Pasture Streams Research Articles

Anthropogenic Disturbance and Edge Effects on Anuran Assemblages Inhabiting Cloud Forest Fragments in Colombia

Anurans have been affected by Andean forest fragmentation and loss, leading to impoverished ensembles along the pasture-forest gradients. We evaluated edge effects on tropical cloud forest anurans in three landscapes with a different degree of anthropogenic disturbance, and their relationships with eight environmental variables. On each landscape we set up 12 permanent transects (two per habitat type: streams in pastures, pastures, the outer limit of the pastures with the forest, the internal edge of the forest, the forest interior and streams inside the forest) and surveyed them using an equal effort. After an effort of 162 man-hours we recorded a total of 251 anurans of eight species. Richness was higher in the forest (seven species) than in pasture (four species). Most species of the ensemble (75%) inhabited the forest habitats, whereas 50% of the anuran species inhabited the pasture. Slope and distance to streams influenced the distribution of all modeled Pristimantis species, although this genus exhibits direct development and doesn’t require bodies of water for its reproduction. The canopy cover had a high explanatory power (through beta regressors) on the presence of anuran species in the microhabitat; whereas temperature and relative humidity did not, suggesting that temperature and relative humidity may be less important for frog distributions along pasture-forest gradients in cloud forest landscapes. The anthropogenic disturbance in the landscape, rather than the edge effects, affects the presence of two rare species (Pristimantis ptochus and Pristimantis kelephas) suggesting that these species display a high degree of sensitivity to the transformation of the cloud forest. Future cloud forest management research needs to incorporate anthropogenic disturbance effects, because it strongly interacts with edge and matrix effects, affecting the habitat quality and the persistence of species in the landscape.

Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers

Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using 15N additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream. These three streams were within several km of each other and on similar soils. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO3 −) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH4 + uptake length, higher uptake rates into organic matter components and a shorter 15NH4 + residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added 15NH4 +) but not in the second-order pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added 15N in organic matter compartments and exported 53% (15NH4 + = 34%; 15NO3 − = 19%). In contrast, the second-order pasture stream retained 75% of added 15N, predominantly in grasses (69%) and exported only 4% as 15NH4 +. The fate of tracer 15N in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported (15NH4 + = 9%; 15NO3 − = 6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first- and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.

Grazing Management and Microclimate Effects on Cattle Distribution Relative to a Cool Season Pasture Stream

Abstract Because of concerns about the impact of grazing management on surface water quality, a 3-yr study was conducted to determine grazing management and microclimate impacts on cattle distribution relative to a pasture stream and shade. Three treatments, continuous stocking with unrestricted stream access (CSU), continuous stocking with restricted stream access (CSR), and rotational stocking (RS), were evaluated on six 12.1-ha cool-season grass pastures stocked with 15 fall-calving Angus cows ( Bos taurus L.) from mid-May through mid-October of each year. On 2 d · mo −1 from May through September of each year, a trained observer in each pasture recorded cattle position and activity every 10 min from 0600 to 1800 hours. In years 2 and 3, position of one cow per pasture was recorded with a Global Positioning System (GPS) collar at 10-min intervals 24 h · d −1 for 2 wk · mo −1 from May through September. In week 2 of collar deployment in May, July, and September, cattle had access to off-stream water. Ambient temperature, black globe temperature, relative humidity, and wind speed were recorded at 10-min intervals and temperature humidity (THI), black globe temperature humidity (BGTHI), and heat load (HLI) indices were calculated. Based on GPS collars, mean percentage of time cows in CSU pastures were in the stream (1.1%) and streamside zone (10.5%) were greater ( P P

Leaf‐litter breakdown in pasture and deciduous woodland streams: a comparison among three European regions

