An experimental study of the role of gradient and substratum in the distribution of two stream-dwelling triclads, Crenobia alpina (Dana) and Polycelis felina (Dalyell) in North Wales

A laboratory stream is described which has simulated steep- and shallow-gradient bottoms whose gradients can be varied. It was found that Polycelis felina, the principal triclad of shallow-gradient streams had a much reduced ability to colonize the simulated steep-gradient bottoms when compared to Crenobia alpina, which is the principal triclad of the steep-gradient bottoms in the field. P. felina is unable to move in the steep-gradient bottoms to any great extent, while C. alpina showed considerable ability to do this. When a reduction in flow was made this did not increase the proportion of P. felina to be found in the steep-gradient regions. The triclads were also tested for possible acclimatory responses and it was found that their responses were more fundamentally based. On the basis of the above evidence it is suggested that the absence of P. felina from the steep-gradient regions of streams in North Wales is due to a behavioural inability to tolerate the current regimes present in the steep-gradient bottoms.     

Predators and planariid competitors of the triclad Phagocata vitta (Dugés)

When Phagocata vitta, Crenobia alpina and Polycelis felina were exposed separately to each of seventeen potential invertebrate predators in the laboratory, only two stonefly species, Dinocras cephalotes and Perlodes microcephala, fed on the three triclad species, whilst the trichopteran Rhyacophila dorsalis ate the last two triclads. On exposing pairs of triclad species to D. cephalotes, significantly more P. felina than Ph. vitta were consumed, whereas similar numbers were eaten in each of the other two triclad combinations. Cannibalism and interspecific predation by triclad species were not observed. It is concluded that predation is unlikely to have a major influence in determining the observed distribution and abundance of triclad species in a Welsh study stream which harbours low numbers of effective predators.The de Wit model of competition was used to examine the competitive relationships between Ph. vitta, and C. alpina and P. felina, using chironomids or tubificid worms as food. In mixed cultures of Ph. vitta and P. felina fed on tubificids a stable equilibrium existed within the range of relative densities used in the experiments, whereas Ph. vitta was competitively superior to C. alpina in cultures fed on each of the food types, and to P. felina fed on chironomids. However, in theory, an equilibrium could occur when 10 or 6–7 times as many Ph. vitta as P. felina and C. alpina respectively are in the culture, when intraspecific rather than interspecific competition would become more important. Where the three triclad species coexist in the Welsh study stream, they are in similar numbers. This could imply that food is not limiting, with no consequent interspecific competition, or that the laboratory experiments were too simplistic to allow any interpretation of the field situation.     

Distribution, ecology, mode of reproduction and karyology of freshwater planarians (Platyhelminthes; Turbellaria; Tricladida) in the springs of the central Pyrenees

The distribution, reproductive modality and karyology of populations of freshwater planarians were studied together with physico-chemical variables m 149 springs in the central Pyrenees Diversity was limited to four species (Dugesia (Dugesia) subtentaculata, Polycelis felina. Crenobia alpina and Phagocata vitta), already known from streams and lakes Planarians were found rarely (only a third of springs contain them), m low-moderate abundance, and any single spring did not harbour more than one species The distribution of potential predators and preys among the springs sampled was fairly uniform So. we assume that whereas predation is unlikely to influence triclad distribution, the oligotrophic character of these habitats may explain, through food shortage and ensuing interspecific competition, the presence of a single species in each spring Besides interspecific competition, the distribution of each species can also be influenced by particular abiotic parameters Temperature and current velocity appeared to be the dominant factors, the first separating C alpina from the rest of species, and the second favouring the presence of P felina in a large set of springs These factors also seem to determine planarian distribution in streams and rivers Other environmental factors such as calcium bicarbonated water (for D subtentaculata) and preference for karst systems (for Phagocata vitta) are of importance Some peculiarities of distribution, like the scarcity of C alpina in reophile habitats and the thermal tolerance of Phagocata vitta, may be due both to competition from P felina and to historical factors in the former and to morphological (presence of pigment) and ecological (epigean habits) features m the latter, respectively Karyological analyses demonstrated triploidy in all populations of D subtentaculata. a polyploid origin for C alpina and the finding in Phagocata vitta of a new polyploid series leading to the highest chromosome number in turbellarians (14n = 238) Furthermore, the basic chromosome number of C alpina (n = 21) and Phagocata vitta (n = 17) differs from those found in northern European populations (n = 7 in both, respectively) This suggests that under the specific names C alpina and Phagocata vitta more than one species actually occurs, this being a problem for further studies Whereas polyploidy has often been related to living in cold, harsh habitats, asexual reproduction has been linked to environments where biotic stress is low The preeminence of asexual reproduction (by fissiparity) and polyploid forms in freshwater planarians inhabiting the unproductive, biotically unsaturated springs of the central Pyrenees adds new evidence to strengthen such links     

