Zealand Streams Research Articles

Prey selection and dietary overlap of shortfinned (Anguilla australis) and longfinned (A. dieffenbachii) eels during summer in the Horokiwi Stream, New Zealand

Benthic macroinvertebrates were collected and the diets of shortfinned eels (Anguilla australis) and longfinned eels (Anguilla dieffenbachii) inhabiting a small coastal New Zealand stream were examined to investigate intraspecific and interspecific trophic partitioning and prey selection. Our results showed intraspecific segregation of preferred prey among three size classes of juvenile shortfinned eels, but there was significant overlap between different size classes of longfinned eels. All size classes of both species of eel fed nocturnally on similar prey, and so there was no indication of temporal or trophic segregation. Therefore, habitat separation is assumed to be the main mechanism to reduce interspecific competition in these two co‐occurring species of eels.

Growth, diet, movement, and abundance of adult banded kokopu (Galaxias fasciatus) in five Coromandel, New Zealand streams

Banded kokopu populations were surveyed seasonally over 1 year in five Coromandel streams to determine the influence of food supply on diet, growth, abundance, and movement. Streams with high benthic invertebrate abundance and drift of aquatic origin had high fish abundance. However, terrestrial drift was the dominant food for adult banded kokopu over all seasons and streams, on average comprising 75% of their diet by number and 89% by weight. Annual growth rates of tagged individuals ranged from 3 mm to 16.5 mm, with smaller fish growing faster than large fish. Low growth rates of large fish were consistent with the recapture of a fish that had only grown from 195 mm to 215 mm in 7 years. Our observations confirmed the strong association between banded kokopu, pool habitat, and cover, and also indicated that high fish abundance restricted growth rates and movement and increased interaction.

Temporal variation of N and P uptake in 2 New Zealand streams

Temporal variation of nutrient uptake in streams may be large because nutrient uptake is driven by many factors that vary substantially over time. Although many studies have compared nutrient uptake among streams, the range and variation of nutrient uptake within streams is known only for a few streams and a few nutrients. We investigated the monthly variation of NH4+, NO3−, and PO43− uptake in 2 New Zealand streams over 1 y. To measure uptake, each nutrient was added individually along with a conservative tracer (Cl−) into each stream on 3 successive days in each month. Ambient nutrient concentrations were low and nutrients were efficiently removed from the water column, with maximum uptake velocities (vf) of 71, 12, and 11 mm/min for NH4+, NO3−, and PO43−, respectively. Nutrient uptake varied considerably during the year (CV = 37–109%), with shortest nutrient uptake lengths (Sw) and highest vf generally in spring and summer months. The range of vf occurring within the streams spanned 25 to 89% of the range of vf among other streams. The range of uptake rates (U) within the streams was lower, accounting for 2 to 40% of the range among other streams. Variation in Sw was largely explained by changes in velocity and effective depth. Physical factors (temperature, transient storage) and chlorophyll a were generally poor predictors of vf and U. There was little correlation in uptake among nutrients, suggesting different factors were responsible for uptake of each nutrient. Our results show that the range and variation of nutrient uptake within some streams can be large. Within-stream variation should be considered when comparing among streams and may be useful for understanding what factors drive nutrient uptake in streams.

Comparison of coarse particulate organic matter retention in meandering and straightened sections of a third-order New Zealand stream

Four types of coarse particulate organic matter (CPOM), 100 mm and 200 mm lengths of wooden dowel, paper strips and Eucalyptus leaves, were released in either straightened or naturally meandering sections of a third-order stream in lower North Island, New Zealand. The distance travelled until retained and the fate of each CPOM type were recorded over a range of discharges (0.10–0.77 m3 s−1) in stream sections with differing riparian characteristics (forest, willow and pasture). In meanders, CPOM travelled a shorter distance until retention and a higher proportion of released CPOM was retained compared to straightened reaches. CPOM travelled further until retention in pasture sections. Snags (vegetation and debris) were more important than rocks and eddies as retention structures, but more items were trapped by eddies in straightened reaches than in meanders. Leafy CPOM (paper strips and Eucalyptus) were retained more than wooden dowels, with paper strips behaving in a similar fashion to Eucalyptus leaves. Longer dowels were retained more than shorter ones. Changes to the channel form of this stream have significantly affected the rate and nature of CPOM retention which in turn could impact on stream ecosystem structure and productivity. Copyright © 2005 John Wiley & Sons, Ltd.

