David and Goliath – a story of bark beetles

Bark beetles (Scolytinae) are small beetles a few millimeters in size. Their larva develop under tree bark eating the phloem, xylem, and cambium layers. Certain species cause extensive forest damage by killing healthy trees, while others only impact weakened individuals. The eating patterns (called galleries) and the trees’ defensive reactions cause disturbances in the nutrient and water cycling within the trunks. The trees literally dry to death.

Bark beetles can be detected by the gallery patterns they leave on tree trunks. These patterns are species-specific, and often very beautiful. The patterns can be used to recognize infestations and begin warding off the worst damage. Then again, the gallery patterns cannot be seen until the tree bark falls off.

Bark beetles also have a secret weapon: wood-staining fungi. This group of fungi includes several species that damage wood or cause serious diseases to trees. Bark beetles and wood-staining fungi have developed various relationships such as the ambrosia beetles that spread certain fungi species into their galleries to farm them for food. Wood-staining fungi benefit from the bark beetles transporting them to new trees, and have developed exceptionally sticky spores that attach to adult beetles as they are preparing to disperse. Bark beetles also benefit: the fungi weaken new tree individuals, giving adult bark beetles the opportunity to infest and lay their eggs in these trees.

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A possible wood-staining fungus is spreading beneath the bark of a birch infested by the birch bark beetle. ©Stella Thompson

It’s hard to believe that tiny beetles and even more minuscule fungi can kill gigantic trees. Situations where a bark beetle or fungi spreads to a new geographical region among lumber are particularly devastating. The new host trees have no immunity or defense mechanisms against this new organism and the alien species spreads like wildfire.

Dutch elm disease is a prime example of this. Ophiostoma ulmi, a fungus killing elm shoots spread from Asia initially to Europe and then, fueled by the post-World War I reconstruction boom, from Europe to North America in lumber. European elm species coped with the disease slightly better than their North American cousins. European elms also died, but the spread of the disease around Europe took several decades and finally the outbreak waned. 10–40% of the elms died, depending on the country in question. The situation was very different in North America. The American elm (Ulmus americana), a very popular urban and ornamental tree, formed large forests in the eastern areas of the continent. It narrowly escaped extinction through active eradication and education measures such as campaigns forbidding the transportation of firewood outside infected states. Unfortunately, a new, much more virulent fungus (Ophiostoma nova-ulmi) causing Dutch elm disease spread to Europe and North America during the 1940s. This fungus has caused the near annihilation of elms from several European countries. As of yet Finland has mostly been spared by the disease, but this may change with a warming climate that allows beetles belonging to the Scolytus genus that carry Dutch elm disease to overwinter in more northern regions. These beetles are already found on the northern coast of Estonia and in the Stockholm area of Sweden. The birch bark beetle (Scolytus ratzeburgi), commonly found in Finland, does not spread Dutch elm disease as it has specialized in solely utilizing birch trees.

However, the birch bark beetle spreads the Ophiostoma karelicum -fungus. Trappings

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The presence of birch bark beetles can be detected by their unique eating patterns. ©Stella Thompson

conducted during 2008 and 2009 for a study carried out in Norway, Finland, and Russia revealed the prevalence of O. karelicum: every single birch bark beetle individual carried the fungus, which was also found in each of the beetle’s galleries that were searched. The life cycle and ecology of O. karelicum is very similar to the fungi spreading Dutch elm disease, and the commonness of the fungus and the birch bark beetle means a very high risk of the disease spreading to e.g. North America. The birch species native to North America would most probably have no resistance to the disease.

On the other hand, pitch canker (Gibberella circinata) is a fungus spread by bark beetles, originating in North America, which has now spread to Europe where it causes pine mortality. The Scots pine (Pinus sylvestris), native to e.g. Finland, is especially susceptible, but the disease has not spread as far north as Scandinavia yet.

To make these dynamics even more complicated, several mite species have also been shown to transport or act as the primary hosts of wood-staining fungi. These mites are in turn spread by bark beetles. The relationships and interactions between these three organisms are still poorly understood.

The disease resistance of tree species can be increased through cultivation. American elm cultivars more resistant to Dutch elm disease have been found, and their disease resistance has been further enhanced through cultivation. These cultivars are most probably the reason that elm forests still exist today in North America, although the age and size composition of these forests has changed considerably with the death of the old and large trees. Biological and chemical disease control is also a possibility: fungicides can be injected into live trees to stop the spread of specific diseases. Six fungicides combating Dutch elm disease are currently on the market in the US.

Similar control measures can most probably be developed against O. karelicum. However, widespread injection campaigns are difficult to implement. In the US, Dutch elm disease is mainly controlled by injecting individual ornamental or urban trees. Injection control as an effective eradication measure requires more development before it becomes a feasible tool for preventing damage caused by alien species.

