Logging intensity (trees / hectare)
4.7 Unregulated commercial logging can cause heavy forest damage. The relationship between harvest intensity and collateral damage in tropical forests subjected to unregulated (South-East Asia; closed circles) and regulated (north Queensland, Australia; closed diamonds) logging is illustrated. Broken and unbroken lines were fitted by linear regressions. Collateral damage is defined as the percentage of non-harvested trees (<10 cm diameter) that were inadvertently killed during logging. (Redrawn from Laurance, 2000; data compiled from Crome et al., 1992, with permission from Elsevier.)
is fabulously rich and complex in biodiversity. Yet all is not well with this biotic jewel.
By far the biggest land-use change on the planet, in the shortest time, has been conversion of 16 million square kilometers of forest in 50 years (World Commission of Forests and Sustainable Development, 1999). Tropical forests are being lost at least at 130 000 square kilometers per year, although we must bear in mind that not all is of ancient origin (Mayle et al., 2000). Besides being home to half of the world's species and most of the insects, these forests regulate movement of water across the land, modulate local and regional climates through transpiration, and they play a major role in determining current atmospheric concentration of CO2 through their high above- and below-ground productivity and huge standing crop (Malhi and Grace, 2000).
The root of the problem is that logging is usually the fastest way for corporations and investors to make lots of money quickly in the tropics (Laurance, 2000) (Figure 4.7), although in reality, the proximate causes are diverse and vary from region to region (Geist and Lambin, 2002). And the stakes are high. In developing nations, forest tracts currently allocated for logging are at least 8-10 times larger than the limited areas set aside for parks and reserves (Johns, 1997). Even reserves are not safe, with accidental deforestation from forest fires becoming greater than deliberate conversion in some areas (Cochrane and Schulze, 1999), threatening unburned patches (Siegert et al., 2001).
So, what is to be done? Whitmore (1999) points out the difference between deforestation (complete removal of trees, often with concurrent burning) and timber extraction. While deforestation dramatically changes structural, compositional and functional biodiversity (with concurrent changes even in soil-inhabiting termites (Eggleton et al., 1996)), timber extraction, in contrast, simulates natural gap-forming processes. This is so long as only a few trees per hectare are removed carefully, and damage to the forest floor is minimal. This low-impact logging, encapsulating good practice, is becoming increasingly the norm (Whitmore, 1999). Nevertheless, much research is required relative to management practices.
Forest clearance in Cameroon, and conversion to farm fallow, caused a 50% drop in butterfly and termite species richness (Watt et al., 1997). Clearance and conversion to forest plantation in comparison with complete clearance caused a reduction in butterflies and leaf-litter ants of 15% and of 40-70% in arboreal beetles and termites. These figures are likely to be conservative as more species will probably disappear over time after logging. Furthermore, type of management plays an important role, with partial manual plantation plots having more insect diversity than plantation plots established after complete clearance. Similarly, in Indonesia, termite species and abundance decreased in proportion to intensity of land use. Primary forest had 34 termite species, while cassava gardens had only one (Jones et al., 2003).
Liberation thinning, which is a management method in favour of potential crop trees, seems to most affect the more specialized West African nymphalid butterfly species with smaller geographic ranges, thus risking loss of regional diversity (Fermon et al., 2000). Similarly, selective logging of tropical forests in Indonesia significantly decreases butterfly diversity, at least during the first 5 years (Hill et al., 1995). Furthermore, there is distinct spatial heterogeneity in vegetation structure within these logged forests, which in turn leads to heterogeneity in butterfly abundance corresponding to availability of suitable forest (Hill, 1999). Some of this may be due to natural population dynamics, such as colony foundation and extinction in termites (Eggleton et al., 1996), which underlies the effect of disturbance.
Nevertheless, there is a significant loss of diversity and taxonomic quality with increasing levels of forest disturbance, with some taxa such as moths being more sensitive to these changes than other taxa, such as beetles (Holloway et al., 1992). Indeed, changes in one taxon following disturbance do not necessarily correlate with those of another (Lawton et al., 1998). Certain taxa, such as arboreal dung beetles in Borneo (Davis and Sutton, 1998) and ants in Ghana (Belshaw and Bolton, 1993) can even survive in agricultural areas after removal of primary forest. Such findings however, must not undermine efforts to conserve tracts of primary forest, which may be the last refuge for some species (Fermon et al., 2001). This is emphasized by Castano-Meneses and Palacios-Vargas' (2003) findings that Mexican tropical deciduous forest disturbance results in a decrease in ant density and diversity with a resultant change in the energy recycling in the ecosystem.
4.6.3 Disturbance and maintenance of late successional stages
The significance of having forest heterogeneity in cooler forests is similar to that in tropical forests, with certain insect species preferring a particular level of disturbance (Spagarino et al., 2001). This underscores the importance of rotational or selective logging, which benefits particular species (Kaila et al., 1997), as well as the importance of maintaining patches of virgin forest with sufficient extent of interior conditions to circumvent gradual loss of species through ecological relaxation (Lovei and Cartellieri, 2000). Indeed, virgin patches maybe critical for maintaining local stenotopic species (Trumbo and Bloch, 2000), which are often large-sized, reluctant-to-disperse endemics (Michaels and McQuillan,
1995). This serves to emphasize the importance of maintaining virgin, old-growth forest nodes which are exempt from any form of rotational management. Such patches (the larger, the better (Horner-Devine et al., 2003)) and patch groups need to be represented across a region (Niemela, 1997). Such old-growth forests also represent habitat predictability, essential for many insect species (Nilsson and Baranowski, 1997). Such late successional patches may have special habitats such as soil conditions (Eggleton et al., 1995), litter depth (Lomolino and Creighton,
1996), fungi (0kland, 1996; Jonsell et al., 1999) or simply large logs (0kland et al., 1996; Grove and Stork, 1999; Kelly and Samways, 2003) which particular insect specialists need. In addition, special habitats, such as tree-holes, where particular ecological interactions take place (Fincke et al., 1997), or unique sites, such as Monarch butterfly roosts (Brower et al., 2002), must also be considered. In summary, the precautionary principle of keeping all the parts, especially all the parts of old forests, is essential for the survival of current insect diversity.
Natural spatial heterogeneity of butterflies in primary tropical forest may be obscured by logging (Willott et al., 2000). Such spatial considerations are important because, at least in Borneo, low-intensity logging in forest in close proximity to primary forest does not necessarily reduce the species richness or abundance of butterflies, although assemblage composition is changed. This is possibly because moderate levels of disturbance may increase butterfly diversity and also because the primary forest was not in its final successional stage, owing to earlier disturbance, possibly from drought and fire about 100 years ago. This may be why these results are apparently conflicting with others where there was a reduction in lepidopteran diversity in logged forest (Willott et al., 2000) (Figure 4.8).
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