Forestry effects on stream invertebrate communities

Pakuratahi - Tamingimingi Land Use Study Report. - Chapter 8 - Forestry Effects on Stream Invertebrate Communities

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Chapter 8

Forestry Effects on Stream Invertebrate CommunitiesRussell Death and Fiona Death

IntroductionThe biological communities in New Zealand streams and rivers comprise three distinct groups: the fi sh, the invertebrates (mostly insects, snails and worms) and the epilithon (a microscopic layer including algae, bacteria and fungi growing on the stream bed) (Harding et al., 2004). These three components are linked together in the stream food web and are in turn affected by the environ-mental conditions of the stream and its surrounding catchment. As the habitat characteristics of a stream affect what organisms can live in that stream it is possible to gain an insight into the environmental health of that stream by examining what organisms are found there.

Of all the organisms living in streams invertebrates are the most widely used indicators of environ-mental quality in New Zealand (Stark, 1985; Boothroyd & Stark, 2000) and overseas (Rosenberg & Resh, 1993; Hynes, 1994). Invertebrate taxa show a range of responses to differences in water quality (Stark, 1985). For example, midges and snails (Fig. 1) are generally considered to indicate degraded streams while stonefl ies, mayfl ies and dobsonfl ies (Fig. 2) require stream quality to be high in order for them to survive.

Figure 1. Chironomidae (midge larvae) and Gastropoda (snail). (Photo: S.N.A.C, 2000)

Figure 2. Plecoptera (stonefl y nymph) and Megaloptera (dobsonfl y nymph). (Photo: S.N.A.C, 2000)


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Compared to spot chemical measures, invertebrate indicators of water quality have the advantage of being able to integrate and monitor the effects of a wide variety of potential environmental stressors over an extended period of time (Rosenberg & Resh, 1993). While such a stress may only exist in the environment for a short period of time, its effect on the stream’s ecology may be more long term. In this way, the presence or absence of key invertebrates may be indicative of pollution or a degraded stream, even if the responsible contaminant is no longer present or detectable in the environment. Invertebrates are a product of the food supply (epilithon) and predators (fi sh) of a stream ecosystem and as such they may be the best single indicators of environmental quality in stream systems.

One of the habitat characteristics of a stream that can have a large effect on the stream’s biota is the nature of the vegetation in the catchment that the stream drains (Hynes, 1975; Harding et al., 1998). Changes to this vegetation that arise from forest harvest or conversion to pasture can lead to increases in nutrients, light, temperature, and fi ne sediments, and decreases in water clarity and quality (Campbell & Doeg, 1989; Harding et al., 2000). These changes will in turn affect the kinds of invertebrate that can survive in a stream. With considerable conversion of land in New Zealand to plantation forestry there have been numerous studies that have examined differences in invertebrate communities between forested and pasture streams and between native forest and exotic forest streams (e.g., Rounick & Winterbourn, 1982; Harding & Winterbourn, 1995; Friberg et al., 1997; Harding et al., 1997; Quinn et al., 1997; Townsend et al., 1997; Harding et al., 2000; Quinn, 2000). In general these studies have found dramatic differences between the inver-tebrates living in forest and pasture streams but little difference in streams with differing forest types. The critical determinant of the kinds of invertebrate that can live in New Zealand streams therefore seems to be whether or not there is forest cover over the stream and not what species of tree make up that cover.

The area of plantation management that is likely to have one of the largest impacts on stream invertebrates is therefore forest harvesting when the forest canopy is removed. Surprisingly there appear to be very few published studies of the effects of forest harvesting on stream invertebrates (but see Harding et al., 2000, and Death et al., 2003) although there are obvious and dramatic changes in the stream habitat. The objective of this study was to document the changes in stream invertebrate communities, and physico-chemical characteristics, associated with pine forest clear-felling in streams of Hawke’s Bay. As the removal of riparian vegetation will create stream con-ditions similar to those in pasture streams, we also examined how any changes in invertebrate communities associated with the logging compared with the more permanent changes imposed on many New Zealand pasture streams by pastoral farming. Effects of forestry on fi sh populations are considered in Chapter 9.

Study SitesSamples were collected at 3 sites on the Pakuratahi Stream (although site P2 was dry and not able to be sampled on several occasions) under exotic forestry management and 3 sites on the Tam-ingimingi Stream under pastoral agriculture management (Table 1). Two of the Pakuratahi sites, P1 and P2, are located on separate fi rst order tributaries of the Pakuratahi Stream with site P3 at the base of the forest catchment just upstream of the weir. The Tamingimingi sites, T1, T2 and T3 are in contrast longitudinally placed along the main branch of the Tamingimingi Stream (Fig. 3).

