The late replication banding patterns of chromosomes are highly conserved in the genera Rana,Hyla, and Bufo (Amphibia: Anura)

Late replication banding and C-banding analyses were performed on the metaphase chromosomes of six species and one subspecies of Palearctic water frogs, genus Rana. Although C-banding patterns showed interspecific or intersubspecific variation, late replication banding patterns of all 13 chromosome pairs of these species were homologous. Minor differences of banding patterns were observed only in chromosomes 2, 7 and 13. Close comparison of the late replication banding patterns with those of three non-water frog species of Rana, and one each of Hyla and Bufo, provided important information on interspecific and intergeneric variability. In the Rana species, the banding patterns of all 13 pairs were homologous except for those some regions of 8 pairs. In one species each of Hyla and Bufo that was examined, the six large chromosome pairs (Nos. 1-6) showed banding homologies. Furthermore, among the Rana, Hyla and Bufo species the four large chromosome pairs (Nos. 1-3, 5 of Rana and Hyla, and Nos. 1, 3–5 of Bufo) shared banding homologies. These results show that the large chromosomes have been highly conserved in the evolutionary history of the three genera.     

Synergistic effects of multiple mechanisms drive priority effects within a tadpole assemblage

Mounting evidence suggests that the history of species arrival to a locality can have important effects on species performance but the mechanism(s) through which priority effects are produced is not always clear. Differences in the developmental time of frog tadpoles provide an opportunity to examine mechanisms through which priority effects may influence fitness components of a late arriving taxon. Specifically, tadpoles of the southern leopard frog (Rana) can often require more than a year to complete metamorphosis so they overwinter in a pond and compete with newly deposited tadpoles in the spring. We conducted an experiment in artificial ponds to evaluate mechanisms through which overwintering Rana tadpoles influence fitness components of southern toad (Bufo) tadpoles deposited into ponds during the spring. We found that overwintered Rana reduced Bufo performance while newly hatched Rana that enter a pond simultaneously with Bufo did not. The production of this priority effect was primarily the result of Rana depleting algal resources in a pond during the winter prior to Bufo arrival. The performance of Bufo did not correspond with variation in the abundance of algae present in a pond during the spring and we found evidence to indicate that, in the absence of resource exploitation during the winter, overwintered Rana do not compete strongly with Bufo during the spring when both species co‐occur. When Rana deplete algal resources during the winter, however, interactions between Rana and Bufo during the spring became much more important as both the development rate and survival of Bufo was reduced to a greater extent than what would have been predicted by resource exploitation alone. Our results demonstrate that priority effects can result from early colonists depleting resource availability prior to the arrival of other species which can intensify behavioral/physical interactions between species when they co‐occur.     

Competitive interactions are mediated in a sex‐specific manner by arbuscular mycorrhiza in Antennaria dioica

• Plants usually interact with other plants, and the outcome of such interaction ranges from facilitation to competition depending on the identity of the plants, including their sexual expression. Arbuscular mycorrhizal (AM) fungi have been shown to modify competitive interactions in plants. However, few studies have evaluated how AM fungi influence plant intraspecific and interspecific interactions in dioecious species.
• The competitive abilities of female and male plants of Antennaria dioica were examined in a greenhouse experiment. Females and males were grown in the following competitive settings: (i) without competition, (ii) with intrasexual competition, (iii) with intersexual competition, and (iv) with interspecific competition by Hieracium pilosella – a plant with similar characteristics to A. dioica. Half of the pots were grown with Claroideoglomus claroideum, an AM fungus isolated from the same habitat as the plant material. We evaluated plant survival, growth, flowering phenology, and production of AM fungal structures.
• Plant survival was unaffected by competition or AM fungi. Competition and the presence of AM fungi reduced plant biomass. However, the sexes responded differently to the interaction between fungal and competition treatments. Both intra‐ and interspecific competition results were sex‐specific, and in general, female performance was reduced by AM colonization. Plant competition or sex did not affect the intraradical structures, extraradical hyphae, or spore production of the AM fungus.
• These findings suggest that plant sexual differences affect fundamental processes such as competitive ability and symbiotic relationships with AM fungi.