The influence of juvenile brown trout (Salmo trutta) on habitat use of inanga (Galaxias maculatus) in a stream simulator

Introduced salmonid fishes may have affected native galaxiid fishes in New Zealand by forcing alterations in their use of spatial resources. A stream simulator was used to test the effects of juvenile brown trout (Salmo trutta) (fork length range 66–115 mm) on habitat use of inanga (Galaxias maculatus) (fork length range 51–100 mm) during autumn. The stream simulator consisted of a single run‐riffle‐pool sequence lined with gravel and cobble substrates, and c. 8 m3 of untreated artesian water was recirculated through the stream simulator at a rate of 3 litres s‐1. Stream invertebrates were introduced into the stream simulator to provide food for the fish. Four experiments were run, each consisting of three 1‐day trials using either 10 inanga, 20 inanga, or 10 inanga and brown trout. Counts of inanga visible in each of 12 designated habitat zones were made throughout the day, and proportions of visible inanga were used in repeated measure ANOVAs to test whether the proportion of fish visible varied according to the time of day, the density of inanga, or the presence of trout. Our experiments showed that neither the presence of juvenile brown trout nor the density of inanga had significant influence on inanga habitat use in the stream simulator. We conclude that in New Zealand streams any influence of brown trout on inanga is more likely to result from predation and/or competition for food by larger brown trout.

Habitat-Specific Nitrogen Dynamics in New Zealand Streams Containing Native or Invasive Fish

Streams are important sites of nutrient transport and transformation in the landscape but little is known about the way in which individual taxa or individual habitats (riffles and pools) influence nutrient dynamics within stream reaches. We used 5-week additions of a stable isotope (15NH4Cl) tracer to investigate nitrogen dynamics in pools and riffles of two New Zealand streams, one with native fish (Galaxias depressiceps) and the other with invasive brown trout (Salmo trutta). In New Zealand, brown trout initiate a trophic cascade leading to increased algal biomass that we predicted would lead to higher N uptake and retention. Uptake of NO3, but not ammonium, was greater in the trout stream. Rather than causing a large increase in N demand, trout may induce a reallocation of N uptake and retention among food web compartments in different habitats. The largest differences between streams were apparent in riffles, where most uptake and retention of N occurred. In the trout stream, uptake rate by epilithon in riffles was more than six times greater than uptake rates of any other compartment. In the Galaxias stream, several compartments in both habitats had similar uptake rates. Epilithon also accounted for a larger percentage of the 15N retained in the study reach in the trout stream (51%) than the Galaxias stream (34%). Our results show that an individual predatory taxon (in our case an invader) can influence N dynamics in streams but that the magnitude and location of the impact depend on a range of abiotic and biotic factors involved in N dynamics in streams.

Application of the Index of Biotic Integrity Methodology to New Zealand Freshwater Fish Communities

An index of biotic integrity (IBI) was developed for freshwater fish in New Zealand streams. Data on freshwater fish occurrence for 5497 sites over the entire country were obtained from the New Zealand freshwater fish database for the period 1980-2002. Environmental habitat descriptors for the stream watersheds above or at each of these sites were obtained from an existing river environment classification using a geographic information system. Of the many IBI metrics in use globally, only six were adapted and applied because of differences between the fish fauna of New Zealand and the United States. A number of evaluation methods showed that all six metrics contributed to the overall IBI scores with high levels of consistency. The IBI assessment of sites sampled at different times showed high levels of temporal concordance. The IBI scores differed significantly among the geological areas, the sampling method used, and the year of survey. Overall, the results presented demonstrate the potential for New Zealand freshwater fish to be used to assess river condition at large spatial scales in New Zealand in the absence of specifically selected reference sites. This application demonstrates the effectiveness of the IBI approach even with a fauna of limited diversity and limited ecological specialisation as in the New Zealand fish fauna.