Parvalbumin-immunoreactive proteins in the nervous system of planarians

Summary 1. Using immunological methods, we have identified parvalbumin-like material in the triclade flatworms,Polycelis nigra, Polycelis auriculata, Crenobia alpina, Dugesia tahitiensis, andDugesia polychroa.2. Western immunoblot analysis of these five species revealed heat stable parvalbumin-immunoreactive proteins between 32 kD and 44 kD.3. Proteins at 19 and 32 kD revealed intense labeling with⁴⁵Ca²⁺.4. Double immunodiffusion of planarian supernatants showed complete fusion of precipitates, indicating immunological relatedness of the parvalbuminlike material among the species investigated.5. Immunocytochemical studies exhibit parvalbumin-immunoreactive material exclusively in neurons supporting the notion of an early evolutionary appearance of these proteins in the nervous system.     

Chromosome number and ploidy-level in a Dutch population ofCrenobia alpina Dana (Planaria)

A Dutch population ofCrenobia alpina was found to have a chromosome number of 2n=42. One pair of large chromosomes is acrocentric and probably shows a secondary constriction; one small pair is acrocentric as well. All other chromosomes are metacentric or sub-metacentric; it has not been possible to discriminate with certainty between them. According to the hypothesis ofDahm (1958) populations with, 2n=42 are autohexaploid, the basic genome consisting of seven chromosomes. On basis of the karyotype it was concluded that the Dutch population described here, is not autohexaploid but functionally diploid, perhaps allohexaploid.     

The effects of electrofishing on the invertebrates of a Lake District stream

Summary The experiments were performed in Dale Park Beck, a stony stream in the English Lake District. Two operators electrofished the sampling area (length 20 m in April and July 1970, and 40 m in May 1971) three times (runs 1, 2, 3) in each experiment.Electrofishing caused a marked increase in the number of invertebrates drifting out of the sampling area, and nearly all taxa taken in the bottom samples were also found in the drift samples. The fish shocker was chiefly responsible for the increased drifting of Plecoptera, Ephemeroptera and Gammarus pulex, and these taxa were dislodged from the substratum more easily than Trichoptera, Coleoptera, Diptera and Polycelis felina. The increased drifting of the latter taxa was chiefly due to the disturbance of the substratum by the two operators.Most of the invertebrates drifting from the upstream end of the experimental section returned to the bottom within the sampling area. The invertebrate drift out of the sampling area came chiefly from the downstream end of the section, and was equivalent to a loss of only 5% from the total benthos in the sampling area (losses varied between <1 and 13% for individual taxa).     

Chromosome numbers in three species groups of freshwater flatworms increase with increasing latitude

Polyploidy in combination with parthenogenesis offers advantages for plasticity and the evolution of a broad ecological tolerance of species. Therefore, a positive correlation between the level of ploidy and increasing latitude as a surrogate for environmental harshness has been suggested. Such a positive correlation is well documented for plants, but examples for animals are still rare. Species of flatworms (Platyhelminthes) are widely distributed, show a remarkably wide range of chromosome numbers, and offer therefore good model systems to study the geographical distribution of chromosome numbers. We analyzed published data on counts of chromosome numbers and geographical information of three flatworm “species” (Phagocata vitta, Polycelis felina and Crenobia alpina) sampled across Europe (220 populations). We used the mean chromosome number across individuals of a population as a proxy for the level of ploidy within populations, and we tested for relationships of this variable with latitude, mode of reproduction (sexual, asexual or both) and environmental variables (annual mean temperature, mean diurnal temperature range, mean precipitation and net primary production). The mean chromosome numbers of all three species increased with latitude and decreased with mean annual temperature. For two species, chromosome number also decreased with mean precipitation and net primary production. Furthermore, high chromosome numbers within species were accompanied with a loss of sexual reproduction. The variation of chromosome numbers within individuals of two of the three species increased with latitude. Our results support the hypothesis that polyploid lineages are able to cope with harsh climatic conditions at high latitudes. Furthermore, we propose that asexual reproduction in populations with high levels of polyploidization stabilizes hybridization events. Chromosomal irregularities within individuals tend to become more frequent at the extreme environments of high latitudes, presumably because of mitotic errors and downsizing of the genome.     

