Unproductive Habitats Research Articles

Reaction norms of size and age at maturity of Pomacea canaliculata (Gastropoda: Ampullariidae) under a gradient of food deprivation

Pomacea canaliculata, an apple snail native to South America, has become a serious pest of aquatic crops and a promoter of ecosystem changes in natural wetlands worldwide. Its success as an invader has been attributed to its great phenotypic plasticity in life-history traits. Our aims were to determine the reaction norms of size and age at maturity under a gradient of food deprivation. Full sibling experimental snails were reared in isolation from hatching and maintained until maturity under seven different levels of relative food deprivation based on size-specific ingestion rates. To detect the onset of sexual activity of experimental snails, fully mature virgin snails reared in the laboratory were used as consorts. The reaction norms for age and size at maturity of P. canaliculata showed marked sexual dimorphism. Shell length was the main component of variation in the male reaction norms for both copulation and egg-laying by female consorts, whereas age was the main component of variation for females. Irrespective of the intensity of food deprivation, males mature at the same age at the expense of size, since size is apparently irrelevant in the access to females and male fitness can be maximized through fast maturation. In contrast, a minimum size is required for females to reach maturity, perhaps as a result of their higher reproductive costs. The highly dimorphic reaction norms lead to an increasing lag between male and female maturity as deprivation increases; in temperate regions, males born early in the reproductive season would mature in the same season irrespective of food availability, while most females would have to overwinter before attaining sexual maturity in unproductive habitats or those dominated by unpalatable macrophytes. The great life-history plasticity reported in invaded areas could be a heritage from populations in the native range.

Spatial Patterns and Dynamic Responses of Arctic Food Webs Corroborate the Exploitation Ecosystems Hypothesis (EEH)

According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation-free islands. Corresponding transfer to herbivore-free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co-occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.

From ancient genes to modern communities: the cellular stress response and the evolution of plant strategies

Summary Two major plant strategy theories attempt to explain how phenotype determines community structure. Crucially, CSR plant strategy theory suggests that stress and sporadic resource availability favour conservative phenotypes, whereas the resource‐ratio hypothesis views the spatial heterogeneity of resources as selecting for optimal foraging in chronically unproductive habitats. Which view is most realistic? The ecophysiology literature demonstrates that stress is comprised of two processes: (1) limitation of resource supply to metabolism; and (2) damage to biomembranes, proteins and genetic material (chronic stress). Thus stress is defined mechanistically as the suboptimal performance of metabolism. Adaptations to limitation buffer metabolism against variability in external resource supply; internal storage pools are more consistent. Chronic stress elicits the same ancient cellular stress response in all cellular life: investment in stress metabolites that preserve the integrity and compartmentalization of metabolic components in concert with molecular damage‐repair mechanisms. The cellular stress response was augmented by morphological innovations during the Silurian–Devonian terrestrial radiation, during which nutrient limitation appears to have been a principal selection pressure (sensu CSR theory). The modern stress–tolerator syndrome is conservative and supports metabolism in limiting or fluctuating environmental conditions: standing resource pools with high investment/maintenance costs impose high internal diffusion resistances and limit inherent growth rate (sensu CSR theory). The resource‐ratio hypothesis cannot account for the cellular stress response or the crucial role of ombrotrophy in primary succession. CSR theory agrees with current understanding of the cellular stress response, terrestrial radiation and modern adaptations recorded in chronically unproductive habitats, and is applicable as CSR classification.

A genetic analysis of relative growth rate and underlying components in Hordeum spontaneum

Species from productive and unproductive habitats differ inherently in their relative growth rate (RGR) and a wide range of correlated quantitative traits. We investigated the genetic basis of this trait complex, and specifically assessed whether it is under the control of just one or a few genes that can act as 'master switches' by simultaneously affecting a range of traits in the complex. To address this problem, we crossed two Hordeum spontaneum lines originating from two habitats that differ in productivity. The F3 offspring, in which parental alleles are present in different combinations due to recombination and segregation, was analysed for RGR and its underlying components (leaf area ratio, unit leaf rate, photosynthesis, respiration), as well as a number of other physiological and morphological parameters. For this intra-specific comparison, we found a complex of positively and negatively correlated traits, which was quite similar to what is generally observed across species. A quantitative trait loci (QTL) analysis showed three major and one minor QTL for RGR. Most other variables of the growth-trait complex showed fewer QTLs that were typically scattered over various locations on the genome. Thus, at least in H. spontaneum, we found no evidence for regulation of the trait complex by one or two master switches.

