Distribution of Cottus poecilopus Heckel and C. gobio L (Pisces) in Scandinavia

Andreasson. S. (Department of Animal Ecology, Lund University, Lund. Sweden.) Distribution of Cottus poecilopus Heckel and C. gobio L. (Pisces) in Scandinavia. Zool. Scripta 1(2): 69–78, 1972. –The distribution of the freshwater sculpins Cottus poecilopus Heckel and C. gobio L. in Scandinavia is presented. In southern Scandinavia there are rather defined limits between the species with C. poecilopus in the upper parts of the water courses and C. gobio in the lower reaches. There are few cases of sympatry. In northern Scandinavia there is a broad overlap and coexistence is common in the large rivers. The distribution pattern seems to be partly a result of an interaction between the species. In the South, the sharp limits are set by falls blocking the upstream dispersal of C. gobio thus indicating an exclusion of C. poecilopus in the lower reaches below the falls by the presence of C. gobio there. Where the species meet or overlap there is a habitat segregation.     

Density‐dependent effects of non‐native brown trout Salmo trutta on the species–area relationship in stream fish assemblages

The spatial scale and density‐dependent effects of non‐native brown trout Salmo trutta on species richness of fish assemblages were examined at 48 study sites in Mamachi Stream, a tributary of Chitose River, Hokkaido, Japan. The density of age ≥1 year S. trutta was high in the upstream side of the main stem of Mamachi Stream. Fish species richness increased with increasing area of study sites (habitat size), but the increasing magnitude of the species richness with area decreased with increasing age of ≥1 year S. trutta density. The relationships between age ≥1 year S. trutta, however, and presence–absence of each species seemed to be different among species. Species richness was also determined by location and physical environmental variables, i.e. it was high on the downstream side and in structurally complex environments.     

Age and growth of Siberian sculpin (Cottus poecilopus) and young brown trout (Salmo trutta) in a subalpine Norwegian river

Age, growth and density of Siberian sculpin (Cottus poecilopus) and young brown trout (Salmo trutta) within two sections of River Atna; above Lake Atnsjøen [Section 1 at altitudes between 739 and 715 m] and below Lake Atnsjøen [Section 2 at altitudes between 430 and 370 m] was studied during a 6-year period (1986–91). The water temperature was considerably lower in Section 1 than in Section 2, as the number of days with a water temperature above 10?°C (T _{D ≥ 10?° C} ) from spring to August 1 ranged between 2–26 and 26–52 days, respectively. Juvenile brown trout (age 0+) attained a significantly smaller body size in Section 1 than in Section 2; mean length ±SD was 35 ± 8 mm (ranged 27–46) and 43 ± 7 mm (range 38–46), respectively. In Section 2, there was a highly positive correlation between the body length of 0+ brown trout and mean water temperature in June (p<0.005), and also to some extent in Section 1 (p=0.11). Individuals of age 1+ did not exhibit any such difference, while fish in age group 2+ were larger in Section 1 than in Section 2. By using the number of days with a water temperature between the range 5–10?°C (T _{D ≥ 5 ? 10?° C} ) as test variables, we found a highly positive correlation between the August 1 body length of 0+ brown trout and T _{D ≥ 9}?°C from spring to August 1 in Section 2 (p<0.05), as opposed to T _{D ≥ 7}?°C for trout in Section 1 (p=0.11). Young Siberian sculpin (age 0+ and 1+) also exhibited slower growth in Section 1 than in Section 2, but this was not the case among older specimens. In the year with the lowest temperature measured (1987), no 0+ Siberian sculpin were caught in any of the two sections, indicating that low temperature affects their survival. Both species exhibited large spatial and temporal variation in density. Thus, data on abundance and growth sampled on one occasion at one site can not be regarded as representative for these two fish populations.     