     

The roles of resource availability and competition in mediating growth rates of invasive and native freshwater snails

相似文献   

Friends of mine: An invasive freshwater mussel facilitates growth of invasive macrophytes and mediates their competitive interactions

1. Increasing rates of invasions in ecosystems worldwide necessitate experiments to determine the role of biotic interactions in the success and impact of multiple alien species. Here, we examined competitive and facilitative interactions among various combinations of three widespread and often co-occurring invaders: the zebra mussel Dreissena polymorpha, and the macrophytes Elodea canadensis and Elodea nuttallii.
2. Using a mesocosm-based, factorial experimental design, we assessed the effect of interspecific competition on macrophyte growth rates in the absence and presence at varying biomass of D. polymorpha.
3. Growth rates (wet g/day) of E. canadensis and E. nuttallii were similar when grown in isolation. When grown together, in the absence of D. polymorpha, E. canadensis growth was not significantly reduced in the presence of E. nuttallii and vice versa. In the presence of D. polymorpha (26.0 ± 1 mm), monocultural growth of E. canadensis was largely unaffected, while E. nuttallii growth was strongly enhanced. Low (2.64 g) and medium (3.96 g) mussel biomass led to negative interspecific effects between E. canadensis and E. nuttallii; at high (5.28 g) mussel biomass, the effect of interspecific competition was negated.
4. Overall, D. polymorpha alleviated competitive interactions between the two invasive macrophytes when all three species co-occurred, and substantially enhanced growth of E. nuttallii with increasing mussel biomass, thereby suggesting a possible influence on the relative dominance of these macrophytes in the field.
5. Our study demonstrates how facilitations can cause shifts in dominance among closely related invaders. The consequences of such facilitations for the structure and function of communities remain to be explored generally.

     

Better desiccated than eaten by fish: Distribution of anurans among habitats with different risks to offspring

1. Freshwater species with complex life cycles face a trade-off between the risks of offspring mortality due to desiccation in temporary habitats and due to predators common in long-duration habitats, especially fish. In real-world conditions, intermediate-gradient areas that are highly suitable for some ecological specialists are often limited. I examined the relative significance of drivers of the permanence–predation gradient in habitat selection by pond-breeding anurans.
2. Anuran oviposition was investigated at the community level in fish pond landscapes with only three types of habitat, permanent fish-containing ponds and highly desiccation-prone pools, either fishless or recently emptied of fish and with fish odour (fish cue). Altogether, 65 ponds/pools, interspersed in four clusters, were examined for the presence of egg masses.
3. Bufo bufo was the only species preferring permanent ponds. Egg masses of Rana sp. (Rana arvalis/Rana temporaria) occurred in all fishless pools, and less frequently in the ponds. Bufotes viridis and long larval period species Pelobates fuscus and Hyla orientalis oviposited almost exclusively in fishless pools. Fish-cue pools were avoided by all species. Anuran richness was higher in fishless pools than in permanent ponds. Species distribution between the two habitats was nested, with both common and rare species occurring in fishless pools.
4. The results indicate the primacy of offspring predation risk over hydroperiod constraints in oviposition decision-making by fish-intolerant anurans. The absence of some species from ponds and pools with fish or fish cues shows that non-consumptive interactions may better explain the scarcity of anuran larvae in waters dominated by fish than actual consumption. The strict avoidance of fish habitats despite their proximity to fishless patches indicates fine-scale assessment of predation risk. However, rigid habitat selection against fish predation on offspring may prove maladaptive if habitats with high desiccation risk are the main alternative and are indiscriminately preferred. Maintenance of fishless wetlands with a hydroperiod sufficient to allow completion of metamorphosis should be a conservation priority for anuran diversity in areas where fishless habitats are limited.

     

Complex competitive interactions in four species of Lucilia blowflies

Abstract.