Emergence, survival and initial growth of North American prairie forbs and British meadow forbs and grasses in low-productivity urban “waste” soils

SummaryTwenty-two species of North American and British forbs naturally associated with dry, unproductive habitats were sown as mono-cultures into pots containing mixes of brick rubble, sand and sub-soil. Three grass species native to Britain were also included as a contrast. Individual species exhibited considerable variation in emergence characteristics; both temporally, and as a percentage of seed sown. Geographical origin had no marked effect on emergence characteristics, although it appears that American species are more sensitive to moisture stress at germination. Grasses demonstrated significantly greater survival than either British or North American forbs at the group level of comparison and produced the highest biomass in the first growing season. North American forbs did not differ significantly from British forbs in terms of weight of first year growth. Overall, brick rubble proved to be the least satisfactory substrate in terms of emergence, survival and initial growth. Sand and sand:subsoil mixes supported high seedling emergence, survival and satisfactory first year growth. The results are discussed in relation to the physical and chemical properties of the four substrates, and an assessment is made of the implications of the study for future work.

Species richness, relative abundances and habitat use in local assemblages ofSorex shrews in Eurasian boreal forests

Using a large body of observational data on the occurrence ofSorex shrews in boreal forests, we test two models that predict the structure of small mammal communities along a gradient of increasing habitat productivity. Tilman’s (1982) model predicts a humped curve of species richness along productivity gradients. In contrast, we found a linear increase in species richness with increasing logarithm of the pooled density of shrews, which we use as a measure of habitat productivity for shrews. The model of Hanski and Kaikusalo (1989) assumes a trade-off between exploitative and interference competitive abilities, and it predicts that the size structure of small mammal communities should shift from the dominance of small species (superior in exploitative competition) in unproductive habitats to the dominance of large species (superior in interference competition) in productive habitats. Shrew assemblages show such a shift. Though it is not possible to draw definite conclusions about the role of interspecific competition from our observational data, the changing size structure of local shrew assemblages with increasing habitat productivity is a predictable feature of their community structure.

Effects of herbivory on competition intensity in two arctic‐alpine tundra communities with different productivity

The effects of long‐term (11 yr) exclusion of vertebrate herbivores on competition intensity and plant community structure were studied using manipulative field experiments in two arctic‐alpine plant communities with contrasting productivity: an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed, the exclusion of herbivorous mammals resulted in a significant increase in the biomasses of vascular plants and cryptogams, whereas no corresponding response was observed on the tall herb meadow. The intensity of competition, measured with a neighbour removal experiment, did not differ significantly between three of the four habitat×treatment combinations – snowbed exclosures, meadow exclosures and open meadow plots – but was significantly lower on open snowbed plots. Our results thus suggest that the low competition intensity in the unproductive snowbed is caused by herbivorous mammals, which tend to depress plant biomass in relatively unproductive habitats. When herbivorous mammals have been excluded for a sufficiently long time to allow the build‐up of plant biomass even in unproductive habitats, between‐habitat differences in competition intensity disappear.

Seedling emergence, survival and initial growth of forbs and grasses native to Britain and central/southern Europe in low productivity urban “waste” substrates

Eleven forbs and three grasses native to Britain and eight forbs native to central and southern Europe were sown as mono-cultures into pots containing mixes of brick rubble, sand and sub-soil. All of the forbs are naturally associated with dry, unproductive habitats. The purpose of the study was to inform subsequent field experiments into the creation of florally attractive meadows on waste substrates associated with urban developments. Forbs of more southerly or continental distribution were included to see whether they possessed superior tolerance of drought prone substrates. Geographical origin had less of an effect on emergence characteristics than did plant type with native grasses demonstrating significantly greater emergence (p < 0.001) than native or non-native forbs. Native grasses demonstrated significantly higher survival (p = 0.009) than either native or non-native forbs. Native grasses also produced signficantly higher biomass (p = 0.024) than native forbs in the first growing season, with non-native forbs not significantly different from native grasses or native forbs. At this group level of comparison substrate only had a significant effect at p = 0.05 on seedling dry weight. The results are discussed in relation to the physical and chemical properties of the substrates, and an assessment is made of the implications of the study for future works.

