan, are they more likely to fall?
Brisbane is much less prone to typhoons than Hong Kong, so Brisbane Box may not be adapted to this kind of situation. The main issue is that trees in a plantation are usually planted very close together, consequently they grow up tall and skinny, competing for light. The individual trees are quite weak and cannot withstand high winds so it's not surprising that some of them fall over.
Historically these exotic trees were planted as pioneer species as they can withstand the very exposed desert-like conditions on deforested or constructed slopes. Another example is Acacia confusa, Taiwan Acacia, which was widely planted in Hong Kong in the 60s, 70s and 80s. Now most of those trees are coming to end of their lives.
If properly managed these pioneer exotics can usefully serve as ‘nurse’ trees providing a better microclimate for native trees to grow. After they’ve served their purpose they can be thinned out in favour of natives. The trouble is, this kind of management is rarely if ever carried out, resulting in ecologically deprived monocultures.
(from left) Brisbane Box plantation on Lung Fu Shan, Taiwan Acacia
5. So should we worry about the trees falling? Both in the city and in the countryside?
The priority must be human safety. If a tree poses a risk near where people circulate, then you need to take it very seriously, even if it means removing the tree.
It makes sense to put more effort into managing the big trees in Kowloon Park for example because of the amenity value of the trees and the proximity of people, but it’s simply not realistic to maintain all the Taiwan Acacias on every hillside in that way. In that case priority then should be on trees near roadside footpaths, cycle tracks, playgrounds etc.
If trees fall in inaccessible areas in Country Parks away from footpaths, then it’s part of a natural process. The decomposing trunks and branches feed the soil micro-organisms, building up soil to nourish the next generation of trees. So that’s fantastic! Let that process continue!
I have heard of names and idioms about trees for many years, such as ’It takes ten years to grow trees and a hundred years to rear people’ and ‘A thousand year Ginkgo Tree’ etc. Just wonder if a tree can die from ageing?