Banksia nectar and pollen: Dietary items affecting the abundance of the common blossom bat,Syconycteris australis,in southeastern Australia

Abstract The relationship between the abundance of the common blossom bat (Syconycteris australis), a nectar/pollen specialist, and the availability of its food sources was investigated across nine Banksia integrifolia heathland sites in coastal New South Wales, Australia. An index of bat abundance was more highly correlated with direct estimates of food density (productive inflorescences) than indirect estimates of food (Banksia trees and total inflorescences). Furthermore, standing crops of nectar-energy per hectare were better predictors of bat abundance than pollen-nitrogen per hectare. Adult male mass did not differ significantly across sites of differing food availability, suggesting that the corresponding increase in bat abundance with food reduces the amount of fat an individual can deposit. Although males and females responded similarly to food, adult numbers were more strongly correlated with food supply and they were more likely to be recaptured than juveniles. Energy requirements of blossom bats exceeded availability in four out of nine sites. Flying foxes were heard feeding in large numbers at the remaining five sites, where energy was superabundant for S. australis. In contrast to the conclusions of previous studies of megabats, these results provide the basis for a prediction that nectar-energy, rather than pollen-nitrogen, is the primary limiting factor for populations of the blossom bat.     

Changes in benthic macroinvertebrate abundance and lake isotope (C,N) signals following biomanipulation: an 18-year study in shallow Lake Vaeng,Denmark

Change in the abundance of benthic macroinvertebrates and the stable isotope composition (C, N) of benthic invertebrates and zooplankton in Lake Vaeng, Denmark, was investigated over an 18-year period following biomanipulation (removal of cyprinids). During the first nine years after biomanipulation, the lake was clear and submerged macrophytes were abundant; after this period, a shift occurred to low plant abundance and high turbidity. Two years after the biomanipulation, total density of benthic macroinvertebrates reached a maximum of 17042 (±2335 SE) individuals m⁻² and the density was overall higher when the lake was in a clear state. Redundancy analysis (RDA) suggested macrophyte abundance and total nitrogen (TN) concentration were the dominant structuring forces on the benthic macroinvertebrate assemblage. Stable isotope analysis revealed that δ¹³C of macroinvertebrates and zooplankton was markedly higher in years with high submerged macrophyte abundance than in years without macrophytes, most likely reflecting elevated δ¹³C of phytoplankton and periphyton mediated by a macrophyte-induced lowering of lake water CO₂ concentrations. We conclude that the strong relationship between macrophyte coverage and δ¹³C of macroinvertebrates and cladocerans may be useful in paleoecological studies of past changes in the dynamics of shallow lakes, as change in macrophyte abundance may be tracked by the δ¹³C of invertebrate remains in the sediment.     

Effects of macrophyte heterogeneity and food availability on structural parameters of the macroinvertebrate community in a Pampean stream

Environmental heterogeneity in natural ecosystems influences several parameters at the population and community levels. In freshwater ecosystems, habitat heterogeneity can be provided by macrophyte species with different structural shapes. Previous studies suggest that aquatic plants with more complex architectures will support higher number, biomass, and taxon richness of macroinvertebrates than plants with simpler shape. We investigated the influence of macrophyte structural heterogeneity (quantified by fractal dimension) and food availability (represented by epiphytic biomass) on several parameters (number of individuals, biomass, body size distribution, taxon richness, and diversity) of the macroinvertebrate community in a Pampean stream. Four submerged macrophyte species (Egeria densa, Elodea ernstae, Ceratophyllum demersum, and Stuckenia striata) and associated macroinvertebrates were sampled in late spring, summer, and autumn. Plants were photographed and fractal dimension was estimated from the images by the box-counting method. Fractal dimension was independent of plant surface area per unit of macrophyte biomass and differed significantly among species. Mean fractal dimension varied between 1.29 and 1.62, and increased following the sequence E. densa → S. striata → E. ernstae → C. demersum. Macrophyte species with higher fractal dimension supported a greater abundance of macroinvertebrates, especially those of small body size (500–1,000 μm); but fractal dimension was unrelated to macroinvertebrate biomass, richness, and diversity. However, overall animal biomass was significantly associated to the epiphytic abundance. Consequently, macrophyte heterogeneity influences macroinvertebrate density and body size distribution, while animal biomass depends on epiphytic food resources provided by plants.     