Growth rate and biomass partitioning of wildtype and low‐gibberellin tomato (Solanum lycopersicum) plants growing at a high and low nitrogen supply

The growth‐promoting effects of gibberellins (GAs) on plants are well documented, but a complete growth analysis at the whole plant level on plants with an altered GA biosynthesis has never been reported. In the present work, the relative growth rate (RGR), biomass partitioning and morphological parameters of wildtype (Wt) tomato (Solanum lycopersicum L. cv. Moneymaker) plants were compared with those of isogenic (gib) mutants with a reduced biosynthesis of gibberellins. GA deficiency reduced RGR and specific leaf area (SLA, leaf area per unit leaf mass) and increased the net assimilation rate (NAR, the rate of biomass increment per unit leaf area). Despite the free access to nitrogen in the rooting medium, the low‐GA mutants had a much higher root mass ratio (RMR, the root mass per unit plant biomass) than the Wt, suggesting that the mutants were disturbed in their growth response to nitrate supply. The experiment was repeated at a low exponential nitrate supply, which forced all plants to grow at the same low RGR. The persistence of the differences in RMR at low N‐supply indicated that the high RMR of the mutants was a direct effect of low GA, which was independent of nitrate supply. Because the low N‐supply increased the RMRs of all genotypes to the same extent, the response of RMR to N‐supply does not seem to depend on GA. Although many of the traits of the slow growing GA mutants were very similar to those of inherently slow growing plant species from unproductive habitats, gibberellins are unlikely to be a main determinant of a plant's potential RGR.

Investigation on the fundamental niche of a rare species: an experiment on establishment of Luronium natans

The rare aquatic plant Luronium natans (L.) Rafin. is a representative of the Littorelletea, i.e. plant communities growing typically in unproductive habitats with oligo- to meso-trophic, slightly acid to circumneutral soft waters. Apart from morphological and physiological adaptations to these particular habitat conditions, Littorelletea communities are characterised by their narrow ecological amplitude. Despite the fact that L. natans is reported to be limited to habitats with low calcium content, it is abundant in two of the numerous potential yet calcareous habitats in the floodplains of the Upper Rhone, and its tributary, the Ain River (France). The present study examines whether the restriction to such a low number of habitats is linked to particular abiotic conditions therein. It also aims to identify the range of floodplain habitats that belong to the species’ fundamental niche, i.e . those where, after arrival, it would be able to establish in the absence of interspecific competition. L. natans was therefore transplanted to sites along a gradient of sediment composition, and allowed to establish during a competition-free period. Extensive physico-chemical analyses were carried out on sediment, surface and interstitial water of the transplantation sites. The species’ fundamental niche appeared very large, since establishment was successful in 3/4 of the transplantation sites. The magnitude of establishment success differed greatly, but only redox-potential was significantly correlated with establishment success. Abiotic parameters could not provide a general explanation of complete establishment failure either. The experiment demonstrated that the restriction of L. natans distribution in these floodplains is not caused by a lack of habitats with suitable water and sediment chemistry.

The exploitation of heterogeneity by a clonal plant in habitats with contrasting productivity levels

Summary 1 We investigated habitat exploitation by the rhizomatous grass Elymus repens in a heterogeneous environment in both productive and unproductive habitats. We tested whether there was selective entry of rhizomes into favourable microsites or random exploration of the habitat, and whether the benefits of selective exploitation were higher in productive than unproductive habitats. 2 Trays were divided into four quadrants around a common central area. Homogeneous environments were created by planting vegetation throughout the trays; heterogeneous environments were created by planting only centre and two opposite quadrants. Contrasting productivity levels were established by fertilizing half of the trays of each environment type. A single rhizome fragment of E. repens was planted in the centre of the trays and allowed to exploit the trays for two growing seasons. 3 Elymus repens effectively exploited habitat heterogeneity at both productivity levels. However, only unproductive trays showed preferential growth of rhizomes into bare quadrants. 4 In the productive trays individual rhizomes that grew into bare quadrants showed a significantly greater production of shoots and total biomass than those that grew into vegetated quadrants. Smaller, non‐significant effects were observed at lower productivity. 5 Increased growth of primary rhizomes, once they had entered a bare quadrant, with or without selective growth into these patches, led to similar increases in heterogeneous compared to homogeneous trays at the two productivity levels (3.0 vs. 2.7). Heterogeneity was not therefore more effectively exploited at high productivity. 6 We suggest that sectorial transport of nutrients through the rhizomes, resulting in directional outgrowth of bud meristems into the bare quadrants, may have been responsible for the selective placement of rhizomes in the unproductive trays. Our results suggest that selective entry of rhizomes into favourable microsites may complement morphological plasticity in enabling rhizomatous plants to forage in heterogeneous environments.