Summary1. Human land‐use has altered catchments on a large scale in most parts of the world, with one of the most profound changes relevant for streams and rivers being the widespread clearance of woody riparian vegetation to make way for livestock grazing pasture. Increasingly, environmental legislation, such as the EU Water Framework Directive (EU WFD), calls for bioassessment tools that can detect such anthropogenic impacts on ecosystem functioning.2. We conducted a large‐scale field experiment in 30 European streams to quantify leaf‐litter breakdown, a key ecosystem process, in streams whose riparian zones and catchments had been cleared for pasture compared with those in native deciduous woodland. The study encompassed a west–east gradient, from Ireland to Switzerland to Romania, with each of the three countries representing a distinct region. We used coarse‐mesh and fine‐mesh litter bags (10 and 0.5 mm, respectively) to assess total, microbial and, by difference, macroinvertebrate‐mediated breakdown.3. Overall, total breakdown rates did not differ between land‐use categories, but in some regions macroinvertebrate‐mediated breakdown was higher in deciduous woodland streams, whereas microbial breakdown was higher in pasture streams. This result suggests that overall ecosystem functioning is maintained by compensatory increases in microbial activity in pasture streams.4. We suggest that simple coefficients of breakdown rates on their own often might not be powerful enough as a bioassessment tool for detecting differences related to land‐use such as riparian vegetation removal. However, shifts in the relative contributions to breakdown by microbial decomposers versus invertebrate detritivores, as revealed by the ratios of their associated breakdown rate coefficients, showed clear responses to land‐use.

The effect of repeated stressor episodes on algal communities in pasture streams

The effects in streams of anthropogenic stressors associated with agricultural intensification can be expected to vary with time, reflecting periodic movements of stock, fertilisation episodes and storm-related inputs. The present study focuses on the cumulative effects of augmented nutrients (N+P, three levels, for 1 month) coupled with episodes of increased bed sediment cover by comparing algal community responses to one, two or three stressor applications in nine sheep pasture streams in New Zealand. Algae were investigated for 1 month before and for 5 months during the manipulations. Algal taxonomic richness decreased with nutrient enrichment plus sediment addition, whereas the pollution-tolerant Achnanthidium increased, indicating reduced water quality. Sediment addition by itself had few negative effects on the algae, possibly because the sediment retention time during each 1-month experimental phase was reduced considerably (to 7–26 days) owing to floods. Algal communities (total algal cell density and densities of the common taxa Navicula spp. and filamentous green algae) reacted more strongly to multiple nutrient/sediment addition episodes than to rare or singular episodes. These results indicate a cumulative impact of anthropogenic stressors on a key component of the stream ecosystem. The effectiveness of management may be improved by focusing on the avoidance of repeated stressor events rather than the occurrence of rare episodes.

Land use, habitat integrity, and aquatic insect assemblages in Central Amazonian streams

The distribution and composition of aquatic insect communities in streams at a local scale are considered to be primarily determined by environmental factors and interactive relationships within the system. Here, we evaluated the effects of forest fragmentation and forest cover changes on habitat characteristics of streamlets (igarapes) in Amazonian forests and on the aquatic insect communities found there. We also developed a habitat integrity index (HII) based on Petersen’s protocol (1992) to evaluate physical integrity of these streamlets and to determine its efficiency to interpret the environmental impacts on this system. We studied 20 small streams at the Biological Dynamics of Forest Fragments Project (BDFFP INPA/SI) study areas, Central Amazonia, 80 km north of Manaus, Amazonas State, Brazil. The vegetation cover was estimated by using LANDSAT images and classified in the following categories: exposed soil, pastures, secondary forests (capoeiras), and primary forests. Stream habitat features were evaluated by using a HII based on visual assessment of local characteristics. Aquatic insects were sampled in four major stream substrates: litter deposited in pools or backwaters, litter retained in riffles, sand, and marginal banks. Stream habitat characteristics were significantly correlated to land use and riparian forest condition. Overall aquatic insect richness and Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness were significantly lower in pasture streams, and their taxonomic composition differed significantly from streams in forested areas. However, these metrics were not significantly correlated to the stream HII. Taxonomic composition of bank insect assemblages changed significantly between streams with low and high values of HII. There was no significant relationship between the proportion of primary forest cover and the faunal metrics. Only drastic changes in the vegetal cover seem to induce significant changes in the aquatic insect community. Matrix habitat heterogeneity, distance to forest fragments, the presence of areas of secondary forest, and the intrinsic capacity to disperse in many of the insect groups may have contributed to attenuate the effects of habitat disturbance on aquatic insect assemblages in streamlets.