• 1 The effects of intraspecific and interspecific larval competition on larval survival, adult size, adult longevity and fecundity were quantified in four species of coexisting Lucilia blowflies: illustris, silvarum, sericata and caesar.
• 2 There was a general negative effect of increasing density on larval and adult survival, size and fecundity. Additionally, complex species-specific and frequency-dependent responses were identified, which were not expected in these biologically and morphologically closely similar species.
• 3 Lucilia illustris, the numerically dominant species in the natural community, was a superior competitor to L.silvarum at intermediate densities but an inferior competitor at high density. Such nonlinear responses may be related to differences in the life histories and larval behaviour of the species (bigger eggs and more contest-type outcome of competition in L.silvarum).
• 4 We parameterized a model of interspecific competition on a subdivided resource in an attempt to reconcile the conflicting results on larval competitive abilities and the abundances of the species in the field. Using laboratory and field-estimated parameter values the model predicted coexistence of L.illustris and L.silvarum and the observed numerical dominance of the former species. The average densities of flies in the field are limited to relatively low levels, apparently preventing L.silvarum (the superior competitor at high density) from dominating and excluding L. illustris.

     

A comparative study of melatonin production in the retina,pineal gland and Harderian gland of Bufo viridis and Rana esculenta

1. The circadian patterns of melatonin and of its synthesizing enzyme N-acetyltransferase (NAT) were investigated in the serum, retina, pineal gland and Harderian gland (HG) of two amphibian species, Bufo viridis and Rana esculenta.2. Serum melatonin levels showed no diurnal fluctuations in Bufo viridis, whereas, in Rana esculenta, they exhibited a circadian rhythm, with the highest values occurring during the night. Retina melatonin exhibited characteristic circadian patterns in both species, with the highest values occurring during the day, in Bufo, and the highest concentrations occurring at night in Rana.3. In the retina, NAT activity peaked at night in both amphibians, but in Bufo the levels were up to 30 times higher than in Rana. In the HG and in the pineal gland, NAT activity showed different patterns in the two species with no diurnal variations in Bufo, and characteristic circadian rhythms in Rana.4. In the HG and pineal gland of both species, melatonin was only occasionally detectable over the 24-hr period.5. This is the first report exploring melatonin production in Bufo viridis and Rana esculenta. In our experimental conditions, marked differences emerged between the two species.     

Effect of arbuscular mycorrhiza on inter- and intraspecific competition of two grassland species

We were interested in the role of arbuscular mycorrhiza (AM) in the competition between plants of different sizes. A pot experiment of factorial design was established, in which AM root colonization and competition were used as treatments. Five-week-old Prunella vulgaris seedlings were chosen as target plants (i.e. plants whose response to competition was studied) and the following (13 replicates of each) were used as neighbours: (1) a large, 10-week-old P. vulgaris, (2) two P. vulgaris seedlings, and (3) a large, 10-week-old Fragaria vesca. In the experiment where small neighbours were grown together with small target plants, competition did not reduce target plant weight significantly, compared to the other two treatments. The competitive effects of large neighbours were significant, regardless of species (both older neighbours reduced the weights of target plants similarly), but there was a clear difference between intra- and interspecific competition when plants were mycorrhizal. In intraspecific competition with a large neighbour, the target plant shoot weight was reduced 24% when inoculated with AM. Thus, AM amplified rather than balanced intraspecific competition. In interspecific competition with old F. vesca, the shoot weights of target plants were 22% greater when inoculated with AM than when non-mycorrhizal. The results showed that, for given soil condition, AM might increase species diversity by increasing competitive intraspecific suppression and decreasing the interspecific suppression of small plants by larger neighbours.     

Experimental removals reveal dietary niche partitioning facilitates coexistence between native and introduced species

相似文献   

Niche partitioning and the effect of interspecific competition on microhabitat use by two sympatric galaxiid stream fishes

