Induced responses in three tropical dry forest plant species – direct and indirect effects on herbivory

Induced changes in plant quality are hypothesized to reduce herbivore numbers and subsequent damage to the plant. The resultant decrease in herbivory may be due to direct negative impacts on herbivores, through the reduction in foliage quality as food, or due to indirect effects of plant-induced traits interacting with the third trophic level, increasing predation and parasitism rates on herbivores. The relative importance of induced responses as direct and/or indirect defenses has not been evaluated in natural systems. Moreover, few studies have evaluated the influence of early-season damage on late-season herbivory in natural systems, particularly in the tropics. The presence of induced responses and subsequent impact on folivory as a consequence of early-season damage were evaluated in three plant species ( Croton pseudoniveus , Bursera instabilis and Piper stipulaceum ) in a tropical dry forest in Mexico. A two-factorial experiment was applied to determine if induced responses influenced subsequent herbivory directly, by reducing foliage quality, or indirectly, through their interaction with parasitoids and predatory arthropods. Plants from all three species with reduced early-season damage had higher herbivory rates through the rest of the growing season, compared to plants that were damaged during leaf expansion. Chemical analyses showed that early-season damage induced the production of total phenolics and condensed tannins for C. pseudoniveus and B. instabilis , respectively. The mechanism by which these compounds affected subsequent herbivory was most likely by directly reducing foliage quality as food for herbivores, given that predatory arthropods and parasitoids had no effects on herbivory in this study. I conclude that early-season damage in these three species influenced later-season herbivory through the induction of plant responses that may act to reduce plant quality as food for herbivores.     

Disentangling direct and indirect effects of a legume shrub on its understorey community

Direct and indirect interactions among plants contribute to shape community composition through above‐ and belowground processes. However, we have not disentangled yet the direct and indirect soil and canopy effects of dominants on understorey species. We addressed this issue in a semi‐arid system from southeast Spain dominated by the legume shrub Retama sphaerocarpa. During a year with an exceptionally dry spring, we removed the shrub canopy to quantify aboveground effects and compared removed‐canopy plots to open plots between shrubs to quantify soil effects, both with and without watering. We added a grass removal treatment in order to separate direct from indirect shrub effects and quantified biomass, abundance, richness and composition of the forb functional group. With watering, changes in forb biomass were primarily driven by indirect shrub effects, with contrasting negative soil and positive aboveground indirect effects; changes in forb abundance and composition were more influenced by direct shrub soil effects with contrasting species composition between open and Retama patches. As community composition was different between open and Retama patches the indirect effects of Retama on forb species did not concern forbs from the open community but forbs from Retama patches. Indirect effects are, thus, important at the functional group level rather than at the species level. Without watering, there were no significant interactions. Changes in species richness between treatments were weak and seldom significant. We conclude that shrub effects on understorey forbs are primarily due to their influence on soil properties, directly affecting forb species composition but indirectly affecting the biomass of the forbs of the Retama patches, and only with sufficient water.     

Impact of floral traits on the reproductive success of epiphytic and terrestrial tropical orchids

We investigated the relationship between habit, population size, floral traits and natural fruit set levels of 23 tropical orchid species of south-east Bangladesh. We showed that epiphytic orchids had lower fruit set levels than terrestrial species and that habit explained much of the variation in floral traits among the orchids. We compared our results with data from 76 other species occurring in the study area and hypothesize that a suite of floral and population characteristics present in tropical orchids combine in epiphytes to reduce their reproductive success. Characteristics which, in addition to their habit, are associated with low reproductive success are small population size, small inflorescences, non-sectile pollinia and self-incompatibility. Several of these characteristics were phylogenetically conserved and we predict that epiphytes might therefore generally have lower fruit set levels than recorded in terrestrial species. Nectar rewards are uncommon in tropical orchids and nectarless species have displays of larger flowers, which may represent an adaptation to increase pollinator attraction, although other rewards such as oils, waxes and pseudo pollen may replace nectar. We suggest that, like many temperate orchids, a high proportion of tropical orchids may lack floral rewards and be pollinated by deceit.     

