Grazing by a native and an exotic crayfish on aquatic macrophytes

1. We compared grazing by native noble crayfish ( Astacus astacus ) and the exotic signal crayfish ( Pacifastacus leniusculus ) on seedling or well-established macrophytes.
2. In a pool experiment, seedlings of emergent Scirpus lacustris and floating-leaved Potamogeton natans were heavily grazed by adult signal crayfish, whereas established plants of the same species sustained only minor damage.
3. In a preference experiment two submerged macrophytes ( Chara vulgaris and Elodea canadensis ), and both seedlings and established plants of S. lacustris and P. natans , were presented pairwise to signal and noble crayfish. There was no significant difference in preference by the two crayfish species. Chara vulgaris was preferred to all other plants presented, established plants of S. lacustris and P. natans were never preferred, and seedlings of P. natans were preferred to established P. natans .
4. An aquarium experiment was conducted in which the consumption of Chara by signal and noble crayfish was measured in relation to water temperature. Signal crayfish consumed significantly more Chara than noble crayfish, especially at higher temperatures.
5. Our results indicate that the signal crayfish is the more voracious grazer especially at higher temperatures. There may be negative effects on vegetation (emergents and floating-leaved, as well as submerged species) when the signal crayfish is introduced. Chara species are particularly susceptible, since they are preferred by crayfish and the genus includes a large number of rare species. Stocking of crayfish therefore could lead to the decline or removal of submerged species in order of crayfish feeding preferences and could prevent the spread or cause a decline of emergent and floating-leaved vegetation.     

The effect of aquatic plant chemistry and morphology on feeding selectivity by the crayfish, Orconectes virilis

SUMMARY. 1. Feeding preference trials conducted with male and female crayfish ( Orconectes virilis ) showed that out of ten submersed aquatic plant species, Chara sp. and Lemna trisulca were the preferred food sources, followed by Myriophyllum exalbescens or Utricularia vulgaris .
2. Analysis of plant fibre, alkaloid, total phosphorus, total and available nitrogen, and organic content showed that crayfish feeding preferences were not related ( P >0.5) to plant fibre or alkaloid content but were negatively correlated ( P <0.05) with plant nutrient content such that species of low nutritional value ( Chara and Lemna ) were consistently preferred.
3. Despite this preference for low nutrient food, daily intake rates by crayfish for phosphorus, nitrogen and organic matter were greatest when feeding on Lemna or Chara due to the large quantity consumed.
4. We suggest that feeding preferences of O. virilis are not determined by plant chemistry but, rather, relate to ease of handling small, short, bottom-dwelling plants such as Chara and Lemna as compared with large erect, roseate or floating-leaved forms. Because crayfish are selective herbivores, their introduction into waters where they do not naturally occur could have a dramatic effect on both the species composition and biomass of submersed plants.     

Effects of different macrophyte growth forms on sediment and P resuspension in a shallow lake

The effects of floating-leaved, submerged and emergent macrophytes on sediment resuspension and internal phosphorus loading were studied in the shallow Kirkkojärvi basin by placing sedimentation traps among different plant beds and adjacent open water and by sediment and water samples. All the three life forms considerably reduced sediment resuspension compared with non-vegetated areas. Both among submerged (Ceratophyllum demersum, Potamogeton obtusifolius, Ranunculus circinatus) and emergent (Typha angustifolia) plants, resuspension rate was on average 43% of that in the adjacent open water, while within floating-leaved plants (Nuphar lutea) the corresponding value was 87%. The effects of submerged and emergent vegetation increased in the course of the growing season together with increasing plant density. Among floating-leaved vegetation, such seasonal trend in resuspension effects was not observed. Compared with the non-vegetated area, floating-leaved, submerged and emergent plants reduced internal phosphorus loading on average by 21, 12 and 26 mg m⁻² d⁻¹, respectively. The effects of floating-leaved plants on resuspension-mediated internal phosphosrus loading were thus comparable to the effects of the other two life forms.     