Land management impacts on runoff sources in small Amazon watersheds

AbstractForest clearing and conversion to cattle pasture in the lowland Amazon region has been linked to soil compaction and increased soil water storage, which combine to diminish soil infiltration, enhance quick lateral flows and increase the stream flow response to precipitation. Quantifying the importance of quick surficial flow in response to this land use change requires identification of water sources within catchments that contribute to stream flow. Using an end member mixing analysis approach, potential contributing sources of stream flow were evaluated during an entire rainy season in a forest and a pasture watershed drained by ephemeral‐to‐intermittent streams in the south‐western Amazon. Water yield was 17% of precipitation in the pasture and 0·8% of precipitation in the forest. During the early rainy season, throughfall, groundwater, and soil water contributed 79%, 18%, and 3%, respectively, to total forest stream flow. Over the entire rainy season, throughfall, groundwater, and shallow soil water provided 57%, 24%, and 19%, respectively, of stream flow. In the pasture watershed, overland flow dominated stream flow both in the early (67%) and late (57%) rainy season, with a mean contribution of 60% overland flow, 35% groundwater, and 5% soil water. The uncertainty associated with those estimates was studied using a Monte Carlo approach. In addition to large changes in total surface flow, marked differences were found in the proportions of total stream flow in the second half of the rainy season between the forest and pasture watershed. These results suggest that (1) there is great potential for alteration of the hydrological budgets of larger watersheds as the proportion of deforested land in the Amazon increases, and (2) as more rainfall is diverted into fast flowpaths to streams in established pastures, the potential to deliver water with higher solute concentrations generated by erosion or by bypassing sites of solute removal increases. Copyright © 2007 John Wiley & Sons, Ltd.

Factors influencing retention of coarse particulate organic matter in streams

AbstractInput of terrestrial coarse particulate organic matter (CPOM: leaves and small wood, >1 mm) adds food resources and habitat to streams. Restoration projects often aim to increase CPOM through riparian planting, but this relies on inputs being retained within the restored reach, and quantitative knowledge of factors influencing retention processes is limited. We investigated retention characteristics under base‐flow conditions in 65 stream reaches in pasture, native forests and pine plantations (with and without recent logging) in five contrasting New Zealand regions, using three CPOM analogues, namely 30 cm dowels, soaked ginkgo leaves and waterproof paper triangles. These differed significantly in geometric mean retention distance (Sp), with greatest retention (smallest Sp) of dowels > ginkgos > triangles. Retention efficiency was greater in riffles and glides than in pools. Objects trapping CPOM differed between land uses and regions in predictable ways. For example, in relatively unshaded pasture streams, instream and encroaching vegetation riparian was more important, and wood less important, than in forest. Catchment‐area‐ and width‐specific Sp values were more informative for comparisons amongst sites than deposition velocities (vdep). Catchment‐area‐specific Sp values varied 10‐fold amongst regions and threefold amongst land uses. Stepwise multiple regression models explained 71–74% of variance in Sp for the different CPOM analogues and showed that flow (+ influence; i.e., Sp increases with flow) was the most important factor, followed by submerged wood (−), with periphyton biomass (−) also significant in all models. A comparison with results from Spain showed similar relationships between Sp of plastic CPOM analogues and discharge under base‐flow conditions. Restoring riparian vegetation is expected to ultimately enhance instream CPOM by increasing input and retention by wood. Our findings indicate that this should prioritize small streams if the aim is to optimize the restoration of CPOM influence on instream processes. Copyright © 2007 John Wiley & Sons, Ltd.