Studies on the speciation of the European freshwater planarians Polycelis nigra and Polycelis tenuis based on the analysis of enzyme variation by means of iso-electric focusing

Polycelis nigra Ehrenberg and Polycelis tenuis Ijima differ morphologically and karyologically. No difference, however, was found in the isozyme pattern of malate dehydrogenase and tetrazolium oxidase, indicating a close relationship. Most sibling species differ at half of the loci.It could be deduced that the reproductive behaviour of a single population of Polycelis nigra in a Dutch pond was not panmictic. Two genetically different strains retained their identities during two years of observation. If pseudogamy occurs in this diploid planarian, the presence of heterozygous specimens indicates the absence of a true meiosis.The iso-electrofocusing technique by which these population-genetical studies were carried out, also lends itself to a comparison of overall protein banding patterns. The membrane proteins especially are conservative. The sodium dodecyl sulphate extracted proteins of Polycelis nigra-tenuis, Planaria torva and Phagocata vitta were very similar, while their water soluble proteins were not. This technique may be of great help in taxonomic studies of the higher taxa.     

Spatial and ontogenetic variability in the microhabitat use of stream-dwelling spined loach (Cobitis taenia) and stone loach (Barbatula barbatula)

Ontogenetic and spatial variability in microhabitat use of spined loach Cobitis taenia (Linnaeus), considered as one species for the purposes of this study, and stone loach Barbatula barbatula (Linnaeus) were examined in the River Great Ouse basin, England, using multivariate and habitat suitability methods, including a technique for handling spatial variation in collections of preference curves. Distinct ordinations of spined age classes and stone loach developmental stages, respectively, in canonical correspondence analysis of species × variables × samples relationships suggest that the two species occupy completely different microhabitats; however, young‐of‐the‐year spined loach occurred more often than expected with all developmental stages of stone loach except young larvae. Water velocity and filamentous algae were the most influential microhabitat variables, the latter decreasing in importance with increasing age of both fish species. Preferred water velocities generally decreased with age in spined loach and increased in stone loach, with substratum size generally increasing with fish age in both species. Spatial variation in microhabitat preferences was great in both species but less so in the spined loach, suggesting that limited plasticity in habitat use could account, at least in part, for the latter species’ limited distribution and abundance in the catchment. Preference curves for a species, if generated and verified for all life intervals and all seasons, could be used as a management tool for a given stream or sector of river basin. But preference curves should be generated for each location to ensure that river management decisions with regard habitat and species conservation consider local‐level species requirements. Thus, a multi‐(eco)species and multi‐scale approach is required in habitat suitability assessments.     

Gene flow between populations of two invertebrates in springs

1. Using allozymes, we analysed genetic structure of the freshwater gastropod Bythinella dunkeri and the freshwater flatworm Crenobia alpina. The two species are habitat specialists, living almost exclusively in springs. The sampled area in Hesse (Germany) covers a spatial scale of 20 km and includes two river drainages. From the biology of the two species we expected little dispersal along rivers. However, the possibility exists that groundwater provide suitable pathways for dispersal. 2. In B. dunkeri heterozygosity decreased from west to east. For some alleles we found clines in this geographic direction. These clines generated a positive correlation between geographic distance and genetic differentiation. Furthermore patterns of genetic variation within populations suggested that populations may have been faced with bottlenecks and founder effects. If populations are not in population genetic equilibrium, such founder effects would also explain the rather high amount of genetic differentiation between populations (10%). 3. For C. alpina the mean number of alleles decreased with increasing isolation of populations. Genetic differentiation between populations contributed 19% to the total genetic variation. Genetic differentiation was not correlated to geographic distance, but compared with B. dunkeri variability of pairwise differentiation between pairs of populations was higher in C. alpina. 4. Overall B. dunkeri appears to be a fairly good disperser, which may use groundwater as dispersal pathway. Furthermore populations seem to be not in equilibrium. In contrast C. alpina forms rather isolated populations with little dispersal between springs and groundwater seems to play no important role for dispersal.     