Differences in refuge function for prey and tolerance to crayfish among macrophyte species

The invasive crayfish Procambarus clarkii is an omnivore and an ecosystem engineer whose feeding mechanism has reduced the abundance of many native invertebrates and macrophytes. Since macrophytes provide refuges for aquatic insects, macrophyte depletion by crayfish might have indirect negative effects on animal prey in aquatic habitats. We postulated that the prey refuges provided by macrophytes and macrophyte tolerance to crayfish cutting and feeding vary among macrophyte species. We conducted two experiments to (1) investigate differences in macrophyte refuge function for dragonfly larvae against crayfish, and (2) test the tolerance to crayfish cutting and feeding among macrophyte species. Elodea nuttallii (submerged plant), Potamogeton crispus (submerged plant), and Carex idzuroei (emergent plant) had greater refuge effects than Trapa japonica (floating-leaved plant), an effect that might result from the larger total cover of E. nuttallii, P. crispus, and C. idzuroei, and the hardness of C. idzuroei leaves. Tolerance to crayfish cutting and feeding was greater in C. idzuroei than in the other species. As the macro-invertebrate assemblages in submerged vegetation are more abundant and species-rich than those in emergent and floating-leaf vegetation, conservation of E. nuttallii and P. crispus should be prioritized for restoring native aquatic animals in ecosystems invaded by the introduced crayfish.     

Distribution and quantitative development of aquatic macrophytes in relation to sediment characteristics in oligotrophic Lake Kalgaard, Denmark

SUMMARY. The distribution and quantitative development of aquatic macrophytes have been studied in oligotrophic Lake Kalgaard, Denmark. The vegetation is dominated by isoetid species, which are widely distributed (about 40% of the lake bottom) compared to emergent and floating-leaved macrophytes (about 4%). Littorella uniflora dominates at depths of 0–2 m and Isoetes lacustris from 2.0 to 4.5m. Within the colonization area the mean midsummer biomass of Littorella is 112g organic dry weight m⁻² and that of Isoetes, 66 gm⁻². The total biomass of these two species constitutes 99% of the biomass of submerged macrophytes.
The perennial Littorella shows only small seasonal biomass variations. The vegetational biomass, the above-ground fraction of the biomass, and the weight of individual plants all increased with the organic content of the sediment at water depths from 0 to 0.75 m. At the same time the interstitial concentrations of carbon dioxide, extractable inorganic nitrogen, and exchangeable inorganic phosphorus increased, thus supporting the hypothesis that an increasing organic content of the sediments at this low level creates a physiologically richer medium for the plants.
The isoetid growth form is discussed in relation to the chemical environment of oligotrophic, softwater lakes.     

Macro-invertebrate communities associated with the macrophytes of Lake Vechten: structural and functional relationships

The macro-invertebrates associated with eight species of emergent, floating-leaved and submerged macrophytes were recorded in May-June 1979. The plant species were: Phragmites australis, Sparganium erectum, Typha angustifolia, Polygonum amphibium, Ceratophyllum demersum, Elodea sp., Myriophyllum spicatum and Nitella mucronata.All macrophytes had a high number of macro-invertebrate species in common; however, some animals were also species-specific, particularly those occurring on the emergent plants. As regards the food source and feeding mechanism, the detritivores, periphytiscrapers and omnivores predominated, followed by predatory carnivores. The seston-filtrators also form a significant part of the community biomass. The macrofaunal distribution depended strongly on colonizable plant surface area and vegetation density. The macrofaunal communities of the Elodea, Ceratophyllum and Phragmites stands contributed mainly to the lake's carbon cycle. Being poorly represented by herbivores, the macro-invertebrate communities will affect the lake's primary production only slightly.     

The aquatic macrophytes of Lake Vechten. Species composition,spatial distribution and production

The macrophytic species compositions in Lake Vechten of 1963 and 1979–80 were compared and showed a considerable change. The main vegetation types were mapped in 1973, 1978, 1979 and 1980. All macrophytic communities, i.e. submerged, floating-leaved and emergent vegetation types, declined mainly due to increasing water turbidity, increasing tree-shading and, from 1978 onwards, grazing and trampling by cattle.Production rates, derived from different combinations of measurement and calculation, were compared. The macrophytes contributed only about 7% to the total lake production in 1980.     