Riparian buffers mitigate effects of pine plantation logging on New Zealand streams1. Riparian vegetation structure, stream geomorphology and periphyton

Influences of the riparian zone vegetation characteristics on bank erosion, stream geomorphology, irradiance and periphyton were examined at 28 sites in the Whangapoua area of the Coromandel Peninsula, North Island, New Zealand. Riparian buffers were defined as areas alongside streams that were not managed for production forestry, and which contained native indigenous vegetation or mixed indigenous and introduced flora. Five forest and riparian zone types were selected: (i) harvested pine plantation with a clearcut to stream edge, (ii) harvested pine plantation with a vegetated riparian buffer, (iii) mature pre-harvest pine plantation with a riparian buffer dominated by native vegetation, (iv) mature pre-harvest pine plantation with a riparian zone of plantation pines and a native vegetation understorey, and (v) mature native forest reference sites. Up to eight replicates were selected for each management regime. Species composition and canopy cover were measured from bounded plots, and stream bank vegetation was recorded separately. Bank erosion was measured along the full length of each study reach, and stream water width and bankfull width were measured at ten transects within each study reach. Stream and bank lighting were measured using a canopy analyser. Periphyton standing crop was measured as Chlorophyll a and ash-free dry weight. Although channel width increased with catchment area for all of the study reaches, bank erosion and channel widths were greater at harvested sites where plantation pines occurred at the stream edge (i.e. within the riparian zone), than other forest and riparian treatments. Stream lighting was heavily influenced by the presence of riparian vegetation as well as stream size for these small to moderate size streams. Mature radiata pine in riparian areas of pre-harvest sites provided shading which was similar but less variable than that recorded in the native reference sites. Mature pine forest may be shading out the taller growing native broadleaved canopy, and thus creating a denser 5–11.9 m height class, reducing light penetration. The lack of native species >12 m in height is likely to be due to the length of time which native conifers and hardwoods take to develop as these are the species which normally dominate the greater than 12 m height class. Periphyton biomass was lowest at pre-harvest sites with native riparian buffers present and greatest at clearcut harvested sites with clearcut riparian zones. This work shows that riparian vegetation composition and maturity can influence the physical characteristics of afforested and harvested New Zealand streams.

Riparian buffers mitigate effects of pine plantation logging on New Zealand streams: 1. Riparian vegetation structure, stream geomorphology and periphyton

Influences of the riparian zone vegetation characteristics on bank erosion, stream geomorphology, irradiance and periphyton were examined at 28 sites in the Whangapoua area of the Coromandel Peninsula, North Island, New Zealand. Riparian buffers were defined as areas alongside streams that were not managed for production forestry, and which contained native indigenous vegetation or mixed indigenous and introduced flora. Five forest and riparian zone types were selected: (i) harvested pine plantation with a clearcut to stream edge, (ii) harvested pine plantation with a vegetated riparian buffer, (iii) mature pre-harvest pine plantation with a riparian buffer dominated by native vegetation, (iv) mature pre-harvest pine plantation with a riparian zone of plantation pines and a native vegetation understorey, and (v) mature native forest reference sites. Up to eight replicates were selected for each management regime. Species composition and canopy cover were measured from bounded plots, and stream bank vegetation was recorded separately. Bank erosion was measured along the full length of each study reach, and stream water width and bankfull width were measured at ten transects within each study reach. Stream and bank lighting were measured using a canopy analyser. Periphyton standing crop was measured as Chlorophyll a and ash-free dry weight. Although channel width increased with catchment area for all of the study reaches, bank erosion and channel widths were greater at harvested sites where plantation pines occurred at the stream edge (i.e. within the riparian zone), than other forest and riparian treatments. Stream lighting was heavily influenced by the presence of riparian vegetation as well as stream size for these small to moderate size streams. Mature radiata pine in riparian areas of pre-harvest sites provided shading which was similar but less variable than that recorded in the native reference sites. Mature pine forest may be shading out the taller growing native broadleaved canopy, and thus creating a denser 5–11.9 m height class, reducing light penetration. The lack of native species >12 m in height is likely to be due to the length of time which native conifers and hardwoods take to develop as these are the species which normally dominate the greater than 12 m height class. Periphyton biomass was lowest at pre-harvest sites with native riparian buffers present and greatest at clearcut harvested sites with clearcut riparian zones. This work shows that riparian vegetation composition and maturity can influence the physical characteristics of afforested and harvested New Zealand streams.