1. Numerous interacting abiotic and biotic factors influence niche use and assemblage structure of freshwater fishes, but the strength of each factor changes with spatial scale. Few studies have examined the role of interspecific competition in structuring stream fish assemblages across spatial scales. We used field and laboratory approaches to examine microhabitat partitioning and the effect of interspecific competition on microhabitat use in two sympatric stream fishes (Galaxias‘southern’ and Galaxias gollumoides) at large (among streams and among sites within streams) and small (within artificial stream channels) spatial scales. 2. Diurnal microhabitat partitioning and interspecific competition at large spatial scales were analysed among three sympatry streams (streams with allotopic and syntopic sites; three separate catchments) and four allopatry streams (streams with only allotopic sites; two separate catchments). Electro‐fishing was used to sample habitat use of fishes at 30 random points within each site by quantifying four variables for each individual: water velocity, depth, distance to nearest cover and substratum size. Habitat availability was then quantified for each site by measuring those variables at each of 50 random points. Diet and stable isotope partitioning was analysed from syntopic sites only. Diel cycles of microhabitat use and interspecific competition at small spatial scales were examined by monitoring water velocity use over 48 h in artificial stream channels for three treatments: (i) allopatric G. ‘southern’ (10 G. ‘southern’); (ii) allopatric G. gollumoides (10 G. gollumoides) and (iii) sympatry (five individuals of each species). 3. One hundred and ninety‐four G. ‘southern’ and 239 G. gollumoides were sampled across all seven streams, and habitat availability between the two species was similar among all sites. Galaxias‘southern’ utilised faster water velocities than G. gollumoides in both the field and in channel experiments. Both species utilised faster water velocities in channels at night than during the day. Diet differences were observed and were supported by isotopic differences (two of three sites). No interspecific differences were observed for the other three microhabitat variables in the field, and multivariate habitat selection did not differ between species. Interspecific competition had no effect on microhabitat use of either species against any variable either in the field (large scale) or in channels (small scale). 4. The results suggest that niche partitioning occurs along a subset of microhabitat variables (water velocity use and diet). Interspecific competition does not appear to be a major biotic factor controlling microhabitat use by these sympatric taxa at any spatial scale. The results further suggest that stream fish assemblages are not primarily structured by biotic factors, reinforcing other studies de‐emphasising interspecific competition.     

Competitive interactions limiting the number of species in rock pools: experiments with Sigara nigrolineata

Rock-pool corixids (Hemiptera) Arctocorisa carinata and Callicorixa producta were excluded from small rock pools. First-stage larvae of Sigara nigrolineata, separately or together with larvae of either of the rock-pool species, were then introduced daily at a rate simulating reproduction by typical populations. In all three treatments, the survival of S. nigrolineata larvae was high at the beginning of experiment, 0.39–0.83 between the 1st and 5th stages, but later high mortality was observed. C. producta adults tended to invade experimental rock pools and towards the end of the experiment larvae of this species were dominant. Oviposition rates and developmental rates of the three species are comparable. In laboratory tests, young larvae of S. nigrolineata were more vulnerable to interspecific predation than those of the rock-pool species, and older larvae proved to be less efficient predators. In rock pools, there are no environmental factors preventing successful reproduction by S. nigrolineata. However, competition and predation by the two other rock-pool species seem to prevent successful reproduction. Dispersal in S. nigrolineata is too slow for efficient colonization of rapidly changing rock-pool environments.     

When to cut your losses: Dispersal allocation in an asexual filamentous fungus in response to competition

相似文献   

Invasion windows for a global legume invader are revealed after joint examination of abiotic and biotic filters

• Successful alien plant invasion is influenced by both climate change and plant–plant interactions. We estimate the single and interactive effects of competition and extreme weather events on the performance of the global legume invader Lupinus polyphyllus (Lindl.).
• In three experimental studies we assessed (i) the stress tolerance of seedling and adult L. polyphyllus plants against extreme weather events (drought, fluctuating precipitation, late frost), (ii) the competitive effects of L. polyphyllus on native grassland species and vice versa, and (iii) the interactive effects of extreme weather events and competition on the performance of L. polyphyllus.
• Drought reduced growth and led to early senescence of L. polyphyllus but did not reduce adult survival. Fluctuating precipitation events and late frost reduced the length of inflorescences. Under control conditions, interspecific competition reduced photosynthetic activity and growth of L. polyphyllus. When subjected to competition during drought, L. polyphyllus conserved water while simultaneously maintaining high assimilation rates, demonstrating increased water use efficiency. Meanwhile, native species had reduced performance under drought.
• In summary, the invader gained an advantage under drought conditions through a smaller reduction in performance relative to its native competitors but was competitively inferior under control conditions. This provides evidence for a possible invasion window for this species. While regions of high elevation or latitude with regular severe late frost events might remain inaccessible for L. polyphyllus, further spread across Europe seems probable as the predicted increase in drought events may favour this non‐native legume over native species.