The Tamingimingi catchment has been intensively grazed since the 1900s but the Pakuratahi catchment was left in scrub until planting in Pinus radiata in 1971-72. Road construction and logging operations in the Pakuratahi catchment began in January 1998. Harvesting comprised clear-felling to the stream margins, utilising skyline logging and 52 skidder pads throughout the catchment, although the majority of the wood processing was concentrated at 2 main areas (Eyles, 1999). Following the completion of harvesting in September 1999, replanting of the catchment in Pinus radiata was completed in 2000.


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Table 1. Map co-ordinates and harvest dates for Pakuratahi and Tamingimingi Stream sites

Figure 3. Location of study sites in the Pakuratahi and Tamingimingi catchments. The map scale is approxi-mately 1:55,000

MethodsInvertebrate samplingAt each site, fi ve 0.1 m2 randomly placed Surber samples (250 mm mesh), were collected from rif-fl es, and preserved in 10% formalin. Subsequently, samples were rinsed through a 500 mm mesh sieve and invertebrates retained, identifi ed and counted. Invertebrates were identifi ed to the lowest


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possible taxonomic level using available keys, except Chironomidae which were only identifi ed to family (e.g., Winterbourn, 1973; Winterbourn et al., 2000).

Periphyton (algae)Periphyton abundance was assessed by collecting 5 stones alongside each Surber sample (mean surface area 40- 60 cm2), which were kept dark and cool in the fi eld until freezing back in the laboratory. In the laboratory, pigments were extracted by soaking the stones in 90% acetone for 24 h at 5°C in the dark. Absorbency readings were taken using a Cary 50TM Conc UV-Visible spectro-photometer and chlorophyll was calculated using the method of Steinman and Lamberti (1996).

Physicochemical site characteristicsConcurrent with the biological collections a number of physicochemical characteristics were re-corded at each site. Stream width, depth and velocity (using a Marsh-McBirney TM fl owmeter) were measured in the thalweg at fi ve equidistant points along the sample reach. Temperature and conductivity (automatically adjusted to 25°C) were measured using an Orion 122 conductivity me-ter and pH was measured using an Orion pHTester TM meter. The percentage of riparian vegetation (native, exotic forest, scrub, crop/pasture, tussock and other) and the substrate size composition were visually estimated at each site.

Biotic indicesTaxon richness, total number of animals, Macroinvertebrate Community Index (MCI), Quantitative Macroinvertebrate Community Index (QMCI) and the Berger-Parker Dominance Index were calcu-lated for each replicate at each site.

Biotic indices are widely used to assess stream health using invertebrates as water quality indica-tors. The MCI (Stark, 1985) is an index based on the presence of invertebrate taxa, which has a score (1 to 10) based on their tolerance to organic pollution (1=highly tolerant, 10=highly sensi-tive). Streams with MCI scores greater than 120 are considered ‘pristine’ and streams with scores less than 80 are ‘severely polluted’. The MCI was calculated using the following equation:

where ti is the MCI tolerance score for the ith taxon and S is the total number of taxa collected in the sample.

The QMCI (Stark, 1993) is similar to the MCI, except that the number of animals collected in each species is taken into account. The QMCI was calculated with the following formula:

where ni is the number of individuals in the ith taxon.

The Berger-Parker dominance index (Berger & Parker, 1970) is a simple measure of evenness (or dominance) and is given by:

D = Nmax/Nwhere Nmax= the number of individuals of the most abundant species. Dominance tends to be higher in more impacted streams.

Evaluating effectsThe biological indices calculated above will vary with time, irrespective of any disturbances occur-ring in the stream. Therefore to evaluate any effects of logging we calculated the natural range of these indices in each of the streams prior to harvesting. This was done by calculating 95% confi -


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dence intervals for the index values from samples collected prior to logging. These can be seen as the solid lines in Figure 4. Measures made before logging can be seen to vary within these limits. Values that fall outside these limits can be considered to indicate signifi cant change.

A further verifi cation that the observed changes are a result of the logging and not some other climate or seasonal change can be gained by examining the corresponding patterns in the pasture streams. If similar changes occur in the pasture streams to those in the forest streams then those changes can not be attributed to any effects of forestry operations.

ResultsInvertebrate communitiesNumber of taxa was reduced at sites P1 and P2 but not P3 following logging (Fig. 4). Taxa richness recovered at P1 within 2 years although remained at the lower limits of natural variability. Taxa richness at P2 recovered in less than 1 year but dropped again probably in response to this stream frequently drying out through this period of the study. Taxa richness at the pasture sites remained relatively constant throughout.

The number of animals collected at the Pakuratahi sites in general showed a delayed (1 -2 years) increase after the logging and is only returning to pre-logging levels in 2005 (Fig. 5). However, it should be noted that on many occasions the Tamingimingi pasture sites also showed an increase in the number of animals so increases in 2002/2003 can not be attributed entirely to logging ef-fects.

Community dominance (a measure that often increases in response to disturbance) increased at all 3 forest sites after logging (Fig. 6). Communities at site P3 returned to pre-logging levels within 3 years, within 6 years at P1 but have not yet returned at site P2.

