Conspecific Plants Research Articles

Shared mycorrhizal networks of forest herbs: Does the presence of conspecific and heterospecific adult plants affect seedling growth and nutrient acquisition?

Arbuscular mycorrhizal fungi can colonize the roots of a large number of plant species and individuals, simultaneously creating a network of hyphae connecting the roots of many plants. In this study, I examined the effect of the mycorrhizal network produced by conspecific and heterospecific adult plants on growth and nutrient content of seedlings of Arisaema triphyllum (L.) Schott. and Maianthemum racemosum (L.) Link ssp. racemosum. Seedlings of both plant species were planted so that they could become colonized by the mycorrhizal network or remain uncolonized, and were planted in the presence of a conspecific or heterospecific adult plant. After 2 years of growth, seedlings were harvested and biomass and nutrient content were determined. Biomass of A. triphyllum seedlings was not affected by the identity of the adult plant or colonization by mycorrhizal fungi. Biomass of M. racemosum seedlings was significantly affected by adult plant identity and mycorrhizal colonization. Maianthemum racemosum seedlings grown near a conspecific adult were significantly larger than when grown near a heterospecific adult, and mycorrhizal M. racemosum seedlings grown near a heterospecific adult had significantly less biomass than nonmycorrhizal seedlings. This suggests that the mycorrhizal network of some plants may confer positive or negative effects on mycorrhizal seedlings as compared to nonmycorrhizal seedlings.

Volatile induction of infected and neighbouring uninfected plants potentially influence attraction/repellence of a cereal herbivore

AbstractPathogen infection can induce plant volatile organic compounds (VOCs). We infected ‘McNeal’ wheat and ‘Harrington’ barley with a Fusarium spp. blend (F. graminearum,F. avenaceum and F. culmorum). Both cereals had the greatest VOC induction 14 days after pathogen innoculation, only slightly lower induction occurred at 7 days, but displayed no induction at 1 days. The induced VOC bouquet for both cereals included six green leaf volatiles (GLVs; e.g. (Z)‐3‐hexenol and (Z)‐3‐hexenyl acetate), four terpenes (linalool, linalool oxide, (Z)‐β‐ocimene and (E)‐β‐caryophyllene) and benzyl acetate. Neighbouring, uninfected individuals of both cereals had significant VOC induction when exposed to an infected, conspecific plant. The temporal pattern and VOC blend were qualitatively similar to infected plants but with quantitative reductions for all induced VOCs. The degree of neighbouring, uninfected plant induction was negatively related to distance from an infected plant. Plant VOC induction in response to pathogen infection potentially influences herbivore attraction or repellency. Y‐tube tests showed that herbivorous female and male Oulema cyanella Voet. (Chrysomelidae: Coleoptera) were significantly attracted to (Z)‐3‐hexenal and (Z)‐3‐hexenyl acetate at 300 and 1500 ng/h but were repelled by both GLVs as well as (Z)‐β‐ocimene and linalool at 7500 ng/h. These O. cyanella behavioural responses were significantly at higher concentrations than those emitted by single plants with pathogen‐induced VOCs, so adults might only be able to respond to a dense group of infected plants. Also, O. cyanella dose responses differ from the previously tested congeneric O. melanopus (cereal leaf beetle), which was attracted to three VOCs induced by Fusarium infection of maize, barley and wheat. Future behavioural tests may indicate whether different herbivore dose responses measured with each VOC singly can help to predict attraction or repellency to injured and uninjured VOC bouquets from different host plant species.

Gender specialization in Palicourea demissa (Rubiaceae), a distylous, hummingbird-pollinated treelet