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Problems in paradise: the destruction of Hawaiian species

A few months ago I wrote a post on invasive species in Finland, and in particular on the North American beaver (Castor canadensis). I received a comment on how it is bold (or maybe the commenter meant reckless) to say that almost all invasive species are threatening the native species of the region. I began thinking of this comment, and tried to find some studies that proved that invasive species are beneficial for the subject ecosystem. Unfortunately, I only came up with sad tales. One very devastating example of invasive species is the Hawaiian Islands.

The Hawaiian Islands in the center of the Pacific Ocean are one of the most isolated islands in the world. Their endemic terrestrial species originate from some hundred species that migrated thousands of kilometers over the Pacific Ocean during several millions of years. Because of the immigration bottleneck and isolated evolution, the Hawaiian Islands have become a place for numerous distinctive and fascinating species. But it has also made the fauna and flora of the islands very vulnerable to various disturbances, such as human invasion and human-mediated invasions.

Nowadays almost a quarter of Hawaiian terrestrial species are non-native. Birds have probably suffered the most. Previously there were 11 native goose species in the Hawaiian Islands, but nowadays only one species is left: the nene (Branta sandvicensis), also known as the Hawaiian goose. The same has also happened to the native duck species; just two duck species are left (the Hawaiian duck, Anas wyvilliana and the Laysan duck, Anas laysanensis).

The nene, also known as the Hawaiian goose (Branta sandvicensis), is the only endemic goose species left in the Hawaiian Islands. © Sari Holopainen

The nene, also known as the Hawaiian goose (Branta sandvicensis), is the only endemic goose species left in the Hawaiian Islands. © Sari Holopainen

The main reasons for these extinctions are introduced predators (e.g. the feral cat and mongoose), and feral and game species (e.g. the mouflon, Axis deer and feral pig). There are almost 60 studies on domestic ungulates, but none have demonstrated any positive effects of them on native species. Ungulates stimulate the growth of grass among other things, leading to more grasses and less forest. And all this changes the light regime and fire resistance of an ecosystem. Grazing is therefore destructive to Hawaiian forests and to every native organism living in them. It has also been proven that the invasive vertebrate species of Hawaii have facilitated at least 33 invasive plant species. In addition to damages caused by grazing, feral pigs alter nutrient cycling and accelerate soil erosion.

The main problems caused by feral pigs are alteration to nutrient cycling and acceleration of soil erosion. © Sari Holopainen

The main problems caused by feral pigs are alteration to nutrient cycling and acceleration of soil erosion. © Sari Holopainen

There is still some light at the end of the tunnel, although it might be rather dim. The public has come to aid in the eradication of many species. Scientists and wildlife managers have concurrently begun multi-scale population monitoring, which includes aerial and ground-based visual surveys as well as trail cameras. To intensify and simplify the eradications even further, several hundred kilometers of management fences have been constructed. As an outcome of this some success stories have emerged; the eradication of rabbits and feral goats. Furthermore, the midway islands of Hawaii are now rat and non-native mammal free!

Unfortunately, it has been too late for some Hawaiian ecosystems. A key threshold has been crossed in some regions, and recovery of certain ecosystems may not be possible any longer. The populations of illegally introduced axis deer (Axis axis) have been reduced to some dozens, but their eventual eradication has been problematic, because assessing the number of remaining deers on private properties has proved difficult. The axis deer was introduced to provide game, so private properties owned by hunters act as reservoirs for the deer, from where they can be disperse to clean areas.

The main feral goat eradication was performed in 1980s, and nowadays the Hawaiian Islands are goat free. © Sari Holopainen

The main feral goat eradication was performed in 1980s, and nowadays the Hawaiian Islands are goat free. © Sari Holopainen

To conclude, I still dare say that almost all invasive species threaten native species. Even though some invasive species don’t harm all native species, we are always looking at nature as a complex ecosystem consisting of several species and functions. When introducing an alien species, we will always alter the pristine ecosystem.

Pandemic 3: Alien diseases threaten wildlife

The world is an ever-increasing humdrum of global connections. This has led to an increased risk of pathogens spreading around. Disease outbreak announcements focus on the human species, and rarely do we hear about the epidemics hitting wildlife worldwide. But the blatant truth is that severe infectious diseases recorded in wildlife are globally on the rise. Their impact on biodiversity and ecosystems can be devastating, especially on geographically isolated or endangered species.

Global wildlife trade is on the increase, mainly to serve the purposes of biomedical research, and the pet and food industries. Illegal wildlife trafficking is also increasing. Increased trade and globalization allow for the rapid and uninhibited spread of pathogens. Invasive alien species gain new territories, bringing with them diseases that native species have no immunity for. The expansion of livestock farming is bringing wildlife into closer contact with domestic species, increasing the risk of disease transmission in both ways. Yet the battle against raging infectious epidemics in wildlife is being waged locally, while regional and cross-border efforts remain weak. The most attention has been given to fighting avian influenza, as its spreading to poultry and humans has caused large economical and human implications.