Changes in what invertebrates were found in the streams as a result of logging can be seen clearly in Figure 7. Communities changed from being dominated by a diversity of mayfl y species (black bars) to communities dominated by a high abundance of Chironomidae, Aoteapsyche sp., Elmidae, and Potamopygrus antipodarum. These communities were more similar to those in the streams draining the Tamingimingi pasture catchment. It is diffi cult to judge when these communities may have recovered but it would seem P3 recovered in 4-5 years and P1 and P2 have yet to recover.


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Figure 4. Mean number (±1 SE) of invertebrate taxa collected in 4 Surber samples between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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Figure 5. Mean number (±1 SE) of animals collected in 4 Surber samples between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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Figure 6. Mean number (±1 SE) Berger-Parker Dominance scores collected in 4 Surber samples between De-cember 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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Figure 7. Relative abundance of invertebrates collected in 4 Surber samples between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures

Biological indicesQMCI values (a measure of water quality) dropped at all 3 forest sites after logging (Fig. 8). Site P3 recovered within 3 years, P2 within 6 years but P1 has yet to recover.

MCI (a less sensitive measure of water quality) values dipped at all 3 forest sites but recovered very quickly at all but site P1 which has still not recovered (Fig. 9).

Periphyton (algae) Logging does not seem to increase the amount of algae growing in the stream except at site P2 which returns to pre-logging levels in around 2 years (Fig. 10).

Sand and siltThe percentage of the stream sediment composed of sand and silt increased dramatically at site P1 and P2 after logging and is yet to recover (Fig. 11). The percentage of silt at site P3 was unaf-fected by the logging.


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Figure 8. Mean QMCI (±1 SE) of invertebrate taxa collected in 4 Surber samples between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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Figure 9. Mean MCI (±1 SE) of invertebrate taxa collected in 4 Surber samples between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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Figure 10. Mean number (±1 SE) chlorophyll collected on fi ve stones between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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Figure 11. Percentage of the stream sediment that is sand or silt at 3 pine and 3 pasture streams recorded between December 1996 and May 2005 in 3 exotic forest streams and 3 pasture streams. A dashed vertical line indicates when forest sites were harvested. The dotted line is the mean measure for samples collected prior to logging and the solid lines 95% confi dence intervals for those measures


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ConclusionsThere are many characteristics of the invertebrate communities in forest streams that can be affected by logging including the number of invertebrates, the type of invertebrate and biologi-cal indices of water quality. It can sometimes be diffi cult to interpret these responses as some measures may be affected whereas others are not. For example, the number of taxa (biodiversity) in the streams was relatively unaffected by logging but the QMCI measure of water quality was dramatically affected.

Taking in to account all these different measures of the invertebrate communities it is clear that immediately after logging the kinds of invertebrate living in these streams changed dramatically. This can be seen most plainly in Figure 7 where communities changed from being dominated by mayfl ies to more disturbance tolerant taxa such as midges, beetle larvae and snails. These commu-nities are much more similar to those in the pasture streams draining the Tamingimingi catchment. This effect is also mirrored in many of the biological indices.

It is also clear that the two smallest stream sites P1 and P2 (width 1 m, depth 15 cm, velocity 0.3 m/s) were most severely degraded by the logging as many of the biological characteristics have only just or are yet to return to their pre-logging condition 5 to 6 years afterwards. This can most likely be attributed to a build up of silt and sand in these small streams from logging which has not yet been fl ushed away. In contrast, site P3 that is deeper and swifter (width 1.2 m, depth 30 cm, velocity 0.8 m/s) did not accumulate any noticeable levels of sand and silt and correspond-ingly had a fauna that recovered from the logging operation much more quickly. Increases in light, temperature and nutrients did not seem to lead to any major increase in algal biomass that might have also had a role in altering the invertebrate communities in these streams.

In our opinion, in the future, minimising the effects of logging operations on the invertebrate com-munities of small Hawke’s Bay streams could be best achieved by focusing on efforts to reduce the inputs of sediment to the stream bed. This might include altering logging procedures, careful placement of roads and log processing sites, use of sediment traps and the retention of buffer zones of vegetation. From the research here it would seem that if the aim is to preserve stream bi-ology the most, attention to preventing sedimentation should be given to the smallest streams and the least to the larger streams which can naturally fl ush out much of the sediment input. In many ways this seems to be the converse of what currently occurs. Regardless, one pleasing observation is that all the invertebrate communities in the streams do seem to be recovering from the effects of logging although depending on the stream this may take at least 6 years.

AcknowledgementsWe are especially grateful to the many people who have survived the steep hills of the Pakuratahi and Tamingimingi catchments and been involved in collection and analysis of the data: Jason Gibson, Kimberley Dunning, Erna Zimmermann, Carol Nicholson, Becky Lewis, Alex James, Zoe Dewson and Emily Atkinson.

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