Logging effects on liana diversity and abundance in Central Guyana

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3 RESULTS Pre-logging diversity 3.1 PRE- LOGGING DIVERSITY AND COMPOSITION 3.1.1 Species density and diversity In plots in undisturbed Greenheart-bearing Mixed forest near Mabura Hill a total of 137 valid liana taxa (from here called “species”) were found. If the species in the three species complexes (see section 2.3.4) would be added, the species count is 146. In the two samples used in this study, the number of species was 132 2 (Sample A; individuals >2 m tall in 100 m 2 recording units) and 103 3 (Sample B; individuals > 0.5 m in 25 m 2 recording units). Pibiri, which was most intensively studied, harboured the largest number of species: 101 and 75, respectively (pre-harvest data). Thirty to forty percent of the regional species pool is found in any 1 ha plot (cf. Table 3.1), while c. 5% of the regional species pool is found in any 100 m 2 (25 m 2 for Sample B) recording unit. At the best-studied site (Pibiri 1993), plot level species richness was at about 40%, and subplot level species richness at c. 8% of the local species pool. A floristic analysis of the Mabura Hill region, including the liana flora, was made previously by Ek (1997) and Ek & ter Steege (1998). Table 3.1 Species density of undisturbed liana communities in Greenheart-bearing Mixed forest near Mabura at three spatial scales: regional (all research sites jointly), plots and subplots. Means and standard deviations are provided. The area enumerated per region, plot and subplot differs between Sample A and B. The regional area sampled was 5.75 ha (Sample A) and 1.4375 ha (Sample B) The same data are provided for the pre-harvest census of the largest single site, Pibiri (3.75 and 0.9375 ha, respectively). All undisturbed and control plots were used for this overview. All sites Pibiri pre-harvest Sample A Sample B Sample A Sample B Scale n mean s.d. mean s.d. n mean s.d. mean s.d. Region/Site 132 - 103 - 101 - 75 - Plot 23 39.9 5.8 29.3 5.8 15 41.1 5.3 30.1 5.0 Subplot 575 8.1 3.5 5.3 2.5 375 8.7 3.6 5.7 2.6 A summary of key biodiversity parameters of the liana community in undisturbed Greenheart forest is provided in Table 3.2; the individual plot data are provided in Appendix C. The difference between Sample A and Sample B in Fisher’s α is not real. It is due to the difference in area sampled between the two samples. If compared at equal sample area, Fisher’s α is almost the same (see also 3.1.5). Table 3.2 Summary of key diversity parameters † for the liana community based on 23 undisturbed (pre-harvest and control) plots in Greenheart bearing forest near Mabura Hill. Mean standard deviation and range are given. All indices are expressed per plot. Sample A (height >2 m in 100 m 2 subplots) Sample B (height >0.5 m in 25 m 2 subplots) mean s.d. range mean s.d. range Abundances Species density S 39.9 5.8 27- 50 29.3 5.8 13- 40 Number of individuals N 554 184 169- 957 371 135 126- 678 Diversity indices Fishers’ alpha α 10.1 1.4 6.9- 13.5 7.7 1.7 3.1- 11.6 Shannon-Wiener index H’ 2.4 0.2 1.9- 3.0 2.0 0.3 1.3- 2.6 Shannon’s evenness E 0.6 0.1 0.6- 0.8 0.6 0.1 0.4- 0.8 Dominance indices Simpson’s index 1/D 5.0 2.1 3.4- 11.1 3.6 1.2 2.0- 6.7 Berger-Parker index 1/d 2.4 0.6 1.9- 4.3 1.9 0.4 1.4- 2.9 † excluding species/individuals of uncertain taxonomic status and species complexes. 2 140 including species in species complexes 3 112 including species in species complexes 23
<strong>Logging</strong> <strong>effects</strong> on liana diversity and abundance in Central Guyana 3.1.2 Species-area relations The data in Table 3.1 show that only a relatively small percentage of the total diversity in the region or site is sampled within any 1 ha plot. This suggests that many plots need to be sampled to encounter all species belonging to the regional species pool. This is illustrated in the species-area curves in Figure 3.1, in which the increase in mean expected number of species with enumerated area is given for the undisturbed forest plots. The curve for all 23 plots is above the curve for the pre-harvest census of Pibiri alone (compare left and right hand panels) because the regional species pool (all species found in undisturbed and control plots) exceeds the pre-harvest species pool of Pibiri by 32 species (Sample A, Table 3.1). The increase in the number of species does not level off quickly as more plots are sampled so more species are expected to accumulate if more plots are added. The species–area relations for Sample A and B appear to be different in Figure 3.1, but this is an artefact caused by large differences in the number of individuals per unit area between the two Samples. The curves for Fisher’s α (triangles), which is relatively insensitive to the number of individuals, are almost overlapping. However, with increasing area sampled, Fisher’s α is not as stable as would be expected in communities with random mixing of species, but it increases slowly. This suggests that larger samples are more diverse. This could point to a non-random but patchy occurrence of species, both within Pibiri (right hand panel) and at the regional scale captured in the left hand panel of Figure 3.1. 140 All undisturbed plots Pibiri pre-harvest 50 S 120 100 80 60 40 20 n=7409 n=2711 S, A S, B Alpha, A Alpha, B 40 30 20 10 Fisher's alpha 0 0 1 2 3 4 5 6 Sampled area (ha) 0 0 1 2 3 4 5 6 Sampled area (ha) Figure 3.1 Relation between sampled area and number of liana species, S (left hand axis) and Fisher’s α (right hand axis) for all undisturbed plots (left hand panel, n=23) and for Pibiri’s pre-harvest plots (right hand panel, n=15) near Mabura Hill. The curves are based on means of 50 random samples of 1, 2, 3 …n plots (dots and triangles). The squares in the left hand panel illustrate that – at a sample area of 1.25 ha – the difference in species density between sample A (closed dots) and Sample B (open dots) is related to the larger number of individuals per unit area (insets) of Sample B, not by true differences in diversity (because Fisher’s α of both Samples is almost identical at that point). 3.1.3 Species abundance distribution in Pibiri The distribution of species over abundance classes in the pre-harvest census at Pibiri is shown in Figure 3.2. Classes of doubling abundance (octaves) were defined and all 15 plots were taken together. As many as 20% of all species were represented by a single individual, even though the total population consisted of almost 9000 individuals (in the case of Sample A). One species, Connarus perrottetii, stands out because it was found to be almost nine times as abundant as the second most abundant species. This species made up c. 50% of the preharvest liana population in Pibiri. This dominance is unusual and causes low values of the Berger-Parker index (which is the reciprocal proportion of the commonest species) and the Simpson index (Table 3.2). 24
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Logging effects on liana diversity and abundance in Central Guyana

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