Rainforest litter invertebrates decimated by high severity burns during Australia's gigafires

Climate change is increasing the frequency of extreme fires. In 2019–2020, extreme fires burned 97 000 km² of native vegetation in south-eastern Australia, affecting many areas of rainforest, which has historically burned less frequently. One year post-fires, we surveyed litter macroinvertebrates in 52 temperate rainforest sites. Sites had experienced increasing levels of fire severity (unburnt, medium severity and high severity). We asked how fire severity affected: (1) litter macroinvertebrate habitats; (2) the abundance of litter macroinvertebrate taxa per unit area; and (3) abundance relative to litter habitat (volumetric density). We also estimated the loss of litter macroinvertebrates across rainforests in the study region. High severity burns supported only a fifth of the litter volume and canopy cover as unburnt sites, lower soil moisture and higher herb cover. Medium burns were intermediate. Macroinvertebrate abundance declined with burn severity: high severity burns supported only 26% of the abundance in unburnt sites; medium severity burns supported 80% of that in unburnt sites. Patterns were similar for all taxa, with millipedes declining most. High severity fires resulted in up to 1.90 million fewer macroinvertebrates per hectare; 0.53 million fewer per hectare of medium burn rainforest. Across the study region, we estimate that 60 billion fewer litter macroinvertebrates persisted in temperate rainforests alone. Volumetric densities of many litter macroinvertebrate taxa in high severity burns were marginally higher than in unburnt sites, suggesting nutrients may be more available post-fire, or that persisting individuals become concentrated in the leaf litter. For less desiccation-tolerant groups (e.g., amphipods), density declines with increasing severity may reflect the combined impact of low soil moisture and reduced litter cover. Many taxa persisted following high severity fires, but declines were substantial, and taxa differed in their vulnerability. Longer-term monitoring is required to understand the recovery trajectory and impacts on ecological function.     

Drought, differential mortality and the coexistence of a native and an introduced fish species in a south east Australian intermittent stream

Synopsis The longitudinal distribution ofSalmo trutta andGalaxias olidus, a small salmoniform fish, was mapped over four summers (1985–1988) in the upper reaches of the Lerderderg River, an intermittent stream in central Victoria. Over the four successive summers of the study, the distribution ofS. trutta expanded upstream. Coincident with the expansion ofS. trutta was a contraction in the distribution ofG. olidus upstream. In the summer of 1988, an extended dry spell resulted in high levels ofS. trutta mortality at the upstream limits of their distribution. This suggested that the upstream distribution ofS. trutta could ultimately be constrained by an inability to tolerate prolonged periods of low stream flow and high temperatures that tended to occur in the headwaters of the stream. A study of summer mortality was conducted over three successive summers (1989–1991) at three reaches along the river. The two downstream reaches contained onlyS. trutta, the upstream reach contained bothS. trutta andG. olidus. Salmo trutta mortality was consistently higher at the most upstream reach compared with lower reaches. High levels ofS. trutta mortality at the two downstream reaches were only observed in 1991, a particularly dry year. In contrast,G. olidus survived in scattered small pools throughout the upper reaches of the stream. The results of the study suggest a shifting pattern ofS. trutta andG. olidus distribution determined by the duration of low stream flow periods and summer high temperatures.     

Identification of natural hybrids between Cottus poecilopus,Heckel, 1837, and Cottus gobio,Linnaeus, 1758, at a hybrid zone on the Svratka River (Czech Republic)

Sculpins occasionally form interspecific hybrids in areas of mutual contact. These can be verified genetically and, potentially, morphologically. At the study site (Svratka River, Czech Republic), Cottus gobio are common whereas C. poecilopus represent a geographically isolated alien population. Potential hybrids between these two species were compared with the parent populations to establish simple identification criteria. Analysis of nuclear and mitochondrial DNA confirmed the existence of a new European hybridisation zone. Hybrids were the result of unidirectional crossing, with female C. gobio taking the maternal position. With the exception of the lateral line position, morphometric characteristics and the epidermis structure were similar in many traits, reflecting the contribution of both species, as well as in the number of sacciform cells where hybrids were similar to C. poecilopus. The vertical position of the lateral line allows recognition of hybrids directly in the field, a useful feature for future studies. The occurrence of these hybrids confirms expansion of non‐native C. poecilopus at the expense of native C. gobio in rivers under anthropogenic pressure.     