Components of pollination effectiveness in Psychotria suerrensis,a tropical distylous shrub

In this paper I report components of effectiveness for pollinators of a tropical distylous shrub, Psychotria suerrensis (Rubiaceae), which is visited by a variety of bees, wasps, and butterflies, and by two species of hummingbirds. In the field, I measured the following components of effectiveness: frequency of visits, evenness of visits across plants, and diurnal pattern of visits. I also used flight-cage experiments to compare pollentransfer abilities of euglossine bees and heliconiid butterflies. Euglossine bees visited more frequently, visited earlier in the day, and visited a higher proportion of plants in the population than did other taxa. In flight cage experiments, bees and butterflies transferred similar amounts of pollen overall, but bees transferred significantly more inter-morph (compatible) pollen. For each component measured, euglossine bees appeared to be the most effective pollinators.     

Direct and indirect effects of pollinators and seed predators to selection on plant and floral traits

Although flowering traits are often assumed to be under strong selection by pollinators, significant variation in such traits remains the norm for most plant species. Thus, it is likely that the interactions among plants, mutualists, and other selective agents, such as antagonists, ultimately shape the evolution of floral and flowering traits. We examined the importance of pollination vs pre-dispersal seed predation to selection on plant and floral characters via female plant-reproductive success in Castilleja linariaefolia (Scrophulariaceae). C. linariaefolia is pollinated by hummingbirds and experiences high levels of pre-dispersal seed predation by plume moth and fly larvae in the Rocky Mountains of Colorado, USA, where this work was conducted. We first examined whether female reproduction in C. linariaefolia was limited by pollination. Supplemental pollination only marginally increased components of female reproduction, likely because seed predation masked, in part, the beneficial effects of pollen addition. In unmanipulated populations, we measured calyx length, flower production, and plant height and used path analysis combined with structural equation modeling to quantify their importance to relative seed set through pathways involving pollination vs seed predation. We found that the strength of selection on calyx length, flower production, and plant height was greater for seed predation pathways than for pollination pathways, and one character, calyx length, experienced opposing selection via pollination vs seed predation. These results suggest that the remarkable intraspecific variation in plant and floral characters exhibited by some flowering plants is likely the result of selection driven, at least in part, by pollinators in concert with antagonists, such as pre-dispersal seed predators. This work highlights the subtle but complex interactions that shape floral and vegetative design in natural ecosystems.     

Competition for pollination influences selection on floral traits of Ipomopsis aggregata

Although rarely tested, it is often assumed that interspecific competition results in the divergence of traits related to resource use. Using a plant-pollinator system as a model, I tested the prediction the presence of a competitor for pollination influences the strength and/or direction of pollinator-mediated selection on floral traits. I measured phenotypic selection via female fitness on five floral traits of Ipomopsis aggregata in seven populations. Four contained only conspecifics (I only) and three also contained the competitor Castilleja linariaefolia (C + I). Directional selection via fruits/plant and conspecific pollen deposited/flower on corolla length was positive and significantly stronger in C + I populations. This difference in selection was apparently driven by interpopulation variation in the degree to which reproduction of I. aggregata was pollen limited. Consistent with expectations of interspecific competition, I. aggregata plants in C + I populations received less conspecific pollen per flower and set fewer seeds per fruit and fruits per plant than those in I only populations. Ipomopsis aggregata's corollas were also significantly longer in C + I populations, suggesting that there had been a response to a similar selective regime in past generations. Phenotypic correlations between corolla length and width, which determine the variation in I. aggregata's flower shape, were significantly weaker in C + I populations. These data suggest that competition for pollination can influence the strength of selection on and patterns of correlations among floral traits of I. aggregata. If I. aggregata populations with and without competitors for pollination are linked by gene flow, then measuring selection in competitive and noncompetitive environments maybe necessary to accurately predict how floral traits will evolve.     