Channel morphology and sediment origin in streams draining the Georgia Piedmont

Summary Urbanization is common across much of the US However, the Southeast, including the Georgia Piedmont, is developing much faster than other regions ( USDA, NRCS, 2004 ). Consequently, water resources in the Middle Chattahoochee Watershed of western Georgia are threatened by increased sedimentation from extensive urban development as well as from other land covers such as livestock grazing and silviculture. A 2-yr study was developed to assess sediment transport and origin across 16 watersheds draining urban, developing, pastoral, managed forest and unmanaged forest landscapes. Total suspended solids (TSS) and total dissolved solids (TDS) yields and sediment rating curves were measured concomitantly with channel morphometry measurements in each stream. Urban streams featured the lowest baseflow concentrations, but sediment concentrations rose rapidly during stormflow in urban streams. Detailed cross-sections assessed channel stability and showed that pasture streams were the most unstable streams during stormflows. Finally, sediment source tracking was performed in a subset of intermittent streams using amorphous to crystalline ratios of iron to estimate the fraction of sediment coming from instream vs. landscape sources. Artificial stormflows were generated to mobilize bed sediment for the development of an instream sediment signature. If these ratios differed during natural events, it was inferred the differences were due to sediments mobilized from the terrestrial landscape. Results indicated that higher ratios of amorphous:crystalline Fe occurred during artificial floods (urban = 0.60 and unmanaged forest = 0.14) than natural stormflows (urban = 0.08 and unmanaged forest = 0.03) in watersheds dominated by urban and unmanaged forest land cover, suggesting that crystalline (i.e., terrestrial) sources of Fe were transported to the stream during rainfall events.

Impacts of fine sediment addition to tussock, pasture, dairy and deer farming streams in New Zealand

Summary1. Increased fine sediment input caused by agricultural development is expected to act as a stressor for stream ecosystems. In a large‐scale field experiment, we added fine river sand to 50‐m reaches of three second‐order streams in each of four categories of catchment development (ungrazed tussock grasslands, grazed pasture, dairying and deer farming) and measured the responses of macroinvertebrates and aquatic moss.2. Before addition, fine sediment cover differed between land uses, being lowest in tussock (7%), intermediate in pasture (30%) and dairy (47%) and highest in deer streams (88%). Sediment addition increased cover by one land‐use category (e.g. augmented sediment cover in tussock streams was similar to pre‐existing cover in pasture streams), and cover remained high in impact reaches (compared with controls) throughout the 5‐week experiment. Sediment addition did not change concentrations of phosphate, nitrate and ammonium, which were generally highest in dairy streams and lowest in tussock streams.3. Aquatic mosses (most common in tussock, absent in dairy and deer), invertebrate density (highest in deer, lowest in tussock), taxon richness (highest in pasture, lowest in deer) and diversity (highest in pasture and tussock, lowest in dairy and deer) all differed between land uses. Sediment addition resulted in reductions of moss cover, invertebrate taxon richness and richness of Ephemeroptera, Plecoptera and Trichoptera in impact relative to control reaches.4. The impact of sediment addition was strongest in pasture streams where pre‐existing sediment cover was moderate and richness and diversity of the invertebrate community highest. However, even in the already sediment‐rich and species‐poor deer streams, density of one common taxon was reduced significantly by sediment addition, and another two were affected in the same way in dairy streams, the second‐most intense land use.5. Our experiment has disentangled the impact of sediment addition from other concomitant land‐use effects that could not be reliably distinguished in previous research, which has mainly consisted of correlative studies or unrealistically small‐scale experiments.

Deforestation alters the hydraulic and biogeochemical characteristics of small lowland Amazonian streams

AbstractWe investigated how clearing of the tropical rainforest for cattle pasture along small, terra firme lowland streams in the Brazilian Amazon influenced stream hydraulic characteristics, solute concentrations and uptake of dissolved inorganic nitrogen and phosphorus. Measurements of stream channel structure, hydraulic properties and nutrient uptake were obtained from short‐term conservative and non‐conservative solute injections and an in‐stream transport model in two watersheds with pairs of similar‐sized forest and pasture streams in Rondônia. The pasture stream channels were deeper and had a lower cover of sandy bottom habitat and a higher cover of aquatic grass habitat than the forest streams. The pasture streams had larger transient storage zones, higher ratios of transient to channel storage and a shorter hydraulic uptake length than the forest streams. The pasture streams had lower concentrations of dissolved oxygen and NO3− and higher concentrations of dissolved Fe2+. Forest streams exhibited no NO3− uptake, but one pasture stream took up NO3− while the other did not. Uptake of NH4+ was low and variable among streams. Uptake velocities and rates of PO43− were greater in pasture streams compared to those in forest streams. In all streams, uptake lengths for NO3−, NH4+ and PO43− were long and indicated generally lower rates of uptake than in most comparably sized temperate forest streams. Uptake lengths or velocities were not correlated with stream transient storage, suggesting that other factors, such as hypoxia in pasture streams, controlled nutrient uptake. The structural differences are typical for the region, suggesting that deforestation may be altering stream hydrology and biogeochemistry over many thousands of kilometres of primary and secondary stream channels in the Amazon Basin. A better understanding of the extent and distribution of altered hydraulic and biogeochemical function in small streams is required to assess the importance of these changes for larger river networks in the Amazon basin. Copyright © 2006 John Wiley & Sons, Ltd.