Distyly has been interpreted as a mechanism that promotes cross-pollination among conspecific plants and as one of the routes leading to the evolution of dioecy. In one of the possible evolutionary pathways, pollinators may disrupt intermorph pollen flow, and, as a consequence, floral morphs may gradually specialize as either male or female (functional dioecy). Natural patterns of pollen deposition and fruit and seed production were estimated in Palicourea demissa (Rubiaceae) and used as parameters to assess functional gender differences between floral morphs. Pollen flow was asymmetrical in P. demissa. Long-styled flowers were more effective than short-styled flowers in pollen deposition towards compatible stigmas, whereas short-styled flowers were more effective in legitimate pollen receipt. Accordingly, short-styled plants produced more fruits and viable seeds than long-styled plants. The contributions of male and female function to the potential functional gender were equivalent in both morphs. However, the realized functional gender deviated significantly from the potential functional gender in both morphs, in which short-styled plants were more successful through their female function, but long-styled plants through their male function. If pollinators disrupt the complementarities of pollen transfer between the two morphs (asymmetric pollen flow), the expression of a more profitable gender is expected in each morph. Thus, our results support the hypothesis that dioecy may evolve in distylous populations through the gradual specialization of each morph as either male or female.

Color and Scent: How Single Genes Influence Pollinator Attraction

A major function of angiosperm flowers is the recruitment of animal pollinators that serve to transfer pollen among conspecific plants. Distinct sets of floral characteristics, called pollination syndromes, are correlated with visitation by specific groups of pollinators. Switches among pollination syndromes have occurred in many plant families. Such switches must have involved coordinated changes in multiple traits and multiple genes. Two well-studied floral traits affecting pollinator attraction are petal color and scent production. We review current knowledge about the biosynthetic pathways for floral color and scent production and their interaction at the genetic and biochemical levels. A key question in the field concerns the genes that underlie natural variation in color and scent and how such genes affect pollinator preference, reproductive isolation, and ultimately speciation.

Photosynthesis and productivity in heterogeneous arctic tundra: consequences for ecosystem function of mixing vegetation types at stand edges

Summary1. Arctic vegetation tends to be spatially heterogeneous and can have large areas of mixed ‘transition zone’ vegetation between stands dominated by a single or few species. If plant photosynthesis and growth within these transition zones differs significantly from main vegetation stands, and if transition zones are not considered when extrapolating stand‐level findings to larger scales in space, then transition zones will provide considerable error to landscape‐level estimates of gross primary productivity (GPP).2. In a heterogeneous sub‐Arctic tundra landscape, we undertook a detailed assessment of plant and ecosystem photosynthesis and plant growth in stands dominated by the short‐stature evergreen dwarf shrub Empetrum hermaphroditum, the deciduous dwarf shrub Betula nana, the taller deciduous shrub Salix glauca and also the transition zones between them.3. Our findings show that plants in transition zones towards taller and more productive vegetation types frequently showed reduced shoot growth, equal or reduced light‐saturated photosynthesis (Pmax) and other typical shade responses (e.g. increased leaf chlorophyll and leaf area per mass) when compared with conspecific plants in main stands where the species is dominant. Critically, whole‐ecosystem GPP per leaf area was 20–40% lower in transition zones than in main vegetation stands as a consequence. A modelling analysis suggests that the under‐productivity of some transition zones results from the lack of a clear ‘winner’ in the competition for light, such that active leaves of some species are shaded by relatively inactive leaves of others.4. These findings highlight how biotic interactions can considerably influence plant performance to the extent that productivity of mixed vegetation (transition zones) cannot be predicted from their main stands either side. How the consequences of mixing vegetation relate to mechanisms in biodiversity‐function theory is discussed.5. Synthesis: Our work shows that the productivity of transition zones of arctic vegetation is considerably lower than may be estimated from the main stands on either side. This reduced GPP in transition zones, therefore, must be considered when modelling carbon fluxes at the landscape scale and suggests that the impact of transition zones on ecosystem function needs further investigation in heterogeneous landscapes, where they make up a significant proportion of the land cover.

Competition versus facilitation: conspecific effects on pollinator visitation and seed set in the iris Lapeirousia oreogena

Ecological interactions between conspecific plants can range from facilitative to competitive depending on the spacing and abundance of individuals. Competitive interactions are expected when plants flower en‐masse and availability of pollinators is limited. We tested this prediction using Lapeirousia oreogena, a mass‐flowering South African iris that is pollinated by a single species of long‐proboscid fly. Controlled hand‐pollination experiments showed that L. oreogena is self‐compatible, but reliant on pollinator visits for seed set. Seed production per flower showed a significant negative relationship with patch size (and the correlated measure of number of individuals per patch), but was not affected by flower density or distance to neighbouring patches. There was a tendency for fly abundance to increase with patch size, but the rate of visits to individual flowers by flies was not affected by patch size. Seed set of hand‐pollinated flowers did not differ for plants in and out of dense patches, indicating that the large differences in seed set among patches were likely to reflect pollinator visits, rather than the genetic or physiological capacity of plants to produce seeds. The reduced fecundity of L. oreogena in large patches with a greater numbers of flowers is consistent with the idea that plants with highly specialized pollination systems can experience intra‐specific competition for pollination.