Effects of florivory on floral volatile emissions and pollination success in the wild parsnip

In addition to reducing fitness by consuming reproductive structures, florivores may also reduce plant fitness by altering interactions with pollinators. To date, the effects of florivore activity on the volatile profile of flowers and subsequent attractiveness to pollinators have not been extensively investigated. In this study, we had three specific objectives: to determine the impact of florivory by the parsnip webworm Depressaria pastinacella on the floral volatile profile of the wild parsnip Pastinaca sativa, to ascertain the mechanisms by which florivory changes the volatile profile, and to estimate the consequences of florivory on visitation by pollinators and eventual seed set. An overall indirect effect of webworms on seed set, that is, the effect of infestation on pollination success, was not detected. However, this overall lack of indirect effect masks the heterogeneity among individual plants. For seven of 14 plants examined, pollination success was altered by webworms, and in four of these plants the alteration in pollination success was consistent with webworm-altered visitation. Webworms significantly altered floral fragrance, in particular causing disproportionate increases in the emissions of octyl esters. Additionally, volatiles from webworm frass, which contains large amounts of the octyl ester metabolite n-octanol, may alter the floral fragrance in ways that change attractiveness of flowers to pollinators. This study suggests that the effects of florivores on plant fitness are not limited to the removal of floral units but may also involve alterations in floral volatile composition, through damage-induced release and detoxification of particular constituents, that affect visitation and pollination success. Handling Editor: Steve Johnson. An erratum to this article can be found at     

The consequences of direct versus indirect species interactions to selection on traits: pollination and nectar robbing in Ipomopsis aggregata

Organisms experience a complex suite of species interactions. Although the ecological consequences of direct versus indirect species interactions have received attention, their evolutionary implications are not well understood. I examined selection on floral traits through direct versus indirect pathways of species interactions using the plant Ipomopsis aggregata and its pollinators and nectar robber. Using path analysis and structural equation modeling, I tested competing hypotheses comparing the relative importance of direct (pollinator-mediated) versus indirect (robber-mediated) interactions to trait selection through female plant function in 2 years. The hypothesis that provided the best fit to the observed data included robbing and pollination, suggesting that both interactors are important in driving selection on some traits; however, the direction and intensity of selection through robbing versus pollination varied between years. I then increased my scope of inference by assessing traits and species interactions across more years. I found that the potential for temporal variation in the direction and intensity of selection was pronounced. Taken together, results suggest that assessing the broader context in which organisms evolve, including both direct and indirect interactions and across multiple years, can provide increased mechanistic understanding of the diversity of ways that animals shape floral and plant evolution.     

The independent and combined effects of floral traits distinguishing two pollination ecotypes of a moth‐pollinated orchid

Identifying traits and agents of selection involved in local adaptation is important for understanding population divergence. In southern Sweden, the moth‐pollinated orchid Platanthera bifolia occurs as a woodland and a grassland ecotype that differ in dominating pollinators. The woodland ecotype is taller (expected to influence pollinator attraction) and produces flowers with longer spurs (expected to influence efficiency of pollen transfer) compared to the grassland ecotype. We examined whether plant height and spur length affect pollination and reproductive success in a woodland population, and whether effects are non‐additive, as expected for traits influencing two multiplicative components of pollen transfer. We reduced plant height and spur length to match trait values observed in the grassland ecotype and determined the effects on pollen removal, pollen receipt, and fruit production. In addition, to examine the effects of naturally occurring variation, we quantified pollinator‐mediated selection through pollen removal and seed production in the same population. Reductions of plant height and spur length decreased pollen removal, number of flowers receiving pollen, mean pollen receipt per pollinated flower, and fruit production per plant, but no significant interaction effect was detected. The selection analysis demonstrated pollinator‐mediated selection for taller plants via female fitness. However, there was no current selection mediated by pollinators on spur length, and pollen removal was not related to plant height or spur length. The results show that, although both traits are important for pollination success and female fitness in the woodland habitat, only plant height was sufficiently variable in the study population for current pollinator‐mediated selection to be detected. More generally, the results illustrate how a combination of experimental approaches can be used to identify both traits and agents of selection.     