Incorporation of marine‐derived nutrients from petrel breeding colonies into stream food webs

Summary1. Stable isotope ratios of aquatic invertebrates, aquatic mosses and leaves of riparian plants were used to determine whether marine‐derived nutrients from breeding colonies of the Westland petrel (Procellaria westlandica) were incorporated into the food webs of small streams in New Zealand.2. The δ15N signatures of all plants and animals examined were higher by 3.6–4.6‰ in small streams draining catchments with petrel colonies than in nearby streams where petrels were absent. δ13C values of leaves from terrestrial plants were also enriched by about 2‰ where petrels were present, but the carbon ratios of aquatic species were depleted in 13C, rather than enriched, suggesting that any marine signal was over‐ridden by isotopic shifts related to photosynthetic fractionation.3. A high marine‐nitrogen signal was maintained along the 3 km length of Scotchman Creek with the δ15N values of leptophlebiid mayflies and predatory insects ranging from 7.4–9.5 and 9.2–11.9‰, respectively.4. Most nutrients derived from petrels are likely to be translocated to streams via the soil, which they enter in the form of excreta, spilled food, feathers, dead chicks, and abandoned eggs. However, because changes in δ15N values are brought about by soil processes such as volatilisation of ammonia, nitrification and denitrification, it is difficult to predict the exact isotope signature of nitrogen entering a stream. Tentative estimates of the proportion of marine‐derived nitrogen in stream biota, calculated using a mass‐balance approach, ranged from 28–38%.5. Our findings indicate that marine nutrients transported inland by seabirds can be incorporated into the food webs of streams. In pre‐human times when there were many more seabird colonies on mainland New Zealand than exist today, marine‐derived nutrients introduced by birds may have had significant effects on nitrogen cycling and the productivity of New Zealand streams.

Riparian buffers mitigate effects of pine plantation logging on New Zealand streams: 2. Invertebrate communities

The influences on forest stream invertebrate communities of riparian forest type (native/exotic Pinus radiata) and logging, with or without native forest riparian buffers, were investigated at 28 stream sites on Coromandel Peninsula, New Zealand. Stream reaches were surveyed under summer, baseflow conditions in six riparian/forest vegetation types: native forest, mature pine plantations with pines planted to the stream edge, mature pine plantations with native forest in the riparian area, clearcut pine plantations, and logged pine plantations with patch buffers of native forest vegetation (upstream areas clearcut) or continuous buffers along the perennially flowing stream length. Multivariate analyses showed that clearcut reaches differed in invertebrate community structure from pine and native forested reaches, and from logged reaches with continuous riparian buffers. Communities at patch buffer sites were intermediate between these groups. Amongst the common taxa, mayflies were the most sensitive to clearcut logging, with three species less abundant at clear-cut and/or patch buffer sites; only the algal-piercing caddis Oxyethira albiceps (Hydroptilidae) responded positively to logging. Clearcut reaches had lowest diversity, taxon richness, relative abundance and numbers of the sensitive mayfly, stonefly and caddisfly taxa, and index of biotic integrity. In contrast, sites that had been logged leaving continuous buffers did not differ in these biometrics from those in intact native or mature plantation forest, indicating that buffers greatly reduced disturbance associated with logging. Logged sites with patch buffers had biometric values intermediate between clearcut and forested/continuous buffered reaches, indicating less protection from logging impact. Correlation and multiple regression analyses showed that logging impacts are strongly related to increases in periphyton biomass and water temperature, associated with changes in stream lighting, and increased channel instability/fine sediment. The findings indicate that late-rotation exotic pine plantations can support very similar stream invertebrate communities to native forests, and highlight the benefit of retaining forested buffers along stream riparian areas to avoid harvesting impacts on stream habitat and invertebrate communities.