Local survival rates of the pied flycatchers (Ficedula hypoleuca) and the great tits (Parus major) in an air pollution gradient

We studied the local survival of pied flycatcher (Ficedula hypoleuca) and great tit (Parus major) females under heavy air pollution stress. Females were trapped from three distance zones around a copper smelter in the town of Harjavalta, southwestern Finland, from 1991 to 1995. The main pollutants in this area are heavy metals (Cu, Ni, Zn, Pb) and sulphuric oxides. Local survival rate (affected by mortality + emigration) was estimated in three distance zones using capture-recapture models. Survival and recapture probabilities were separately estimated from breeding to next breeding for F. hypoleuca and by half year trapping periods for P. major. The local survival rate of F. hypoleuca females showed a decreasing, although marginally significant, trend towards the polluted area. No indication of such an effect was found in P. major. Immigrant breeders formed about 89% of the population of F. hypoleuca females in our study area. Possible reasons for the low survival rate in F. hypoleuca are discussed. We suggest that the low local survival rate of F. hypoleuca females is caused by higher emigration from the low quality and unproductive habitat rather than by the direct mortality of adult birds due to heavy metals. Better wintering conditions next to the human inhabitation may compensate for the possible detrimental effects of pollutants on the P. major population.

Lemming Grazing and Structure of a Snowbed Plant Community: A Long-Term Experiment at Kilpisjärvi, Finnish Lapland

Eight years after establishing exclosure fences on an alpine snowbed, the habitat preferred by wintering Norwegian lemming (Lemmus lemmus), the vegetation had profoundly changed. The moss-dominated vegetation had expanded in the exclosures and haircap mosses (Polytrichaceae spp.) had formed about a three times thicker bryophyte carpet than in the open plots. This was due to the vertical growth of the mosses in undisturbed conditions. Some graminoid species able to grow through the moss carpet were also more abundant in the exclosures. The low-statured snowbed species (hepatics, some prostrate vascular plants) were virtually absent in the exclosures. Bryophyte species with good colonizing ability were only found on open plots. The number of plant species in open plots (19) was higher than that within the exclosures (11). This was most likely due to increased utilization of space by dominant species in the absence of grazing. These changes took place even though lemming density was low throughout the study period, which emphasizes the importance of lemming grazing in snowbed habitats. The results conform with the predictions of exploitation ecosystems hypothesis which stresses the importance of natural grazing in unproductive habitats.

Breeding systems in flowering plants and the control of variability

Plants have three basic means of reproduction, by outcrossing, by selfing, and asexually. In most plant populations, at least two and often all three of these options are everpresent, so that individuals adopt mixed mating strategies at evolutionarily stable strategy (ESS) threshholds. Because mating systems are genetically controlled and affect genotype structure, they are liable to feedback. Productive habitats with a large standing crop are more likely to favour outcrossing, while unproductive habitats may favour asexuality or selfing, so that mating systems may change through seral development, even within the same species. Outcrossing tends to break up linkage disequilibria, but may also favour the creation of adaptive linkage groups. Mechanisms whereby male sexual selection, small population size and selfing can influence the genetic structure of populations are examined.

Photosynthetic responses to variable light: a comparison of species from contrasting habitats.