Herbivore-induced maize volatiles serve as priming cues for resistance against post-attack by the specialist armyworm Mythimna separata

Plants show a diversity of defense strategies against a wide range of herbivores. The emission of a specific blend of volatiles in response to herbivory (HIPVs – herbivore-induced plant volatiles) plays a great ecological role in indirect protection of the plants by attracting natural enemies of herbivores and priming their neighboring plants that can cause poorer foraging habitats on the ‘receiver’ plants. Maize plants showed induction of a blend of volatiles including monoterpenes, sesequiterpenes, homoterpenes, green leaf volatiles, and indole, in response to the specialist herbivore Mythimna separata. Conspecific plants placed downwind of infested maize plants showed reduced larval development after exposure to HIPVs, compared to those exposed to volatiles from uninfested plants. These findings indicate that exposure of maize plants to HIPVs primed these plants for enhanced defense responses to specialist herbivores of Poaceae.

Specific Regulation of Pyrethrin Biosynthesis in Chrysanthemum cinerariaefolium by a Blend of Volatiles Emitted from Artificially Damaged Conspecific Plants

Plants emit specific blends of volatile organic compounds (VOCs) in response to mechanical wounding. Such induced VOCs have been shown to mediate in plant and interplant communication, yet little is known about the time- and dose-response relationships in VOC-mediated communications. Here, we employed young seedlings of Chrysanthemum cinerariaefolium to examine the effects of volatiles emitted by artificially damaged seedlings on the biosynthesis of the natural insecticides pyrethrins in intact conspecific plants. Wounded leaves emitted (Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate and (E)-β-farnesene as dominant wound-induced VOCs. Exposing intact seedlings to a mixture of these VOCs at concentrations mimicking those emitted from wounded seedlings, as well as placing the intact seedlings next to the wounded seedlings, resulted in enhanced pyrethrin contents in the intact seedlings. Thus we quantified mRNA transcripts of 1-deoxy-D-xylulose 5-phosphate synthase (DXS), chrysanthemyl diphosphate synthase (CPPase), 13-lipoxygenase (13-LOX) and allene oxide synthase (AOS) genes in intact seedlings exposed to the VOC mixture to show that DXS and 13-LOX gene expression reached a maximum at 3 h, whereas CPPase and AOS reached it at 6 h. Interestingly, both increasing and decreasing the VOC mixture concentrations from those observed on injury reduced the expression of DXS, CPPase and AOS genes to the control level. Also, separating the VOC mixture into individual components eliminated the ability to enhance the expression of all the biosynthetic genes examined. This is the first study showing that the wound-induced VOCs function as a blend to control the biosynthesis of second metabolites at specific concentrations.

Conspecific plant–soil feedbacks reduce survivorship and growth of tropical tree seedlings