How does plant population density affect the biomass of Ravenna grass?

Ravenna grass, Tripidium ravennae (L.) H. Scholz, is known to produce an abundance of biomass, but how plant density affects its biomass potential remains unknown. The objectives were to determine the effects of plant density on biomass yield; plant growth traits; biomass?carbon, nitrogen, and ash concentrations; heating value; nitrogen removal; and sucrose concentration in leaves and culms. The treatments consisted of five plant densities (1,250; 2,500; 5,000; 10,000; and 20,000 plants per hectare) in a randomized complete block design with four blocks. Plots were nonirrigated, unfertilized, and harvested once during the dormant season each year. Data were collected from 2015?2019. Dependent variables that varied with plant population density (p < .05) were biomass yield, number of reproductive culms per plant, reproductive culm diameter, reproductive culm sucrose concentration, and nitrogen removal with biomass. Biomass yield ranged from 5.6 to 16.3 Mg/ha for plant densities of 1,250–20,000 plants per hectare, respectively. Combined over years, nonlinear regression of the data showed the equation for biomass yield to plateau at 16.2 Mg/ha at a plant density of 10,640 plants per hectare. As plant density increased, the number of reproductive culms per plant, culm diameter, and culm sucrose concentration significantly decreased. At 1,250 plants per hectare, the number of reproductive culms per plant, culm diameter, and culm sucrose averaged 70, 10.2 mm, and 63.2 g/kg, respectively. Nitrogen removed with biomass significantly increased as biomass yield increased with plant density. At a density of 10,000 and 20,000 plants per hectare, the amount of nitrogen removed annually in the harvested biomass averaged 88 kg/ha. The data suggest that 10,000 plants per hectare would produce the greatest annual biomass yields; however, research is needed to determine the nutrient requirement for Ravenna grass to sustain biomass production at that density.     

The abundance,distribution and structural characteristics of tree‐holes in Nothofagus forest,New Zealand

Tree‐holes provide an important microhabitat that is used for feeding, roosting and breeding by numerous species around the world. Yet despite their ecological importance for many of New Zealand's endangered species, few studies have investigated the abundance or distribution of tree‐holes in native forests. We used complementary ground and climbed tree surveys to determine the abundance, distribution and characteristics of tree‐holes in undisturbed Nothofagus forest in the Lewis Pass, New Zealand. We found that hole‐bearing trees were surprisingly abundant compared with many other studies, including Australian Eucalyptus species and American beech. In fact, we estimated as many as 3906 tree‐holes per hectare, of which 963 holes per hectare were potentially large enough to provide roost sites for hole‐nesting bats in New Zealand, while only eight holes per hectare were potentially suitable for specialist hole‐nesting birds. This was of great interest as primary cavity‐excavating animals are absent from New Zealand forests, compared with North America and Australia. Moreover, tree‐hole formation in New Zealand is likely to be dominated by abiotic processes, such as branch breakage from windstorms and snow damage. As has been found in many other studies, tree‐holes were not uniformly distributed throughout the forest. Tree‐holes were significantly more abundant on the least abundant tree species, Nothofagus fusca, than on either N. menziesii or N. solandri. In addition to tree species, tree size was also an important factor influencing the structural characteristics of tree‐holes and their abundance in this forest. Moreover, these trends were not fully evident without climbed tree surveys. Our results revealed that ground‐based surveys consistently underestimated the number of tree‐holes present on Nothofagus trees, and illustrate the importance of using climbed inspections where possible in tree‐hole surveys. We compare our results with other studies overseas and discuss how these are linked to the biotic and abiotic processes involved in tree‐hole formation. We consider the potential implications of our findings for New Zealand's hole‐dwelling fauna and how stand dynamics and past and future forest management practices will influence the structural characteristics of tree‐holes and their abundance in remnant forest throughout New Zealand.     