The distribution and habitat preference of ostracods (Crustacea: Ostracoda) in a coastal sand-dune lake, Loch of Strathbeg, north-east Scotland

SUMMARY. 1. Ostracods were a numerically abundant component of the benthos >0.1 mm in size (up to 20% of the total) in the Loch of Strathbeg.
2. Distinct assemblages of species were associated with some habitats: Cypridopsis vidua (Müller) and Herpetocypris reptans (Baird) with Eleocharis palustris (L.) beds; Cypria uphtalmica (Jurine), Candona Candida Müller and Cypria exsculpta (Fischer) with Phragmites communis Trin. beds; and Limnocythere inopinata (Baird) with open sand. In contrast, no distinct ostracod assemblage was associated with submerged beds of Chara aspera Detharding, or Myriophyllum spicatum (L.).
3. The number of ostracod species and ostracod diversity increased from open sand through macroalgal and submerged macrophytes to emergent macrophyte habitats and was associated with increasing number of types of detritus.
4. Laboratory experiments suggested that substratum particle size played a dominant role in determining the distribution of L. inopinata. This species preferred sandy sediments. The distributions of both H. reptans and C. vidua were affected by food supply and H. reptans also preferred fine-grained sediments. The absence of these taxa from some habitats where food supply and sediment particle size conditions were suitable suggested that unmeasured factors played a role. A possible predatory exclusion of H. reptans from such habitats was indicated by a negative association of this species with a predatory mite.     

Seed Bank Variation Along a Water Depth Gradient in a Subtropical Lakeshore Marsh,Longgan Lake,China

Although zonation patterns of the standing vegetation along a water depth gradient in wetlands have been well described, few studies have explored whether such patterns also occur in the seed bank. This study examined patterns of the seed bank along a water depth gradient in three vegetation types (submerged zone, floating-leaved zone, and emergent zone) of a subtropical lakeshore marsh, Longgan Lake, China. Submerged zone is the deepest water and never exposed its soil to air, and was dominant by submerged species; floating-leaved zone is waterlogged soil even during drawdown and was dominant by Nelumbo nucifera; emergent zone is rarely exceeded 1 m water depth during the wet season (summer and autumn), and the marsh soil was usually exposed to air during drawdown (winter and spring), and is dominant by Zizania latifolia, Polygonum hydropiper and Scirpus yagara. It was found that many species were ubiquitous in the seed bank. Frequency of distribution and densities of the dominant species, however, varied significantly from zone to zone. A total of 22 species was recorded in submerged zone, 20 in floating-leaved zone, and 29 in emergent zone. Germinated seedling density was 1,580, 8,994 and 20,424 seedlings m⁻² in submerged zone, floating-leaved zone, and emergent zone, respectively. Submerged and floating-leaved species were significantly abundant in the submerged zone, while the emergent species were found predominantly in the emergent zone. A fern species, Ceratopsis pterioides, was the most abundant seedling in seed banks from all three zones. A TWINSPAN dendrogram and CCA ordination diagram clearly showed separation of species among sites with the emergent zone being well separated from the submerged zone and floating-leaved zone, thus revealing marked zonal patterns in species distributions in the seed bank. This pattern of zonation reflected the pattern in the standing vegetation.     

Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.     

黄河三角洲湿地水生植物组成及生态位

通过大范围的野外调查和室内测定,运用Levins及Pianka公式,对近代黄河三角洲湿地水生植物的构成及主要物种的生态位进行了研究.结果表明:调查样方内共有水生植物34 种,隶属于16科26属;各调查区域间物种组成和群落类型具有一定的差异性.区域形成的年份越久,其物种数和群落类型就越丰富;挺水植物中以芦苇的生态位宽度最大,为0.905,藨草的最低,为0.127; 沉水植物中轮藻、穗花狐尾藻、川蔓藻及小茨藻的生态位宽度较大,分别为0.783、0.673、0.669和0.640,金鱼藻的最低,仅为0.100.龙须眼子菜、大茨藻和苦草间,马来眼子菜和菹草间生态位重叠值较大,说明这几种植物具有相似的环境适应能力.物种的生态位宽度与该物种的分布范围及耐盐区间的大小存在显著的相关性,由此可以断定土壤和水体盐度是导致黄河三角洲湿地水生植物生态位分化的关键因子.     