Photosynthetic responses to variable light were compared for species from habitats differing in light availability and dynamics. Plants were grown under the same controlled conditions and were analysed for the kinetics of photosynthetic induction when photon flux density (PFD) was increased from 25 to 800 μmol m-2s-1. Gas exchange techniques were used to analyse the two principal components of induction, opening of stomata and activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). On average, 90% of the final photosynthetic rate was attained after 7 min for obligate shade plants (two species), 18 min for fast-growing sun plants (seven species from productive habitats) and 32 min for slow-growing sun plants (nine species from unproductive habitats). The rapidity of response of the shade plants was explained by stomata remaining more open in the low-light period prior to induction. This was also observed in two species of deciduous trees, which therefore resembled shade plants rather than other fast-growing sun plants. The slow response of the slow-growing sun plants was the result of lower rates of both Rubisco activation and stomatal opening, the latter being more important for the final phase of induction. The lower rate of Rubisco activation was confirmed by direct, enzymatic measurements of representative plants. With increasing leaf age, the rate of stomatal opening appeared to decrease but the rate of Rubisco activation was largely conserved. Representative species were also compared with respect to the efficiency of using light-flecks relative to continuously high light. The shade plants and the slow-growing sun plants had a higher efficiency than the fast-growing sun plants. This could be related to the presence of a higher electron transport capacity relative to carboxylation capacity in the former group, which seems to be associated with their lower photosynthetic capacities. Representative species were also compared with respect to the ability to maintain the various induction components through periods of low light. Generally, the fast-growing sun plants were less able than the other two categories to maintain the rapidly reversible component. Thus, although the rate of induction appears to be related to the ecology of the plant, other aspects of photosynthetic dynamics, such as the efficiency of using lightflecks and the ability to maintain the rapidly reversible component, seem rather to be inversely related to the photosynthetic capacity.

Functional Types: A Comment on Steneck and Dethier (1994)

May I echo the plea by Steneck and Dethier (1994) for deployment of functional types in analyses of community and ecosystem structure. In particular, I support their attempt to recognise functional types by relating the spatial distribution of organisms to independent measurements of potential productivity and disturbance. This approach is an advance on methods which rely upon subjective assessments and complements the experimental approach in which communities and ecosystems are allowed to assemble under strictly controlled conditions of productivity and disturbance (Heal and Grime 1991, Campbell and Grime 1992). Despite broad agreement in method and conclusions, an interesting difference is apparent between the classification of algal functional types of Steneck and Dethier (1994: Fig. 7c) and the general model of plant and animal functional types in Grime (1989: Fig. 1). Both models propose that only half of the rectangular matrix corresponding to orthoganally opposed gradients in productivity and disturbance is tenable but there is a clear difference with respect to the functional type said to be associated with conditions of high productivity and severe disturbance. In Grime (1989), fast-growing ephemerals of high fecundity (ruderals) occupy the high productivity-high disturbance corner of the matrix whereas Steneck and Dethier (1994) allocate this position to disturbance-tolerant perennial organisms and propose that ephemeral algae occupy habitats of moderate productivity and with intermediate levels of disturbance. There are several possible explanations for these divergent interpretations: (1) Marine algae may exhibit a fourth primary strategy not represented in terrestrial vegetation. (2) C-S-R theory may have failed to recognise a fourth primary strategy (disturbance-tolerance) occurring in both aquatic and terrestrial environments. (3) Steneck and Dethier (1994) may have misinterpreted the selection forces associated with disturbance-tolerance. In order to consider these three hypotheses it is necessary first to add a dimension omitted from the discussion of Steneck and Dethier (1994). This is the integral role of disturbance-tolerance in stress-tolerance (Whittaker 1975, Grime 1977). The argument that defence against disturbance achieves maximum expression in the plants of unproductive habitats has been stated as follows:

Effects of N‐supply on the rates of photosynthesis and shoot and root respiration of inherently fast‐ and slow‐growing monocotyledonous species

Are there intrinsic differences in the rates of photosynthesis, shoot‐ and root‐respiration between inherently fast‐ and slow‐growing monocotyledons at high and low nitrogen supply? To analyze this question we grew 5 monocotyledons, widely differing in their inherent relative growth rate at high and low nitrogen supply in a growth room. Nitrate was exponentially added to the plants, enabling us to compare inherent differences in plant characteristics, without any effect of species differences in the ability to take up nutrients.At high nitrogen supply, the fast‐growing species from productive habitats had a higher photosynthetic nitrogen use efficiency and rate of root respiration than the slow‐growing ones from unproductive habitats. Only minor differences were observed in their rates of photosynthesis and shoot respiration per unit leaf area. At low nitrogen supply, the rates of photosynthesis and shoot‐ and root respiration decreased for all species, even though there were no longer any differences in these processes between inherently fast‐ and slow‐growing species. The photosynthetic nitrogen use efficiency increased for all species, and no differences were found among species.Differences in the photosynthetic nitrogen use efficiency among species and nitrogen treatments are discussed in terms of the utilization of the photosynthetic apparatus, whereas differences in respiration rate are discussed in terms of the energy demand for growth, maintenance and ion uptake and their related specific respiratory energy costs. It is concluded that the relatively high abundance of slow‐growing species compared to fast‐growing ones in unproductive habitats is unlikely to be explained by differences in rates of photosynthesis and respiration or in photosynthetic nitrogen use efficiency.