Mitigation of acidified salmon rivers – effects of liming on young brown trout Salmo trutta

In southern Norway, 22 acidified rivers supporting anadromous salmonids were mitigated with lime to improve water quality and restore fish populations. In 13 of these rivers, effects on Salmo trutta and Salmo salar densities were monitored over 10–12 years, grouped into age 0 and age ≥ 1 year fish. These rivers had a mean annual discharge of between 4·9 and 85·5 m³ s^?1, and six of them were regulated for hydro‐power production. Salmo salar were lost in six of these rivers prior to liming, and highly reduced in the remaining seven rivers. Post‐liming, S. salar became re‐established in all six rivers with lost populations, and recovered in the seven other rivers. Salmo trutta occurred in all 13 study rivers prior to liming. Despite the improved water quality, both age 0 and age ≥ 1 year S. trutta densities decreased as S. salar density increased, with an average reduction of >50% after 10 years of liming. For age 0 year S. trutta this effect was less strong in rivers where S. salar were present prior to liming. In contrast, densities of S. trutta increased in unlimed streams above the anadromous stretches in two of the rivers following improved water quality due to natural recovery. Density increases of both age 0 and age ≥ 1 year S. salar showed a positive effect of river discharge. The results suggest that the decline in S. trutta density after liming is related to interspecific resource competition due to the recovery of S. salar. Thus, improved water quality through liming may not only sustain susceptible species, but can have a negative effect on species that are more tolerant prior to the treatment, such as S. trutta.     

Abundance and distribution of a brown trout (Salmo trutta, L.) population in a remote high mountain lake

Brown trout (Salmo trutta) population of a remote high mountain lake (Lake Redó, Pyrenees, 2240 m above sea level) were studied during autumn using hydroacoustic techniques. This acoustic technique was for the first time used on fish at such high altitude in Spain. Sampling using multimesh nets fish catches and echosounding recording were carried out in September 1998. Mean density of fish was estimated to be 1.82 fish per 1000 m³ (597 fish ha^–1). The results exhibited mainly a littoral habitat, with the brown trout being preferentially in the 10–25 m deep layer, where the water was warmer and the richness and diversity of macroinvertebrates higher. The sampling by hydroacoustic technique found a length frequency range of fish higher than the multimesh gill nets but both of them estimated similar fish proportion for the common length range. The more frequent target strength (TS) for the population was –38 dB (TS range –37 to –39 dB). Good correlation was found between TS frequency distribution obtained by echosounding and that predicted by means of a model based on the log of the fish total length from multimesh gill nets captures.     

Occupancy and density of a habitat specialist and a sympatric generalist songbird species in Tasmania

Patterns of distribution and abundance of species are dependent on their particular ecological requirements. Taking specialisation into account is important for interpreting population parameters. Here, we evaluate population parameters of an endangered habitat specialist, the forty‐spotted pardalote (Pardalotus quadragintus; dependent on white gum Eucalyptus viminalis in south‐eastern Tasmania), and a sympatric congeneric habitat generalist, the striated pardalote (Pardalotus striatus). We used occupancy models to estimate occupancy of both species, and distance sampling models to estimate population density and size on North Bruny Island. Within their shared habitat (i.e. white gum forest), we also fitted hierarchical distance sampling models to estimate density in relation to fine‐scale habitat features. We show that forty‐spotted pardalotes only occurred in forests where white gums were present, with a mean density of 2.7 birds per hectare. The density of forty‐spotted pardalotes decreased in areas with abundant small trees and trees with dead crowns, but they increased in areas where larger white gums were abundant. The striated pardalote was widespread, but where white gums were present, they occurred at 2.1 birds per hectare, compared to 0.6 birds per hectare in forests where white gums were absent. Within white gum habitat, the relative abundance of forty‐spotted pardalotes and dead trees had a positive effect on the density of striated pardalotes while small trees had a negative effect. Our study reveals that although widespread, the generalist is most abundant in the limited areas of habitat suitable for the specialist, and this indicates the need of future research to look at whether this pattern of occurrence exacerbates competition in resource depleted habitats.     