太湖常见浮叶植物和沉水植物的光合荧光特性比较

利用水下饱和脉冲荧光仪(DIVING-PAM),原位观测太湖常见的水生植物菱、荇菜、苦草、黑藻的最大光化学量子产量(F_v/F_m)及在暗适应和光适应条件下的快速光曲线.结果表明:浮叶植物菱和荇菜光系统Ⅱ的潜在最大光合作用能力高于沉水植物苦草和黑藻,其最大量子产量分别为0.837、0.831、0.684和0.764;菱和荇菜的最大电子传递速率、半饱和光强分别高于沉水植物苦草和黑藻,且在光适应条件下尤为显著.     

Submerged macrophyte zonation in a Mediterranean salt marsh: a facilitation effect from established helophytes?

Abstract. The relationship between communities of submerged annual macrophytes (predominately Chara spp.) and environmental characteristics is studied in three habitats with different dominant perennial species (Arthrocnemum, Juncus, Scirpus) and areas of bare soil. The distribution of submerged annual macrophytes is significantly dependent on two independent environmental factors: a dominant gradient of flooding/salinity, and a secondary gradient of nutrients related exclusively to the combined abundance of helophytes of the genus Scirpus (S. maritimus and S. litoralis). The results suggest that these emergent helophytes (1) are able to modify water column parameters (such as concentration of nitrates, phosphates, potassium, and bicarbonates) which are important for the communities of submerged macrophytes, and (2) play a fundamental role in the generation of secondary sources of environmental variability which, superimposed on the main gradient of flooding/salinity, favours the appearance of new compositional equilibria in such communities. The existence of a process of facilitation is discussed by which the emergent helophytes induce changes in nutrient availability that would favour relatively nutrient‐demanding charophyte species (such as Chara connivens and Nitella hyalina), altering the established relationships with other coexisting charophytes (such as Chara canescens and C. galioides) that dominate in the absence of the facilitating species. Nevertheless, the increased nutrient concentration associated to the presence of helophytes would not introduce significant changes in the total biomass of submerged aquatic macrophytes.     

新疆开都河大型水生植物的氮磷化学计量特征及其影响因素

氮 (N)、磷(P)是最常见的限制植物生长的矿质元素。水生植物所需营养主要来源于水体和沉积物,植物体内的元素组成能够反映淡水生态系统的营养供应状况。开都河是新疆一条流经高山草甸和荒漠绿洲的重要内陆河。本研究采集了开都河大型水生植物和水、沉积物样品,分析了不同生活型和发育史的植物物种叶的氮磷化学计量特征,探讨了上下游、不同土地利用类型下植物、水体和沉积物化学计量特征的差异和关联。结果表明: 植物叶的N、P平均含量分别为24.9 mg·g^-1和2.49 mg·g^-1,氮磷质量比值(N/P)平均为12.6。不同生活型植物叶的N、P含量及N/P存在显著差异:沉水植物N、P含量均显著高于浮叶和挺水植物;浮叶植物N/P为19.2,显著高于其他两类植物,表明其生长可能受P的限制。上游水生植物叶的N、P含量均高于下游。同时,河流上游的沉积物和水体中的N含量均高于下游。与农耕区河段相比,草地区河段水生植物的叶N和P含量较高,可能与草地土壤有机质丰富和放牧产生的排泄物有关,说明放牧对开都河水生态系统化学计量特征的影响大于农耕。过度放牧可能使开都河上游水质变差,改变水生态系统的氮磷养分平衡,并最终影响特定区域的生物地球化学循环过程。     

Feeding of the exotic Louisiana red swamp crayfish,Procambarus clarkii (Crustacea,Decapoda), in an African tropical lake: Lake Naivasha,Kenya

The Louisiana red swamp crayfish, Procambarus clarkii, has had a major impact on aquatic ecosystems in Europe and America where it has been introduced for human consumption. In Lake Naivasha, where it was introduced in 1970, it is suspected of causing the complete disappearance of native floating-leaved and submerged plants by 1982 and again by 1996; recovery of submerged plants occurred in between, concurrent with a decline in P. clarkii. Examination of the feeding of P. clarkii by microscopic analysis of gut contents collected from the lake confirmed information from both its native and exotic ranges; that it is an omnivore with green plants and insects the predominant items recognised. Feeding experiments confirmed this omnivory, with a preference from herbivory over carnivory. They further showed the importance of native plants as opposed to exotic water hyacinth Eichhornia crassipes. It is concluded that P. clarkii could well have accounted for the observed elimination of native plant species in Lake Naivasha; the cyclical nature of populations of submerged plant species and of P. clarkii in inverse proportion to each other lead us to hypothesise that P. clarkii is a `keystone' species in the lake ecosystem.     

