4. SERIAL PATTERNS OF CHANGE IN PERMANENT TRANSECTS 1 Statement of the problem
4.2 Methods
4.2.3 Scale
Serial data from sites 2a, 2b, 3b, 4a and 4b were transformed into matrices of species-by- cumulative area. The smallest scale was equal to the area of the first quadrat(1 m2) and the largest scale was equal to the sum of all quadrats in the transect (25 m2). Turnover was determined by subtracting species presence values from earlier and later surveys, producing change profiles at the scales from 1 to 25 m2•
Change in species local frequency between two observations can be positive or negative values. Negative values show a decline in frequency, positive values show increases and zeros denote no change in species frequency over time. The measurement of absolute change is calledsensitivity(after Critchley& Poulton 1998) and is calculated by summing the modulus of change values (e.g. -1+1=2) down each column (Appendix D). The inability to detect change is called 'blindness' and is calculated by counting the number of zeros in each column. Optimum scale was originally based on a parabolic principle for nested quadrats whereby changes in species local frequency are detectable in both directions (Critchley&
Poulton 1998). Changes in local frequency that are closest to the midpoint of the transect area, i.e. 12.5 m2,was considered the optimum scale. Optimum scale can only be determined from change profiles that show changes in opposite directions. Effective optimum scale should reflect changes values that are higher than the change values at the 25 m2scale.
Absolute change and blindness data were analysed with Genstat 5 (Anon. 1998) using linear and exponential models.
4.3 Results 4.3.1 Effective sampling intensity
Trends in the raw data suggest that the 25 m transect (B) had greater similarity to the 30 m transect (A) than it did to both 20 (C) and 15 m (D) transects (Figure 4.2). Chromolaena populations were comparatively similar in the 1992/93 biological year, namely 4.1, 5.2,3 and 2 m-2in A, B, C and D respectively. All transects showed increases in species richness over time coinciding with annual burns and declining chromolaena populations. The dip in richness during the 1996/97 is attributed to a monitoring season covering only four months (Section 3.1.2).
Statistical differences between the 25 and 30 m sample sizes were negligible throughout the study period (Figure 4.3). The 30 m transect yielded only six additional species from six years of data. Transects of 20 and 15 m were repeatedly inferior to the transects of 25 m.
The relation between richness and sample size also appears consistent. Species richness increased at the same rate across all transects shown by the parallel lines. Twenty-five metre transects remained the most area-effective sample size throughout the study period. The null hypothesis of no significant difference in species richness over time in transects longer than 25 m is upheld.
4.3.2 Species-area curves and minimal area 4.3.2.1 Species-by-area relations
Species richness increased with increasing sample area at all sites over time (Figure 4.4).
Curvature (R) in species-by-area relations were nonparallel over time (P <0.05). Exponential curve-fitting analysis (Equation 4.1) established that curvature was not different (P >0.05) at Site 3b only. The null hypothesis of no change in species-area curves over time was therefore falsified.
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Figure 4.2: The relation between species richness and transect length. The graph shows raw data measured from 1991/92 to 1997/98. Arrows denote burning events during the study period.
Numbers next to markers cite transect populations of Chromolaena odorata.
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Figure 4.3:The relation between species richness and transect length. The graph shows observed values (markers) and lines of best fit(r= 0.753 ± 0.051). Significant differences in species richness are depicted by the error bar(1). Slope differences were not significant (P 0.671).
The temporal dynamics reflected in the species-area curves varied from site to site. Site 2a had 33 species in 1991190 and maintained steady increases over the study period, reaching 61 species in 1997/98. Site 2b and 3b followed the same pattern of sustained increases in species richness over time. A shorter monitoring period in 1996/97 resulted a decline in species richness being recorded at Site 3 (Figure 4.4c). Initial chromolaena density appears to have had a residual impact on the temporal dynamics of species-area curves. Sites 4a and 4b, initially dense in chromolaena, produced curves with inconsistent directions of change.
Gains in species richness during the first half of the study were unable to maintain the impetus of directed change and both sites showed signs of non-equilibrium towards the end of the study.
Exponential curves of species/area-by-Iocation data yielded similar results to the species/area- by-time analyses. Species richness in 25 m2transects were summarised using fitted values of species-area curves from the first two and last two field surveys (Figure 4.5). Species
richness varied across sites over time (P<O.OOl). Increases in richness occurred where chromo1aena populations declined under routine veld burning. Chromolaena popu1ations were not effectively suppressed by fire at Site 4b.
4.3.2.2 Minimal area
Asymptotes occurred consistently beyond the length oftransects and the physical boundaries of each site (Figure 4.4). The difference between the asymptote and transect richness (see Figure 4.4 caption, last sentence) increased over time at sites 2a, 2b and 3b. Asymptotes fluctuated strongly at sites 4a and 4b but differences in richness still increased at 4a before the site showed signs of failure to restore itself in 1997/98. Transects were only partially able to reveal an appropriate minimal areas of 90 to 95% (Table 4.2). Directions of change in proportions of asymptotes contained within transects could not be determined under
conditions of increasing or fluctuating heterogeneity. The proportion of asymptotes revealed by 25 m2transects did not seem significantly different overalL
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(d) Site 4a: curvature (R) P<O.OOl, r2= 0.99±0.01 (e) Site 4b: curvature (R) PO.06, r2= 0.99±0.01
Figure 4.4: The relation between species richness (S) and time (T) with increasing area at five localities. Individual curves show increasing species richness with increasing number and area of quadrats. P-values are given for changes in curvature (R) over time. Correlation coefficients(r)are also provided. Monospecific infestations are horizontal straight lines. Species-area curves fluctuated over time, either increasing or decreasing, depending on site condition. Asymptotes (A) are given for each curve (x, y intercepts; difference in richness of A - 25 m).
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Figure 4.5: Summary of exponential curve fitting analyses of species-by-area data comparing species richness (S) and location (sites) inpermanent transects of25 m2 over the study period. The 1994/95 and 1995/96 seasons were not analysed.
Significant differences in species richness are depicted by the error bar(1).
Arrows denote burning events during the study period. Numbers next to markers cite populations ofChromolaena odorata. Monospecific stands ofC. odorata occurred at sites 2b, 4a and 4b in 1990/91.
Table 4.2 Percentage of asymptotes] contained within fixed-belt-transects (25 m2) at five coastal grassland sites
Site Biological year
90/91 91/92 92/93 93/94 94/95 95/96 96/97 97/98 Mean
2a 97 92 79 85 88.3±7.9
2b 86 95 85 88.7±5.5
3b 86 85 83 87 85 85.2±1.5
4a 90 79 74 66 91 80.0±10.7
4b 76 51 77 73 83 72.0±12.3
1Figure 4.4