Reproductive ecology of Dendrocoelum lacteum (Turbellaria) in a rapid stream in southern Sweden and comparisons with a lake population

Rapid streams in southern Sweden are often inhabited by the triclad Dendrocoelum lacteum. Outside Fennoscandia, this species is mainly restricted to lentic habitats. The normal food refuge for D. lacteum, the isopod Asellus aquaticus, is seldom found in the stream habitats, and probably the prey here is the amphipod Gammarus pulex. With respect to spatial and trophic niche components, it seems that D. lacteum has taken the place of Crenobia alpina in southern Sweden.Two parameters of reproduction, namely production of cocoons and of hatchlings, both peaked approximately one month later in a stream than in an adjacent lake. This difference was attributed to a low temperature regime in the stream. Number of hatchlings per cocoon and cocoon sterility were higher in the stream than in the lake. Similar cocoon and hatchling output per adult was found in the two habitats, indicating a similar reproductive effort for the two triclad populations. I suggest that D. lacteum, by virtue of its choice of microhabitat, viz., under stones, is not so vulnerable to the more severe stream environment.     

Geographical distribution of Polycelis (Seidlia) auriculata in Japan

Polycelis (Seidlia) auriculata is endemic to mountain districts of Japan, from the central part of Honshû to the area of the Daisetsu Mts of Hokkaidô. In northern Japan, it sometimes occurs in cold-water biotopes of lowland areas. The progenitor of P. auriculata appears to have been the oldest immigrant into northern Japan among the Japanese Polycelis species, entering through a northern route as a preglacial faunal element. P. auriculata now shows a discontinious distribution in northern Japan. By virtue of its geographical and vertical distribution, ecological niche, variation in anatomy of the copulatory apparatus, and cytodemes, this species appears to be in the process of transformation.     

Measuring thermal behavior in smaller insects: A case study in Drosophila melanogaster demonstrates effects of sex,geographic origin,and rearing temperature on adult behavior

Measuring thermal behavior in smaller insects is particularly challenging. In this study, we describe a new horizontal thermal gradient apparatus designed to study adult thermal behavior in small insects and apply it using D. melanogaster as a model and case study. Specifically, we used this apparatus and associated methodology to examine the effects of sex, geographic origin, and developmental rearing temperature on temperature preferences exhibited by adults in a controlled laboratory environment. The thermal gradient established by the apparatus was stable over diurnal and calendar time. Furthermore, the distribution of adult flies across thermal habitats within the apparatus remained stable following the period of acclimation, as evidenced by the high degree of repeatability across both biological and technical replicates. Our data demonstrate significant and predictable variation in temperature preference for all 3 assayed variables. Behaviorally, females were more sensitive than males to higher temperatures. Flies originating from high latitude, temperate populations exhibited a greater preference for cooler temperatures; conversely, flies originating from low latitude, tropical habitats demonstrated a relative preference for higher temperatures. Similarly, larval rearing temperature was positively associated with adult thermal behavior: low culture temperatures increased the relative adult preference for cooler temperatures, and this response was distinct between the sexes and for flies from the temperate and subtropical geographic regions. Together, these results demonstrate that the temperature chamber apparatus elicits robust, predictable, and quantifiable thermal preference behavior that could readily be applied to other taxa to examine the role of temperature-mediated behavior in a variety of contexts.     

Activity rhythms and cyclical changes of microhabitat preferences in the intertidal pipefish Nerophis lumbriciformis (Pisces: Syngnathidae)

As an inhabitant of the intertidal zone, Nerophis lumbriciformis predictably should show a clear rhythmic activity, with its maximum closely correlated with the high tide period, as observed in several other rocky intertidal fish species. We investigated the rhythmic activity patterns of N. lumbriciformis and analysed to what extent specific substratum preferences may be linked to water level and, ultimately, to the particular activity rhythm patterns of the fish. To determine N. lumbriciformis substratum preferences, two different experiments were conducted. The first evaluated substratum preferences from among a group of the four most common substrates found in the sample area. The second experiment evaluated the fish's maintenance of that substratum preference, after alteration of the water level in a simulated ebb tide. From the available substrata, N. lumbriciformis displayed a clear preference for algae until the water level began to decrease. Then, the preference radically shifted from algae to boulders. N. lumbriciformis showed a clear rhythmic activity, affected by both tidal and circadian periodicity, producing asymmetrical activity peaks. Unlike the rhythms of other rocky intertidal fish species, the maximum activity peaks were not centred at the expected high tide period. The adaptive explanation for the apparently peculiar activity peaks appears to be related to the particular substratum preferences of N. lumbriciformis. The detected circatidal period seems, consequently, closely related to small migrations between substrata, whilst the observed increase in diurnal activity, independent of the tidal cycle, may be related to predatory activity, since N. lumbriciformis is a visual feeder. Electronic Publication     

