Measuring disturbance in tropical forests: a critique of the use of species–abundance models and indicator measures in general

Abstract

Recent papers have considered the merits or otherwise of using species-abundance models to detect disturbance in tropical forests (Hill et al. 1995; Hamer et al. 1997; Nummelin 1998; Hill & Hamer 1988). The purpose of this paper is to argue: (1) that there are simpler, more reliable ways of assessing forest disturbance; and (2) that the use of species-abundance models to assess whether a forest is disturbed or not ignores the different types, degrees and scales of disturbance that exist, each of which potentially have a different impact on forest biodiversity. Tropical forest biodiversity is undeniably threatened. Although estimates on the number of species in tropical forests vary greatly, there is a widespread assumption that at least half the world's species are concentrated in only 7% of the terrestrial land mass (e.g. Stork 1988; Myers 1990; Bibby et al. 1992; World Conservation Monitoring Centre 1992). As Hill et al. (1995), Hamer et al. (1997) and many others point out, tropical forest species are threatened by logging and forest clearance for a range of purposes, from the establishment of small farms as part of the practice of shifting cultivation to the large scale clearance of forest for pasture and plantation crops of exotic timber tree species, rubber, oil palm, and many other crops. Shifting cultivation and logging have been practiced in tropical forests for thousands of years, but the area partially or wholly cleared for these purposes has increased dramatically during the twentieth century (Phillips 1997). Thus, many authors rightly argue that there is an urgent need for more research on the impact of forest disturbance on biodiversity (e.g. Phillips 1997; Watt et al. 1997). In fact Hill & Hamer (1998) justify the development of methods of assessing whether or not a forest is disturbed as a precursor to the measurement of the impact of forest disturbance on tropical forest communities. Hill et al. (1995) first suggested that species-abundance models could be used to indicate forest disturbance after finding that butterfly data from an unlogged area of forest on Buru, Indonesia, fitted a log-normal distribution, whereas the data for an area of logged forest did not. Nummelin (1998) tested the applicability of species-abundance models as indicators of disturbance by studying ladybirds (Coccinellidae), tortoise beetles (Cassidinae), dung beetles and plants in forest near Kibale, Uganda. Nummelin found that the species-abundance distributions fitted a log-normal distribution irrespective of whether they came from logged or unlogged forest. In a more recent paper on Indonesian butterflies (and therefore not available to Nummelin at the time of preparing his paper), Hamer et al. (1997) again showed that speciesabundance data collected in Sumba, Indonesia, were affected by forest disturbance, although in this case only the butterflies in the most disturbed of three disturbed forest sites sampled showed a log-normal distribution. Hill & Hamer (1998) respond to Nummelin by listing a number of requirements that should be met for assessing the usefulness of species-abundance models as indicators of forest disturbance. The purpose of this paper is not primarily to prolong the discussion on the usefulness of species-abundance models as indicators of disturbance. However, it is worth noting that the restrictions in the use of species-abundance models described by Hill & Hamer imply that this technique cannot be regarded as an easy 'instantaneous indicator of forest disturbance' (Hamer et al. 1997), but at best must be applied cautiously. In addition, although Hill et al. (1995) found that the presence of a log-normal distribution distinguished disturbed from undisturbed forest, data from only one of the three disturbed forest sites sampled by Hamer et al. (1997) showed a log-normal distribution. A more important point, however, is that it is surely unnecessary to sample butterflies to assess whether or not a forest has been disturbed. It is, of course, possible to detect extreme disturbance in the form of complete deforestation, both from the ground and from satellite (e.g. INPE 1997). Less severe disturbance in the form of selective logging may also be assessed through the presence of logging roads and trails; the soil compaction caused by the removal of logs resulting in poor growth of vegetation over long periods of time (e.g. Pinard, Howlett & Davidson 1996; Guariguata & Dupuy 1997; Whitman, Brokaw & Hagan 1997). For example, Nummelin (1998) noted that '1520 years after logging, logged areas were still easily recognizable due to the large gaps in the canopy' and