两种沉水植物对间隙水磷浓度的影响

为研究两种根系特征的沉水植物在生长过程中对间隙水中磷浓度的影响,选取根系较多的沉水植物苦草和根系相对较少的沉水植物黑藻作为实验材料,监测底泥中间隙水各形态磷含量及环境因子的变化,探讨不同根系特征沉水植物对间隙水中磷的影响。结果表明:黑藻和苦草实验组沉积物间隙水中各形态磷的浓度均呈不同程度的降低,黑藻和苦草对于稳定水质,减少底泥中磷向水中转移具有明显的效果;沉水植物不同,底泥间隙水中溶解性总磷(DTP)和溶解性活性磷(SRP)存在明显差异。实验结束时黑藻组和苦草组间隙水中DTP的浓度分别为0.24,0.01 mg/L,SRP的浓度分别为0.22 mg/L,0.004 mg/L。间隙水中磷的形态主要以DTP和SRP为主,溶解性有机磷(DOP)的含量相对较低。沉水植物对间隙水中磷的吸收是降低间隙水中磷含量的重要原因,苦草的吸收能力大于黑藻。沉水植物根系通过降低底泥p H值,提高氧化还原电位(Eh)的方式抑制了底泥中磷的释放。     

Above- and belowground competition between two submersed macrophytes

Several studies have shown that submerged macrophytes provide a refuge for zooplankton against fish predation, whereas the role of emergent and floating-leaved species, which are often dominant in eutrophic turbid lakes, is far less investigated. Zooplankton density in open water and amongst emergent and floating-leaved vegetation was monitored in a small, eutrophic lake (Frederiksborg Slotssø) in Denmark during July–October 2006. Emergent and floating-leaved macrophytes harboured significantly higher densities of pelagic as well as plant-associated zooplankton species, compared to the open water, even during periods where the predation pressure was presumably high (during the recruitment of 0+ fish fry). Zooplankton abundance in open water and among vegetation exhibited low values in July and peaked in August. Bosmina and Ceriodaphnia dominated the zooplankton community in the littoral vegetated areas (up to 4,400 ind l^?1 among Phragmites australis and 11,000 ind l^?1 between Polygonum amphibium stands), whereas the dominant species in the pelagic were Daphnia (up to 67 ind l^?1) and Cyclops (41 ind l^?1). The zooplankton density pattern observed was probably a consequence of concomitant modifications in the predation pressure, refuge availability and concentration of cyanobacteria in the lake. It is suggested that emergent and floating-leaved macrophytes may play an important role in enhancing water clarity due to increased grazing pressure by zooplankton migrating into the plant stands. As a consequence, especially in turbid lakes, the ecological role of these functional types of vegetation, and not merely that of submerged macrophyte species, should be taken into consideration.     

六种沉水植物的克隆生长特征

为构建种群动态模型以指导沉水植被修复工程实践, 研究采用同质园实验方法对6种常见沉水植物(竹叶眼子菜(Potamogeton wrightii)、眼子菜(P. distinctus)、光叶眼子菜(P. lucens)、穿叶眼子菜(P. perfoliatus)、扭叶眼子菜(P. intortifolius)和苦草(Vallisneria natans)的克隆生长模式进行了连续观测研究, 获取了分株形成速率、空间扩张速率、株高增加速率等种群扩张动态参数,及分株数、间隔子长度、分株高度等克隆构件特征参数。结果表明, 6种沉水植物的分株数从28d开始增长, 其中苦草的分株形成速率最高, 平均为1.09株/d, 分株形成最大速率出现在55d之后; 穿叶眼子菜和扭叶眼子菜的分株形成速率低于苦草, 但是高于竹叶眼子菜、眼子菜和光叶眼子菜, 最大速率出现在41d之后。虽然苦草的分株最多, 但是分株的株高最低, 其株高增长速率均值为0.2 cm/d。眼子菜属物种中竹叶眼子菜和眼子菜株高增长速率最高, 光叶眼子菜的株高增长速率和分株形成速率都最低。克隆系占据面积随时间的扩张速率为穿叶眼子菜(113.22 cm2/d)>扭叶眼子菜(71.70 cm2/d)>苦草(35.48 cm2/d)>竹叶眼子菜(12.09 cm2/d)>眼子菜(3.07 cm2/d)>光叶眼子菜(0.53 cm2/d)。此外, 研究还发现眼子菜属植物普遍表现出匍匐茎上“节”的形成, 而苦草则不具备这种特性, 匍匐茎“节”的形成及随之形成的不定根在眼子菜属植物空间扩张过程中具有重要的生态功能, 并在种群构建方面与苦草等其他物种发生分异。基于眼子菜属植物匍匐茎上的“节”可以形成跳跃性的分株, 在种群面积扩张方面更具优势; 而苦草形成分株的数量更多、速度更快, 在提高种群密度保障种群稳定方面更有优势。     

