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Results

Thirty-seven years of freshwater fish and crustacean presence/absence data were obtained from the NZFFD. That was all entries on flowing water dating from January 1970 to June 2007. This consisted of 22,546 sites, over a broad geographic coverage (Fig. 1).

Figure 1 Locations of the 22,546 sites in the NZFFD from 1970 – 2007

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Figure 1 Locations of the 22,546 sites in the NZFFD from 1970 – 2007

Sampling trends

The number of sites added to the database has increased over time but there were different patterns of increase related to land cover. The number of sites sampled in exotic forest, urban and scrub land cover showed a gradual increase, approximately doubling every decade. Sampling indigenous forest sites increased at a much greater rate but peaked in the 1990s, while pasture sites increased exponentially over the entire period. Between 2000 and 2007 the number of pasture sites sampled was more than all the other classes combined (Fig. 2).

Figure 2 Sites added to the database (NZFFD) over decades by land-cover type (Note: 2000s includes 2000 to 2007)

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Figure 2 Sites added to the database (NZFFD) over decades by land-cover type (Note: 2000s includes 2000 to 2007)

Index of biotic integrity scores in relation to land use

The average IBI score was significantly higher at indigenous forest and scrub sites than the other land cover classes, and tussock was significantly lower than all other land-cover classes (ANOVA F7,22538 = 247; P < 0.0001) (Table 1; Fig. 3). Pasture sites had the next lowest scores but were not significantly different from urban, exotic and unvegetated (bare land) sites.

Table 1 Descriptive statistics for IBI scores by River Environment Classification (REC) class

REC class Pasture Urban Exotic forest Bare land Tussock Indigenous forest Scrub
Mean 29.68 30.33 30.47 31.4 17.98 36.22 36.51
Median 32 34 32 40 0 38 40
Standard deviation 17.62 16.69 19.93 20.09 19.89 17.85 19.27
Standard error 0.18 0.49 0.55 0.88 0.37 0.24 0.56
Number of sites 9,932 1,167 1,319 522 2,806 5,530 1,194

Figure 3 Average IBI score for all sites grouped by River Environment Classification (REC) land-cover class (whiskers = Standard Error).

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Figure 3 Average IBI score for all sites grouped by River Environment Classification (REC) land-cover class (whiskers = Standard Error).

Temporal trends in biotic integrity scores 1970 - 2007

The IBI scores for all sites show there has been a significant decline in indigenous freshwater biodiversity over the past 37 years. This decline was significant for both years and decades (Table 2) with the biggest reduction in the last decade (Fig. 4). To assess which of the land-cover classes contributed to this decline the different classes were analysed separately. Indigenous forest sites showed a significant increase for both years and decades, peaking in the 1990s (Fig. 5; Table 2). Pasture sites showed a significant decrease in IBI scores for both years and decades especially in the last decade (Fig. 6; Table 2). The sites in scrub land cover showed no significant trend over decades but did for years (Fig. 7; Table 2). Urban sites showed a significant decline in IBI scores over the 37 years for both years and decades (Fig. 8; Table 2). The exotic forest sites showed a dip in the 1990s but there was no significant linear trend for both years and decades (Fig. 9; Table 2). Tussock sites showed a significant decline in IBI scores for both years and decades (Fig 10; Table 2).

Table 2 Results of regression analyses for all sites and land cover classes using IBI scores for years and decades. Note: trend is significant if P-value is less than 0.05 (ns = not significant).

REC land-use class Direction of change Number of sites All years Decades
F-value P-value F-value P-value
All sites negative 22545 191.2 0.0001 223.7 0.0001
Pasture negative 9931 92.0 0.0001 118.4 0.0001
Tussock negative 2805 21.1 0.0001 38.83 0.0001
Indigenous positive 5529 41.5 0.0001 24.7 0.0001
Urban negative 1157 29.6 0.0001 19.9 0.001
Scrub negative/ns 1193 3.9 0.047 1.21 0.27
Exotic ns 1318 2.4 0.13 0.09 0.77

Figure 4 Average decadal IBI score for all sites (number of sites inside bars, whiskers = Standard Error)

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Figure 4 Average decadal IBI score for all sites (number of sites inside bars, whiskers = Standard Error)

Figure 5 Average decadal IBI scores for River Environment Classification (REC) land-cover Indigenous forest sites (number of sites inside bars, whiskers = Standard Error).

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Figure 5 Average decadal IBI scores for River Environment Classification (REC) land-cover Indigenous forest sites (number of sites inside bars, whiskers = Standard Error).

Figure 6 Average decadal IBI scores for River Environment Classification (REC) land-cover pasture sites (number of sites inside bars, whiskers = Standard Error)

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Figure 6 Average decadal IBI scores for River Environment Classification (REC) land-cover pasture sites (number of sites inside bars, whiskers = Standard Error)

Figure 7 Average decadal IBI scores for River Environment Classification (REC) land-cover scrub sites (number of sites inside bars, whiskers = Standard Error)

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Figure 7 Average decadal IBI scores for River Environment Classification (REC) land-cover scrub sites (number of sites inside bars, whiskers = Standard Error)

Figure 8 Average decadal IBI scores for River Environment Classification (REC) land-cover urban sites (number of sites inside bars, whiskers = Standard Error)

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Figure 8 Average decadal IBI scores for River Environment Classification (REC) land-cover urban sites (number of sites inside bars, whiskers = Standard Error)

Figure 9 Average decadal IBI scores for REC land-cover exotic forest sites (number of sites inside bars, whiskers = Standard Error)

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Figure 9 Average decadal IBI scores for REC land-cover exotic forest sites (number of sites inside bars, whiskers = Standard Error)

Figure 10 Average decadal IBI scores for River Environment Classification (REC) land-cover tussock sites (number of sites inside bars, whiskers = Standard Error)

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Figure 10 Average decadal IBI scores for River Environment Classification (REC) land-cover tussock sites (number of sites inside bars, whiskers = Standard Error)

Changes in number of species at resampled reaches

A number of reaches in the database had been sampled more than once over the 37 years and these were analysed to look for changes. This could be done using the number of species as they were directly comparable, which negated the need to use the IBI. The number of sites that had more or fewer species in 2000 – 2007 compared to the 5-year periods between 1970 and 1999 were counted. There were consistently more sites sampled on the same reach that had less species than more species in the 2000s, when compared to the 5-year periods between 1975 and 1999 (Fig. 11). The only exception was the 1970 – 1975 period when more sites gained species than lost species over time. However, there were only 40 sites that were sampled in 1970 – 1975 that changed from the 2000s.

Figure 11 Comparison between the numbers of species at reaches sampled before and then again in 2000s

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Figure 11 Comparison between the numbers of species at reaches sampled before and then again in 2000s

Using rank correlation analysis there were very few significant differences found in IBI scores over time for the stream reaches that had multiple sampling events. However, there were more significant positive than negative changes for native vegetation, no differences at exotic forest, but more negative correlations at pasture and urban sites (Fig. 12).

Figure 12 Significant Spearman rank correlations for IBI scores over time for stream reaches with multiple sampling events by River Environment Classification land cover

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Figure 12 Significant Spearman rank correlations for IBI scores over time for stream reaches with multiple sampling events by River Environment Classification land cover

 

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