     

Composition of the excitatory drive during swimming in two amphibian embryos: Rana and Bufo

It has recently been shown that spinal neurons in Xenopus embryos receive cholinergic and electrotonic excitation during swimming, in addition to the well documented excitatory amino acid (EAA)-mediated excitation. We have now examined the composition of the excitatory drive during swimming in embryos of two further amphibian species, Rana and Bufo, which have somewhat different motor patterns. Localised applications of antagonists show that presumed motoneurons in Rana and Bufo embryos receive both cholinergic and EAA input during swimming. There is also a further chemical component which is blocked by Cd²⁺ and a small Cd²⁺-insensitive component, which is usually non-rhythmic. Rhythmic Cd²⁺-insensitive components, presumed to be phasic electrotonic potentials, were only seen in a small proportion of Bufo neurons and in no Rana neurons. While EAA and cholinergic inputs therefore appear to be consistent features of excitatory drive for swimming in amphibian embryo motoneurons, electrotonic input apparently occurs less commonly. Antagonist specificity was tested using applied agonists in Rana. Results of these tests also suggested that the further, unidentified Cd²⁺-sensitive component seen during swimming could represent an incomplete block of AMPA receptor-mediated excitation.Abbreviations AMPA -Amino-3-hydroxy-5-methyl- 4-isoxazolepropionic acid - CNQX 6-Cyano-7- nitroquinoxaline-2,3-dione - D-AP5 D(-)-2- Amino-5-phosphonopentanoic acid - DHE Dihydro--erythroidin - DMPP 1,1-dimethyl-4- phenylpiperazinium - HEPES N-[2-hydroxyethyl] piperazine-N-[2-ethanesulphonic acid] - NMDA N-methyl-D-aspartic acid     

Competition and habitat suitability: small-scale segregation underpins large-scale coexistence of key species on temperate rocky shores

Identifying the mechanisms that underpin species richness is one of the central issues of community ecology. On rocky shores in north-western Europe, two key limpet species coexist on the mid- and low shore but are segregated at small scales with respect to habitat. Short-term and long-term experiments were done to test whether habitat suitability or habitat-specific competition drives the small-scale segregation of these species and therefore underpins their coexistence at larger spatial scales. In a controlled short-term experiment, Patella vulgata was transplanted onto open rock and into pools that either contained Patella ulyssiponensis and/or their mucus or from which P. ulyssiponensis and/or their mucus had been removed. After 2 days, P. vulgata remained in all experimental plots in similar numbers irrespective of treatment indicating that there was no negative response to P. ulyssiponensis, mucus or the pool habitat. In a long-term experiment, cage enclosures containing both species were set up in pools and on open rock over a 6-month period. P. vulgata grew equally well on both open rock and in pools but suffered higher mortality in pools. P. ulyssiponensis showed lower growth rate and higher mortality on open rock than in pools. P. ulyssiponensis exhibited increased growth in higher intraspecific densities on open rock and reduced growth in higher intraspecific densities in pools, indicating some degree of intraspecific facilitation on open rock and intraspecific competition in pools. There was no evidence of interspecific competition either in the short term or in the long term. Results revealed that habitat suitability was the mechanism causing segregation of these species at smaller spatial scales enabling them to coexist at larger spatial scales. Conflicting results in the short-term and long-term experiments highlight the importance of considering the correct temporal extent for experimental tests of hypotheses.     