Summary 1. The Janzen–Connell (J–C) Model proposes that host‐specific enemies maintain high tree species diversity by reducing seedling performance near conspecific adults and promoting replacement by heterospecific seedlings. Support for this model often comes from decreased performance for a species at near versus far distances from conspecific adults. However, the relative success of conspecific versus heterospecific seedlings recruiting under a given tree species is a critical, but untested, component of the J–C Model. 2. In a shade‐house experiment, we tested plant–soil feedbacks as a J–C mechanism in six tropical tree species. We assessed effects of conspecific versus heterospecific cultured soil extracts on seedling performance for each species, and we compared performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. Additionally, we tested whether soil microbes were creating these plant–soil feedbacks and whether low light increased species vulnerability to pathogens. 3. Among 30 potential comparisons of survival and mass for seedlings grown in conspecific versus heterospecific soil extracts, survival decreased in seven and increased in two, whereas mass decreased in 13 and increased in 1. To integrate survival and growth, we also examined seedling performance [(mean total mass × mean survival time)/(days of experiment)], which was lower in 16 and higher in 2 of 30 comparisons between seedlings grown with soil extract cultured by conspecific versus heterospecific individuals. Based on performance within a soil extract, conspecific seedlings were disadvantaged in 15 and favoured in 7 of 30 cases relative to heterospecific seedlings. 4. Species pairwise interactions of soil modification and seedling performance occurred regardless of sterilization, suggesting chemical mediation. Microbes lacked host‐specificity and reduced performance regardless of extract source and irradiance. 5. Synthesis. These results, along with parallel research in temperate forests, suggest that plant–soil feedbacks are an important component of seedling dynamics in both ecosystems. However, negative conspecific feedbacks were more prevalent in tropical than temperate species. Thus, negative plant–soil feedbacks appear to facilitate species coexistence via negative distance‐dependent processes in tropical but not temperate forests, but the feedbacks were mediated through chemical effects rather than through natural enemies as expected under the J–C Model.

Species properties and recovery from disturbance: Forest herbs buried by volcanic tephra

AbstractQuestion: How do forest herb species differ from each other in their spatial and temporal dynamics during recovery from volcanic disturbance, and how are dynamics related to species traits?Location: Northeast of Mount St. Helens, Washington, USA.Methods: Following deposition of volcanic tephra in 1980, we measured herb density and cover in permanent 1‐m2 plots during 1980‐2005 in three old‐growth forests with differing tephra depths. For 26 species, we calculated the frequency of plots with residuals (individuals that survived the eruption) versus re‐establishment on the tephra, timing of establishment, turnover, influence of nearby conspecific plants, importance of three components of cover increase, and relationship of flowering frequency to succession.Results: Tephra depth affected species behavior. Deep tephra produced fewer residuals, a greater increase in shoot size, and more shoot turnover; favored species that established late; and allowed establishment of early seral herbs. Nearby presence of conspecifics increased permanence of first establishment and rate of plot occupancy. Most species spread significantly in deep tephra but not in shallow tephra. Among species, frequency of flowering increased with fewer residuals, later establishment, and higher turnover. Species behavior seldom differed among growth forms based on leaf longevity and vegetative spread.Conclusion: Population dynamics at the small‐plot scale differed from those of the entire population. The timing and permanency of establishment and mechanism of expansion differed among species and with tephra depth. There was some consistency among species with similar habitat breadth and degree of flowering, but little consistency associated with the usual growth form classification.

Relationship between heavy metal concentrations of herbaceous plants and soils at four Pb-Zn mining sites in Yunnan, China

This paper studied the relationship between heavy metal concentrations of herbaceous plants and soils at four Pb-Zn mining sites in Yunnan, China. 50 herbaceous plant samples of 9 plant species from 4 families and 50 soil samples were collected and then ana1yzed for the tota1 concentrations of Pb, Cd, and Zn. The results showed that the average concentrations of Pb, Cd, and Zn in soil samples were 3772.83, 168.81, and 5385.65 mg/kg, respectively. The average concentrations of Pb, Cd, and Zn were 395.68, 28.14, and 1664.20 mg/kg in the shoots, and 924.12, 57.25, and 1778.75 mg/kg in the roots, respectively. Heterospecific plants at the same site and conspecific plants at various sites had different average levels of Pb, Cd, and Zn, both in the shoots and the roots. Enrichment coefficients of Pb, Cd, and Zn were greater than 1 in 2, 3, and 9 herbaceous plant samples, respectively. Translocation factors of Pb, Cd, and Zn were greater than 1 in 10, 17, and 25 herbaceous plant samples, respectively. In all 50 samples, the concentrations of Pb, Cd, and Zn between the shoots and the roots, the shoots, and the soils, and the roots and the soils had significant positive relationships.