Competition and Facilitation in the Capuchin–Squirrel Monkey Relationship

In Guyana, the range of the brown capuchin, Cebus apella, meets the range of its congener, the wedge‐capped capuchin, C. olivaceus, with the two species exhibiting a mutually exclusive patchy distribution. Squirrel monkeys, Saimiri sciureus, and C. apella form ubiquitous interspecific associations, but the reason remains debatable. With a large biogeographic field study, we tested the degree to which the distribution and abundance of Cebus and S. sciureus is determined by habitat type, fruit availability, and geography (i.e., determinants of the fundamental niche) relative to interspecific interactions, such as competition and facilitation (i.e., the realized niche). We used the competition between the two capuchin monkeys as a natural experiment that subjected S. sciureus to C. apella and C. olivaceus ‘treatments’. Using spatial regression models and principal components analysis, we found that S. sciureus was associated with seasonally flooded forests, and was correlated with fruit abundance and diversity (fundamental niche), but was also correlated with C. apella density even when accounting for habitat and fruit availability (realized niche). Saimiri sciureus density was unrelated to C. olivaceus density. Cebus apella was associated with a variety of forest types, but particularly included disturbed and edge habitats such as logged forests, seasonally flooded forests, and upland savanna, in addition to mature forest. Cebus apella was also positively correlated with S. sciureus density and negatively correlated with the density of C. olivaceus. In contrast, C. olivaceus avoided riparian areas and was associated with mixed‐height forests on sloped mountainous terrain. In sum, interspecific interactions such as competition between species of Cebus and facilitation between C. apella and S. sciureus were as important as habitat and fruit availability in determining the distribution and abundance of these primates.     

Impacts of acidification on brown trout Salmo trutta populations and the contribution of stocking to population recovery and genetic diversity

Anthropogenic acidification in SW-Scotland, from the early 19th Century onwards, led to the extinction of several loch (lake) brown trout (Salmo trutta) populations and substantial reductions in numbers in many others. Higher altitude populations with no stocking influence, which are isolated above natural and artificial barriers and subjected to the greatest effect of acidification, exhibited the least intrapopulation genetic diversity (34% of the allelic richness of the populations accessible to anadromous S. trutta). These, however, were characterised by the greatest interpopulation divergence (highest pairwise D_EST 0.61 and F_ST 0.53 in contemporary samples) based on 16 microsatellite loci and are among the most differentiated S. trutta populations in NW-Europe. Five lochs above impassable waterfalls, where S. trutta were thought to be extinct, are documented as having been stocked in the late 1980s or 1990s. All five lochs now support self-sustaining S. trutta populations; three as a direct result of restoration stocking and two adjoining lochs largely arising from a small remnant wild population in one, but with some stocking input. The genetically unique Loch Grannoch S. trutta, which has been shown to have a heritable increased tolerance to acid conditions, was successfully used as a donor stock to restore populations in two acidic lochs. Loch Fleet S. trutta, which were re-established from four separate donor sources in the late 1980s, showed differential contribution from these ancestors and a higher genetic diversity than all 17 natural loch populations examined in the area. Genetically distinct inlet and outlet spawning S. trutta populations were found in this loch. Three genetically distinct sympatric populations of S. trutta were identified in Loch Grannoch, most likely representing recruitment from the three main spawning rivers. A distinct genetic signature of Loch Leven S. trutta, the progenitor of many Scottish farm strains, facilitated detection of stocking with these strains. One artificially created loch was shown to have a population genetically very similar to Loch Leven S. trutta. In spite of recorded historical supplemental stocking with Loch Leven derived farm strains, much of the indigenous S. trutta genetic diversity in the area remains intact, aside from the effects of acidification induced bottlenecks. Overall genetic diversity and extant populations have been increased by allochthonous stocking.     