Effects of nitrogen availability on competition between Bromus tectorum and Bouteloua gracilis

Exotic plant invasions are a serious concern for land managers and conservationists. There is evidence that increased nitrogen availability favors exotic species and decreased nitrogen availability favors non-weedy native species. This study was conducted to test the effect of nitrogen availability on competition between two grass species with contrasting life histories, cheatgrass (Bromus tectorum), a North American exotic, and blue grama (Bouteloua gracilis), a North American native. We investigated the effects of nitrogen availability and competition on aboveground biomass, belowground biomass, height, and % nitrogen tissue concentrations by growing the two species in the greenhouse under five levels of nitrogen and six levels of competition. Nitrogen availability affected competition between Bromus tectorum and Bouteloua gracilis. At the lowest level of N availability, neither species was affected by competition. As N availability increased, aboveground biomass gain of Bromus was more negatively affected by intraspecific competition relative to interspecific competition while the opposite occurred for Bouteloua. At the competition level at which each species gained the most aboveground biomass, Bromus had a linear response to increasing N availability while the response of Bouteloua was asymptotic. Our results do provide some support for the theory that fast growing exotic species have a rapid response to nutrient enrichment while native non-weedy species do not, and that low N levels can reduce competitive pressure from the exotic on the native.     

Established native perennial grasses out-compete an invasive annual grass regardless of soil water and nutrient availability

Competition and resource availability influence invasions into native perennial grasslands by non-native annual grasses such as Bromus tectorum. In two greenhouse experiments we examined the influence of competition, water availability, and elevated nitrogen (N) and phosphorus (P) availability on growth and reproduction of the invasive annual grass B. tectorum and two native perennial grasses (Elymus elymoides, Pascopyrum smithii). Bromus tectorum aboveground biomass and seed production were significantly reduced when grown with one or more established native perennial grasses. Conversely, average seed weight and germination were significantly lower in the B. tectorum monoculture than in competition native perennial grasses. Intraspecific competition reduced per-plant production of both established native grasses, whereas interspecific competition from B. tectorum increased production. Established native perennial grasses were highly competitive against B. tectorum, regardless of water, N, or P availability. Bromus tectorum reproductive potential (viable seed production) was not significantly influenced by any experimental manipulation, except for competition with P. smithii. In all cases, B. tectorum per-plant production of viable seeds exceeded parental replacement. Our results show that established plants of Elymus elymoides and Pascopyrum smithii compete successfully against B. tectorum over a wide range of soil resource availability.     

Seasonal variation in the strength of interference competition among headwater stream predators

1. Vertebrate communities in headwater streams are assumed to be regulated through competitive and predatory interactions. Although documented predation is rare, studies regularly report competitive dominance by fish that, as larger competitors reliant on aquatic habitat, exclude semi-aquatic salamanders to marginal stream habitat. However, it is unclear whether fish interact with stream-breeding salamanders through indirect effects such as competition for resources (e.g. food or cover) or fear (i.e. threat of predation) nor is it known whether these interactions are consistent through time.
2. This study used a novel caging approach to determine if competitive outcomes between a headwater fish and salamanders were regulated primarily through resource depletion (exploitative competition) or behavioural avoidance (interference competition).
3. We paired banded sculpin (Cottus carolinae) and larval red salamanders (Pseudotriton ruber) of similar body size in independent flow through mesocosms with intra- and inter-specific pairs allowed to interact physically or non-physically. The experiment was repeated in the autumn and in the spring when stream salamander larvae begin to transform into terrestrial juveniles.
4. Banded sculpin negatively influenced growth of red salamanders regardless of whether they were allowed to physically interact, suggesting interference competition and behavioural avoidance. This asymmetrical effect was strongest in the spring when salamanders underwent metamorphosis at higher rates in the presence of fish. However, in the autumn, the effects were more balanced between the two species with salamanders impacting fish through exploitative competition.
5. By studying the temporal relationships between two competitors and using a caging method novel to competition studies, we established that the outcomes of competition are dependent on season and may vary in type relative to the timing of life-history events. For this community, these results suggest that outcomes of competition are highly dependent on season and could indicate a biotic mechanism maintaining headwater salamander distributions through source–sink dynamics. Our results also suggest that, in this species interaction, it may be unwarranted to assume that the outcomes of competition at one time represent the complex relationships regulating community interactions.