Sediment texture mediates high-flow effects on lotic macroinvertebrates

We conducted 2 experiments to examine how sorting, shape, and size of sediment grains affected drift and mortality of the mayfly Deleatidium spp., the snail Potamopyrgus antipodarum, and the stone-cased caddisfly Pycnocentrodes aeris in ramped flows in a large, outdoor, recirculating flume. These macroinvertebrates are common to many New Zealand streams and have different flow habitat preferences, body morphologies, and motilities. In the 1st experiment, we quantified taxon drift and mortality and sediment erosion in well-sorted gravel (i.e., high grain-size homogeneity), moderately sorted cobble, and a poorly sorted mixture of gravel and cobble. In the 2nd experiment, we examined the same response variables in angular (rough), oblate (flat, elliptical), and spherical gravel or cobble. Grain sorting significantly affected drift, and this effect was taxon-specific. Drift was highest from moderately sorted cobble, largely because of the relatively high number of dislodged snails. Overall, caddisflies had the highest propensity to drift, regardless of sorting treatment, whereas mayflies had the lowest. Caddisflies generally remained on tops of rocks in ramped flows, and their dislodgment was linked to velocity increases rather than sediment erosion. In contrast, dorsoventrally flattened Deleatidium clung to rock undersides. The result of clinging to sediment undersides to reduce dislodgment was significantly higher mortality compared to the other 2 taxa, particularly in well-sorted gravel with relatively low transport thresholds. The addition of cobble to gravel (i.e., poorly sorted sediments) decreased sediment erosion and significantly reduced mayfly mortality. Grain shape also significantly affected sediment transport thresholds and hence taxon responses. Erosion of angular gravel equaled that of spherical cobble, and no angular cobble eroded in ramped flows. Drift by dislodgment was relatively low in angular gravel and cobble, even for caddisflies. Mortality was again greater for mayflies than for snails and caddisflies, particularly in spherical and oblate gravel that tended to erode. Our results suggest that the interplay between sediment textural properties that affect transport thresholds, and species behaviors and morphologies, influence the overall effectiveness of sedimentary flow-refugia.

Phosphorus and selected metals mobilized from two commercial fertilizers into overland flow during border irrigation

Metals, present as impurities, may be used to trace phosphorus (P) derived directly from fertilizers in field studies. This paper reports a study that compared P to strontium, copper, zinc, cadmium, lead, chromium and nickel concentrations in overland flow from border irrigated fields, before and after fertilization with commercial diammonium phosphate (DAP), or single superphosphate (SSP) blends. The concentrations of P and various metal species in overland flow were within the guidelines for potable water. However, with the possible exception of chromium, all samples, including pre-fertilization samples, were outside the trigger values for Australian and New Zealand streams. Changes in the cadmium and strontium concentrations were related to changes in the total dissolved P (TDP) concentrations following fertilization. This suggests that these metals may be used for tracing P derived directly from fertilizers. However, the background concentrations of cadmium and strontium in overland flow varied between farms and the decay in P concentrations in the two irrigations following fertilizer application were not reflected in cadmium and strontium concentrations. These appear to be major limitations on the use of cadmium and strontium for tracing P derived directly from fertilizers.

Population structure, individual movement, and growth rate of shortjaw kokopu (Galaxias postvectis) in two North Island, New Zealand streams

The shortjaw kokopu (Galaxias postvectis), although widely distributed throughout New Zealand, is only known from a few sites and generally with extremely low population numbers. Two sites in Taranaki (Katikara Stream and Stony River tributary) were found to contain relatively high densities of shortjaw kokopu. These sites were monitored at 6‐monthly intervals over 3½ years to gather site fidelity and growth rate data. Half‐way through the sampling period the riparian vegetation alongside the Katikara Stream site was destroyed. Although this was unfortunate, it allowed examination of the impact of a disturbance event of this nature on shortjaw kokopu growth and behaviour. The density of shortjaw kokopu in Katikara Stream was about twice that in the Stony River tributary, whose population was characterised by fewer but larger individuals. Growth rate was significantly greater for smaller fish, and varied between the two sites. Fish in the Katikara Stream were generally site loyal with only a small percentage of mobile fish; however, greater movement from sites was noted immediately after the disturbance event.