Phylogenetic affinity of arbuscular mycorrhizal symbionts in Psilotum nudum

Many lineages of land plants (from lycopsids to angiosperms) have non-photosynthetic life cycle phases that involve obligate mycoheterotrophic arbuscular mycorrhizal (AM) associations where the plant host gains organic carbon through glomalean symbionts. Our goal was to isolate and phylogenetically identify the AM fungi associated with both the autotrophic and underground mycoheterotrophic life cycle phases of Psilotum nudum. Phylogenetic analyses recovered 11 fungal phylotypes in four diverse clades of Glomus A that form AM associations with P. nudum mycoheterotrophic gametophytes and autotrophic sporophytes, and angiosperm roots found in the same greenhouse pots. The correspondence of identities of AM symbionts in P. nudum sporophytes, gametophytes and neighboring angiosperms provides compelling evidence that photosynthetic heterospecific and conspecific plants can serve as the ultimate sources of fixed carbon for mycoheterotrophic gametophytes of P. nudum, and that the transfer of carbon occurs via shared fungal networks. Moreover, broader phylogenetic analyses suggest greenhouse Psilotum populations, like field-surveyed populations of mycoheterotrophic plants, form AM associations with restricted clades of Glomus A. The phylogenetic affinities and distribution of Glomus A symbionts indicate that P. nudum greenhouse populations have the potential to be exploited as an experimental system to further study the physiology, ecology and evolution of mycoheterotrophic AM associations.

Resource and non‐resource root competition effects of grasses on early‐ versus late‐successional trees

Summary This study assessed the effects of resource (i.e. nutrients) and non‐resource (i.e. interference for space) competition from fine roots of competing grasses on the growth, morphology and architecture of fine roots of four tree species of varying successional status: Populus deltoides ¥ P. balsamifera (a hybrid), Betula papyrifera, Acer saccharum and Fraxinus americana. We tested the general hypothesis that tree fine‐roots are affected by both below‐ground resource and non‐resource competition from non‐self plants, and the more specific hypothesis that this effect is stronger in early‐successional tree species. The experiment was conducted in split‐containers where half of the roots of tree seedlings experienced either below‐ground resource competition or non‐resource competition, or both, by grasses while the other half experienced no competition. The late‐successional tree species A. saccharum and F. americana were mostly affected by resource competition, whereas the early‐successional P. deltoides¥balsamifera and B. papyrifera were strongly affected by both resource and non‐resource competition. Non‐resource competition reduced fine‐root growth, root branching over root length (a measure of root architecture) and specific root length (a measure of root morphology) of both early‐successional species. Synthesis. This study suggests that early‐successional tree species have been selected for root avoidance or segregation and late‐successional tree species for root tolerance of competition as mechanisms to improve below‐ground resource uptake in their particular environments. It also contradicts recent studies showing perennial and annual grasses tend to overproduce roots in the presence of non‐self conspecific plants. Woody plants, required to grow and develop for long periods in the presence of other plants, may react differently to non‐self root competition than perennial or annual grasses that have much shorter lives.

Density‐dependence at multiple scales in experimental and natural plant populations

Summary The survival and reproduction of individual plants may be related to the distribution of conspecific neighbours with which they interact. We used 2 years’ data from a field experiment to relate the reproduction and survival of focal Silene latifolia plants to the numbers of conspecific plants and flowers within a range of distances (0.16–150 m) from the focal plants. Thus we consider spatial scale as a continuously‐varying component of population density. Because the data come from a single continuous population, we fitted regression models in two stages in order to remove spatial autocorrelation induced by environmental factors before modelling density effects. As well as single‐scale models, we fitted multiple‐scale models to explore interaction kernels. We statistically standardized plant size in some analyses to isolate pollination effects. Reproductive output per plant generally decreased with greater densities of plants and flowers; for example there was a negative effect of female flowers within a 70‐m radius in the second year. However, at low female densities there was a positive effect of male flowers at a scale of 13 m in the first year. Seed‐set per flower decreased with greater densities of flowers in the second year (at radii < 10 m). Seed predation was greater when more female flowers were present within 5–7 m. Plants’ survival probability increased with density within 0.28 m for all but the smallest plants. We also collected data on the pattern and reproductive success of S. latifolia plants in a natural population nearby and applied similar methodology. Here the negative effects of female flower densities occurred at finer scales and gave way to positive effects above 2 m, contrasting with some patterns from the experiment. Synthesis. In a sessile population interacting with mobile biota, the effects of density on individual fitness may vary with the way in which density is measured. This, together with the methodological challenges for analysing spatial data from continuous populations, should be considered when designing and interpreting studies of spatial density‐dependence. Taking these issues into account, we may begin to explain the spatial patterns of natural populations.