Fencing Pasture Streams, Ponds, and Wetlands

WEC-210, a 3-page illustrated fact sheet by William M. Giuliano, encourages the fencing of riparian zones to exclude grazing livestock and provide better habitat for wildlife. It describes the benefits to landowners of several fencing programs of federal, state, and private organizations. Published by the UF Department of Wildlife Ecology and Conservation, May 2006.

Should I Fence the Streams, Ponds, and Wetlands in My Pastures?: Fencing pasture streams, ponds, and wetlands can improve fish and wildlife habitat and benefit agricultural landowners.

Fencing pasture streams, ponds, and wetlands can improve fish and wildlife habitat and benefit agricultural landowners. DOI:10.2458/azu_rangelands_v28i1_Giuliano

Should I Fence the Streams, Ponds, and Wetlands in My Pastures?

Fencing pasture streams, ponds, and wetlands can improve fish and wildlife habitat and benefit agricultural landowners.

Interaction of Press and Pulse Disturbance on Crayfish Populations: Flood Impacts in Pasture and Forest Streams

Interactions between press and pulse disturbances can significantly impact benthic macroinvertebrate (crayfish) populations. Press disturbances such as land-use change can make crayfish more vulnerable to the detrimental effects of pulse disturbances, by changing the habitats available to crayfish. The impact of a pulse disturbance, a major flood (1 in 28 years return period), on crayfish (Paranephrops planifrons) was significantly greater in pasture than native forest streams. Population densities of crayfish in three forest and three pasture streams had been measured seasonally for 2 years prior to the flood and many crayfish had been marked with individually identifiable tags. Low numbers of marked crayfish were recaptured after the flood in the forest streams, but no marked crayfish were identified in any pasture stream. Crayfish densities in one pasture stream declined from an average of 5 m−2 prior to the flood to <1 m−2 soon after the flood and it took 3 years for the population to show evidence of recovery. Macrophytes and cobbles, the dominant habitats of crayfish in pasture streams, did not appear to provide stable refugia during the flood. Habitat stability was linked to the riparian zone in forest streams where undercut banks, tree roots, and pools were important habitats for crayfish. Frequent pulse disturbance could affect population persistence of refugia-dependent species when the pressure of land-use change affects the stability of habitats, but this may only be evident over long time scales.

Influences of land use and stream size on particulate and dissolved materials in a small Amazonian stream network

We investigated the influences of forest or pasture land use and stream size on particulate and dissolved material concentrations in a network of second to third order streams in Rondonia, in the Brazilian Amazon. During the dry season, a second order stream originating in pasture had lower concentrations of dissolved oxygen and nitrate, higher concentrations of chlorophyll, total suspended solids, particulate organic carbon, particular organic nitrogen, ammonium, and phosphate than a second order stream originating in forest. Where the second order forest stream exited forest and entered pasture, concentrations of dissolved oxygen dropped from 6 mg/L to almost 0 mg/L and nitrate concentrations dropped from 12 μM to 2 μM over a reach of 2 km. These changes indicated a strong influence of land use. During the rainy season, differences among reaches of all particulate and dissolved materials were diminished. Concentrations of oxygen, chlorophyll, total suspended solids, particulate organic carbon and nitrogen, nitrate, ammonium, and phosphate in the third order pasture stream more closely resembled the second order forest stream than the second order pasture stream, suggesting that conditions in the channels of larger pasture streams more strongly control concentrations of these materials. If this pattern is widespread in stream networks of regions that consist of a mosaic of forest and pasture lands, it may have important consequences for understanding the effects of deforestation on larger rivers of the Amazon Basin. This would indicate that the effects of forest clearing on the concentrations of many suspended and dissolved materials will be most easily detected in very small streams but potentially difficult to detect in larger streams and rivers.