A long‐term record of Nothofagus dominance in the southern Andes,Chile

Abstract The general model of regeneration dynamics in Nothofagus forests of southern South America could have value in community ecology if predictive relationships between disturbance history, functional traits and site attributes could be identified. Examined here is the proposal that on favourable sites shade‐intolerant Nothofagus are likely not to survive in competition with shade‐tolerant, broad‐leaved evergreen taxa of temperate rain forests, and persistence, thus, is dependent on periodic coarse‐scale disturbance. Comparison of stand dynamics of three old‐growth Nothofagus forests at different elevations in the southern Andes, Chile where deciduous Nothofagus alpina dominates the upper canopy, and examination of the life history trade‐offs of this variation were made. Stem density of all stems ≥5.0 cm d.b.h. was 233–303 stems per hectare, and basal area was 123.9–171.0 m²ha^?1. Maximum lifespan of N. alpina was found to be greater than ca 640 years, exceeding all previously reported ages for this species in the region. Forests had a stable canopy composition for this long‐term, but some appeared to lack effective regeneration of N. alpina in recent years. Regeneration of N. alpina was generally greater in disturbed stands and higher elevation than in undisturbed stands and at lower elevation. Recruitment emerged to be strongly affected by competitive over‐ and understorey associates. There was a gradient of increasing dependence of N. alpina on disturbance towards the more productive end of the environment gradients, and hence less dependence of N. alpina on disturbance for its regeneration towards higher elevation. The study confirms that changes in forest composition may be explained by processes occurring in accordance with the predictions of the existing model of Nothofagus regeneration dynamics, providing stronger evidence specifically directed at mid‐tolerant N. alpina, and by factoring out regeneration dynamics on favourable sites. Thus, for N. alpina, trait differences probably contribute to the competitive advantage over its associates in productive habitats, and may be linked to small‐to‐intermediate‐sized disturbances which inevitably occur as older trees die, enabling N. alpina to persist in forests and therefore maintain species coexistence for the long‐term.     

Interference competition among native and invader amphipods

Aquarium experiments were used to study indications of interference competition, such as substratum choice shifts, swimming activities and mortality of invasive and indigenous gammarids in each other's presence. The more recent invaders Gammarus tigrinus and Dikerogammarus villosus were more likely to prefer stone substratum, whereas the native Gammarus pulex and an earlier invader Gammarus roeseli were found more frequently in the water layer. Sand was the least likely substratum to be chosen by any of the species. G. pulex and G. roeseli did not alter their substratum preference in each other's presence. In the presence of D. villosus, G. pulex shifted towards smaller stones and increased its swimming activities, whereas D. villosus did not change its behaviour in the presence of G. pulex. These shifts may indicate interference competition, with D. villosus being the stronger competitor. The greatest shifts in substratum preference arose when one species had occupied a substratum before the other one was introduced, especially when D. villosus was already present before G. pulex was introduced, possibly indicating pre-emptive competition. Swimming activities of G. pulex increased in the presence of D. villosus, whereas D. villosus spent little time swimming. Mortality was comparable between the different experiments without any indication of predation. The effect of Intra Guild Predation (IGP) may not be reflected adequately by short-time experiments as moults occurred seldom during the experiments. Although no IGP was observed during our experiments, habitat shifts occurred, which may indicate that competitive interactions are apparent before IGP starts. Such shifts may serve to avoid intraguild competition.     