Spawning site selection, egg development, and larval drift of Galaxias postvectis and G. fasciatus in a New Zealand stream

Twelve Galaxias postvectis (shortjaw kokopu) and four G. fasciatus (banded kokopu) spawning sites were found on the margins of the Katikara Stream, Taranaki, New Zealand. This is the first time G. postvectis spawning sites have been documented. G. fasciatus spawning sites discovered in this study confirm previous observations elsewhere in New Zealand. These spawning sites were all found out of water, variable distances from the base flow water level amongst stony substrate, vegetation, and debris. Most G. fasciatus appeared to lay their eggs, and subsequently hatch, c. 3 weeks earlier than G. postvectis, which spawned from 9 to 17 May 2001. G. brevipinnis (koaro) larvae were also identified drifting downstream in late May and mid June 2001. Deposition of eggs and subsequent hatching were found to be associated with elevated flows.

Breakdown of tussock grass in streams along a gradient of agricultural development in New Zealand

Summary 1. We measured the breakdown rate of tussock grass in 12 New Zealand streams in catchments that provided a gradient of agricultural development. We also examined the microbial and invertebrate communities associated with decomposing tussock litter.2. Pristine streams in the study had low concentrations of dissolved inorganic nitrogen (<10 μg L−1) and dissolved reactive phosphate (<3 μg L−1), whereas streams in the most developed catchments had high concentrations of nitrate (>2500 μg L−1) and phosphate (35 μg L−1), as well as greater amounts of suspended sediment and fine sediment covering the streambed.3. Breakdown rate and microbial respiration were significantly related across the sites, and both were positively related to concentrations of nitrate and phosphate. Fungal biomass, measured as ergosterol, was positively related to microbial respiration and was also higher at sites with higher concentrations of nutrients. Total and shredding invertebrates were most abundant at the sites with high nutrient concentrations, but abundance of shredding invertebrates was not significantly related to breakdown rate. Amphipods were the most common shredding invertebrate at most sites, but probably did not contribute greatly to high rates of breakdown in streams in agricultural catchments.4. With the exception of one site, nutrients from agricultural development appeared to have larger positive effects on litter breakdown than negative effects from sedimentation. Litter breakdown can serve as a functional measure of ecosystem health in streams, but caution should be exercised when a stress, such as land use, can have both positive (nutrients) and negative (sedimentation) effects.

Hydrologic and hydraulic control of macrophyte establishment and performance in streams

Macrophytes play a key role in many unshaded lotic ecosystems, but little is known of the factors controlling their presence, abundance, and composition. Macrophyte abundance, diversity, and composition were studied in 15 New Zealand streams to test the hypotheses that the presence and development of macrophytes in lotic systems is primarily controlled by the hydrologic regime (frequency of high‐velocity flood events) and that the interflood spatial distribution and performance of taxa in more stable systems is strongly influenced by local hydraulic conditions (depth/velocity/sediments). Both hypotheses were supported by our results. We found that the abundance and diversity of macrophytes decreased as flood disturbance frequency increased (r2 = 0.52, P = 0.002 for abundance; r2 = 0.53, P = 0.022 for diversity) and that vegetation was absent in streams with more than ~13 high‐flow disturbances per year. An experiment in an ecohydraulics flume identified that the main mechanism causing these effects was not stem breakage at high water velocity but probably uprooting associated with bed sediment erosion. We found that plants with high propagule production constituted a greater proportion of the vegetation in more flood disturbed streams than in stable streams, suggesting that this species trait is important for the maintenance of macrophyte communities in flood prone streams. Distinct velocity, depth, and substrate particle size habitat preferences were displayed by four common species in the study streams. None of the macrophytes showed overlapping preferences for all three habitat variables, suggesting coexisting of the species in streams by physical niche separation. These results significantly expand our understanding of the role of flow regimes in determining lotic ecosystem structure and functioning.