Experimental Assessment of Factors Limiting Seedling Recruitment of an Amazonian Understory Herb

ABSTRACTUnderstory herbs are an important and species‐rich component of tropical forests, but little is known about factors limiting recruitment of these herbs. In a Peruvian flood plain forest, we studied seedling recruitment of the widespread clonal herb Heliconia metallica. We analyzed natural recruitment and experimentally added seeds, excluded predators and removed the understory vegetation at occupied and unoccupied sites to test whether seed limitation, predation, disturbance, and the presence of mature conspecific plants influence seedling recruitment. The number of naturally recruited seedlings was higher at flooded sites and close to flowering H. metallica ramets. Predation caused considerable seed loss shortly after sowing. Seed survival strongly increased with the openness of a site, but was not affected by the presence of mature conspecifics. Seed addition increased the number of seedlings at both occupied and unoccupied sites. Two years after sowing, the number of established seedlings and their size increased with light availability at a site. In experimentally disturbed plots, light availability, seedling establishment, and survival were higher. The effect of disturbance on seedling survival was stronger at occupied sites, probably due to strong light competition from mature H. metallica plants. Herbivore damage on leaves of seedlings was lower at occupied sites, but seedlings grew faster at unoccupied sites. We conclude that seedling recruitment of H. metallica depends on natural disturbance and is limited by both seed availability and the presence of mature conspecific plants.

Response of the shot-hole borer of tea, Xyleborus fornicatus (Coleoptera: Scolytidae) to conspecifics and plant semiochemicals

The responses of the tea shot-hole borer beetle, Xyleborus fornicatus, to conspecific and host plant odours were tested in an olfactometer. Males showed greater attraction to unmated females than to mated females and mated females showed a negative density dependent response with extremely significant repulsion shown when 10 mated females were present. Plants of the susceptible tea cultivar TRI2025 were significantly more attractive to females than those of the less susceptible cultivar TRI2023, although stem extracts of the susceptible cultivar were not found to be attractive to beetles. Beetles were found to be attracted to ethanol and to the known tea volatiles eugenol, hexanol, α- and β-pinene, geraniol, and methyl salicylate. The attraction of the tea volatiles was found to be enhanced when they were mixed with ethanol, indicating that ethanol has a broad synergistic effect. In a study on phagostimulant effects of tea extracts, sugars, and caffeine, the methanol extract of cultivar TRI2025 and the sugars glucose and sucrose were found to be attractive, and caffeine repulsive, to beetles. Pellets containing the extract were found to be preferred over those containing glucose. Beetles were attracted to pellets containing glucose/inositol mixture with glucose:inositol in the 3:1 ratio found in the susceptible cultivar TRI2025, being preferred to the mixture in 5:1 ratio found in stems of the less susceptible cultivar TRI2025. Both volatile and nonvolatile constituents of tea appear to play a role in attracting the female beetle to the plant.

High nocturnal CO2 emanation guides the sand fly Phlebotomus papatasi to sugar‐rich plants

Abstract In arid areas, Phlebotomus papatasi obtains essential carbohydrates by feeding on green tissues of plants. There is a great variation in sugar content, metabolic rates and dark respiration between conspecific plants, and also between branches and leaves of the same plant. It is hypothesized that high nocturnal release of CO2 may guide the sand flies to rich sugar sources. Comparisons of Ph. papatasi feeding on branches of different plant species demonstrate a significant positive correlation between the level of sugar in leaves, the rate of sand fly feeding on them and the size of ingested sugar meals. Mean nocturnal CO2 emanation of low‐sugar Ricinus communis branches is 26.5 ppm and that of sugar rich branches is 86.3 ppm above room level. Low‐sugar Capparis spinosa branches release 45.0 ppm and the emission from sugar rich branches is 76.0 ppm above room level. Branches with similar emissions, placed behind net partitions, are used in no‐choice orientation experiments: R. communis branches releasing high CO2 levels are approached by 53.7% of the flies compared with 3.0% of flies that orientate to low CO2 branches. Capparis spinosa with high CO2 emission are approached by 9.8% of the flies compared with 0.95% of flies that orientate to low CO2 branches.