Competition in Bembicium auratum (Gastropoda) and its effect on microalgal standing stock in mangrove muds

Summary Field experiments were undertaken on the intertidal gastropod Bembicium auratum, which is one of the dominant organisms in sheltered bays and mangroves in New South Wales. Animals were caged at various densities ranging from natural levels to about four times normal density. Increased density resulted in increased mortality (particularly of juveniles), reduced body weight, and (less conclusively) a decline in growth rate. Chlorophyll levels of the mangrove mud within the cages were monitored as an index of food availability, and increased rapidly in the control cages in the absence of Bembicium, remained steady at normal Bembicium densities, and declined at higher densities. Parallel experiments were conducted on the effects of substratum on Bembicium. One series of animals was allowed access to a hard substratum and the other only to mud. Body weights and survivorship were proportionally lower in animals deprived of a hard substratum. Juveniles were particularly susceptible to a shortage of food and more dependent on a hard substratum than adults. The population structure also suggests a high juvenile mortality but high adult survival and longevity. Bembicium is very abundant low on the shore but numbers decline and body weights increase higher up. Chlorophyll levels and amounts of hard substratum also decline up the shore. Possibly juveniles settle selectively (or survive better) low on the shore where oysters provide a dense hard substratum. Adults are less dependent on the hard substratum and may disperse up the shore, explaining the patterns of abundance and size.As Bembicium limits its own food supply and seemingly is limited by it, predation is unlikely to play an important role in restricting population density.     

Distribution of native legumes (Leguminoseae) in frequently burned longleaf pine (Pinaceae)-wiregrass (Poaceae) ecosystems

Legume species distribution and abundance and selected environmental variables were quantified across a complex gradient (varying in both water-holding capacity and fertility) for frequently burned longleaf pine (Pinus palustris)–wiregrass (Aristida stricta) ecosystems. Legumes were present in all months; however, abundance peaked in June and was minimal after killing frosts in October. Legume species were prominent in the flora (43 species encountered) ubiquitous (94% of 2-m² subplots had at least one legume species), and abundant (nearly 120 000 stems/ha). Although most species were widely distributed throughout the gradient, Lespedeza angustifolia was distinctly associated with the more hydric end of the gradient, while both Petalostemon pinnatum and Galactia microphylla were located in the more xeric extreme. The percentage variation in species that could be accounted for by environmental variation was low (27%). Of the variation that could be accounted for, a number of environmental variables were important, including soil moisture, pine basal area (i.e., light), and bivalent base cations (e.g., Ca²⁺). Although gradients in resource availability among sites did not affect the distribution of species or abundance of legumes strongly, variation in resources are likely to regulate N₂-fixation rates of the various native legume species, and thereby affect ecological functions such as maintenance of N capital and productivity.     

Variation in Microbial Community Composition and Culturability in the Rhizosphere of Leucanthemopsis alpina (L.) Heywood and Adjacent Bare Soil Along an Alpine Chronosequence

We compared the size, culturability, diversity, and dominant species similarity of the bacterial communities of Leucanthemopsis alpina (L.) Heywood rhizosphere and adjacent bare soil (interspace) along a chronosequence of soil development time (5, 50, and 70 years) in the forefield of the Dammaglacier (Switzerland). We found no evidence that the size of the bacterial community was significantly affected by either soil age or the presence of L. alpina. In contrast, the proportion of the bacterial community that could be cultured on nonselective agars, and which was taken as an indication of the proportion of r-selected populations, was significantly higher in the 50- and 70-year-old soils than in the 5-year-old soil, and was also significantly higher in the rhizosphere of L. alpina at all time points. RDA indicated significant correlations between the increased culturability of the bacterial community over time and increasing concentrations of labile N, and between the increased culturability in the rhizosphere and increased concentrations of labile C and N. HaeIII-amplified ribosomal DNA (rDNA) restriction analysis of a library of 120 clones of 16S rDNA revealed 85 distinct phylotypes. Hurlbert's probability of interspecific encounter (PIE) values derived from this library ranged from 0.95 to 1.0, indicating a very high genetic diversity. There was no significant difference in the PIE values of rhizosphere and interspace communities. Detrended correspondence analysis (DCA) of 16S ribosomal RNA (rRNA) denaturing gradient gel electrophoresis (DGGE) community profiles clearly distinguished the rhizosphere from the interspace community in the 5-year-old soils and also clearly distinguished between these communities and the rhizosphere and interspace communities of the 50- and 70-year-old soils. However, 16S rRNA DGGE revealed little difference between rhizosphere and interspace communities in the 50- and 70-year-old soils. The relative similarity of the 16S rRNA profiles strongly reflected labile carbon and nitrogen availability. Overall, our results suggest that improved C and N availability in the rhizosphere of L. alpina increases the size of r-selected bacterial species populations, but that the influence of L. alpina depends on soil age, being maximal in the youngest soils and minimal in the oldest. The reduced influence of L. alpina in the older soils may reflect a feedback between improved nutrient availability and reduced rhizodeposition.