Parental effects and gender specialization in a tropical heterostylous shrub

Abstract.— Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense , thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene.     

Effects of floral herbivory, limited pollination, and intrinsic plant characteristics on phenotypic gender in Sanicula arctopoides

Sexual expression in hermaphroditic plants is often a function of environmental factors affecting individuals before or during flowering. I tested for the effects of floral herbivory and lack of pollination in early umbels on the relative proportions of hermaphroditic and staminate (male) flowers produced on later umbels by Sanicula arctopoides, a monocarpic, andromonoecious perennial. Neither floral herbivory or lack of early pollination had a significant effect on the ratio of the two floral morphs, but the probability of producing staminate flowers on late umbels was strongly and positively related to plant size measured just prior to floral initiation and prior to herbivory. Plant size was also negatively correlated with flowering date. I suggest that producing staminate flowers on late umbels should benefit large early-blooming plants more than small late-blooming plants because more mating opportunities occur during the period when these flowers release pollen. Although herbivory did not cause labile changes of sex, whole plant phenotypic gender was still strongly affected by various forms of treatment. Sex-biased herbivory or lack of pollination rendered plants more or less phenotypically male, depending on which tissues were affected. Deer and pollen-feeding mites preferentially remove male tissues while hymenopteran seed predators preferentially remove female tissues. I conclude that combinations of herbivores could have counteracting or compounding effects on plant gender, and these effects may change the rankings of male and female reproductive success within populations. Received: 20 February 1996/Accepted: 30 July 1996     

Strong direct and inconsistent indirect effects of fishing found using stereo-video: Testing indicators from fisheries closures

Candidate indicators of the direct and indirect effects of fishing can be developed by investigating fisheries closures. We tested a suite of such indicators in areas open to fishing but with suspected differences in effort, using baited remote underwater stereo-video methods. In particular, we predicted that greater fishing would result in decreased biomass of high risk target species and indirectly increase the biomass of small-bodied non-target species. As predicted, the biomass of target species was found to be greater in areas of lower fishing effort and in deeper waters. However, no indirect effects of fishing were detected and any community-level effects were driven by differences in the biomass of target species. In particular, assemblage length class analysis of covariance (ANCOVA), size spectra analysis and the abundance-biomass comparison (ABC) method did not provide any evidence of indirect effects of fishing. The magnitude of the differences in fishing effort between the two areas sampled, may be sufficient to significantly affect target fisheries species, but insufficient to lead to indirect effects on non-target populations. It is also possible that the predicted indirect effects do not occur in this assemblage, due to weak trophic linkages between species. Differences observed using the ABC method were attributed to variation in the abundance of large herbivorous fishes, which are not fished. We also found assemblage length class ANCOVA and size spectra to be insensitive to the direct effects of fishing where large numbers of non-target individuals are sampled along with fished species. We suggest diet studies and comparisons across stronger gradients in fishing pressure to further investigate the indirect effects of fishing in this assemblage.     

Reproductive success in varying light environments: direct and indirect effects of light on plants and pollinators

Plant populations often exist in spatially heterogeneous environments. Light level can directly affect plant reproductive success through resource availability or by altering pollinator behavior. It can also indirectly influence reproductive success by determining floral display size which may in turn influence pollinator attraction. We evaluated direct and indirect effects of light availability and measured phenotypic selection on phenological traits that may enhance pollen receipt in the insect-pollinated herb Campanulastrum americanum. In a natural population, plants in the sun had larger displays and received 7 times more visits than plants in the shade. Using experimental arrays to separate the direct effects of irradiance on insects from their response to display size, we found more visits to plants in the sun than in the shade, but no association between number of visits each flower received and display size. Plants in the sun were not pollen limited but pollen-augmented shade flowers produced 50% more seeds than open-pollinated flowers. Phenological traits, which may influence pollen receipt, were not under direct selection in the sun. However, earlier initiation and a longer duration of flowering were favored in the shade, which may enhance visitation in this pollen-limited habitat.     