Habitat preferences and habitat restoration options for small‐bodied and juvenile fish species in the northern Murray–Darling Basin

Degradation of instream habitats in the northern Murray–Darling Basin has occurred through numerous stressors, including siltation, clearing of bankside vegetation, intrusion of livestock and impacts of pest species. A better understanding of habitat preferences of native fish species could help guide future instream habitat restoration actions. The habitat choices of seven native fish species, juvenile Murray Cod (Maccullochella peelii), juvenile Golden Perch (Macquaria ambigua ambigua), juvenile Silver Perch (Bidyanus bidyanus), adult Murray–Darling Rainbowfish (Melanotaenia fluviatilis), adult Olive Perchlet (Ambassis agassizii), adult Un‐specked Hardyhead (Craterocephalus stercusmuscarum fulvus) and adult carp gudgeons (Hypseleotris spp.) were tested in preference troughs to help inform potential habitat restoration actions in the Condamine catchment. Each species was given a choice between pair combinations of open sandy habitat, submerged macrophytes, emergent plants and rocky rubble. Habitat preferences varied between species. Murray Cod, Golden Perch, carp gudgeons and Olive Perchlets preferred structure over open sandy habitat, whilst juvenile Silver Perch, Un‐specked Hardyhead and Murray–Darling Rainbowfish did not avoid open sandy habitats. Juvenile Murray Cod preferred rocky rubble habitat over all other habitat choices. Use of complex rock piles to provide nursery habitat for Murray Cod populations is a potential restoration option. Introduction of rock could also benefit Golden Perch and carp gudgeons. Use of emergent plants, submerged macrophytes and rocky rubble for habitat restoration all appear to have merit for one or more species of small‐bodied fishes or juvenile stages of larger sized fishes. Rocky rubble or floating attached macrophytes could be viable restoration options in areas too turbid to establish submerged macrophytes. These habitat interventions would complement existing actions such as re‐snagging and provision of fish passage to assist with sustainable management of native fish populations.     

Invasions and niche width: does niche width of an introduced crayfish differ from a native crayfish?

1. Human activities have promoted the spread of species worldwide. Several crayfish species have been introduced into new areas, posing a threat to native crayfish and other biota. Invader success may depend on the ability to utilise a wide variety of habitats and resources. Successful invaders are generally expected to have broader niches and to be more plastic than non-invasive species.
2. Using stable isotope ratios of carbon and nitrogen we compared the niche widths of native noble crayfish and introduced signal crayfish, a successful invader of Swedish streams. The calculation of niche width took account of between-site differences in basal resource isotope signature ranges. We also assessed whether population density, prey biomass or prey diversity affected niche width.
3. At the species level, signal crayfish had twice the niche width of noble crayfish. However, individual populations of noble crayfish and signal crayfish in Swedish streams had similar niche widths. This suggests that signal crayfish has greater plasticity with respect to habitat utilisation and feeding than noble crayfish. Niche width in both species correlated positively with benthic invertebrate biomass and diversity, indicating that animal food sources are important for crayfish.
4. We find that assessing niche width in relation to invader success can be a useful tool trying to predict the impact of invasions on different scales. The findings in this study suggest that invaders and natives will have a similar impact on the stream scale whereas the invader will have a larger impact on the regional scale due to the ability to utilise a wider range of streams.