Contribution of physiological and morphological plant traits to a species' competitive ability at high and low nitrogen supply : A hypothesis for inherently fast- and slow-growing monocotyledonous species.

Why do inherently fast-growing species from productive habitats generally have a higher rate of biomass production in short-term low-nitrogen experiments than slow-growing species from unproductive habitats, whereas the opposite is found in long-term experiments? Is this mainly due to inherent differences in biomass allocation, leaf characteristics or the plants' physiology? To analyse these questions we grew five monocotyledonous species from productive and unproductive habitats in a climate chamber at both high and low nitrogen supply. Nitrate was supplied exponentially, enabling us to compare inherent differences in morphological and physiological traits between the species, without any interference due to differences in the species' ability to take up nutrients. At high nitrogen supply, we found major inherent differences in specific leaf area and nitrogen productivity, i.e. daily biomass increment per unit plant nitrogen, where-as there were only small differences in net assimilation rate, i.e. daily biomass increment per unit leaf area, and biomass partitioning. We propose that the higher specific leaf area and nitrogen productivity of inherently fast-growing species are the key factors explaining their high abundance in productive habitats compared with inherently slow-growing ones. At low nitrogen supply, the net assimilation rate was decreased to a similar extent for all species, compared with that at high nitrogen supply. The nitrogen productivity of the inherentlyfast-growing species decreased with decreasing nitrogen supply, whereas that of the inherently slow-growing species remained constant. There were no inherent differences in nitrogen productivity in this treatment. At this low nitrogen supply, the inherently fast-growing species invested relatively more biomass in their roots that the slow-growing ones did. The inherently fast-growing species still had a higher specific leaf area at low nitrogen supply, but the difference between species was less than that at high nitrogen supply. Based on the present results and our optimization model for carbon and nitrogen allocation (Van der Werf et al. 1993a), we propose that the relatively large investment in root biomass of fast-growing species is the key factor explaining their higher biomass production in short-term experiments. We also propose that in the long run the competitive ability of the slow-growing species will increase due to a lower turnover rate of biomass. It is concluded that the plant's physiology (net assimilation rate and nitrogen productivity), only plays a minor role in the species' competitive ability in low-nitrogen environments.

Importance of Habitat Productivity Differences, Competition and Predation for the Migratory Behaviour of Arctic Charr

Landlocked Arctic charr (Salvelinus alpinus) inhabiting the oligotrophic Lake Visjon (NW Sweden) grew very slowly in the 10-20 cm size interval, which suggests a competitive bottleneck. Fish in the upper part of this interval showed a propensity to leave the lake. Migration took place during spring freshets and was directed upstream. The migrants had a significantly (p < 0.05) lower condition factor than residents, indicating that they were less successful in the competition for food, and that the current feeding situation of a fish affected its migratory behaviour. Migrants entered many of the tributaries of L. Visjon, but a majority of the migrants ended up in unproductive habitats and returned downstream shortly after the ascent

Distribution of a Restricted Locally Abundant Species: Effects of Competition and Nutrients on Coreopsis lanceolata

-To determine if there is a trade-off between competitive ability and growth in unproductive conditions in Coreopsis lanceolata L. (Asteraceae), two studies were carried out. In one, a survey of natural populations quantified environmental parameters and plant traits. The results indicated that these habitats are characterized by low soil moisture and low nutrient availability; densities of C. lanceolata were negatively correlated with densities of other species. The second study was a transplant garden experiment in which three levels of nutrients and two levels of competition were manipulated. One garden was adjacent to a natural population and the second was in a more productive area where C. lanceolata did not occur. Plants grew, survived and reproduced better in the area where C. lanceolata did not occur if competitors were removed. Nutrient effects were transient and only affected growth soon after application. The trade-off between competitive ability and growth in unproductive habitats may be responsible for the distribution of C. lanceolata and other restricted locally abundant species which often are habitat specialists.