Natural controls and human impacts on stream nutrient concentrations in a deforested region of the Brazilian Amazon basin

This study documents regional patterns in stream nitrogen and phosphorus concentrations in the Brazilian state of Rondonia in the southwestern Amazon basin, and interprets the patterns as functions of watershed soil properties, deforestation extent, and urban population density. The survey includes 77 different locations sampled in the dry and wet seasons, with a watershed size range from 1.8 to 33,000 km2 over a total area of approximately 140,000 km2. A sequential regression technique is used to separate the effects of natural watersheds properties and anthropogenic disturbance on nutrients and chloride. Natural variation in soil texture explains most of the variance in stream nitrate concentrations, while deforestation extent and urban population density explain most of the variance in stream chloride (Cl) and total dissolved nitrogen (TDN) concentrations. Stream TDN, total dissolved phosphorus (TDP), particulate phosphorus (PP) and Cl concentrations all increase non-linearly with deforestation extent in the dry season after controlling for natural variability due to soil type. Stream nutrient and Cl disturbances are observed only in watersheds more than 66–75% deforested (watershed area range 2–300 km2), suggesting stream nutrient concentrations are resistant to perturbation from vegetation conversion below a 66–75% threshold. In heavily deforested watersheds, stream Cl shows the largest changes in concentration (12 ± 6 times forested background), followed by TDP (2.3 ± 1.5), PP (1.9 ± 0.8) and TDN (1.7 ± 0.5). Wet season signals in Cl and TDP are diluted relative to the dry season, and no land use signal is observed in wet season TDN, PN, or PP. Stream TDN and TDP concentrations in non-urban watersheds both correlate with stream Cl, suggesting that sources other than vegetation and soil organic matter contribute to enhanced nutrient concentrations. Small, urbanized watersheds (5–20 km2) have up to 40 times the chloride and 10 times the TDN concentrations of forested catchments in the dry season. Several large watersheds (∼1000–3000 km2) with urban populations show higher Cl, TDN and TDP levels than any small pasture watershed, suggesting that human impacts on nutrient concentrations in large river systems may be dominated by urban areas. Anthropogenic disturbance of dry-season stream Cl and TDN is detectable in large streams draining deforested and urbanized watersheds up to 33,000 km2. We conclude that regional deforestation and urbanization result in changes in stream Cl, N and P concentrations at wide range of scales, from small pasture streams to large river systems.

Scale and the detection of land‐use effects on morphology, vegetation and macroinvertebrate communities of grassland streams

Summary1. Land‐use studies are challenging because of the difficulty of finding catchments that can be used as replicates and because land‐use effects may be obscured by sources of variance acting over spatial scales smaller than the catchment. To determine the extent to which land‐use effects on stream ecosystems are scale dependent, we designed a whole‐catchment study of six matched pairs (pasture versus native tussock) of second‐order stream catchments, taking replicate samples from replicate bedforms (pools and riffles) in each stream.2. Pasture streams had a smaller representation of endemic riparian plant species, particularly tussock grasses, higher bank erosion, a somewhat deeper layer of fine sediment, lower water velocities in riffles, less moss cover and higher macroinvertebrate biodiversity. At the bedform scale, suspendable inorganic sediment (SIS) was higher in pools than riffles and in pasture streams there was a negative relationship between SIS and the percentage of the bed free of overhanging vegetation. Differences between stream reaches (including any interactions between land use and stream pair) were significant for SIS, substrate depth and characteristics of riparian vegetation. There were also significant differences between replicate bedforms in the same stream reaches in percentage exotic species in overhanging vegetation, percentage moss cover, QMCI (Quantitative Macroinvertebrate Community Index – a macroinvertebrate‐based stream health index) and macroinvertebrate density.3. Significant differences among stream reaches and among replicate bedform units within the same reach, as well as interactions between these spatial units and land‐use effects, are neither trivial nor ‘noise’ but represent real differences among spatial units that typically are unaccounted for in stream studies. Our multi‐scale study design, accompanied by an investigation of the explanatory power of different factors operating at different scales, provides an improved understanding of variability in nature.