Effect of pollination success on floral longevity in the orchid Calypso bulbosa (Orchidaceae)

The lifespan of an individual flower is often affected by pollination success. Species differ regarding whether male function (pollen removal), female function (pollen deposition), or both trigger floral senescence. We studied senescence in the singleflowered, deceptive orchid Calypso bulbosa by manipulating the degree of male and female reproductive success. We found that deposition of any amount of pollen resulted in dramatic changes in shape and color within 4 d, whereas unmanipulated flowers and those that had had pollinia removed remained unchanged for 8-11 d after treatment. Selection may favor the reproductive function that is less easily satisfied as the trigger for senescence, because a flower that senesces after accomplishment of this function is likely to have already succeeded at the more easily satisfied one. Deceptive (i.e., rewardless) flowers are more likely to satisfy male than female function since the latter requires that a pollinator be fooled twice, first to pick up pollen and second to deposit it. A survey of naturally pollinated Calypso showed that male function, pollinium removal, was more likely to occur than female function, deposition (95% vs. 66% of visited flowers); thus floral senescence in Calypso is triggered by achievement of the function less likely to succeed. Studies of senescence triggers in species in which female function is more likely to be achieved than male are necessary to further test this hypothesis.     

Leaf herbivory and drought stress affect floral attractive and defensive traits in Nicotiana quadrivalvis

Adaptive phenotypic plasticity allows sessile organisms such as plants to match trait expression to the particular environment they experience. Plasticity may be limited, however, by resources in the environment, by responses to prior environmental cues, or by previous interactions with other species, such as competition or herbivory. Thus, understanding the expression of plastic traits and their effects on plant performance requires evaluating trait expression in complex environments, rather than across levels of a single variable. In this study, we tested the independent and combined effects of two components of a plant’s environment, herbivory and water availability, on the expression of attractive and defensive traits in Nicotiana quadrivalvis in the greenhouse. Damage and drought did not affect leaf nicotine concentrations but had additive and non-additive effects on floral attractive and defensive traits. Plants in the high water treatment produced larger flowers with more nectar than in the low water treatment. Leaf damage induced greater nectar volumes in the high water treatment only, suggesting that low water limited plastic responses to herbivore damage. Leaf damage also tended to induce higher nicotine concentrations in nectar, consistent with other studies showing that leaf damage can induce floral defenses. Our results suggest that there are separate and synergistic effects of leaf herbivory and drought on floral trait expression, and thus plasticity in response to complex environments may influence plant fitness via effects on floral visitation and defense.     

Direct and indirect selection on floral pigmentation by pollinators and seed predators in a color polymorphic South African shrub

The coexistence of different color morphs is often attributed to variable selection pressures across space, time, morph frequencies, or selection agents, but the routes by which each morph is favored are rarely identified. In this study we investigated factors that influence floral color polymorphisms on a local scale in Protea, within which approximately 40% of species are polymorphic. Previous work shows that seed predators and reproductive differences likely contribute to maintaining polymorphism in four Protea species. We explored whether selection acts directly or indirectly on floral color in two populations of Protea aurea, using path analysis of pollinator behavior, nectar production, seed predation, color, morphology, and maternal fecundity fitness components. We found that avian pollinators spent more time on white morphs, likely due to nectar differences, but that this had no apparent consequences for fecundity. Instead, the number of flowers per inflorescence underpinned many of the reproductively important differences between color morphs. White morphs had more flowers per inflorescence, which itself was positively correlated with nectar production, seed predator occurrence, and total long-term seed production. The number of seeds per plant to survive predation, in contrast, was not directly associated with color or any other floral trait. Thus, although color differences may be associated with conflicting selection pressures, the selection appears to be associated with the number of flowers per inflorescence and its unmeasured correlates, rather than with inflorescence color itself.     