Amphibian populations have been decimated globally by the fungus Batrachochytrium dendrobatidis (Bd), up to the extinction of several species. Local extinction rates of up to 40% of infected species have been recorder in the tropics. Such levels will have significant effects on foodwebs and their functioning. Crayfish plague and squirrel parapoxvirus are both examples of infectious diseases that have decimated native populations after being spread by alien species. Squirrel parapoxvirus is spread by grey squirrels, which act as immune carriers. The disease has only been recorded in Britain, where the mortality rate of untreated infected red squirrels is 100%. Crayfish plague has spread to Europe approximately 150 years ago, and despite this lengthy contact, native crayfish species still do not exhibit any resistance to the disease. Bird trichomonosis has affected several European finch species in recent years, locally bringing their populations down by as much as 35%.

Wildlife disease outbreaks are often underreported; information on location, affected species, and severity is lacking, and the availability of the reports leaves much to desire. The World Organisation for Animal Health (OIE) has launched an information system that will gather disease outbreak incidences together. However, they concentrate on diseases with economic or human impacts, and the control of purely wildlife-specific diseases is left to the policies of individual countries and regions (e.g. the EU).

Many invasive species strategies and wildlife trade and animal health policies (such as those of the EU and US) include short sections on combating wildlife epidemics, but biodiversity policies often do not recognize the issue. Policies mentioning disease outbreak control also leave a lot to be desired, as the severity that diseases can impact wildlife populations is not fully detailed. Global and regional prevention, monitoring, and regulation must be implemented to effectively combat biodiversity decline caused by wildlife disease outbreaks. One successful example of this is the eradication of rabies from Finland using vaccination baits spread into nature. The disease was common during the early 1900s, but has last been recorded in wildlife in 1989. Spreading awareness is often left to NGOs, which attempt to raise public knowledge, often successfully. Save The Frogs Day is an educational and conservational happening that has spread to 59 countries since its establishment in 2009. During the last Year of the Frog (2008), zoos across the world raised money for combating BT.

Aliens are among us

The EU announced a new alien species strategy just a few weeks ago. The strategy uses a three-step hierarchical approach: 1) preventing species from entering and spreading in a country, 2) early detection and eradication, and 3) long-term control and spreading prevention. What are the most efficient actions to preventing the spread and/or eradication of alien species?

 

Alien species are considered one of the greatest threats to biological diversity. Their success lies in rapid reproduction, good tolerance of different environments and a high dispersal ability. So what to do? Prevention is the key. One of the most efficient examples of a country with a very strict controlling program is Australia. The county controls everything that tries to enter through its international borders. Even the dirt on the soles of your shoes is investigated for microscopic aliens. This is no wonder as prevention is the most cost-effective method against invasive alien species. The costs caused by alien species are over 1 400 billion Euros every year.

 

When an alien species has spread, there are several methods for eradicating it. One can try to hunt down every individual or use pesticides, predators or pathogens against the alien. There are few successful examples of eradications, such as rat eradication in Tahanea Atol to save the endangered Tuamotu sandpiper Prosobonia cancellata, but most of the time the process is difficult if not completely unsuccessful. Pesticides used against plant pathogens or pest insects can seep into the soil and pollute near ecosystems as well as fresh water supplies. Predators can be used to control alien species population levels, but on the other hand finding a predator specialized in one particular species is very difficult. So the downside is that the predator will probably wipe out several other species also.

 

Are modern-day eradication strategies timely? What if we invest a lot of effort and money into eradication, and the alien species eventually spreads naturally and unaided into the ecosystem due to climate change? This has already happened in Spain, where the human-introduced ruddy duck Oxyura jamaicensis (originally from North America) threatened the rare white-headed duck Oxyura leucocephala. The Spanish government and EU first tried to exterminate the ruddy duck populations but then gave up, as it was quite clear that the species might spread naturally into the ecosystem within just a matter of years.


Alien species don’t just influence nature. They also affect the societies and economies that we live in. The pine wood nematode Bursaphelenchus xylophilus would have tremendous effects on Finnish forest industry if it were to invade Finland. The pine wood nematode is originally from North America, where local pine species have evolved a resistance to the nematode. The pine wood nematode is a real threat to Finland, as it has already spread to Portugal and the EU commission has restricted the export of Portuguese conifers. If the nematode would spread to Finland, the consequences could be devastating not only to Finnish forest industry, but to Finnish society as a whole. The total value of exports by the Finnish forest industry was 10.8 billion Euros in 2010, which is approximately 20% of the total export of Finland. In light of all the negative effects, can the aliens ever be friendly? And can we afford to be friendly in return?