Foresters fear outbreaks of bark beetles, particularly in spruce-dominated stands. Climate change is expected to increasingly foster the ideal conditions for bark beetle gradations. The European spruce bark beetle (Ips typographus) in particular benefits from higher temperatures by developing more generations per year. In addition, drier summers lead to drought stress of their host trees (spruces), and storms, which will possibly strike more often, will trigger outbreaks.
Foresters combat bark beetles by rapidly clearing away uprooted trees after storms as a preventive measure and through the salvage logging of infested standing trees. However, bark beetle populations are also controlled by natural predators such as woodpeckers, predatory and parasitic insects, mites, fungi and microorganisms. The interrelationships are extremely complex and also depend on abiotic factors. It is difficult to predict how climate change will affect the millennia-old ecosystem between bark beetles and their antagonists.
However, natural enemies should most certainly be further taken into account and supported in forest management. Woodpeckers are one of the bark beetle’s conspicuous enemies; the three-toed woodpecker (Picoides tridactylus, fig. 1) in particular plays a decisive role in the reduction of bark beetle populations.
Studies conducted in the US over 40 years ago showed that woodpeckers gather in areas with high bark beetle populations following disturbances. The number of foraging three-toed woodpeckers can rise considerably as numbers of bark beetles increase, a trait much less pronounced in other types of woodpecker. Up to 22.2 woodpeckers per hectare were counted during bark beetle gradations in the US (mostly between August and December when young birds fly away from their nesting habitat). In the Canton of Schwyz, between 14 and 16 three-toed woodpeckers were recorded on a hectare of forest infested by bark beetles in the summer of 1994. The density of breeding birds per forest area can rocket by a factor of between 2 and 21.
Analyses of the faeces and stomach content of three-toed woodpeckers provide information about their diet. Adult three-toed woodpeckers predominantly feed on bark beetles, longhorned beetles and spiders. During breeding season adult birds mainly eat longhorned beetles, while outside this period they primarily prey on adult bark beetles and their larvae. In contrast, the nestlings are frequently fed with spiders and longhorned beetle larvae. The consumption of bark beetles therefore fluctuates over the year, reaching its lowest point during breeding season.
In Germany’s Berchtesgaden National Park, bark beetles made up over 80% of the diet of adult three-toed woodpeckers, while this figure amounted to 99% in the US during bark beetle outbreaks – primarily during wintertime. In Finland, bark beetles accounted for 97% of insects eaten before breeding season. In general, the greater the beetle population, the greater the amount of bark beetles consumed by three-toed woodpeckers.
At -12° C, a three-toed woodpecker needs to consume 3,200 bark beetle larvae per day to satisfy its energy requirements; at 10° C they still need to eat 1,700 larvae. At the start of breeding season woodpeckers reduce their consumption of bark beetles, choosing instead more calorie-rich prey, such as the larger longhorned beetle larvae.
A Swiss study estimates the number of bark beetles devoured at all stages of development (from larva and pupa to beetle) at approximately 670,000 per year and per woodpecker, meaning that Switzerland’s entire population of three-toed woodpeckers eats an estimated 1.7 billion bark beetles per year. In comparison, between 1984 and 1999 5,000 to 25,000 bark beetle traps trapped on average 85 million bark beetles per year, peaking at 137 million after Vivian in 1992. This means that three-toed woodpeckers are much more efficient than beetle traps, especially during latency periods.
As well as devouring beetle larvae directly, three-toed woodpeckers indirectly kill beetle broods when searching tree trunks for food. These woodpeckers’ predominant foraging technique is bark scaling by , using their beaks to strip off countless pieces of bark to search for food underneath.
If they find a tree that offers plenty of food, the bird can strip the bark off of virtually the entire tree in a short space of time (fig. 2). This exposes the beetle broods to adverse weather and they die from dehydration or unfavourable temperatures, for example. Fungi also infest breeding galleries. Bark beetle broods in sections of bark that have fallen to the ground are in turn prey for other birds or small mammals.
When bark is thinner owing to sections being stripped away, this can trigger increased parasitisation of bark beetle larvae. In this case, parasitic wasps with short ovipositor are able to parasitise the larvae beneath the bark, which they would not be able to reach under normal, thicker bark. This indirect destruction of bark beetles brought about by three-toed woodpeckers bark scaling is more considerable than direct consumption. Three-toed woodpeckers can therefore play a key role in controlling bark beetle populations in forests dominated by conifers.
High population densities of three-toed woodpeckers serve a dual purpose. As efficient predators and cause of the indirect destruction of bark beetles, they keep these populations in check during latency periods and help to control beetle gradations. Of course, these birds alone cannot prevent outbreaks of bark beetles.
What are the main features of a habitat suitable for three-toed woodpeckers? In Central and Northern Europe, occurrences of three-toed woodpeckers correlate with the proportion of forests that are more than a century old, a high ratio of thicker trees and a high volume of deadwood (fig 3). A study conducted in the Canton of Schwyz indicated that spruces account for approximately 97% of trees providing food, of which 90% are dying or dead, and have an average diameter of approximately 40 cm.
A study from six locations in Switzerland used a bioenergetic model to investigate the number of dead trees needed to cover three-toed woodpeckers’ food requirements. If there are at least 14 dead standing trees with over 21 cm DBH per hectare, there is a 90% probability that a three-toed woodpecker will find enough food. This corresponds to approximately 5% dead standing trees (or a basal area of over 1.6 m2 of dead standing trees per hectare or a volume over 18 m3 snags per hectare). The deadwood volume of standing and lying trees should total at least 33 m3 per hectare. The same average volume of deadwood (30 m3 per hectare) was also found for the habitats of three-toed woodpeckers in Germany’s Berchtesgaden National Park.
Forest management should take advantage of the cost-free work of the three-toed woodpecker in the fight against bark beetles. This means consistently providing woodpeckers with suitable habitats so that they have a sufficiently large basic population during beetle gradations. First and foremost, this involves providing enough suitable trees for food (i.e. dying or dead spruces) to ease food shortages in the winter months. How effective the control in a local infestation spot is depends primarily on how many young woodpeckers can reach the affected spot. Woodpeckers have a greater impact when they are at the site of the outbreak from the outset. Their influence is strongest in forests where they both live year-round and breed.
Translation : TTN Translation Network