Floral traits and pollination systems in the Caatinga, a Brazilian tropical dry forest

BACKGROUND AND AIMS: Pollination is a critical stage in plant reproduction and thus in the maintenance and evolution of species and communities. The Caatinga is the fourth largest ecosystem in Brazil, but despite its great extent and its importance few studies providing ecological information are available, with a notable lack of work focusing on pollination biology. Here, general data are presented regarding the frequency of pollination systems within Caatinga communities, with the aim of characterizing patterns related to floral attributes in order to make possible comparisons with data for plant communities in other tropical areas, and to test ideas about the utility of syndromes. This paper also intends to provide a reference point for further studies on pollination ecology in this threatened ecosystem. METHODS: The floral traits and the pollination systems of 147 species were analysed in three areas of Caatinga vegetation in northeastern Brazil, and compared with world-wide studies focusing on the same subject. For each species, floral attributes were recorded as form, size, colour, rewards and pollination units. The species were grouped into 12 guilds according to the main pollinator vector. Analyses of the frequencies of the floral traits and pollination systems were undertaken. KEY RESULTS: Nectar and pollen were the most common floral resources and insect pollination was the most frequent, occurring in 69.9 % of the studied species. Of the entomophilous species, 61.7 % were considered to be melittophilous (43.1 % of the total). Vertebrate pollination occurred in 28.1 % of the species (ornithophily in 15.0 % and chiropterophily in 13.1 %), and anemophily was recorded in only 2.0 %. CONCLUSIONS: The results indicated that the pollination systems in Caatinga, despite climatic restrictions, are diversified, with a low percentage of generalist flowers, and similar to other tropical dry and wet forest communities, including those with high rainfall levels.     

Contrasting effects of land‐use changes on herbivory and pollination networks

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Micro-organisms behind the pollination scenes: microbial imprint on floral nectar sugar variation in a tropical plant community

Background and Aims

Variation in the composition of floral nectar reflects intrinsic plant characteristics as well as the action of extrinsic factors. Micro-organisms, particularly yeasts, represent one extrinsic factor that inhabit the nectar of animal-pollinated flowers worldwide. In this study a ‘microbial imprint hypothesis’ is formulated and tested, in which it is proposed that natural community-wide variation in nectar sugar composition will partly depend on the presence of yeasts in flowers.

Methods

Occurrence and density of yeasts were studied microscopically in single-flower nectar samples of 22 animal-pollinated species from coastal xeric and sub-humid tropical habitats of the Yucatán Peninsula, Mexico. Nectar sugar concentration and composition were concurrently determined on the same samples using high-performance liquid chromatography (HPLC) methods.

Key Results

Microscopical examination of nectar samples revealed the presence of yeasts in nearly all plant species (21 out of 22 species) and in about half of the samples examined (51·8 % of total, all species combined). Plant species and individuals differed significantly in nectar sugar concentration and composition, and also in the incidence of nectar yeasts. After statistically controlling for differences between plant species and individuals, nectar yeasts still accounted for a significant fraction of community-wide variance in all nectar sugar parameters considered. Significant yeast × species interactions on sugar parameters revealed that plant species differed in the nectar sugar correlates of variation in yeast incidence.

Conclusions

The results support the hypothesis that nectar yeasts impose a detectable imprint on community-wide variation in nectar sugar composition and concentration. Since nectar sugar features influence pollinator attraction and plant reproduction, future nectar studies should control for yeast presence and examine the extent to which microbial signatures on nectar characteristics ultimately have some influence on pollination services in plant communities.