Resistance, resilience, and patchiness of invertebrate assemblages in native tussock and pasture streams in New Zealand after a hydrological disturbance

We generated hydrological disturbances to investigate the role of disturbance in New Zealand streams in two land uses: native tussock grasslands and exotic pasture catchments. We tested whether physical differences in streambed structure confer higher resistance and resilience in tussock sites than in pasture sites. We also investigated changes in patchiness (at spatial scales larger than 0.06 m2) caused by the disturbance. Invertebrate abundance decreased immediately after the disturbance. Species density remained unchanged, but species richness (rarefied) increased. Eight days after the disturbance event, abundance and species richness (rarefied) were similar to those of samples collected immediately before the disturbance. Resistance (measured as decrease in abundance) and resilience (measured as recovery within 8 days) did not differ significantly between the land uses. Patchiness increased in both stream types immediately after the disturbance but decreased to predisturbance levels after 8 days. Disturbance caused a redistribution of individuals among patches, some receiving individuals, others losing individuals, and some remaining unchanged. Our results conform with predictions of the patch dynamics concept and are consistent with results of studies of natural disturbance caused by floods.

Are impacts of an exotic predator on a stream food web influenced by disturbance history?

Predatory species have been introduced to habitats spanning a wide range of environmental conditions. To better understand the consequences of predation in natural communities we need to examine how variations in abiotic factors modify the influence of predation. The effects of introduced predators may vary amongst habitats if natural disturbance affects the abundance and taxonomic composition of consumers and their resources, or the predator alters recolonisation after disturbance. We tested whether a bed-moving disturbance altered subsequent interactions involving native and introduced predatory fish, invertebrate grazers and algae in experimental channels within a New Zealand stream. Disturbance reduced the abundance of invertebrates by 84%, and induced mortality of Conoesucidae caddisflies. However, the relative abundance of taxa changed little immediately following the disturbance. Invertebrate communities recovered following disturbance in fishless channels and those with native galaxiids (Galaxias vulgaris), and were almost indistinguishable from undisturbed fishless controls after 2 weeks. Invertebrate abundance declined and algal abundance increased in channels with exotic brown trout (Salmo trutta) and their effect was strongest in previously disturbed channels. However, predators and disturbance only had interactive effects on grazer emigration rates. Trout affected grazers through direct consumption (e.g. Conoesucidae caddisflies), and induced higher emigration rates of grazers from channels via drift (e.g. the mayfly Deleatidium). The effects of predatory trout and galaxiids combined differed in disturbed and stable channels. The observed combined effects of predatory trout and galaxiids on invertebrate grazers were lower than expected in stable channels partly due to low emigration rates of Conoesucidae, whereas emigration of grazers was higher than expected in the disturbed channels. The biomass of algae was higher than expected in disturbed channels with both predators. Collectively, our results indicate that predator substitutability and the non-lethal effects of introduced predators varied depending on disturbance history, but their effects on the biomass of grazers and algae did not.

Spatial patterns in lotic invertebrate community composition: is substrate disturbance actually important?

Twenty-five forest streams were sampled in August 1994 in Te Urewera National Park, New Zealand, to examine the effect of substrate disturbance on invertebrate community structure. Stream size, flow permanence, and riparian cover were more influential than substrate disturbance in affecting invertebrate composition. Three community types were distinguishable based on these three factors: small (<1 m wide), intermittent streams were dominated by Chironomidae; larger (12–15 m wide), open streams were dominated by Chironomidae, Plecoptera, and Ephemeroptera; and intermediate-sized (1–10 m wide) streams with continuous riparian cover were dominated by mayflies and caddisflies. Periphyton biomass was negatively affected by substrate disturbance but not to the same degree as reported by others studying unshaded streams. This may explain why the influence of substrate disturbance on community composition was less than that of stream size, flow permanence, and riparian cover. The key effect of substrate disturbance on postdisturbance community composition in these light-limited New Zealand streams appears to be the removal of animals rather than food loss. Thus, differences between communities that experience high flows and those that do not are far less than they might be in unshaded streams in which the food base is more severely affected by substrate disturbance.