Influence from Hydrological Modification on Energy and Nutrient Transference in a Deltaic Food Web

Alterations of hydrology are known to trigger changes in coastal ecosystems, such as the composition and abundances of local flora and fauna. It is less well known how these alterations lead to changes in nutrient and energy transfers in these systems. We used comparisons of stable isotope signatures (δ13C, δ15N, and δ34S) in the tissues of conspecific plants and animals collected on each side of the Mobile Bay Causeway to determine the extent to which the causeway may have altered energy and nutrient exchange between the Mobile–Tensaw Delta and upper Mobile Bay. While the δ13C signatures of most plants and animals varied irrespective of their location relative with causeway location, their δ15N and δ34S signatures were almost always more enriched south of the causeway, indicating significant alterations of trophic linkages within this estuarine food web. Dual isotope plots and mixing model analyses indicated that while terrestrial and floating plants were trophically important to consumers north of the causeway, submerged aquatic vegetation was more important to consumers south of the causeway. Although limited in spatial and temporal scale, our results preliminarily show that there are noteworthy differences in stable isotope signatures most likely due to the Mobile Bay Causeway altering energy and nutrient transference.

The plot thickens: does low density affect visitation and reproductive success in a perennial herb, and are these effects altered in the presence of a co-flowering species?

Plants may experience reduced reproductive success at low densities, due to lower numbers of pollinator visits or reduced visit quality. Co-occurring plant species that share pollinators have the potential to facilitate pollination by either increasing numbers of pollinator visits or increasing the quality of visits, but also have the potential to reduce plant reproductive success through competition for pollination. I used a field experiment with a common distylous perennial (Piriqueta caroliniana) in the presence and absence of a co-flowering species (Coreopsis leavenworthii) in plots with one of four different distances between conspecific plants. I found strong negative effects of increasing interplant distance (related to conspecific density) on several components of P. caroliniana reproductive success: pollinator visits to plants per plot visit, visits received by individual plants, conspecific pollen grains on stigmas, outcross pollen grains on stigmas, and probability of fruit production. Although P. caroliniana and C. leavenworthii share pollinators, the co-flowering species did not affect visitation, pollen receipt or reproductive effort in P. caroliniana. Pollinators moved very infrequently between species in this experiment, so floral constancy might explain the lack of effect of the co-flowering species on P. caroliniana reproductive success at low densities. In co-occurring self-incompatible plants with floral rewards, reproductive success at low density may depend more on conspecific densities than on the presence of other species.

Factors affecting seed predation of Eriotheca gracilipes (Bombacaceae) by parakeets in a cerrado fragment

a b s t r a c t Psittacids are important pre-dispersal seed predators. However, little is known about the parameters that may determine seed predation rates by these birds, such as plants’ characteristics and microhabitat. Eriotheca gracilipes (Bombacaceae) is a semi-deciduous tree widely distributed in the Brazilian cerrado. The fruits are dehiscent pods and the seeds are wind-dispersed. Some individuals lose their leaves during the fruiting season, getting very conspicuous. Here we tested the hypothesis that the absence of leaves in E. gracilipes during the fruiting season may increase pre-dispersal seed predation by psittacids. We also tested the hypotheses that (1) seed predation intensity increases with increasing plant size and (2) number of fruits, (3) seed predation decreases with the increasing number of conspecific plants in a range of 15 m, and (4) seed predation intensity is lower in plants with higher vegetation cover over their crowns. The small parakeet Brotogeris versicolurus was the only species observed preying upon the seeds of E. gracilipes. The percentage of fruits damaged by the parakeets ranged from 0 to 100% (66.98 ! 43.11%, n 72) among the different plants. Our data give weak support to the hypothesis that the absence of leaves may facilitate plants and/or fruits detection by the parakeets. However, seed predation intensity was significantly affected by crop size. The hypothesis that conspecific fruiting plants surrounding the studied individuals may reduce predation rate was not supported. Nevertheless, trees without higher vegetation cover over their crowns were significantly affected by increased seed predation. This suggests that seed predation by parakeets can be a potential selective factor influencing fruit crop sizes in E. gracilipes.