Fungal hotspots of insect diversity in boreal forests

The fact that many species of insect are found in association with wood-decaying macrofungi has long been known, but these have often been clumped together with other invertebrates associated with dead and decaying wood. Up to a thousand species of fly and the same number of beetles are believed to be associated with these two habitats in Fennoscandia alone. A review of recent major advances in our knowledge of species-richness, host specificity and the

Boreal Forest Biodi Versity Hufson
Figure 6.8 Ant hill beneath Norway spruce Picea abies forest at Hall, near Ullanger, central Sweden. Bilberry Vaccinium myrtillus and seedlings of rowan Sorbus aucuparia are present in the field layer. (Photograph by Andrew J. Packham.)

rarity of the insects directly involved with wood-decaying fungi is provided by Komonen (2003). Species of boreal bracket fungi (Fig. 6.9) which have been investigated in this way include Amylocystis lapponica, Fomes fomentarius, Fomitopsispinicola, Fomitopsis rosea and Piptoporus betulinus. All were associated with considerable numbers of insect species, 172 being recorded on the birch polypore Piptoporus betulinus in Canada. The largest number so far listed is for Polyporellus squamosus, which is associated with 246 species of beetles. The perennial bracket fungus Fomitopsis pinicola is more common on conifers, including Norway spruce Picea abies, than on broadleaved trees. This species of fungus is unusual in that clear droplets of fluid are exuded from the young fruiting body.

Fomitopsis Pinicola

Figure 6.9 Left: Bracket fungi on the dead and fissured trunk of a common beech Fagus sylvatica in central Europe. Mosses cover the trunk base. Right: The hoof or tinder bracket fungus Fomes fomentarius. A specimen of this bracket fungus was found growing on a Bedford willow Salix fragilis ssp. russelliana by Shrawardine Pool, Shropshire, UK in 2004. Though often found on birch in Scotland and northern England, this fungus is rare further south and had been recorded only once before in Shropshire. Hoof fungus was at one time thought to be common in England owing to confusion with Ganoderma applanatum, which causes a serious and damaging heart rot in ageing beeches. (Drawn by John R. Packham.)

Figure 6.9 Left: Bracket fungi on the dead and fissured trunk of a common beech Fagus sylvatica in central Europe. Mosses cover the trunk base. Right: The hoof or tinder bracket fungus Fomes fomentarius. A specimen of this bracket fungus was found growing on a Bedford willow Salix fragilis ssp. russelliana by Shrawardine Pool, Shropshire, UK in 2004. Though often found on birch in Scotland and northern England, this fungus is rare further south and had been recorded only once before in Shropshire. Hoof fungus was at one time thought to be common in England owing to confusion with Ganoderma applanatum, which causes a serious and damaging heart rot in ageing beeches. (Drawn by John R. Packham.)

Some 36 insect species were found to be associated with this fungus in Norway, of which six were rare and five had a strong preference for this particular fungal host.

The collection of fungal fruiting bodies for laboratory rearing has enabled researchers to be much more certain about relationships between particular insects and the fungi with which they are associated. It turns out that the fruiting bodies of particular fungal species support a very similar insect fauna over wide geographical areas, whereas those of other fungal species are dissimilar. Even ecologically and taxonomically related fungal species growing on the same fallen log may host almost completely different faunas. It is also the case that most primary fungivores (fungus-consuming animals) are associated with parasitic wasps and flies, many of which are extremely host-specific. The combined host specificity of many primary fungivores and their parasitoids is significant; it is thought that microhabitat specialists are more prone to extinction than habitat generalists.

Old-growth forests favour high insect diversity; indeed some rare insects are absent from managed forests. This may be because, like the moth Agnothosia mendicella, they specialize on fungal species occurring primarily in old-growth forest, or because they require the common fungi in which they develop to be in mature managed or old-growth forest, as in the case of the beetle Cis quad-ridens. It is also important that the areas of old-growth forest available should be reasonably large. In terms of insect conservation it is better to have a few large areas of old-growth forest than many scattered fragments. Our increasing knowledge of the factors controlling forest biodiversity in general, and that of insects in particular, has practical applications. Normal forestry practice has traditionally regarded decaying wood as being insanitary, harbouring diseases and pests. It is now clear that its removal markedly decreases biodiversity. Such wood should be allowed to remain in situ if some of the rare insects and other organisms are to be saved for posterity (see Section 7.7).

6.5.3 Resilience of tropical rain-forest bird populations to habitat degradation: does complexity beget stability?

Conservationists everywhere are faced with the problem of how to maintain biodiversity, and also prevent the extinction of as many species as possible, in the face of habitat loss and modification caused by humans. This problem is nowhere more acute than it is in the tropical rain forests, that are both more diverse and being more rapidly degraded than any other biome. In the mid-twentieth century MacArthur (1955) and Elton (1958) promoted the view that community complexity begat stability, that the more complex the ecosystem, the more resistant it was to change. By the 1970s it was becoming increasingly obvious that the opposite was the case; an increasing number of studies showed that the overall tendency was for inherent stability to decrease as complexity increases (May, 1972). In fact, as explained further in Section 9.5, stability begets complexity; it is the stability of the tropical environment that has led to the high species diversity by allowing each species to occupy a narrower niche safely. As Danielsen (1997) reminds us, the fact that communities in climatically stable regions are often both fragile and complex, while those where large and erratic climatic changes occur are relatively robust and simple, are in agreement with this. In view of this, one might expect communities in areas whose ecoclimatic histories were unstable to be more resilient to change than those that evolved in tropical rain forests of palaeoecologically stable regions.

Existing bird studies related to selective logging (and hence the removal of trees used by particular birds) in Asia and of habitat fragmentation in Latin

America did not enable this question to be answered with confidence. Studies of selective logging and habitat fragmentation in Africa, on the other hand, seem to confirm this assumption. This difference is not surprising as there are many problems in such investigations. The great diversity of the natural habitat (in which there are often relatively few common trees but many unusual ones), difficulties of access, lack of suitable controls in the majority of investigations, use of various census methods and inadequate descriptions of disturbance regimes, all make comparisons difficult. Two things in particular stand out from our existing knowledge. The first is the enormity of the damage to the biodiversity of complex tropical rain-forest faunas caused by habitat destruction and fragmentation, and the second is the way this contrasts with the simpler and more resilient communities of the temperate regions. Despite the widespread conversions of European landscape by humans, there have been comparatively few extinctions apart from some megaherbivores and large carnivores. However, these large animals have undoubtedly played an important role in forest ecology in Europe (see Section 5.9).

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