Evidence for effects of Spartina anglica invasion on benthic macrofauna in Little Swanport estuary,Tasmania

Spartina anglica is an exotic perennial grass that can rapidly colonise the intertidal zone of temperate estuaries and lagoons. Consequently, there is considerable concern about its impact on estuarine flora and fauna. This study provides the first investigation of ecological impacts by S. anglica in Australia. The objective was to investigate the impacts of S. anglica on benthic macroinvertebrate communities inhabiting mudflat and native saltmarsh habitats at Little Swanport estuary, Tasmania. The null hypothesis that species richness and species abundance of benthic macroinvertebrates in exotic S. anglica marsh does not differ from adjacent native saltmarsh and mudflat habitats was tested. Eighteen species and 3716 macroinvertebrates were collected from 60 intertidal core samples in three habitats. Species richness, total abundance of invertebrates, crustacean abundance and mollusc abundance of mudflat communities were significantly (P < 0.05) lower when compared to those inhabiting adjacent S. anglica marsh and native saltmarsh. However, species richness and total abundance of invertebrates of native saltmarsh and S. anglica marsh did not differ significantly. Ordination of macroinvertebrate data clearly separated mudflat sites from vegetated sites but showed remarkable similarity between exotic and native vegetated sites.     

Root and stem partitioning of Pinus taeda

We measured root and stem mass at three sites (Piedmont (P), Coastal Plain (C), and Sandhills (S)) in the southeastern United States. Stand density, soil texture and drainage, genetic makeup and environmental conditions varied with site while differences in tree size at each site were induced with fertilizer additions. Across sites, root mass was about one half of stem mass when estimated on a per hectare basis. Stem mass per hectare explained 91% of the variation in root mass per hectare, while mean tree diameter at breast height (D), site, and site by measurement year were significant variables explaining an additional 6% of the variation in root mass per hectare. At the S site, the root:stem ratio decreased from 0.7 to 0.5 when mean tree D increased from 10 to 22 cm. At the P and C sites, where mean root:stem ratios were 0.40 and 0.47, respectively, no significant slope in the root:stem to mean tree D relationship was found over a more narrow range in mean tree D (12–15 and 12–18 cm, respectively). Roots were observed in the deepest layers measured (190, 190, and 290 cm for the P, C, and S sites, respectively); however, the asymptotically decreasing root mass per layer indicated the bulk of roots were measured. Root growth relative to stem growth would need to change with increased mean tree D to explain the results observed here. While these changes in growth rate among plant components may differ across sites, stem mass alone does a good job of estimating root mass across sites.     

Dietary ontogeny and niche shift to piscivory in lacustrine brown trout Salmo trutta revealed by stomach content and stable isotope analyses

The feeding ecology and ontogeny of a large size range of brown trout Salmo trutta in Lake Fyresvatnet, southern Norway, were examined by stomach content and stable isotope analyses. According to the stomach contents, the S. trutta changed their diet at c. 30 cm total length (L_T). The smaller size classes fed on benthic invertebrates and surface insects, whereas larger S. trutta (>30 cm) fed mainly on whitefish Coregonus lavaretus. A similar, but more gradual shift to piscivory in the size range 25–30 cm was found when using the stable isotope mixing model SIAR to reveal dietary ontogeny. The δ¹³C isotopic signature confirmed that S. trutta independent of size predominantly relied upon benthic energy sources, suggesting that the littoral zone was the primary foraging habitat for both invertebrate and piscivorous feeders. The δ¹⁵N values and trophic position increased with predator length, ranging from an average of 3·60 for small‐sized S. trutta (<15 cm) to 4·15 for large‐sized fish (>35 cm). The S. trutta exhibited a relatively slow growth rate during the predominant invertebrate feeding stages up to 7 years of age and 28 cm L_T, whereas fish above this size and age displayed a rapid growth rate of 9–11 cm year^?1, demonstrating the profitability of piscivorous feeding.