Effect of Pinus radiata logging on stream invertebrate communities in Hawke's Bay, New Zealand

Invertebrate communities and associated environmental characteristics were monitored at three Pinus radiata and three pasture stream sites in the Pakuratahi and Tamingimingi Stream catchments, New Zealand, respectively, at nine irregular intervals between December 1996 and April 2001. The Pakuratahi sites were logged between May 1998 and September 1999. Following logging the Pakuratahi Stream invertebrate communities changed from being dominated by a diversity of mayfly species to communities dominated by a high abundance of Chironomidae, Aoteapsyche sp., Elmidae, Ostra‐coda, and Potamopygrus antipodarum. Invertebrate communities that developed following the pine forest harvesting closely resembled those at pasture stream sites in the adjoining Tamingimingi catchment. Invertebrate communities at the pasture stream sites were dominated throughout the study by the same taxa as in the post‐harvest pine sites, except immediately following a storm in July 1997 when mayflies became proportionally more abundant. Biotic indices of water quality, such as the Macroinvertebrate Community Index and Quantitative Macroinvertebrate Community Index, reflected the change in invertebrate communities at the Pakuratahi sites after harvesting, shifting from impact “sensitive” taxa to more “tolerant” taxa. In April 2001 (1.5–2.5 years after harvesting) invertebrate communities had not recovered to their pre‐harvest structure. Recovery of invertebrate communities from a natural disturbance, a major storm in July 1997, was much more rapid (5 months) than the recovery observed from forest harvesting, however. An increase in streambed fine sediment may have been primarily responsible for the changes to invertebrate communities following forest harvesting.

Land‐use influences macroinvertebrate community response following a pulse disturbance

Summary1. We tested the hypothesis that interactions between disturbance types can influence invertebrate community response and recovery in two streams draining pasture (press‐pulse disturbance) and native forest (pulse disturbance) catchments before and after a one‐in‐28‐year flood. We also sampled drift and adult insects to gain insights into the relative importance of these two postdisturbance recolonisation pathways.2. Taxa numbers and total density declined markedly at the forested site after the flood, but there was a delayed response at the pasture site, reflecting greater initial resistance to this pulse disturbance among taxa adapted to the underlying press disturbance.3. Community composition was less stable at the pasture site where per cent abundance of taxa was highly variable prior to the flood and over the 2‐year postflood sampling period. After the flood, the pasture stream fauna was more heavily dominated by vagile taxa, including several chironomid species and hydroptilid caddisflies.4. Taxa numbers and densities recovered to preflood levels within 5–7 months at both sites, but a range of taxa‐specific responses was observed that took up to 18 months to recover to preflood densities. Community stability at the pasture site had not returned to preflood composition by 2 years postflood.5. Changes in drift densities of several common stream invertebrates at the pasture site reflected postflood changes in benthic densities and seasonally low drift in winter. Terrestrial invertebrates dominated drift at the pasture site for 3 months postflood whereas Ephemeroptera were most common at the native forest site.6. Flight patterns of selected adult aquatic insects showed a strongly seasonal pattern. Abundance of adults at the pasture site in the second year following the flood increased in line with the recovery of the non‐Dipteran benthic fauna. Significant upstream flight occurred for several caddisfly species at the native forest site, and weakly directional or downstream flight was evident for most common Plecoptera and Ephemeroptera.7. This study indicates that the magnitude and duration of responses to major pulse disturbances can depend on the presence or absence of an underlying press disturbance. This finding has implications for monitoring, and suggests that a knowledge of disturbance history beyond 2 years may be required to interpret mechanisms contributing to observed land‐use impacts.